Synthesis and electrochemical study of aromatic manganese carbonyl complexes and applications in hydrogen storage & proton reduction catalysis By Wei Dai B.S. Unitersity of Science and Technology of China 2006 A Dissertation Submitted in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy in the Department of Chemistry at Brown University Providence, Rhode Island May 2012 © Copyright 2012 by Wei Dai This dissertation by Wei Dai is accepted in its present form by the Department of Chemistry as satisfying the dissertation requirement for the degree of Doctor of Philosophy. Date_____________________ ___________________________________ Dwight A. Sweigart, Advisor Recommended to the Graduate Council Date_____________________ ___________________________________ Shouheng Sun, Reader Date_____________________ ___________________________________ Eunsuk Kim, Reader Approved by the Graduate Council Date_____________________ ___________________________________ Peter Weber, Dean of the Graduate School iii VITA Wei Dai was born on Jan. 12, 1984 in lianyungang, Jiangsu. He attended the USTC from Sep. 2002 to June 2006, graduating with a B. S. in Chemistry Department. He has been a research assistant in Dr. Yi Xie’s lab since summer in 2004, and has co-authored a few publications in peer-reviewed journals. In 2006, he was admitted to the Department of Chemistry of Brown University. In December of 2006, he began to work toward the degree of Doctor of Philosophy in the area of organometallic electrochemistry under the supervision of Professor Dwight A. Sweigart. During this time, he held positions as research and teaching assistant. During his 5 years at Brown, he published 1 paper and 2 more in process. He won the poster price in 2010 of the chemistry department. Right now he is exited about the new discovery of proton couple reduction catalyzed by aromatic manganese complexes. He will continue his academic career as an electrochemist in organometallics. iv Publications at Brown University 1. Wei Dai, Sang Bok Kim, Robert D. Pike, Christopher L. Cahill and Dwight A. Sweigart*. “Electrochemical study of manganese and rhenium arene complexes (C6R6)M(CO)3+ (R= Me, Et)”. Organometallics 2010 29(21), 5173-5178 2. Wei Dai and Dwight A. Sweigart*. “Electrocatalytic proton reduction by aromatic manganese carbonyl complex”. Manuscript in preparation 3. Wei Dai, Robert D. Pike, Dwight A. Sweigart*. “Direct synthesis and characterization of (η6-aniline) manganese tricarbonyl cation and its analogs”. Manuscript in preparation 4. Weibin Li, Gerald Kagan, Huan Yang, Chen Cai, Russell Hopson, Wei Dai, Dwight A. Sweigart and Paul G. Williard*. “Accurate Formula Weight Determination in Physically Separated Systems by Diffusion Coefficient−Formula Weight Correlation”. Organometallics 2010 29(6), 1309-1311 v Acknowledgements First of all, I am grateful to have Dr. Sweigart as my advisor. I knew nothing about organometallics before joining his group, he teached me electrochemistry of organometallics which is a very useful technique. He inspired me with great ideas about new directions of my study. Without his fully support, I could not survive through these five years. Secondly, I would like to express my gratitude to my research committee members: Dr. Sun, I appreciate your kind help as my reference, for your helpful advise in my study; Dr. Suggs and Dr. Kim, I would like to thank you for been my committee in ORP and RPD. And I wish you all the best in the future. Thirdly, I would like to thank Dr. Risen for being my reference. He is truly a nice professor to the chinese community in chemistry department. He phone interviewed me before I came to Brown and teached me advanced inorganic chemistry in my first year. You are truly missed, Bill. Also, I want to thank Lynn, Ginni, Gen, Eric, Allen. Lynn, thank you for being so kind to me. Ginni, thank you for your help for all these five years. Gen, you are missed by the whole department. Even though our group is not a rich group financially, and we haven’t been to many different places for the conferences, but we have been to a few places many many times, and we had our fun as a team in sun valley, davenports, boss’s house. Not many graduate students can really share a life with their advisors, we did and enjoyed it. Boss, to me, you are like a father. You are a person who always has a place in my vi heart for as long as I live. I wish that you have a good health and great happiness in the future. I wish all the students from Sweigart group best luck. Finally, I would like to thank my mom and dad for being there with me whenever I need them. The only thing is, I missed them so much…… vii Abstract of “Synthesis and electrochemical study of aromatic manganese carbonyl complexes and applications in hydrogen storage & proton reduction catalysis” by Wei Dai, Ph.D., Brown University, May 2012. The scope of understanding of aromatic manganese reductive electrochemistry was extended. Five internally connected projects will be introduced. My studies primarily focused on homogeneous and heterogeneous electrochemistry of organomanganese complexes and possible application in electrocatalysis, specifically, proton reduction catalysis. The mechanism of homogeneous reductive electrochemistry of [(η6-HMB)Mn(CO)3]+ was investigated: it undergoes 1-electron irreversible reduction at room temperature and 2-electron reversible reduction at low temperature. And for the first time, we observed reversibility of 2-electron reduction at room temperature by altering the electrochemical environment. An important reduction intermediate [(η6-HMB)Mn(CO)2]- was believed to catalyze proton reduction in strong acidic condition. The proton coupled reduction mechanism was discussed, based on which, optimization of this new class of Mn-catalyst was also proposed. Followed by a different mechanism, polyarene analog, (η6-naphthalene) Mn(CO)3+ can be used to transfer and store electric energy to stable “metal hydride” complex and releases the hydrogen when needed. A one-step synthetic method for direct synthesis of (η6-aniline) manganese tricarbonyl cation and its analogs was discovered. The aniline complex is an important precursor for heterogeneous electrochemistry study. The purpose of heterogeneous electrochemistry study is to achieve the reductive electrochemical viii reversibility by covalent attachment of electrode surface with arene manganese tricarbonyl moiety. Physical separation of manganese compound may stop the dimerization after one-electron reduction and result in reductive reversibility. One of my projects is focused on covalent attachment of arene manganese tricarbonyl using diazonium-base chemistry and collecting & analyzing e-chem signal from the surface functionalized electrode. The η6-aromatic manganese dicarbonyl anion is strong nucleophile. It would be interesting to check the reaction between the manganese anion with [(η6-HMB)Re(CO)3]PF6. We want to explore the possibility of forming the heterodinuclear dimmer with a novel core structure of Mn-Re metal-metal bond. There is no report about the study of Mn-Re metal-metal bond; furthermore, heterodinuclear complex has potential applications in molecular catalysis. ix Table of contents Chapter 1. Synthesis and electrochemistry study of aromatic manganese carbonyl complexes and their applications in hydrogen storage, proton reduction catalysis: General introduction ································································ 1 1.1 Overview·························································································································· 2 1.2 Electrochemistry of aromatic manganese carbonyl complex and its analogs ·················· 3 1.3 Proton reduction catalyzed by aromatic manganese carbonyl complexes ······················· 5 1.4 Direct synthesis of (η6-aniline) manganese tricarbonyl cation and its analogs ················ 8 1.5 Surface electrochemistry of aromatic manganese tricarbonyl complexes using diazonium attachment and physical attachment.·································································· 10 1.6 Synthesis and characterization of heterodinuclear complex with the core structure of Mn-Re metal-metal bond. ···································································································· 12 1.7 References······················································································································ 14 Chapter 2. General experiments ················································································ 17 2.1 Inert atmosphere work···································································································· 17 1. Nitrogen gas ··································································································· 17 2. Air-free technique··························································································· 17 1). Schlenk techniques ···················································································· 17 2). Glove box and Glove bag ·········································································· 17 3). Schlenk line. ······························································································ 18 x 3. Compound Storage ························································································· 18 2.2 Instruments and measurements ······················································································ 19 1. Liquid infrared spectroscopy measurement ···················································· 19 2. Solid infrared spectroscopy measurement ······················································ 20 3. In-situ Infrared (IR) optic probe ····································································· 21 4. Mass Spectroscopy ························································································· 22 5. Nuclear Magnetic Resonance (NMR)····························································· 22 2.3 Electrochemical method································································································· 23 1. Electrolyte ······································································································ 23 2. Synthesis of electrolyte TBAPF6 and {NBu4[B(C6F5)4]} ······························ 23 3 Test the purity of the electrolyte ······································································ 24 4. Solvent ··········································································································· 24 5. Preparation of reference electrode1 ································································· 24 6. Work Station and three-electrode system························································ 25 7. Hardware installation and instruction ····························································· 26 8. Software installation and instruction ······························································ 26 9. Conduct cyclic voltammetry at room temperature ·········································· 27 10. Conduct cyclic voltammetry at low temperature ·········································· 29 11. Interpretation of the cyclic voltammetry······················································· 29 12. The mechanism of the cyclic voltammogram (Scheme 2-1): ························ 30 13. Digital simulation of cyclic voltammgram8 ·················································· 32 14. Bulk electrolysis ··························································································· 32 xi 2.4 References······················································································································ 34 Chapter 3. Electrochemical Study of Manganese and Rhenium Arene Complexes (C6R6)M(CO)3+ (R = Me, Et)··································································· 35 3.1 Introduction···················································································································· 36 3.2 Experimental Section ····································································································· 38 1. Solvents ·········································································································· 38 2. Synthesis ········································································································ 38 3. Electrochemistry····························································································· 40 1). Cyclic voltammetry ··················································································· 40 2). Bulk electrolysis ························································································ 41 4. Crystallography ······························································································ 41 3.3 Results and Discussion··································································································· 42 1. Manganese Complexes ··················································································· 42 1). Cyclic voltammetry ··················································································· 42 2). Bulk electrolysis (BE)················································································ 43 3). Manganese dicarbonyl complexes ····························································· 48 4). Reversibility of [(η6-C6Me6)Mn(CO)3]BF4 ················································ 50 2. Rhenium Complexes ······················································································ 52 3.4 Conclusions···················································································································· 55 3.5 Supporting Information·································································································· 57 1. Digital simulation ··························································································· 57 xii 2. In-situ IR Spectroscopy and Bulk Electrolysis ··············································· 59 3.6 X-ray crystal structure data ···························································································· 63 3.7 References······················································································································ 83 Chapter 4. Proton reduction catalyzed by aromatic manganese carbonyl complexes 86 4.1 Introduction···················································································································· 87 4.2 Experiments ··················································································································· 93 1. Synthesis ········································································································ 93 2. Protonation of (η5-hydronaphthalene)Mn(CO)3 ·············································· 95 3. Electrochemistry····························································································· 95 4. Further Experiments ······················································································· 95 4.3 Result and discussion ····································································································· 96 1. Aromatic manganese dicarbonyl ···································································· 96 2. Aromatic manganese dicarbonyl ···································································· 96 3. Ion analog······································································································· 99 4. Electrochemical formation and protonation of arene-Mn-hydride complex·· 100 5. Solvent effect································································································ 105 6. Cp-Mn-NO system ······················································································· 105 7. Optimazation ································································································ 105 8. Electronic energy stored as chemical bonding mediated by olyarene-Mn(CO)3+ ····················································································································· 109 9. Hydrogen slow release mediated via (η5-H monoarene)-Mn(CO)3 ················111 xiii 4.4 Conclusions·················································································································· 112 4.5 References···················································································································· 114 Chapter 5. Direct synthesis of (η6-aniline) manganese tricarbonyl cation and its analogs····································································································117 5.2 Experiments ················································································································· 120 1. Synthesis ······································································································ 120 2. Polymerization reaction and diazotization reaction ······································ 121 3. Infrared Spectroscopy··················································································· 121 4. NMR ············································································································ 123 5. Mass spectrometry: electron spray ionization and fragmentation. ················ 125 5.3. Result and discussion ·································································································· 127 1. Reaction mechanism····················································································· 128 2. Scope············································································································ 131 3. Aromatic amine protection — remote protection·········································· 133 5.4 Conclusions·················································································································· 133 5.5 X-ray crystal structure data ·························································································· 135 5.6 References···················································································································· 148 Chapter 6. Surface electrochemistry of aromatic manganese tricarbonyl complexes using diazonium attachment and physical surface modification ············ 151 6.1 Introduction·················································································································· 151 xiv 1. The following methods might stop the dimerization:···································· 153 1). Excess CO ······························································································· 153 2). Physical attachment ················································································· 153 3). Chemical attachment ··············································································· 153 6.2 Experimental ················································································································ 157 1. Synthesis ······································································································ 157 2. Surface modification of glassy carbon electrode via reduction of diazonium salt ····················································································································· 160 3. To test the electrochemical signal from the modified electrode. ··················· 160 4. Physical attachment ······················································································ 160 6.3 Result and discussion ··································································································· 161 1. Side reaction································································································· 162 2. Direct diazotization ······················································································ 163 3. Physical attachment ······················································································ 165 6.5 References···················································································································· 169 Chapter 7. Synthesis and characterization of heterodinuclear complex with core structure of Mn-M metal-metal bond····················································· 171 7.1 Introduction·················································································································· 171 7.2 References···················································································································· 172 xv List of Figures Figure 1-1. Crystal structure of compound 31 ·························································· 10 Fiture 2-1. Glove box (left) and schlenk line (right). ················································ 18 Figure 2-2. Liquid infrared cell················································································· 20 Figure 2-3. Bolt, barrel, mortar and pestle for solid IR1············································ 21 Figure 2-4. In-situ IR probe with external detector and its tip2 ································· 21 Figure 2-5. PGSTAT100 potentiostat/ galvanostat with a standard three electrode working system ·································································································· 26 Figure 2-6. GPES software, which controls the PGSTAT100 potentiostat/ galvanostat. ··························································································································· 28 Figure 2-7. Representation of reduction of an electrochemical active species in the solution.·············································································································· 29 Figure 3-1. CVs of 1.0 mM [(η6-C6Me6)Mn(CO)3]BF4 (1a) at +14 oC (red), -20 oC (blue) and -40 oC (black) in CH2Cl2/0.10 M Bu4NPF6 under N2. The working electrode was a 2.0 mm diameter platinum disk, and the scan rate was 0.50 V s-1. A ferrocene internal standard was added (left).··················································· 42 Figure 3-2. X-ray structures of [(C6Et6)Mn(CO)3]BF4 (1b) and [(C6Et6)Re(CO)3]PF6 (2b). The average M-C(arene) bond length is 2.22(1) A for Mn and 2.35(1) A for Re. ······················································································································ 45 Figure 3-3. CVs of 1.0 mM [(η6-C6Et6)Mn(CO)3]BF4 (1b) at +20 oC (red) and -30 oC (black) in CH3CN/0.10 M Bu4NPF6 under N2. The working electrode was a 2.0 xvi mm diameter platinum disk, and the scan rate was 0.50 V s-1. A ferrocene internal standard was added (left).··················································································· 47 Figure 3-4. Electrochemistry of [(η6-C6Me6)Mn(CO)2THF]BF4 and (η6- C6Me6)Mn(CO)2Cl. The CV of both complexes was conducted in 0.1M TBAPF6 dichloromethane solution at room temperature, and scan rate was 0.5V/s. Glassy carbon electrode was used. (1) CV in black is 1mM [(η6-C6Me6)Mn(CO)3]BF4. (2) CV in blue is [(η6-C6Me6)Mn(CO)2THF]BF4 with unknown concentration. (3) CV in red is (η6-C6Me6)Mn(CO)2Cl with unknown concentration. ·························· 49 Figure 3-5. CVs of 1.0 mM [(η6-C6Me6)Mn(CO)2THF]BF4 at +20 oC (black) and -60 o C (red) in CH2Cl2/0.10 M Bu4NPF6 under N2. The working electrode was a 2.0 mm diameter platinum disk, and the scan rate was 0.50 V s-1.···························· 50 Figure 3-6. Cyclic voltammetry of 1mM 1a in 0.1M TBAPF6 dichloromethane solution with the protection gas of (1) CO (2) N2 on glassy carbon electrode at room temperature. Scan rate is 0.50V/s. ····························································· 51 Figure 3-7. CVs of 1.0 mM [(η6-C6Et6)Re(CO)3]PF6 (2b) at +20 oC (red) and -46 oC (black) in CH2Cl2/0.10 M Bu4NPF6 under N2. The working electrode was a 2.0 mm diameter platinum disk, and the scan rate was 0.50 V s-1. A ferrocene internal standard was added (left).··················································································· 54 Figure 3-8. Reductive electrochemistry of 1a··························································· 56 Figure 3-s1. Simulations A). 2a in CH2Cl2 at R.T. ; B). 2a in CH2Cl2 at -40 oC ; C). 2b in CH2Cl2 at R.T. ; D). 2b in CH2Cl2 at -46 oC ; E). 1a in CH2Cl2 at R.T. ; F). 1a in xvii CH2Cl2 at -40 oC. Pt was used as the electrode. Scan rate was 0.5V/s. The concentration of each compound is 1mM/ 0.1M Bu4NPF6. Red line is simulated CV, black line is actual CV. ················································································ 58 Figure 3-s2. A). In-situ IR spectra for the bulk electrolysis process of [1a]BF4 in CH2Cl2 with 0.1M Bu4NPF6 under N2 at R.T. Applied potentical -1.2V : Changes are labeled by red arrows. B). corresponding CV of 1mM [1a]BF4 and 1mM ferrocene (left) in CH2Cl2 with 0.1M Bu4NPF6 under N2 at R.T.························ 59 Figure 3-s3. A). In-situ IR spectra for the bulk electrolysis process of [1a]BF4 in CH2Cl2 with 0.1M Bu4NPF6 under N2 at -60 oC. Applied potential -1.4V : Changes are labeled by red arrows. B). corresponding CV of 1mM [1a]BF4 and 1mM ferrocene (left) in CH2Cl2 with 0.1M Bu4NPF6 under N2 at -60 oC····················· 60 Figure 3-s4. In-situ IR spectra for the bulk electrolysis process of [1a]BF4 in CH2Cl2 with 0.1M Bu4NPF6 under N2 at -60 oC. Applied potential +0.1V: Changes are labeled by red arrows.························································································· 60 Figure 3-s5. Bulk electrolysis [1a]BF4 in CH3CN + 0.1M Bu4NPF6 under N2 at RT. Apply -1.2V for about 15 mins.·········································································· 61 Figure 3-s6. In-situ IR spectra for the bulk electrolysis process of [1b]BF4 in CH2Cl2 with 0.1M Bu4NPF6 under N2 at -60 oC. Applied potential -1.5V: Changes are labeled by red arrows.························································································· 62 Figure 3-s7. Overlapped CV of 1mM [2a]PF6 and 1mM ferrocene (left) in CH2Cl2 with 0.1M Bu4NPF6 under N2 from room temperature to -60 oC. Pt was used as working electrode. ············································································································ 62 xviii Figure 4-1. Reaction equilibrium of proton reduction and hydrogen oxidation. ······· 87 Figure 4-2. Proposed hydrogen bonding and hyterolytic cleavage via an Fe2S2 ferredoxin ··········································································································· 88 Figure 4-3. Synthesis of (η6-hexamethylbenzene) manganese dicarbonyl complex using trimethylamine N-oxide ············································································ 97 Figure 4-4. Synthesis of (η6-hexamethylbenzene) manganese dicarbonyl anion.······ 97 Figure 4-5. Protonation of (η6-hexamethylbenzene) manganese dicarbonyl anion. ·· 98 Figure 4-6. Reduction electrochemistry of compound 1 changes from 1-electron without acid to 2-electron with the presence of acid.········································ 102 Figure 4-7. CVs of [(η6-HMB)Mn(CO)3]PF6 with and without the presence of HBF4 in CH2Cl2. Cyclic voltammetry (1). In red, blank solution of dichloromethane with only 0.1M TBAPF6 present. (2). In blue, 50mM HBF4 with 0.1M TBAPF6 in dichloromethane solution. (3). In green, 1mM [(η6-HMB)Mn(CO)3]PF6 in the presence of no acid. (4). In light blue, 1mM [(η6-HMB)Mn(CO)3]PF6 in the presence of 6.6mM HBF4. Glassy carbon electrode was used and scan rate was 50mV/s, 0.1M TBAPF6 was used as electrolyte. HBF4 acid is in the form of HBF4 · O(CH2CH3)2. Nitrogen was used to bubble through the solution before experiment and bubble above the solution during the experiment. ··················· 103 Figure 4-8. CVs of [(η6-HMB)Mn(CO)3]PF6 with and without the presence of HBF4 in acetonitrile. Cyclic voltammetry (1). In red, 1mM [(η6-HMB)Mn(CO)3]PF6 in the presence of no acid. (2). In blue, 6.6mM HBF4 with 0.1M TBAPF6 in xix dichloromethane solution. Glassy carbon electrode was used and scan rate was 50mV/s, 0.1M TBAPF6 was used as electrolyte. HBF4 acid is in the form of HBF4 · O(CH2CH3)2. Nitrogen was used to bubble through the solution before experiment and bubble above the solution during the experiment. ··················· 104 Figure 4-9. Synthesis method of [(η5-methylcyclopentadienyl)Mn(CO)2NO]BF4 ·· 107 Figure 4-10. Cyclic voltammetry of 1.29mM [(η5-methylcyclopentadienyl)Mn(CO)2- NO]BF4 with (blue line) and without the presence (red line) of 100mM TFA. Glassy carbon electrode is used and scan rate was 50mV/s, 0.1M TBAPF6 was used as electrolyte. 0.1M trifluoroacetic acid was used as acid source. Nitrogen was used to bubble through the solution before experiment and bubble above the solution during the experiment.······················································································ 107 Figure 4-11. CVs of 1mM [(η6- naphthalene)Mn(CO)3]PF6 and 1mM ferrocene with the presence of HBF4·Et2O in different concentrations: (a). Red line, no HBF4 (b). Blue line, 1.1mM HBF4 (c). Green line, 2.2mM HBF4 (d). Black line, 3.3mM HBF4. No further change was observed when up to 6.6 mM HBF4 was treated.···············································································································111 Figure 5-1. Crystal structure of compound 3 ·························································· 121 Figure 5-2. IR of the following complexes in acetonitrile ······································ 122 Figure 5-3. NMR of the following complexes in deuterated acetonitrile ················ 124 Figure 5-4. Mass spectrometry (electron spray ionization) of the following complexes in 50% acetonitrile/50% H2O solution······························································ 126 xx Figure 5-5. Nucleophilic reactions pathways of (η6-chlorobenzene) Mn(CO)3+. Nucleophiles can react with (η6-chlorobenzene) Mn(CO)3+ cation by several alternative pathways which depend on the nature of the nucleophile, solvent, temperature.······································································································ 127 Figure 5-6. How Aniline Manganese Tricarbonyl was made ·································· 128 Figure 5-7. Control experiment between compound 1 and MTT ···························· 129 Figure 5-8. Chemical property of aniline after coordination of manganese moiety 130 Figure 5-9. Decrease of the basicity of (η6-aniline) Mn(CO)3+ ······························· 130 Figure 5-10. Reversible deprotonation···································································· 131 Figure 6-1. Mechanism of electrochemistry of (η6-HMB)Mn(CO)3+ ······················ 152 Figure 6-2. CV of 1mM compound 1 at room temperature on Pt electrode. 1mM ferrocene was used as an internal potential standard. Scan rate is 0.50V/s.4 ····· 152 Figure 6-3. Surface modification of Tin dioxide with ruthenium complex ············· 155 Figure 6-4. Continuous five scans of cyclic voltammetry of (η6-p-C6H5NHC6H4-)Mn(CO)3+ modified glassy carbon electrode in blank dichloromethane solution with 0.1M TBAPF6 at room temperature. The scan rate is 5V/s. ············································································································· 165 Figure 6-5. CVs of solid [(η6-C6Me6)Mn(CO)3]PF6 (1) deposited from a 1 mM acetone solution onto a 3.0 mm diameter glassy carbon working electrode by evaporation at 20 oC. The CV medium was 1.0 M KCl in water under N2. The scan rate was 0.50 V s-1. The first scan is shown in red and the second is shown in xxi black.················································································································ 166 Figure 6-6. CVs of solid [(η6-C6H6)Mn(CO)3]PF6 deposited from a unknown concentration acetone solution onto a 3.0 mm diameter glassy carbon working electrode by evaporation under nitrogen at 20 oC. The CV medium was 1.0 M KCl in water under N2. The scan rate was 0.05 V s-1. The first scan is shown in red and the second is shown in blue.································································· 167 xxii List of Schemes Scheme 1-1. Mechanism of (η6-HMB)Mn(CO)3+ reduction electrochemistry.··········· 4 Scheme 1-2. Proposed mechanism of proton reduction catalyzed by (η6-hexamethylbenzene) manganese dicarbonyl anion. ········································ 6 Scheme 1-3. Proposed mechanism of proton reduction catalyzed by [(η5-methylcyclopentadienyl)Mn(CO)2NO]BF4 ··················································· 7 Scheme 1-4. Optimization of (η5-Cp)Mn(CO)2NO+ procatalyst. ································ 7 Scheme 1-5. Polyarene manganese tricarbonyl mediated proton coupled electron storage (“hydride” storage) and hydrogen slow release. ······································· 8 Scheme 1-6. Mechanism of direct synthesis of (η6-aniline) Mn(CO)3+ and its analogs (R = alkyl, or m-NH2 group)················································································· 9 Scheme 1-7. Surface modification of electrode by aromatic manganese tricarbonyl cation through diazonium attachment & slow hydrogen release as side reaction.12 Scheme 1-8. Proposed scheme of formation of Mn-Re metal metal bond················· 13 Scheme 2-1. Simplified scheme of cyclic voltammogram. ······································· 31 Scheme 4-1. The mechanism of hydrogen oxidation and production catalyzed by [Ni(PR2NR’2)2]2+. Mechanism for proton reduction is shown anticlockwisely; Mechanism for hydrogen is shown clockwisely. ················································ 88 Scheme 4-2. Proposed mechanism of proton reduction catalyzed by (η5-C5H5) xxiii (CO)2Mn=C=C=CPh2························································································· 90 Scheme 4-3. Homolytic (left) and heterolytic (right) mechanism for proton reduction catalyzed by a organometallic compound. ·························································· 91 Scheme 4-4． Mechanism of (η6-HMB)Mn(CO)3+ reduction electrochemistry······· 92 Scheme 4-5. [(η5-C5H5)Fe(CO)2]2 (Fp2) reduction followed by catalytic reduction of proton to hydrogen by (η5-C5H5)Fe(CO)2- (Fp-) ··············································· 100 Scheme 4-6. Proposed mechanism of proton reduction catalyzed by (η6-hexamethylbenzene) manganese dicarbonyl anion. ···································· 101 Scheme 4-7. Proposed mechanism of proton reduction catalyzed by [(η5-methylcyclopentadienyl)Mn(CO)2NO]BF4 ··············································· 108 Scheme 4-8. Optimization of (η5-Cp)Mn(CO)2NO+ procatalyst ····························· 109 Scheme 4-9. Proposed mechanism of slow hydrogen release, “hydride” generation and storage mediated by (η6- polyarene) manganese tricarbonyl cation. ···········110 Scheme 4-10. Proposed mechanism of slow hydrogen release mediated via eta6 monoarene manganese tricarbonyl cation. (R = H)············································112 Scheme 5-1. Mechanism of direct synthesis of (η6-aniline) Mn(CO)3+ and its analogs (R = alkyl, or m-NH2 group)············································································· 129 Scheme 6-1. Mechanism of surface modification using diazonium attachment. ····· 155 Scheme 6-2. Surface modification of electrode by aromatic manganese tricarbonyl cation through diazonium attachment & slow hydrogen release as side reaction. xxiv ························································································································· 157 Scheme 6-3. Reaction pathway for indirect diazotization without changing hapticity. ························································································································· 164 xxv List of Tables Table 3-s1．Simulation data based on actual CV of each compound (unit of k1s, k2s is cm/s; unit of D is cm2/s). ···················································································· 58 Table 3-C1-1. Crystal data and structure refinement for [1b]BF4. ·························· 64 Table 3-C1-2. Atomic coordinates ( x 10^4) and equivalent isotropic displacement parameters (A^2 x 10^3) for [1b]BF4. U(eq) is defined as one third of the trace of the orthogonalized Uij tensor.············································································· 65 Table 3-C1-3. Bond lengths [A] and angles [deg] for [1b]BF4. ······························ 66 Table 3-C1-4. Anisotropic displacement parameters (A^2 x 10^3) for [1b]BF4. The anisotropic displacement factor exponent takes the form: -2 pi^2 [ h^2 a*^2 U11 + ... + 2 h k a* b* U12 ]······················································································ 69 Table 3-C1-5. Hydrogen coordinates ( x 10^4) and isotropic displacement parameters (A^2 x 10^3) for [1b]BF4. ·················································································· 70 Table 3-C2-1. Crystal data and structure refinement for [2b]PF6. ·························· 71 Table 3-C2-2. Atomic coordinates ( x 10^4) and equivalent isotropic displacement parameters (A^2 x 10^3) for [2b]PF6. U(eq) is defined as one third of the trace of the orthogonalized Uij tensor.············································································· 72 Table 3-C2-3. Bond lengths [A] and angles [deg] for [2b]PF6. ······························ 74 Table 3-C2-4. Anisotropic displacement parameters (A^2 x 10^3) for [2b]PF6. The anisotropic displacement factor exponent takes the form: -2 pi^2 [ h^2 a*^2 U11 + ... + 2 h k a* b* U12 ]······················································································ 80 xxvi Table 3-C2-5. Hydrogen coordinates ( x 10^4) and isotropic displacement parameters (A^2 x 10^3) for [2b]PF6.··················································································· 82 Table 5-C1-1. Crystal data and structure refinement for 3 (R = m-NH2) ················ 136 Table 5-C1-2. Atomic coordinates ( x 104) and equivalent isotropic displacement parameters (Å 2x 103) U(eq) is defined as one third of the trace of the orthogonalized Uij tensor. ················································································ 137 Table 5-C1-3. Bond lengths [Å] and angles [°] ·················································· 138 Table 5-C1-4. Anisotropic displacement parameters (Å 2x 103)for p1bar. The anisotropic displacement factor exponent takes the form: -2　2[ h2a*2U11 + ... + 2 h k a* b* U12 ]································································································· 140 Table 5-C1-5. Hydrogen coordinates ( x 104) and isotropic displacement parameters (A2x 103) ········································································································· 141 Table 5-C1-6. Hydrogen bonds ············································································ 142 Table 5-C2-1. Crystal data and structure refinement for 3 (R = H). ····················· 143 Table 5-C2-2. Atomic coordinates ( x 104) and equivalent isotropic displacement parameters (A2x 103) for3 (R = H). U(eq) is defined as one third of the trace of the orthogonalized Uij tensor.··········································································· 144 Table 5-C2-3. Bond lengths [A] and angles [º] for 3 (R = H). ···························· 145 Table 5-C2-4. Anisotropic displacement parameters (A2x 103)for p21onc. The anisotropic displacement factor exponent takes the form: -2　2[ h2a*2U11 + ... + 2 xxvii h k a* b* U12 ]································································································· 147 xxviii List of Charts Chart 3-1. Complexes relevant to this study.·························································· 38 Chart 5-1. Aniline bearing heteroatom: electron donating (a-e) group and electron withdrawing group (f).······················································································ 132 xxix List of Formula Formula 4-1. The peak current in a cyclic voltammogram containing only one species is described by above fomula at 25 °C where ip is the peak current, n is the number of electrons transferred, A is the electrode area, D is the diffusion coefficient of the species, v is the scan rate and C* is the bulk concentration of the species. n is the number of electrons transferred. ······································································· 102 xxx Chapter 1 Synthesis and electrochemistry study of aromatic manganese carbonyl complexes and their applications in hydrogen storage, proton reduction catalysis: General introduction + 1e- - CO + Mn Mn Mn 22 OC NO OC NO OC NO OC OC 20 21 + 1e- + Mn H +H - H2 Mn _ OC CO 28 OC CO CO CO 24 Mn Mn OC NO OC NO + H+ + H+ 28' H 23 SLOW - H2 + H+ + 2e- + 2e- Mn + _ Mn + Mn OC NO OC CO CO - 2e- OC CO CO 25 CO atmosphere room temperatre + 2e- + 2e- complex 11,14,15 + 1e- + 1e- CH + 2e- Mn + E1 E2 Mn 2C OC CO OC CO Low Temperature Mn - l2 CO CO OC CO CO Mn 1 2 OC CO Cl 6 - CO 11 O + . Et 2 Mn + + 1e- F4 + CO 1 HB OC _ CO Mn Mn CO 9 OC CO OC CO OC CO + Mn Mn 4 + 1 3 OC CO OEt2 +H + + H+ TFA 14 CO CO Mn Mn Mn OC H CO - H2 CO OC Mn OC CO Mn 13 CO OC CO OTf 5 7 15 1 1.1 Overview It is well known that the origins of organomanganese chemistry are found in the time of the discovery of ferrocene, and following development of organometallic chemistry has been in step with the rapid expansion of most other transition metals. However, the organometallic chemistry of manganese was only initiated in the late 1930s and early 1940s, and the first example is phenylmanganese species, of which the investigation was incomplete and unsubstantiated. The development of the organomanganese chemistry dates essentially from 1954. Dimaganese decacarbonyl is the first fully characterized manganese compound. Methylcyclopentadienyl manganese tricarbonyl (MMT), which was marketed initially in 1958 as a supplement to the gasoline additive tetraethyl,1 is the most wildly used organomanganese complex in the industry. Until late 1960s, the first η6 arene manganese tricarbonyl was synthesized. Not surprisingly, due to the strong electron withdrawing ability, manganese tricarbonyl strongly changed the reactivity of the coordinated arene ring. For example, arene can readily undergo nucleophilic addition when coordinated to the manganese tricarbonyl moiety.2a-g This electrophilic activation leads to two important applications: (1). Organic synthesis3-4: Manganese coordinated arene rings can be further functionalized by desired nucleophilic group. (2). Remote activation: (A). C-C activation.5-6 Cleavage of a strained C-C bond in the four-membered ring in biphenylene (BP) is enormously facilitated by coordination of the electrophilic fragment Mn(CO)3+ to one of the aromatic rings. (B). C-S 2 activation.7-11 Coordination of Mn(CO)3+ to a carbocyclic ring of sterically congested thiophenes activates a C-S bond to regiospecific insertion of platinum. (C). C-O activation.12 In my group, the study of arene manganese tricarbonyl is not only one of the most important research areas, but also the most long lasting research topic. The past group members have thoroughly investigated synthetic scope13 and chemical properties of monoarene and polyarene manganese tricarbonyl cations.14 There is another important aspect deserves great attention, however not being well developed —— homogeneous and heterogenous electrochemistry of monoarene manganese tricarbonyl and its analogs, it will be discussed in chapter 3 and 6. Also, in chapter 4, the proton reduction catalyzed by aromatic manganese carbonyl complexes will be discussed. I also broaden the synthetic scope of aromatic manganese, and report the first direct synthesis method of (η6-aniline) Mn(CO)3+ and its analogs (chapter 5). 1.2 Electrochemistry of aromatic manganese carbonyl complex and its analogs The homogeneous electrochemistry of monoarene manganese tricarbonyl and dicarbonyl was investigated: Compared with the far greater reactivity of the polyarene manganese tricarbonyl cations,14 monoarene manganese tricarbonyl complex [(η6-HMB)Mn(CO)3]PF6 and its analogs exhibit quite stable properties. It is reduced irreversibly by one electron at much more negative potential to give 19e- unstable radical at room temperature, the free radicals will dissociate one carbonyl from the 3 + Mn(CO) 3 moiety followed by dimerization to afford a green color bimetallic compound with a metal-metal bond.15 However, when the temperature is low enough, the reduction is 2-electron chemically reversible with η4-HMB Mn(CO)3- as the product. Slow heterogeneous charge transfer accompanies the formation of the ring-slippage contribute to the large peak separation in the cyclic voltammogram. The electrochemical properties of other analogs, such as HMB Re(CO)3+ PF6- was also studied via combining CV, bulk electrolysis and in-situ optic IR techniques together. Scheme 1-1. Mechanism of (η6-HMB)Mn(CO)3+ reduction electrochemistry. Interestingly, under the protection of carbon monoxide, the same compound [(HMB)Mn(CO)3]PF6 exhibits chemically reversible cyclic voltammetry curve at room temperature. It is common to observe ring-slipped polyarene manganese tricarbonyl anions at room temperature which have already been synthesized and 4 characterized by x-ray16-17. It is the first time to observe the formation of ring slippage monoarene manganese tricarbonyl anion at such a high temperature. Chemical reduction carried out under CO might afford isolable compound 6 (η4-HMB) Mn(CO)3- anion. The structure information might be interesting. 1.3 Proton reduction catalyzed by aromatic manganese carbonyl complexes Hexamethylbenzene manganese tricarbonyl cation [(η6-HMB)Mn(CO)3]PF6 is very stable towards acid. However it is not after it is reduced. Since its reduction intermediate ((η6-HMB)Mn(CO)2-) would react with proton to generate ((η6-HMB)Mn(CO)2H), which is also reactive towards acid. This metal hydride [(η6-HMB)Mn(CO)2H] complex would be protonated to afford hydrogen and [(η6-HMB)Mn(CO)2L] (L is the solvent or counter anion of the acid). The stability and reactivity of the complex [(η6-HMB)Mn(CO)2H] has been previously studied by Eyman.18 The [(η6-HMB)Mn(CO)2L] can be reduced by two electrons and react with proton to finish the catalytic cycle. It has been proved that, (η6-HMB)Mn(CO)2+ does catalyze the proton reduction reaction after it is reduced in dichloromethane with HBF4 presents. Inspired by it, we synthesized another aromatic manganese carbonyl cation: methylcyclopentadienyl manganese dicarbonyl nitrosyl [methylcyclopentadienylMn(CO)2NO+], which can be reduced at more positive reduction potential compared with [(η6-HMB)Mn(CO)3+]. Not surprisingly, it does show similar proton coupled reductive electrochemistry at -0.3V (Vs Ag/AgCl). 5 Herein, we want to report the first proton reduction catalyzed by aromatic manganese carbonyl complex based on metal hydride mechanism. These aromatic manganese derivatives are promising catalysts for proton reduction. We are working on optimizing the efficiency of this class of catalyst. Scheme 1-2. Proposed mechanism of proton reduction catalyzed by (η6-hexamethylbenzene) manganese dicarbonyl anion. 6 Scheme 1-3. Proposed mechanism of proton reduction catalyzed by [(η5-methylcyclopentadienyl)Mn(CO)2NO]BF4 R R + 1e- R - CO R'2HN + R'2HN R'2HN Mn Mn Mn OC OC NO OC NO OC NO 22' OC 20' 21' + 1e- R R + H+ R'2N _ 24' H H Mn + R'2HN Mn NO OC NO OC base Hbase+ + H+ Hbase+ 23' Proton - H2 base + H2 - 2e- R + 2e- Hydrogen R'2HN R + Oxidation Mn OC NO R'2N H Mn NO 25' OC Scheme 1-4. Optimization of (η5-Cp)Mn(CO)2NO+ procatalyst. Moreover, η5-hydronaphthalene complex, such as polyarene (η5- C10H9) Mn(CO)3 or monoarene (η5- C6H7) Mn(CO)3, could react with proton to liberate dihydrogen gas and generate (η6-naphthalene) Mn(CO)3+ cation. Specifically, 7 (η6-naphthalene) Mn(CO)3+ can be used to transfer and store electric energy to stable “metal hydride” complex and releases the hydrogen when needed. Scheme 1-5. Polyarene manganese tricarbonyl mediated proton coupled electron storage (“hydride” storage) and hydrogen slow release. 1.4 Direct synthesis of (η6-aniline) manganese tricarbonyl cation and its analogs Arene manganese tricarbonyl is one of the most useful compounds in organomanganese chemistry. Four methods have been applied to the synthesis of arene manganese complexes, but the direct synthesis of (η6-aniline) manganese tricarbonyl complex remains challenging13 because of the basicity of amine group. Traditionally, manganese complexes of this kind could only be obtained through 8 nucleophilic substitution of chloroarene manganese tricarbonyl by primary or secondary amines,19 which involves side reactions and usually in an overall low yield. Herein we report a direct synthetic method of making (η6-aniline) manganese tricarbonyl complexes in moderate to high yield. The synthesis follows the mechanism shown below: one equivalent of manganese (MTT) links to three equivalents of amine to yield 29, which reacts with a second MTT to produce 30. Complex 30 spontaneously cleavages to the desired product 31. The X-ray structure of 31 (R = H, m-NH2) was determined. The objective of direct synthesis of π-bonded aniline mangaeses complex is two folds: (1) synthesis of aniline analogs with desired functional group (2) precursor of heterogeneous electrochemistry using diazonium attachment (see chapter 6). R R R R R R + R R Mn CO NH 2 CO spontaneous NH 2 MTT OC NH 2NHH22N NH 2NH H 22N cleavage Mn + Mn + OC CO OC Mn + Mn + OC CO CO CO OC CO CO CO OC MTT 29 30 31 Scheme 1-6. Mechanism of direct synthesis of (η6-aniline) Mn(CO)3+ and its analogs (R = alkyl, or m-NH2 group) 9 31 (R = H) 31 (R = m-NH2) Figure 1-1. Crystal structure of compound 31 Direct synthesis method of (η6-aniline) Mn(CO)3+ was discovered, which provided us a new and much simplified synthetic route toward activation of aniline and organic synthesis of aniline analogs with desirable functional groups. Furthermore, (η6-aniline) Mn(CO)3+ is an important precursor for surface modification of the electrode using diazonium attachment. 1.5 Surface electrochemistry of aromatic manganese tricarbonyl complexes using diazonium attachment and physical attachment. The purpose of heterogeneous electrochemistry study is to achieve the electrochemical reductive reversibility by covalent attachment of arene manganese tricarbonyl using diazonium-base chemistry or simply physical adsorption attachment. Physical separation of manganese compound may stop the dimerization after one-electron reduction and result in reductive reversibility. Organometallic electrode20 is a new concept in the area of heterogeneous electrochemistry. One of my projects is focused on covalent attachment of arene manganese tricarbonyl using 10 diazonium-base chemistry and collecting and analyzing e-chem signal from the surface functionalized electrode.21 Not being alike aniline by itself, manganese tricarbonyl functionalized aniline complex (η6-aniline) Mn(CO)3+ could not be diazotized by nitrite. So we change its hapticity from eta6 to eta5 to make the π system more electron rich. (η5- aminocyclohexadienyl) manganese tricarbonyl was synthesized and diazotized to afford (η5-cyclohexadienyldiazonium) manganese tricarbonyl complex. This cyclohexadienyl manganese tricarbonyl diazonium complex can be reduced to liberate nitrogen and form a bond between the cyclohexadienyl group and electrode surface. The substituted hydrogen could be removed by treating with CPh3+ or strong acid thereafter. Herein, we want to report a unique method for surface modification of electrode by (η6-aromatic) manganese tricarbonyl cation. We want to compare its reductive electrochemistry before and after electrode surface modification, and we expect to see that the chemical reversibility shows up. No preliminary data has been acquired yet. We are still working on the purification and characterization of the intermediates. (η6-C6H5NHC6H4NH2)Mn(CO)3+ was synthesized from nucleophilic substitution of (η6-C6H5Cl)Mn(CO)3+ by p-phenylenediamine, then it was diazotized to afford (η6-C6H5NHC6H4N2)Mn(CO)32+. We can see partial reversibility shows up in the CV time scale. In the meanwhile, taking advantage of the solubility of arene manganese tricarbonyl complex, we have done their heterogeneous electrochemistry by physical attachment. 11 (η5-aminocyclohexadienyl) manganese tricarbonyl mediated slow hydrogen release is the side reaction when it is treated with acid. Other (η5-hydrocyclohexadienyl) manganese tricarbonyl analogs show similar properties of hydrogen release. The amine group of (η5-C6H6NH2)Mn(CO)3 facilitates the hydrogen release reaction. H H R H R H + H NH2 NH3 Mn Mn OC CO CO OC CO CO Diazotization NaBH4 Nitrite H R R H NH2 N N Mn + Mn OC CO CO OC CO H2 CO R N N CO Mn + Mn CO OC CO +e- CPh3+ CO electrode CO CO R Mn CO CO R Scheme 1-7. Surface modification of electrode by aromatic manganese tricarbonyl cation through diazonium attachment & slow hydrogen release as side reaction. 1.6 Synthesis and characterization of heterodinuclear complex with the core structure of Mn-Re metal-metal bond. The η6-aromatic manganese dicarbonyl anion is strong nucleophile, it was proposed to be the important intermediate which catalyzed the proton reduction 12 reaction (chapter 4). By introducing the dicarbonyl anion with the tricarbonyl cation stoichiometrically, Eyman reported the formation of an aromatic homodinuclear manganese complex with a Mn-Mn bond and bridged carbonyl ligands.18 It would be interesting to check the reaction between the same manganese anion with [(η6-HMB)Re(CO)3]PF6. We want to explore the possibility of forming the heterodinuclear dimmer with a novel core structure of Mn-Re metal-metal bond. There is no report about the study of Mn-Re metal-metal bond; furthermore, heterodinuclear complex has potential applications in molecular catalysis. CO CO Mn Mn CO OC Mn + OC CO CO + 1e - + -C O + 1e- _ Mn Mn OC CO OC CO + Re + OC CO CO CO CO Mn Re CO OC Scheme 1-8. Proposed scheme of formation of Mn-Re metal metal bond 13 1.7 References 1. Information about Methylcyclopentadienyl manganese tricarbonyl, please refer to wikipedia at http://en.wikipedia.org/wiki/Methylcyclopentadienyl_manganese_ tricarbonyl 2. (a) Rose-Munch, F.; Rose, E. Eur. J. Inorg. Chem. 2002, 1269. (b) Auffrant, A.; Prim, D.; Rose-Munch, F.; Rose, E.; Schouteeten, S.; Vaissermann, J. Organometallics 2003, 22, 1898. (c) Prim, D.; Andrioletti, B.; Rose-Munch, F.; Rose, E.; Couty, F. Tetrahedron 2004, 60, 3325. (d) Ku¨ndig, E. P.; Pape, A. Top. Organomet. Chem. 2004, 7, 71. (e) Sweigart, D. A.; Reingold, J. A.; Son, S. U. Comprehensive Organometallic Chemistry III; Crabtree, R. H., Mingos, D. M. P., Eds.; Elsevier: Oxford, U.K., 2006; Vol. 5, Chapter 10, pp 761-814. (f) Jacques, B.; Chavarot, M.; Rose-Munch, F.; Rose, E. Angew. Chem., Int. Ed. 2006, 45, 3481. (g) Jacques, B.; Chanaewa, A.; Chavarot-Kerlidou, M.; Rose-Munch, F.; Rose, E.; Ge´rard, H. Organometallics 2008, 27, 626. 3. Anthony J. Pearson* and Paul R. Bruhn J. Org. Chem. 1991. 56, 70927097 4. Pearson, A. J and Shin, H. Tetrahedron 1992 48, 7527-7538 5. X. Zhang, G. B. Carpenter and D. A. Sweigart, Organometallics, 1999,18, 4887 6. M. Oh, K. Yu, H. Li, E. J. Watson, G. B. Carpenter and D. A. Sweigart, Adv. Synth. Catal., 2003 345, 1. 7. K. Yu, H. Li, E. J. Watson, K. L. Virkaitis, G. B. Carpenter and D. A. Sweigart, Organometallics, 2001 20, 3550 8. C. A. Dullaghan, X. Zhang, D. L. Greene, G. B. Carpenter, D. A. Sweigart, C. 14 Camiletti and E. Rajaseelan, Organometallics, 1998 17, 3316. 9. X. Zhang, C. A. Dullaghan, E. J. Watson, G. B. Carpenter and D. A. Sweigart, Organometallics, 1998, 17, 2067. 10. C. A. Dullaghan, X. Zhang, D. Walther, G. B. Carpenter and D. A. Sweigart, Organometallics, 1997 16, 5604. 11. C. A. Dullaghan, S. Sun, G. B. Carpenter, B. Weldon, and D. A. Sweigart, Angew. Chem. Int. Ed. Engl., 1996, 35, 212. 12. X. Zhang, E. J. Watson, C. A. Dullaghan, S. M. Gorun and D. A. Sweigart, Angew. Chem. Int. Ed. Engl., 1999 38, 2206. 13. J. Derek Jackson, Sharon J. Villa, Deborah S. Bacon, and Robert D. Pike* Organometallics 1994, 13, 3972-3980 14. Reingold, J. A.; Virkaitis, K. L.; Carpenter, G. B.; Sun, S.; Sweigart, D. A.; Czech, P. T.; Overly, K. R. J. Am. Chem. Soc. 2005, 127, 11146. 15. Neto, C. C.; Baer, C. D.; Chung, Y. K.; Sweigart, D. A. Chem. Commun. 1993, 816. 16. Jacqueline M. Veauthier, Albert Chow, Gideon Fraenkel, Steven J. Geib and N. John Cooper* Organometallics 2000, 19, 3942-3947 17. Jacqueline M. Veauthier, Albert Chow, Gideon Fraenkel Steven J. Geib and N. John Cooper* Organometallics 2000, 19, 661-671 18. Peter J. Schlom, Ann M. Morken, Darrell P. Eyman,’ Norman C. Baenziger, and Steven J. Schauer Organometallics 1993,12, 3461-3467 19. Peter L. Pauson and John A. Segal, J.C.S. Dalton 1975 1677-1682 15 20. Jannie C. Swarts, Derek Laws, and William E. Geiger* Organometallics 2005, 24, 341-343 21. Derek R. Laws, John Sheats, Arnold L. Rheingold and William E. Geiger*, Langmuir 2010, 26(18), 15010–15021 16 Chapter 2 General experiments 2.1 Inert atmosphere work 1. Nitrogen gas Nitrogen is the most inexpensive and wildly used inert gas applied to the protection and synthesis of air and moisture sensitive organometallics complex. The nitrogen gas was passed through a drierite (CaSO4 97% and CoCl2 3%). The apparatus is basically a T-type connection with one end connected to the nitrogen valve and an exit containing a gas bubbler to monitor the flow rate of the dried nitrogen gas and to balance the pressure in the protected system. In some cases which require more inert atmosphere; nitrogen will be replaced by argon (99.998%). 2. Air-free technique 1). Schlenk techniques The two most common types of air-free technique involve the use of a glovebox and a Schlenk line. The two methods share one similarity: glassware is pre-dried in ovens prior to use. After the connectivity to the schlenk line, they should be flame-dried to remove adsorbed water. 2). Glove box and Glove bag Glove box provides direct inner atmosphere under argon’s protection. It has a copper made catalyst to remove oxygen, and molecular sieves to remove water by adsorbing it in the molecular sieves' pores. According to the needs of the chemist, the atmosphere can be nitrogen and vacuum. You can put equipments inside of the glove 17 box, such as balance. Glove bag is a much simplified glove box, it has the advantage of much cheaper but is usually a poorer substitute because it is more difficult to purge, and less well sealed. 3). Schlenk line. The Schlenk line is the most commonly-used air free technique developed by Wilhelm Schlenk. It consists of a dual manifold with several ports. One manifold is connected to a source of purified inert gas, usually nitrogen, while the other is connected to a high-vacuum pump. Special stopcocks allow chemist to select vacuum or inert gas without placing the sample on a separate line. The high vacuum is often used to remove the last traces of solvent from a sample. Vacuum gas manifolds often have many ports and lines, and with care it is possible for several reactions to be run in the same time. Fiture 2-1. Glove box (left) and schlenk line (right). 3. Compound Storage 18 In the case of air sensitive chemicals, we usually seal it with parafilm and keep it in the nitrogen box with drierite inside. Light sensitive compound will be wrapped with aluminum foil. For longer storage, compounds could be stored in vacuum desiccators. 2.2 Instruments and measurements 1. Liquid infrared spectroscopy measurement The instrument is an ATI Mattson Infinity Series FT-IR spectrophotometer (Mattson Instruments, Madison, Wisconsin). The data was recorded and analyzed with Winfirst Version 3.2 software. We use liquid infrared cell for the measurement of sample in the solution. The cell is made of two CaF2 (International Crystal, Inc., Garfield, New Jersey) crystal plates. The sample was dissolved in proper solvent at relatively low concentration, and was transferred into the liquid cell. Make sure there is no bubble in the test window of the cell. Sample was run at room temperature using 16 scans. After testing each sample, the cell will be thoroughly rinsed by acetone and dried by passing nitrogen through. We have many choices of solvents: acetone, acetonitrile, dichloromethane, ether, hexane…… Different solvents have different cut-off lines; the choice of solvent is limited by solubility of the sample and the cut-off line. In our group, since carbonyl peak (2100~1700 cm-1) is the characteristic of the manganese carbonyl complex and most solvents don’t have any overlap in 2100~1700 cm-1 area, so there is no disturbance from the solvent we use. 19 Figure 2-2. Liquid infrared cell 2. Solid infrared spectroscopy measurement The same instrument was used in solid infrared measurement; however, KBr pellet of the sample was made instead of dissolving sample into the solvent. Mortars and pestles are the ideal tool for grinding a matrix of a solid sample with KBr powder in preparation for making a transparent pellet free from absorption peaks. We mixed small amount of sample together with the proper amount of KBr (purchased from Aldrich, FT-IR grade, and used as is), grinded them into fine uniform powder. We put one bolt on the bottom of the barrel and added proper amount of grinded powder, screwed another bolt on the top by hand initially and later on by ratchet. Hold the ratchet tight for one minute, and then we removed both bolts, leaving the pellet in the middle of the barrel. Ideally, the pellet should be one piece and transparent. 128 scans were used for the IR data collection. 20 Figure 2-3. Bolt, barrel, mortar and pestle for solid IR1 3. In-situ Infrared (IR) optic probe The equipment used for in-situ IR spectroscopic measurements was manufactured by the Remspec Corporation2 and consists of three primary components used in conjunction with a Matteson Infinity Series FTIR. The first part is the probe itself, which was immersed into the sample solution for direct IR measurements during the reaction. The second portion is an external detector to collect information returned from the probe tip. The final portion is the launch module, which is attached to the outside of the FTIR spectrometer and directs the input signal to the probe tip. The three components are connected by 7 fiber optic cables, which are bundled together and sheathed in a flexible steel housing. IR optic probe Liquid Transmission Head (HD-02) Figure 2-4. In-situ IR probe with external detector and its tip2 21 The probe tip itself is fitted with a low temperature head to protect the fiber optic cables from damage due to thermal expansion and contraction when changing temperature. The low temperature head allows for real time in-situ measurements to temperature as low as -90C. The fiber optic cable bundle is butted up against the low temperature head, which consists of a ZnSe crystal and Meldin sheath. Below the end of the ZnSe crystal is a mirror that reflects the signal back through the crystal and eventually to the detector. The path length can be adjusted by screwing in or out the mirror to increase the gap between the crystal tip and the mirror. 4. Mass Spectroscopy Mass spec. was recorded on the Kratos MS-80 spectrometer3 ref 3 by Dr. Shen in chemistry department. ESI was used to test the molecular weight of manganese cation, since most our manganese cations are plus one charged, so there is no need of ionization. For the most cases, ESI mode works very well. Different solvents are available: H2O, acetone, acetonitrile and dichloromethane. 5. Nuclear Magnetic Resonance (NMR) The Chemistry Department currently has four Bruker high field NMR spectrometers: two 300 Megahertz (MHz) instruments and two 400 MHz instruments.4 Approximately 3ml deuterated solvent is needed for a standard sample. For 1D proton NMR, the concentration of the sample should be higher than 0.1mM; For COSY - Proton-proton correlation experiment, the recommend concentration 22 13 should be higher than 10mM; For 1D C NMR experiment, the recommend concentration is 100mM. For 1D phosphorus NMR experiment, the recommend concentration. The NMR tube was used as it was in the lab. 2.3 Electrochemical method 1. Electrolyte Electrolyte is a substance containing free ions that behaves as an electrically conductive medium. For most of the experiments we use tetrabutylammonium hexaflourophosphate (TBAPF6), because it is inert to most compounds, and pretty stable over a wide potential range, furthermore, it is still soluble at relatively low temperature(-60/℃). It was reported by Geiger that, in low polarity solvent (dichloromethane, THF), large anions such as B(C6F5)4- (TFAB)and B(C6H3(CF3)2)4-, will significantly reduce the error in cyclic voltammetry by enhanced conductivity and expanded scan rate range. Typically, the concentration of the electrolyte is 0.1M. 2. Synthesis of electrolyte TBAPF6 and {NBu4[B(C6F5)4]} 32.6g tetrabutylammonium bromide was dissolved in 1L erlenmeyer flask with 450ml distilled water. 21.0 ml 60% hexafluorophosphoric acid was slowly added to the above solution, and a large amount of white precipitate formed instantaneously. Keep stirring for 30mins after all the acid was added. Then the white slurry was filtered through a big 500ml coarse fritted filter using vacuum filtration. The white product was completely dissolved in minimal amount of hot ethanol and allowed to cool down at room temperature and thereafter put into a freezer for further 23 recrystallization. The crystal was collected by filtration and the above procedure was repeated twice more to ensure the purity of the electrolyte. Synthesis of electrolyte {NBu4[B(C6F5)4]}5-6 ref 5-6, tetrabutylammonium tetrakis (pentafluoro -phenyl) borate: One to one equivalent of an aqueous solution of Li[B(C6F5)4]·nEt2O(n =2-3) and a solution of tetrbutylammonium bromide in methanol were combined, white precipitate formed instantaneously. The precipitate was washed with water and dried under vacuum, and recrystallized three times from dichloromethane/Ether. 3 Test the purity of the electrolyte To make sure the synthesized electrolyte is pure. Simply run a cyclic voltammetry of arene chromium tricarbonyl (purchased from aldrich) in dichloromethane (HPLC grade, purchased from Fisher). If nice reversibility was observed at around +0.5V, you can use the electrolyte. The mechanism is, after the oxidation of the chromium complex, it generates a 17e- intermediate which would react rapidly with any impurities in the solution. 4. Solvent The solvent typically used for cyclic voltammetry is HPLC grade methylenechloride (CH2Cl2) purchased from Fisher Chemical. CH2Cl2 is the primary solvent because it will not tend to coordinate to the metal like MeCN and it is fairly inert to most compounds we synthesized. 5. Preparation of reference electrode1 Ag/AgCl electrode and a glass salt bridge were needed. Firmly attach the 24 salt bridge to the open end of the Ag/AgCl electrode and seal the attachment with parafirm. Invert the electrode and fill the glass salt bridge with 0.1 M TBAClO4 in dichloromethane. The tip of the salt bridge is sealed by Teflon shrink wrap with a porous vycor tip. The electrode system will need at overnight to fully settle down and if the solution level changes, more 0.1 M TBAClO4 should be added and the seal should be strengthened. The new reference electrode should be stored in a 0.1 M TBAClO4 dichloromethane solution saturated with LiCl1. 6. Work Station and three-electrode system Our cyclic voltammetry experiments are carried out by the PGSTAT1007 ref 7 work station which is a high voltage potentiostat/ galvanostat with a standard three electrode working system. The reference electrode (RE) is an electrode with stable potential and Ag/AgCl reference electrode is the one we are using. The counter electrode (CE, made of Pt) is used to make sure that current doesn’t go through the reference electrode (the passage of current will affect the potential of the reference E and will also affect the measured potential because of iRs drop, in which Rs is the solution resistance between RE and WE); Working electrode is where the reaction of interest is occurring. 25 Three-electrode system: Working E; Reference E; Counter E Working E: Reference E Counter E GC;Au;Pt Ag/AgCl Pt Figure 2-5. PGSTAT100 potentiostat/ galvanostat with a standard three electrode working system 7. Hardware installation and instruction For cyclic voltammetry, only three plus one connections are needed. Working electrode connection is labeled by “WE” and it is red color. Reference electrode connection is labeled by “RE” and it is blue color. Counter electrode connection is label by “CE” and it is black color. And one more connection is ground connection and it should be connected to the ground all the time to protect the work station and the people working on the station. The ground connection is green color. After each electrode is connected, you will need to press the “on/off” button once, so the work station is powered on. Then press “cell on/off” button, then the electrochemical cell is powered on. 8. Software installation and instruction 26 Simply put the CD (from Metrohm Autolab, version 4.9.007) into the CD player, and follow the instruction displayed to install the software. After you finish, you will see folder “autolab software” from all programs. Click the “autolab software”, you will see four icons: GPES, Interface, Diagnostics, Hardware. 9. Conduct cyclic voltammetry at room temperature A schlenk tube was filled 2/3 with the solvent used in the cyclic voltammetry. Nitrogen gas was bubbled through the solvent from the top of the schlenk tube and exited from the side arm to the three electrode cell. The schlenk tube should be put into a beaker filled with water. The water level should be higher than the solvent level inside the schlenk tube. The above procedure is extremely important for maintaining the consistency of cyclic voltammetry at room temperature. The nitrogen gas saturated with the solvent will maintain the concentration of the solution inside the e-chem cell. Without the protection of the bubbling system, the solvent inside the e-chem cell would evaporate quickly, and the current shown in the cyclic voltammetry will keep increasing as the time going on. 27 Figure 2-6. GPES software, which controls the PGSTAT100 potentiostat/ galvanostat. After the four connections were made and power button was turned on, click the “interface” icon, after 10s, computer should tell you that the interface was connected. Click the “Gpes” icon, you will see the following page, which was made of four important parts: “GPES Manager”, “Manual control”, “Edit procedure”, “Data presentation”. From “GPES Manager”, select “method”, choose “cyclic voltammetry”. From “Edit procedure”, select desired scans numbers, scan range, scan rates. “Manual control” provided a way to manually reduce the experimental error. After clicking the “start” button at the left bottom, an in time cyclic voltammetry will show in the “Data presentation”window. You can run as many scans as you want to. After finished, only the last scan was shown in the window. Both the data and picture file of the CV could be saved after the experiment. For data file, select “file”Æ “save scan as” from the “GPES manager”; for picture file, select 28 “copy”Æ “copy to” from the “Data presentation” window. 10. Conduct cyclic voltammetry at low temperature Usually, dry ice/acetone bath was made as low temperature bath for low temperature CV experiments. By carefully and slowly adding different amount of dry ice to the acetone bath, the temperature of the bath could be controlled. The e-chem cell with three electrodes and a thermometer was kept in the dry ice/acetone bath. The thermometer will indicate the temperature simultaneously. The bubbling system was no longer needed when the temperature was lower than 0 ℃. 11. Interpretation of the cyclic voltammetry In a cyclic voltammetry experiment, a voltage is applied to a working electrode in solution and current flowing at the working electrode is plotted versus the applied voltage to give the cyclic voltammogram (Figure 2). Cyclic voltammetry can be used to study the electrochemical properties of species in solution as well as at the electrode/electrolyte interface. Because any change of the components in the solution is limited in the diffusion layer. R Negative potential Vacant MO Vacant MO + e- Energy level Of electrons Occupied MO Occupied MO O Figure 2-7. Representation of reduction of an electrochemical active species in the solution. By driving the electrode to more negative potentials, the energy of the 29 electrons is raised, and they will eventually reach a level high enough to occupy vacant states on the species in the electrolyte, it is a process of reduction (Figure 3); Meanwhile, if you apply more positive potentials to the electrode, the energy of the electrons will be lowered, finally, when the energy gets lower enough, electrons from the occupied MO(molecular orbital) of the surface redox active molecule will jump to the electrode, it’s a process of oxidation 12. The mechanism of the cyclic voltammogram (Scheme 2-1): 1). For a given bulk solution containing only O: at potentials well positive of the redox potential, there is no net conversion of O to R. (O is oxidant; R is reductant) 2). When redox potential is almost approached, there is a net cathodic current, which increases exponentially with potential due to the exponential potential dependence of the rate of heterogeneous electron transfer. Shown in A 3). When the redox potential is reached at B, and the surface concentrations of O and R are equal at this potential. In the meanwhile, the product R tends to diffuse away from the electrode surface, the O will diffuse towards the electrode. 4). After passing the (cathodic) peak potential C, the current decays as a result of the decrease of O in the interfacial region. The rate of electrolysis (and hence the current) now depends on the rate diffusion of O from the bulk solution to the electrode surface; so the time dependence is t -½. The peak is therefore asymmetric. 5).Then reversing of the direction of the potential scan, the current continues to decay D with t -½ until the potential gets close to the redox potential E, at which point there begins a net reoxidation of R to O which causes an anodic current. 30 6). After the peak potential, the anodic current will decrease as a result of the depletion of R in the interfacial region, just like repeating the reversal process of 4……. A B C D Diffusing away R R R R +e- +e- +e- +e- Diffusing towards electrode O O O O Diffusing towards electrode R R -e- -e- Diffusing away O O F E Scheme 2-1. Simplified scheme of cyclic voltammogram. Bulk electrolysis is to electrochemically reduce or oxidize the redox active species in the solution in bulk amount with basket-like working and counter electrode. It’s different from the cyclic voltammetry. Firstly, it has much larger surface area of working and counter electrode; secondly, it will keep potential at fixed value, not back and forth like the sweeping potential applied in cyclic voltammetry; finally, you have to stir the solution to destroy the diffusion layer, to make sure that all the solution is mixed evenly. One great advantage of our research is: we can use the CO on the moiety of metal-tricarbonyl as a labeling ligand because of its characteristic IR frequency; IR can be easily applied to monitor the change in the bulk solution. Meanwhile, due to 31 the previous tremendous work on Mn and other metal-tricarbonyl complexes, our lab has accumulated large amount of IR data. In-situ optic IR probe is used to detect the characteristic IR frequencies due to the electrochemical reaction. On the other hand, you can simply insert its tip to the solution and the measurement can be done under an atmosphere of N2, CO without exposure to the air from room temperature to as low as -90 ° C. The minimum interval between each spectrum is 30s. So we can easily combine bulk electrolysis techniques (electrochemistry) and in-situ optic IR (Spectroscopy) together by simply inserting the tip of the probe into the bulk solution. 13. Digital simulation of cyclic voltammgram8 Digisim 3.0 from BASi was installed and used for digital simulation of cyclic voltammetry. A dongle from the company was needed when using the digisim software for secure connectivity. 14. Bulk electrolysis Bulk electrolysis is a potential controlled coulometry, which uses three electrode system controlled by potentiostat. During the process of electrolysis, a potential is hold at certain value, the current is monitored all the time, so the overall coulomb can be easily calculated. As the oxidation or reduction going to finish, the current gets smaller, and becomes zero when all the starting material is consumed. Working and counter electrode: the working and counter electrode used for bulk electrolysis are bulky basket electrodes made of platinum. Cell design: The cell for bulk electrolysis is shown below. The working and 32 reference electrode were kept in the left cell; the counter electrode was kept in the other side, there was a salt bridge between them. Figure 2-8. Cell design for bulk electrolysis. During the process of electrolysis, in order to make the solution homogeneous, a medium size stirring bar was needed, and kept stirring during the whole process. If the bulk electrolysis was conducted at room temperature, a similar bubbling system was needed to maintain the concentration of the solution. If the bulk electrolysis was done at low temperature, the dry ice/acetone bath should be used to control the temperature. The bubbling system wasn’t needed. 33 2.4 References 1. Dr. Jeffery. A. Reingold thesis. 2. Remspec Corporation website http://www.remspec.com/ 3. Brown University Chemistry department mass spectrometry facility webpage. http://www.chem.brown.edu/facilities/mass_spec/mass_spec.html 4. Brown University Chemistry department NMR facility webpage. http://www.chem.brown.edu/facilities/NMR/nmr.html 5. Robert J. LeSuer, Catherine Buttolph, and William E. Geiger Anal. Chem., 2004, 76 6. Robert J. LeSuer and William E. Geiger* Angew. Chem. Int. Ed. 2000, 39, No. 1 7. Parameters for Metrohm Autolab PGSTAT100, please go to autolab website at: http://www.nlab.pl/pgstat100_en.html. 8. More information about digisim program, please refer to its webpage at: http://www.basinc.com/products/ec/digisim/ 34 Chapter 3 Electrochemical Study of Manganese and Rhenium Arene Complexes (C6R6)M(CO)3+ (R = Me, Et) Mn Re 35 3.1 Introduction Electrochemical methods have been widely used to study the reactivity of 17- and 19-electron organometallic radicals generated from stable 18-electron precursors.1,2 Electron-transfer-induced ligand dissociation, addition, and substitution reactions, as well as structural rearrangements and catalysis, have been probed by using transient voltammetry, bulk electrolysis, and chemical redox reagents. Variable temperature studies have been especially valuable for understanding the chemical reactions that occur subsequent to electron transfer. The reductive electrochemistry of monocyclic arene and polycyclic arene complexes has been studied in considerable depth.3-7 In some cases, especially with the heavier transition metals, an overall two-electron reduction occurs that is accompanied by arene ring slippage from η6 to η4 bonding. It is thought that the first electron addition (E1o) generates a 19-electron complex and that ring slippage occurs in concert with addition of the second electron (E2o) to afford an 18-electron η4-bonded arene product. The nuclear reorganization associated with the η6 to η4 slippage manifests in two important ways: (1) E2o is near or even positive (anodic) of E1o, with the separation ΔE being significantly temperature dependent and (2) the heterogeneous charge-transfer rate constant for the second electron addition, ks(2), is much smaller than that for the first addition, ks(1). This paper is concerned with the reductive electrochemistry of (η6-arene)M(CO)3+ (M = Mn, Re) complexes. A preliminary report of selected monocyclic arene systems has appeared.6 Recently, a complete electrochemical 36 study of naphthalene-based polycyclic arene complexes (arene)Mn(CO)3+ has been published.7 Cooper and coworkers have shown previously that the generic complex (η6-benzene)Mn(CO)3+ can be reduced chemically at low temperature to afford the nucleophilic (η4-benzene)Mn(CO)3- anion, which undergoes an impressive variety of reactions with electrophilic reagents.5 We report herein a description of the monocyclic arene complexes (η6-C6R6)M(CO)3+ (R = Me, Et; M = Mn, Re). The purpose of this study was to determine the influence of the metal and steric congestion on the nature and stability of the reactants and their reduction products. It is shown that the two rhenium systems, which differ greatly in their reactivity as 18-electron cations, are cleanly reduced to the anionic η4-arene complexes on the voltammetric time scale regardless of solvent or temperature. With manganese, however, the situation is less straightforward, with the reduction products depending on solvent, temperature and steric congestion (R). As indicated in Chart 3-1, reduction of 1 can lead to the anion 3, the ring-coupled product 5, or the dimeric complex 6. The last complex, which was first reported by Eyman and coworkers,8 is produced by CO dissociation from the neutral 19-electron radical (η6-C6R6)Mn(CO)3. While the cation (η6-C6Me6)Mn(CO)3+ is generally more reactive than (η6-C6Et6)Mn(CO)3+, a primary conclusion from the present work is that the transient neutral 19-electron radical displays the opposite reactivity, (η6-C6Et6)Mn(CO)3 >> (η6-C6Me6)Mn(CO)3. It is also concluded that in general the (η6-C6R6)M(CO)3 radicals follow the chemical reactivity order Mn >> Re, which is opposite to the reactivity order found for the 37 (η6-C6R6)M(CO)3+ precursors (Re >> Mn). Chart 3-1. Complexes relevant to this study. 3.2 Experimental Section 1. Solvents Solvents were purchased from commercial sources as HPLC grade. Methylene chloride and acetonitrile solvents were stored and opened under nitrogen. The hexamethylbenzene complexes [1a]BF4 and [2a]PF6 were prepared by literature methods.9 The hexaethylbenzene analogues [1b]BF4 and [2b]PF6 were synthesized by similar procedures. 2. Synthesis 38 [(η6-Hexaethylbenzene)Mn(CO)3]BF4 Acenaphthene manganese tricarbonyl tetrafluoroborate (0.600 g, 1.58 mmol) and hexaethylbenzene (0.581 g, 2.37 mmol) were combined with 17 ml methylene chloride (Fisher Scientific Co.) in a 20 ml pressure tube under nitrogen. The tube was sealed, wrapped in aluminum foil, and placed in a 75 oC oil bath for 2 hrs. The solvent was then removed and the yellow solid residue was washed with diethyl ether to afford the product in 71% yield (0.529 g). A crystal suitable for X-ray analysis was obtained by diethyl ether diffusion into a methylene chloride solution at room temperature Anal. Calcd for C21H30O3MnBF4: C, 53.42; H, 6.40. Found: C, 52.86; H, 6.22. IR (CH2Cl2, cm-1): 2060, 2003. 1 H NMR (CD2Cl2, 300 MHz, room temperature, δ ppm): 2.66 (q, J = 7.5 Hz, 2H), 1.40 (t, J = 7.5 Hz, 3H). [(η6-Hexaethylbenzene)Re(CO)3]PF6. Re(CO)5Br (0.400 g, 0.98 mmol), hexaethylbenzene (1.210 g, 4.92 mmol) and AlCl3 (0.390 g, 2.95 mmol) were mixed in a 50 ml round bottle flask with 25 ml of degassed decane. The mixture was heated at 150 oC for 1 hour. After cooling down, the decane layer was removed. The orange-red solid residue was washed with cyclohexane three times. Then 20 ml of ice water was added, and any residue remaining undissolved after ca. 30 s of shaking was removed by filtration. NH4PF6 (0.19 g, 1.2 mmol) was added to the aqueous solution to precipitate the pale white product. Recystallization was effected twice by dissolving the product in a minimum amount of methylene chloride, and slowly adding diethyl ether. The yield was 65%. A sample suitable for single crystal X-ray crystallographic analysis was obtained by diethyl ether diffusion into a 39 methylene chloride solution at -20 oC. IR (CH2Cl2, cm-1): 2064, 1993. 1 H NMR (CD2Cl2, 400 MHz, room temperature, δ ppm): 2.66 (q, J = 7.5 Hz, 2H), 1.39 (t, J = 7.5 Hz, 3H). 3. Electrochemistry 1). Cyclic voltammetry Voltammetric data were collected under a blanket of nitrogen that was saturated with solvent. The electrolyte was 0.10 M Bu4NPF6, which was synthesized by the metathesis of Bu4NBr and HPF6, recrystallized from hot ethanol, and dried under vacuum. The solvents were HPLC grade CH2Cl2 and CH3CN. Additional purification was not required. Voltammetry at low temperatures utilized a simple slush bath; with a thermocouple probe inserted to monitor the temperature.10 Cyclic voltammetry data were measured with Eco Chemie AUTOLAB potentiostatic instrumentation. A standard three-electrode system was used. The working electrode was a 2 mm diameter platinum disk, and the counter electrode was a platinum wire. The reference was a Metrohm Ag/AgCl electrode filled with CH2Cl2/0.10 M Bu4NPF6 and saturated with LiCl. The reference electrode was separated from the test solution by a salt bridge containing 0.10 M Bu4NPF6 in the solvent in use. Ferrocene was generally added as an internal potential standard. Bulk electrolyses, performed with an EG&G 175 potentiostat and a 179 digital coulometer under an atmosphere of nitrogen, utilized a platinum-basket working electrode and a platinum-mesh counter electrode, which were separated from the test solution by a salt bridge. IR spectra of solutions undergoing electrolysis or chemical 40 reduction were recorded at variable temperatures with a Remspec fiber optic probe. Digital simulation of the proposed mechanisms were performed with Digisim.11 The cyclic voltammetry of (η6-C6Me6)Mn(CO)3+ (1a) in the solid state was probed by dissolving [1a]BF4 in acetone and then adding several drops of the solution to the working area of a 3 mm glassy carbon disk electrode. After complete evaporation of the acetone, the electrode was placed in water containing 1.0 M KCl, with connection to an aqueous Ag/AgCl reference electrode in 3.0 M KCl. The two electrodes were connected with a salt bridge containing 3.0 M KCl in water. 2). Bulk electrolysis Bulk electrolysis is to electrochemically reduce or oxidize the redox active species in the solution in bulk amount with basket-like working and counter electrode. It’s different from the cyclic voltammetry. Firstly, it has much larger surface area of working and counter electrode; secondly, it will keep potential at fixed value, not back and forth like the sweeping potential applied in cyclic voltammetry; finally, you have to stir the solution to destroy the diffusion layer, to make sure that all the solution is mixed evenly. 4. Crystallography X-ray data collection for [1b]BF4 and [2b]PF6 was carried out using a Bruker single-crystal diffractometer equipped with an APEX CCD area detector and controlled by SMART version 5.0. Data were collected at room temperature. Data reduction was performed by SAINT version 6.0 and absorption corrections were applied by SADABS version 2.0. The structures were determined by direct methods 41 and refined on F squared by use of programs in SHELXTL. Most hydrogen atoms appeared in a difference map, or they were inserted in ideal positions, riding on the atoms to which they are attached. 3.3 Results and Discussion 1. Manganese Complexes 1). Cyclic voltammetry Cyclic voltammetry of the hexamethylbenzene complex 1a at a Pt electrode was studied as a function of temperature in CH2Cl2 and in CH3CN Figure 3-1. CVs of 1.0 mM [(η6-C6Me6)Mn(CO)3]BF4 (1a) at +14 oC (red), -20 oC (blue) and -40 oC (black) in CH2Cl2/0.10 M Bu4NPF6 under N2. The working electrode was a 2.0 mm diameter platinum disk, and the scan rate was 0.50 V s-1. A ferrocene internal standard was added (left). Figure 3-1 illustrates the results in CH2Cl2 at two temperatures. The 42 reversible couple on the left of Figure 3-1 is due to the ferrocene added to the cell as an internal reference standard. At +14 oC, the system shows a 1-electron reduction that is chemically irreversible. The product of the reduction is known to be the dimer 6a, as established by the potential of the associated oxidation (-0.29 V).6 We found similar behavior at a Pt electrode in CH3CN solvent.12 As the temperature is lowered, the wave attributable to dimer 6a vanishes and evidence of chemical reversibility appears. Curiously, the reduction peak current does not decrease much with temperature (compare to the ferrocene oxidation current) until about -40 oC, below which the current drops off as may be expected. Our interpretation is that dissociation of CO from the initial reduction product, (η6-C6Me6)Mn(CO)3, becomes relatively slow and a second reduction to the slipped η4-arene complex 3a becomes prominent as the temperature is lowered, which explains why the peak current does not drop off as anticipated. 2). Bulk electrolysis (BE) Bulk electrolysis results are consistent with the CV results. BE of 1a in CH3CN at room temperature produced the green dimer 6a (νCO = 1849, 1680 cm-1), which slowly decomposed. BE in CH2Cl2 at room temperature also produced 6a, which is stable in this solvent. Interestingly, BE in CH2Cl2 at -60 oC did not lead to any 6a. Rather, a mixture of 3a and 5a was produced. It was further observed that at low temperature with no applied potential the η4-3a complex reacted with the starting material η6-1a to yield the η5-dimer 5a. This reaction was previously observed5d during chemical reduction of (η6-benzene)Mn(CO)3+. 43 On the cyclic voltammetric time scale at low temperature it is likely that the only product is the 2-electron reduced complex 3a. Digital simulations agree well with the interpretation that electrochemical reduction of 1a is a 1-electron process at room temperature to give 6a but a 2-electron process at low temperatures to give 3a. The simulations quantitatively reproduce the observed peak potentials and current ratios provided ΔE = E2o – E1o is zero or slightly positive at room temperature and more positive (ca. 250 mV) at low temperature. Additionally, it is required that the heterogeneous charge-transfer rate constant ks(2) be much smaller than ks(1). This order of charge transfer rates is unsurprising in view of the ring slippage that occurs in the second electron reduction step, which exhibits electrochemical irreversibility by virtue of its very slow heterogeneous electron-transfer rate constant. That ΔE becomes more positive as the temperature is lowered was observed with analogous manganese naphthalene complexes and is predictable from consideration of the relative entropy changes that accompany the two reduction steps.7 The peak reduction current shown in Figure 3-1 shifts negative with decreasing temperature. This represents the combined effect of a positive kinetic shift at +14 oC due to chemical irreversibility and a negative shift at -40 oC due to a very small value of ks(2) (estimated as 1 x 10-3 cm/s at +14 oC and 2 x 10-5 cm/s at -40 o C). Simulations indicate that E1o does not change with temperature (-1.35 V) and that E2o increases from -1.25 to -1.10 V, so that ΔE = E2o – E1o increases with temperature, as expected. The hexaethylbenzene manganese complex 1b undergoes reduction reactions 44 that are significantly different in comparison to the hexamethylbenzene complex 1a. Because 1b and the rhenium analogue 2b are new, their X-ray structures were determined. As shown in Figure 3-2, the ethyl substituents point alternatively up and down as may be expected. The result is that the M(CO)3+ moiety is shielded from nucleophilic attack and the cations 1b and 2b are unusually stable for this reason when compared to other (monoarene)M(CO)3+ complexes. This stabilization is especially remarkable for the rhenium complexes (vide infra). Figure 3-2. X-ray structures of [(C6Et6)Mn(CO)3]BF4 (1b) and [(C6Et6)Re(CO)3]PF6 (2b). The average M-C(arene) bond length is 2.22(1) A for Mn and 2.35(1) A for Re. Complex 1b undergoes reduction in CH2Cl2 at room temperature in a chemically irreversible manner to give unknown products. The dimeric product 6b, resulting from CO dissociation from the neutral radical, was not observed. At low temperatures in CH2Cl2, however, chemical reversibility is seen, but with especially 45 slow charge transfer (e.g., ks(2) is simulated to be 3 x 10-6 cm/s at -40 oC). The electrochemical behavior of 1b in CH3CN solvent is easier to interpret, and significant chemical reversibility was observed at all temperatures. Relevant results are given in Figure 3-3. The room temperature voltammogram features two waves, which at -30 oC merge into one wave. The CVs at low temperatures also show a hint of product 6b. Digital simulations indicate that ks(2) at -40 oC for 1b in CH3CN is 4 x 10-6 cm/s, which is about 100 times smaller than that found with 1a. Two waves are seen at room temperature with 1b even though E2o is simulated to be slightly positive of E1o; this is due to the small value of ks(2) (ca. 4 x 10-5 cm/s). BE experiments with 1b were very informative. BE in CH3CN gave the dark green dimer 6b at both room temperature and at -40 oC. The dimer slowly decomposes at room temperature and all CO bands fade away. In CH2Cl2, BE at room temperature gives only CO bands assigned to (C6Et6)Mn(CO)2Cl. At -55 oC in CH2Cl2, BE led to a mixture of dimer 6b and anion 3b, both of which decompose upon heating to -30 oC. That dimer 6b is formed during BE of 1b but not 1a in CH2Cl2 at low temperature suggests that the 19-electron radical follows the order (η6-C6Et6)Mn(CO)3 > (η6-C6Me6)Mn(CO)3 for the rate of CO dissociation. Indeed, 6b can even be seen in trace amounts in the CVs of 1b at low temperatures. Ligand dissociation is enhanced by steric effects is what one would expect. 46 Figure 3-3. CVs of 1.0 mM [(η6-C6Et6)Mn(CO)3]BF4 (1b) at +20 oC (red) and -30 o C (black) in CH3CN/0.10 M Bu4NPF6 under N2. The working electrode was a 2.0 mm diameter platinum disk, and the scan rate was 0.50 V s-1. A ferrocene internal standard was added (left). Of course, the steric congestion can influence all chemical reactions of (η6-C6R6)Mn(CO)3+ including CO dissociation, ring coupling to afford 5, and η6 → η4 ring slippage to accompany a second electron addition when 1 is reduced. Comparison of the present work to published results7 with (η6-naphthalene)Mn(CO)3+ reduction leads to the conclusion that the rate of second electron addition to (η6-arene)Mn(CO)3+ as measured by ks(2) is in the order: naphthalene >> C6Me6 >> C6Et6. This order seems to be reasonable for the relative order of ring slippage. The 47 small ks(2) for the monoarenes in effect provides extra time for the (η6-C6R6)Mn(CO)3 radical to react by pathways other than electron addition, in accordance with the CV and BE results presented herein. The “extra time” is not available to (η6-naphthalene)Mn(CO)3+ and this complex is reduced in a chemically reversible 2-electron manner at all temperatures. The η6 → η4 ring slippage is more facile for the naphthalene complex because the diminishment of its resonance energy upon ring slippage is less severe. This effect manifests as a larger ks(2) and a more positive ΔE = E2o – E1o. The electron transfer rate is indeed larger for polycyclic arenes than for monocyclic arenes. Curiously, however, the thermodynamics as reflected by ΔE = E2o – E1o do not follow this order. Rather, ΔE is surprisingly constant (200 – 300 mV) at low temperatures for all three arene manganese complexes (naphthalene, C6Me6, C6Et6). This observation may mean that the activation barrier for the second electron addition reflects slippage that is not directly in concert with the bending of the arene ring, which is known5 to be 37o in (η4-naphthalene)Mn(CO)3-. Alternatively, it is possible that the actual ring bending is significantly less with the manganese monocyclic arene complexes. 3). Manganese dicarbonyl complexes [(η6-C6Me6)Mn(CO)2THF]+ and (η6- C6Me6)Mn(CO)2Cl was synthesized and discovered that their reductive electrochemistry is mimicking the one of [(η6- C6Me6)Mn(CO)3]+. 48 Figure 3-4. Electrochemistry of [(η6-C6Me6)Mn(CO)2THF]BF4 and (η6- C6Me6)Mn(CO)2Cl. The CV of both complexes was conducted in 0.1M TBAPF6 dichloromethane solution at room temperature, and scan rate was 0.5V/s. Glassy carbon electrode was used. (1) CV in black is 1mM [(η6-C6Me6)Mn(CO)3]BF4. (2) CV in blue is [(η6-C6Me6)Mn(CO)2THF]BF4 with unknown concentration. (3) CV in red is (η6-C6Me6)Mn(CO)2Cl with unknown concentration. After comparing CVs between manganese tricarbonyl complex [1a]BF4 and dicarbonyl complexes, we conclude that, the reductive electrochemistry of all above three compound is the same. They all undergo 1-electron reduction and then ligand cleavage (CO, Cl- or THF) followed by dimerization. Their reduction potential differs from each other because of different electron density of each molecule. However from figure 3-4, it is obvious that they share the very same reduction product 6a, which has the characteristic reversible redox peak located at aournd -0.25V. Manganese 49 dicarbonyl complexes have similar reductive electrochemical behavior at room tempterature. While at low temperature, their electrochemical properties haven’t changed, still maintained 1-electron irreversible reduction with 6a as the product. Figure 3-5. CVs of 1.0 mM [(η6-C6Me6)Mn(CO)2THF]BF4 at +20 oC (black) and -60 oC (red) in CH2Cl2/0.10 M Bu4NPF6 under N2. The working electrode was a 2.0 mm diameter platinum disk, and the scan rate was 0.50 V s-1. CV of [(η6-C6Me6)Mn(CO)2THF]BF4 at both room temperature and low temperature gave characteristic redox peak corresponding to the dimer 6a (figure 3-5). It indicates a much weaker bond between THF and Mn center compared with the bond between CO and Mn, which explains the irreversible electrochemistry of the dicarbonyl complex even at low temperature. 4). Reversibility of [(η6-C6Me6)Mn(CO)3]BF4 Electrochemist prefers reversibility not only because that it indicates the 50 stability under reduction or oxidation, but also the potential application as an electrocatalyst. If the decomposition, in another word, dimerization of the complex 1a could be stopped, the electrochemistry could very possibly be chemically reversible. Enabling the reversibility of complex 1 would widen the scope of our view of electrochemistry of monoarene manganese complex. A redox active complex would be more attractive, might have potential applications in catalysis and sensor. In order to achieve chemical reversibility of 1a, we need to retain the chemical structure unchanged after reduction, thus, stop the cleavage of carbonyl group from Mn center. Excess carbon monoxide was bubbled through the dichloromethane solution of 1a. CV was taken with saturated CO presence (figure 3-6). Figure 3-6. Cyclic voltammetry of 1mM 1a in 0.1M TBAPF6 dichloromethane solution with the protection gas of (1) CO (2) N2 on glassy carbon electrode at room temperature. Scan rate is 0.50V/s. Under nitrogen protection, after 1-electron irreversible reduction, peak corresponding to the dimer 6a was observed, however, under the protection of carbon 51 monoxide, reduction became partially 2-electron reversible reduction, and the peak corresponding to the dimer completely vanished. Under 1atm, excess CO prevents the CO ligand from leaving the manganese center, thus the re-association rate of CO is faster compared with the dissociation The partial reversibility comes from the reaction between 2-electron reduction product 3a and starting material 1a. Since cyclic voltammetry only changes the component of the tiny area on the electrode surface, after 2-electron reduction, 3a will be the only component close to the surface area after reduction, however, in the bulky solution, the only component is 1a. As the generation and diffusion of 3a, ring couple reaction was initiated and resulted in loss of reversibility. Peak current jump indicates that the number of electrons transferred has changed, which was confirmed by the diminish peak current corresponding to the dimer. The importance of this reversibility generation is: it is for the first time ever that, we observe the formation of ring slippaged form of monoarene manganese tricarbonyl anion at such a high temperature. The formation of eta4 anion was only seen at low temperature before and could never be isolated. We are expecting to synthesize this anion by reducing the cation chemically under saturated CO atmosphere in dichloromethane solution. 2. Rhenium Complexes (Arene)Re(CO)3+ complexes have long been known to be orders of magnitude more susceptible to nucleophilic attack at the metal than the corresponding manganese complexes.9e Arene displacement with CH3CN according to eq 1 was examined for complexes 1 and 2. As determined by IR spectrometry, the manganese 52 complexes 1a and 1b show no reactions after days in CH3CN solvent at room temperature. However, eq 1 occurs smoothly with rhenium complex 2a. At 20 oC in neat CH3CN the half-life is 23 min. By comparison, the mesitylene analogue (η6-C6H3Me3)Re(CO)3+ has a half-life of 15 min. With the much more sterically encumbered complex 2b (see Figure 3-2), there was no detectable reaction with neat CH3CN after 5 hr, so that the reactivity must be in the order: C6H3Me3 > C6Me6 >>> C6Et6. This pattern was anticipated, although the extreme inertness of 2b to nucleophilic attack was surprising. (η6-arene)Re(CO)3+ + 3 CH3CN (CH3CN)3Re(CO)3+ + arene (1) The CVs of 2a and 2b in CH2Cl2 were straightforward to interpret because the electron transfers are chemically reversible at all temperatures. Figure 3-7 reveals a typical result for 2b. It can immediately be seen that the electron transfer reactions are very slow. The reduction peak corresponds to two electrons (vide infra) and simulations are consistent with a reduction potential separation ΔE = E2o – E1o at room temperature of 450 mV and 570 V for 2a and 2b, respectively. The charge transfer rate constants for the second electron addition at -40 oC were best simulated to be very slow: ks(2) = 4 x 10-5 cm/s for 2a and 6 x 10-6 cm/s for 2b. Bulk electrolysis in CH2Cl2 was performed only for complex 2a. At -60 oC, the passage of two equivalents of electrons through the solution cleanly produced the η4-anion 4a (νCO = 1930, 1827, 1805 cm-1). The addition of one equivalent electron at -60 oC gave a mixture of η4-4a and η6-2a, which reacted with each other above -10 53 o C to afford an η5 complex (νCO = 2003, 1881 cm-1) that is most likely the rhenium analogue of 5a. BE at room temperature consumed 1.5 electrons and produced an observable mixture of η4-4a and a η5-species. Over time at room temperature the η4-4a complex decomposed. Given these observations, it is virtually certain that chemical reaction of the 19-electron radical (η6-C6R6)Re(CO)3 to yield η5-species is not significant on the CV time scale. Additionally, digital simulations were not influenced by incorporation of the thermodynamically favored disproportionation of (η6-C6Me6)Re(CO)3 to 2a and 4a. Figure 3-7. CVs of 1.0 mM [(η6-C6Et6)Re(CO)3]PF6 (2b) at +20 oC (red) and -46 o C (black) in CH2Cl2/0.10 M Bu4NPF6 under N2. The working electrode was a 2.0 mm diameter platinum disk, and the scan rate was 0.50 V s-1. A ferrocene internal standard was added (left). 54 3.4 Conclusions The complexes (η6-C6R6)M(CO)3+ (M = Mn, Re; R = Me, Et) can be reduced electrochemically to give 19-electron radicals that can accept a second electron to yield ring-slipped (η4-C6R6)M(CO)3- complexes, or can react chemically by ring coupling to afford η5-cyclohexadienyl dimeric species. Alternatively, the radicals can dissociate CO and couple to produce dimeric complexes 6. Regardless of the metal or the R substituent, the second electron addition is characterized by very slow heterogeneous charge transfer and by an E2o value that is positive of E1o. With rhenium as the metal, only chemically reversible reduction is observed on the CV time scale. With manganese, however, the initial 1-electron reduction product can react chemically or, at low temperatures, undergo a second electron addition. Consideration of all of the voltammetric and bulk electrolysis data leads to a number of interesting conclusions. In comparison to the hexamethylbenzene analogues, the effect of the ethyl substituents in 1b and 2b is to protect the metal from nucleophilic attack. As a result, the reactivity towards CH3CN is in the order Me >> Et and Re >> Mn. In contrast, the 19-electron radicals obtained by 1-electron addition react by CO dissociation in the opposite order: Mn >> Re and C6Et6 >> C6Me6. Indeed, a key feature with the rhenium complexes is the great stability of the 19-electron radicals and the consequent reluctance to undergo any chemical reaction on the CV time scale. Similarly, the ring-slipped (η4-C6R6)M(CO)3- is more stable with rhenium, and can be directly observed via bulk electrolysis at room temperature. 55 The reductive electrochemistry of 1a is illustrated in figure 3-8. CO atmosphere room temperatre + 2e- + 1e- + 1e- Mn + E1 Mn E2 OC CO OC CO Low Temperature Mn - CO CO OC CO CO 1 2 6 - CO + + 1e- + CO 1 _ Mn Mn OC CO OC CO CO OC CO Mn 4 + 1 3 CO CO Mn Mn CO OC Mn OC CO CO 5 7 Figure 3-8. Reductive electrochemistry of 1a 56 3.5 Supporting Information 1. Digital simulation All digital simulations were completed using Digisim version 3.0 installed on a desktop. Simulations were done by comparison of ratios of the anodic and cathodic peaks measured from a common base line in both the CV and the simulation output. The simulation results were compared with the actual CV until the experimental result was mimicked as closely as possible. Ohmic polarization was not taken into account and was judged not in general of major importance by according to the behavior displayed by the internal ferrocene standard. Metal Temp sol E1o/V k1s E2o/V k2s α2 E3o/V 105 D Simulated (observed) Ep(V) Re-HMB RT CH2Cl2 -1.43 0.1 -0.98 1.6E-3 - - 1 Ep = -1.36 (-1.36), -0.85 (-0.86) -40 oC CH2Cl2 -1.57 0.04 -0.97 4E-5 0.6 - 0.1 Ep = -1.50 (-1.49), -0.72 (-0.72) Re-HEB RT CH2Cl2 -1.47 0.06 -0.90 7E-4 - - 1 Ep = -1.41 (-1.41), -0.73 (-0.73) -46 oC CH2Cl2 -1.57 0.01 -0.94 6E-6 0.6 - 0.1 Ep = -1.56 (-1.56), -0.60 (-0.60) Mn-HMB RT CH2Cl2 -1.35 1 -1.25 1E-3 - -0.315 1 Ep = -1.225 (-1.22), -0.29 (-0.29) -40 oC CH2Cl2 -1.35 0.1 -1.1 2E-5 0.6 - 0.1 Ep = -1.34 (-1.33), -0.81 (-0.81) RT MeCN -1.35 1 -1.1 3E-3 - -0.31 1 Ep = -1.23 (-1.25), -1.01 (-1.01) -40 oC MeCN -1.44 0.1 -1.04 3E-4 0.6 - 0.1 Ep = -1.35 (-1.35), -0.89 (-0.89) Mn-HEB -40 oC CH2Cl2 -1.3 0.01 -1.1 3E-6 0.6 -0.31 0.1 Ep = -1.30 (-1.29), -1.46 (-1.44), -0.70 (-0.70) RT MeCN -1.2 0.1 -1.15 4E-5 - -0.31 1 Ep = -1.235 (-1.22), -1.485 (-1.47), -0.82 (-0.85) -40 oC MeCN -1.38 0.1 -1.1 4E-6 0.6 -0.31 0.1 Ep = -1.38 (-1.38), -0.72 (-0.73) 57 Table 3-s1．Simulation data based on actual CV of each compound (unit of k1s, k2s is cm/s; unit of D is cm2/s). A B C D E F Figure 3-s1. Simulations A). 2a in CH2Cl2 at R.T. ; B). 2a in CH2Cl2 at -40 oC ; C). 2b in CH2Cl2 at R.T. ; D). 2b in CH2Cl2 at -46 oC ; E). 1a in CH2Cl2 at R.T. ; F). 1a in 58 CH2Cl2 at -40 oC. Pt was used as the electrode. Scan rate was 0.5V/s. The concentration of each compound is 1mM/ 0.1M Bu4NPF6. Red line is simulated CV, black line is actual CV. 2. In-situ IR Spectroscopy and Bulk Electrolysis At room temperature, we fixed the applied potential at -1.2V for 15min, the light yellow color solution of 1a got greener and greener, the in-situ optic IR spectra suggested that, the band which corresponded to the 6a got stronger and stronger (Figure s2A), there was barely anything else (though we got slight amount of the byproduct (HMB) Mn(CO)2Cl)). The CV shown in Figure s2B agrees with the conclusion. When we used CH3CN as the solvent, the result stayed the same. 1921 1973 Transmittance /% 1682 Brown to 1848.3 6a dark green R= Hexamethyl A 2061.50 2001 B Wavenumbers /cm-1 Figure 3-s2. A). In-situ IR spectra for the bulk electrolysis process of [1a]BF4 in CH2Cl2 with 0.1M Bu4NPF6 under N2 at R.T. Applied potentical -1.2V : Changes are labeled by red arrows. B). corresponding CV of 1mM [1a]BF4 and 1mM ferrocene (left) in CH2Cl2 with 0.1M Bu4NPF6 under N2 at R.T. When we lowered the temperature to -60 oC, and applied potential at -1.4V for 25min to the system, the light yellow color (1a) got deeper. In-situ IR suggested to have the ring slippage η4-3a species (1910, 1817, 1786 cm-1) and η5-5a (1990, 1910 59 cm-1) as products. 1951 Transmittance /% 1786 1817 5a 5a 1910 1990 - 2e- Mn + Mn OC CO OC CO CO 2061 CO A 2000 B Wavenumbers /cm-1 Figure 3-s3. A). In-situ IR spectra for the bulk electrolysis process of [1a]BF4 in CH2Cl2 with 0.1M Bu4NPF6 under N2 at -60 oC. Applied potential -1.4V : Changes are labeled by red arrows. B). corresponding CV of 1mM [1a]BF4 and 1mM ferrocene (left) in CH2Cl2 with 0.1M Bu4NPF6 under N2 at -60 oC Keeping the above solution at -60 oC, +0.1V was applied to the system. It was found that all the η4-3a went back to the starting material, shown in Figure s4, which confirmed that at low temperature, the reduction was chemically reversible. Transmittance /% -2e- Unknown 5a ? Wavenumbers /cm-1 Figure 3-s4. In-situ IR spectra for the bulk electrolysis process of [1a]BF4 in CH2Cl2 with 0.1M Bu4NPF6 under N2 at -60 oC. Applied potential +0.1V: Changes are labeled by red arrows. 60 In a control experiment, when the starting material was still present and the transmittance of IR bands for the starting material η6-1a, η5-5a and the η4-3a was almost equal to each other, we stopped applying potensital to the solution and kept the temperature at -60 oC for 20mins. We could clearly see that, the starting material and the η4 reductant were disappearing; in the meanwhile, the IR bands of η5-5a were getting stronger. Thus, it is reasonable to believe that the starting material and the η4 reductant react to afford η5-5a. As the η4 species was synthesized electrochemically from the surface of the bulky working electrode, it diffused away to the bulk solution. Since it has negative charge, it’s easy for it to react with the positive charged starting compound, they can dimerize together to give η5 neutral species 5a. Transmittance /% 1950 1680 6a R= Hexamethyl 2062.5 2002 Wavenumbers /cm-1 Figure 3-s5. Bulk electrolysis [1a]BF4 in CH3CN + 0.1M Bu4NPF6 under N2 at RT. Apply -1.2V for about 15 mins. 61 1969 1786 1670 1911 1844 1820 Transmittance /% Might be η 4 -form 6b R= hexaethyl 2059 2001 Wavenumbers /cm-1 Figure 3-s6. In-situ IR spectra for the bulk electrolysis process of [1b]BF4 in CH2Cl2 with 0.1M Bu4NPF6 under N2 at -60 oC. Applied potential -1.5V: Changes are labeled by red arrows. HMB Re(CO)3+ in CH2Cl2 with TBAPF6 on Pt eletrode with Fc at different Temperature with IR comp. R.T.~ --60/C R.T. I/A -60/C Figure 3-s7. Overlapped CV of 1mM [2a]PF6 and 1mM ferrocene (left) in CH2Cl2 with 0.1M Bu4NPF6 under N2 from room temperature to -60 oC. Pt was used as working electrode. 62 3.6 X-ray crystal structure data X-ray structures of [(C6Et6)Mn(CO)3]BF4 (1b) and [(C6Et6)Re(CO)3]PF6 (2b). The average M-C(arene) bond length is 2.22(1) A for Mn and 2.35(1) A for Re. 63 Table 3-C1-1. Crystal data and structure refinement for [1b]BF4. ____________________________________________________________________ Identification code [1b]BF4 Empirical formula C21 H30 B1 F4 Mn1 O3 Formula weight 553.48 Temperature 293(2) K Wavelength 0.71073 A Crystal system, space group Orthorhombic, P2(1)2(1)2(1) Unit cell dimensions a = 8.9357(18) A alpha = 90 deg. b = 14.007(3) A beta = 90 deg. c = 17.480(4) A gamma = 90 deg. Volume 2187.8(8) A^3 Z, Calculated density 4, 1.680 Mg/m^3 Absorption coefficient 1.215 mm^-1 F(000) 1112 Crystal size 1 x 1 x 1 mm Theta range for data collection 1.86 to 28.31 deg. Limiting indices -11<=h<=11, -18<=k<=18, -23<=l<=22 Reflections collected / unique 18886 / 5297 [R(int) = 0.0701] Completeness to theta = 28.31 98.6 % Absorption correction done Refinement method Full-matrix least-squares on F^2 Data / restraints / parameters 5297 / 0 / 277 Goodness-of-fit on F^2 1.024 Final R indices [I>2sigma(I)] R1 = 0.0442, wR2 = 0.0954 R indices (all data) R1 = 0.0559, wR2 = 0.1000 Absolute structure parameter 0.157(17) Largest diff. peak and hole 0.668 and -0.269 e.A^-3 64 Table 3-C1-2. Atomic coordinates ( x 10^4) and equivalent isotropic displacement parameters (A^2 x 10^3) for [1b]BF4. U(eq) is defined as one third of the trace of the orthogonalized Uij tensor. ________________________________________________________________ x y z U(eq) ________________________________________________________________ F(1) 6620(2) 5712(1) 7527(1) 45(1) Mn(1) 3479(1) 5094(1) 403(1) 16(1) C(1) 1321(3) 4684(2) 958(1) 16(1) O(3) 6585(2) 5777(1) 162(1) 27(1) C(2) 2763(3) 5808(2) -390(1) 19(1) C(3) 1420(3) 2567(2) 1810(1) 22(1) C(4) 1565(3) 5681(2) 1057(1) 15(1) C(5) 2172(3) 2963(2) 1084(1) 19(1) O(2) 2260(2) 6257(2) -871(1) 30(1) C(6) -181(3) 4335(2) 651(1) 19(1) C(7) 3805(3) 4355(2) 1517(1) 17(1) C(8) 5375(3) 5521(2) 223(1) 20(1) C(9) 350(3) 6384(2) 828(1) 18(1) C(10) 4027(3) 5352(2) 1622(1) 17(1) O(1) 3884(2) 3440(2) -633(1) 30(1) C(11) 5437(3) 5701(2) 2010(1) 20(1) C(12) 2451(3) 4028(2) 1180(1) 17(1) C(13) 3835(3) 7662(2) 927(2) 25(1) C(14) 6142(3) 3338(2) 1196(2) 25(1) C(15) 5208(3) 5735(2) 2881(1) 28(1) C(16) 3754(3) 4095(2) -251(1) 22(1) C(17) -303(3) 4246(2) -219(1) 25(1) C(18) 4979(3) 3639(2) 1793(1) 21(1) C(19) -726(3) 6550(2) 1498(1) 22(1) C(20) 2906(3) 6019(2) 1403(1) 17(1) C(21) 3100(3) 7084(2) 1564(1) 19(1) F(3) 6323(2) 4796(1) 8592(1) 38(1) F(2) 5395(2) 6304(1) 8566(1) 29(1) F(4) 4323(2) 5141(2) 7839(1) 51(1) B(1) 5654(4) 5487(2) 8130(2) 28(1) ________________________________________________________________ 65 Table 3-C1-3. Bond lengths [A] and angles [deg] for [1b]BF4. _________________________________________________________________________ F(1)-B(1) 1.398(4) C(4)-C(9) 1.519(3) Mn(1)-C(8) 1.824(3) C(5)-C(12) 1.522(3) Mn(1)-C(16) 1.824(3) C(6)-C(17) 1.529(3) Mn(1)-C(2) 1.825(3) C(7)-C(12) 1.422(4) Mn(1)-C(10) 2.216(2) C(7)-C(10) 1.422(4) Mn(1)-C(4) 2.216(3) C(7)-C(18) 1.530(3) Mn(1)-C(12) 2.219(3) C(9)-C(19) 1.531(3) Mn(1)-C(7) 2.225(2) C(10)-C(20) 1.422(4) Mn(1)-C(1) 2.235(2) C(10)-C(11) 1.512(4) Mn(1)-C(20) 2.235(3) O(1)-C(16) 1.139(3) C(1)-C(12) 1.419(3) C(11)-C(15) 1.537(3) C(1)-C(4) 1.424(3) C(13)-C(21) 1.525(3) C(1)-C(6) 1.526(3) C(14)-C(18) 1.532(4) O(3)-C(8) 1.144(3) C(20)-C(21) 1.529(4) C(2)-O(2) 1.142(3) F(3)-B(1) 1.394(4) C(3)-C(5) 1.539(3) F(2)-B(1) 1.395(4) C(4)-C(20) 1.423(4) F(4)-B(1) 1.382(4) C(8)-Mn(1)-C(16) 91.05(12) C(8)-Mn(1)-C(2) 90.89(11) C(16)-Mn(1)-C(2) 89.51(12) C(8)-Mn(1)-C(10) 84.68(10) C(16)-Mn(1)-C(10) 134.27(11) C(2)-Mn(1)-C(10) 135.94(11) C(8)-Mn(1)-C(4) 133.21(11) C(16)-Mn(1)-C(4) 135.40(11) C(2)-Mn(1)-C(4) 85.31(10) C(10)-Mn(1)-C(4) 67.28(9) C(8)-Mn(1)-C(12) 135.32(11) C(16)-Mn(1)-C(12) 85.57(10) C(2)-Mn(1)-C(12) 133.52(11) C(10)-Mn(1)-C(12) 67.20(9) C(4)-Mn(1)-C(12) 67.28(9) C(8)-Mn(1)-C(7) 100.49(10) C(16)-Mn(1)-C(7) 100.02(10) C(2)-Mn(1)-C(7) 164.94(10) C(10)-Mn(1)-C(7) 37.36(9) C(4)-Mn(1)-C(7) 79.71(9) C(12)-Mn(1)-C(7) 37.32(9) C(8)-Mn(1)-C(1) 163.95(9) C(16)-Mn(1)-C(1) 101.02(10) 66 C(2)-Mn(1)-C(1) 99.68(10) C(10)-Mn(1)-C(1) 79.33(9) C(4)-Mn(1)-C(1) 37.32(9) C(12)-Mn(1)-C(1) 37.17(9) C(7)-Mn(1)-C(1) 67.21(9) C(8)-Mn(1)-C(20) 99.10(10) C(16)-Mn(1)-C(20) 164.96(10) C(2)-Mn(1)-C(20) 101.32(10) C(10)-Mn(1)-C(20) 37.27(9) C(4)-Mn(1)-C(20) 37.28(9) C(12)-Mn(1)-C(20) 79.41(9) C(7)-Mn(1)-C(20) 67.35(9) C(1)-Mn(1)-C(20) 67.12(9) C(12)-C(1)-C(4) 119.6(2) C(12)-C(1)-C(6) 120.9(2) C(4)-C(1)-C(6) 119.5(2) C(12)-C(1)-Mn(1) 70.81(14) C(4)-C(1)-Mn(1) 70.64(15) C(6)-C(1)-Mn(1) 133.49(15) O(2)-C(2)-Mn(1) 177.2(2) C(20)-C(4)-C(1) 120.4(2) C(20)-C(4)-C(9) 119.9(2) C(1)-C(4)-C(9) 119.6(2) C(20)-C(4)-Mn(1) 72.08(15) C(1)-C(4)-Mn(1) 72.04(15) C(9)-C(4)-Mn(1) 131.00(15) C(12)-C(5)-C(3) 109.4(2) C(1)-C(6)-C(17) 116.0(2) C(12)-C(7)-C(10) 119.3(2) C(12)-C(7)-C(18) 120.2(2) C(10)-C(7)-C(18) 120.5(2) C(12)-C(7)-Mn(1) 71.08(14) C(10)-C(7)-Mn(1) 70.95(14) C(18)-C(7)-Mn(1) 132.11(17) O(3)-C(8)-Mn(1) 175.2(2) C(4)-C(9)-C(19) 110.2(2) C(20)-C(10)-C(7) 120.8(2) C(20)-C(10)-C(11) 119.7(2) C(7)-C(10)-C(11) 119.5(2) C(20)-C(10)-Mn(1) 72.09(14) C(7)-C(10)-Mn(1) 71.69(14) C(11)-C(10)-Mn(1) 131.97(17) C(10)-C(11)-C(15) 110.1(2) C(1)-C(12)-C(7) 120.6(2) 67 C(1)-C(12)-C(5) 119.2(2) C(7)-C(12)-C(5) 120.1(2) C(1)-C(12)-Mn(1) 72.02(14) C(7)-C(12)-Mn(1) 71.60(14) C(5)-C(12)-Mn(1) 131.32(17) O(1)-C(16)-Mn(1) 176.3(2) C(7)-C(18)-C(14) 115.5(2) C(10)-C(20)-C(4) 119.3(2) C(10)-C(20)-C(21) 120.8(2) C(4)-C(20)-C(21) 119.9(2) C(10)-C(20)-Mn(1) 70.63(14) C(4)-C(20)-Mn(1) 70.64(14) C(21)-C(20)-Mn(1) 133.13(17) C(13)-C(21)-C(20) 115.6(2) F(4)-B(1)-F(3) 109.8(3) F(4)-B(1)-F(2) 110.2(3) F(3)-B(1)-F(2) 108.9(2) F(4)-B(1)-F(1) 109.4(2) F(3)-B(1)-F(1) 109.2(3) F(2)-B(1)-F(1) 109.3(3) _____________________________________________________________ 68 Symmetry transformations used to generate equivalent atoms: Table 3-C1-4. Anisotropic displacement parameters (A^2 x 10^3) for [1b]BF4. The anisotropic displacement factor exponent takes the form: -2 pi^2 [ h^2 a*^2 U11 + ... + 2 h k a* b* U12 ] _______________________________________________________________________ U11 U22 U33 U23 U13 U12 _______________________________________________________________________ F(1) 50(1) 47(1) 39(1) 5(1) 25(1) 8(1) Mn(1) 15(1) 19(1) 14(1) 0(1) 1(1) 0(1) C(1) 17(1) 21(1) 11(1) 0(1) 3(1) -1(1) O(3) 19(1) 38(1) 25(1) 2(1) 1(1) -5(1) C(2) 16(1) 26(2) 16(1) -1(1) 6(1) 3(1) C(3) 24(1) 19(1) 23(1) 2(1) 3(1) -4(1) C(4) 17(1) 17(1) 13(1) 0(1) 1(1) 0(1) C(5) 18(1) 17(1) 21(1) 1(1) -1(1) -2(1) O(2) 25(1) 40(1) 23(1) 7(1) 3(1) 6(1) C(6) 17(1) 20(1) 19(1) -3(1) -1(1) -2(1) C(7) 18(1) 17(1) 15(1) 1(1) 2(1) 2(1) C(8) 20(1) 25(2) 16(1) 0(1) 0(1) 3(1) C(9) 17(1) 16(1) 19(1) 0(1) 0(1) 0(1) C(10) 16(1) 22(2) 12(1) 2(1) 2(1) 0(1) O(1) 35(1) 28(1) 28(1) -7(1) 10(1) -3(1) C(11) 17(1) 22(2) 20(1) 1(1) -2(1) -3(1) C(12) 18(1) 19(1) 13(1) 0(1) 3(1) 1(1) C(13) 26(2) 19(2) 30(1) 3(1) 1(1) -3(1) C(14) 19(1) 23(2) 33(1) 1(1) 3(1) 5(1) C(15) 31(2) 34(2) 18(1) -1(1) -5(1) -3(1) C(16) 23(2) 24(2) 21(1) 2(1) 4(1) -3(1) C(17) 24(1) 28(2) 21(1) -5(1) -5(1) 0(1) C(18) 20(1) 18(1) 24(1) 3(1) -5(1) 2(1) C(19) 18(1) 26(2) 24(1) -3(1) 2(1) 1(1) C(20) 17(1) 21(2) 14(1) 3(1) 2(1) -3(1) C(21) 20(1) 18(1) 20(1) -3(1) -1(1) -2(1) F(3) 52(1) 26(1) 37(1) 3(1) 3(1) 8(1) F(2) 30(1) 24(1) 34(1) -3(1) 6(1) 2(1) F(4) 39(1) 62(2) 53(1) -22(1) -9(1) -7(1) B(1) 32(2) 26(2) 25(2) -2(1) 7(1) 0(2) _______________________________________________________________________ 69 Table 3-C1-5. Hydrogen coordinates ( x 10^4) and isotropic displacement parameters (A^2 x 10^3) for [1b]BF4. ________________________________________________________________ x y z U(eq) ________________________________________________________________ H(3A) 2058 2678 2243 33 H(3B) 1254 1894 1752 33 H(3C) 479 2884 1888 33 H(5A) 3113 2635 996 22 H(5B) 1530 2854 645 22 H(6A) -954 4769 826 23 H(6B) -388 3714 874 23 H(9A) -200 6136 393 21 H(9B) 802 6985 679 21 H(11A) 5687 6334 1824 24 H(11B) 6261 5277 1889 24 H(13A) 4829 7426 838 37 H(13B) 3884 8321 1075 37 H(13C) 3256 7602 467 37 H(14A) 5643 3057 764 37 H(14B) 6814 2880 1417 37 H(14C) 6697 3887 1031 37 H(15A) 4455 6199 3003 41 H(15B) 6132 5908 3125 41 H(15C) 4895 5118 3060 41 H(17A) -125 4858 -449 37 H(17B) -1287 4029 -353 37 H(17C) 427 3797 -401 37 H(18A) 4467 3071 1973 25 H(18B) 5502 3914 2227 25 H(19A) -1102 5948 1675 33 H(19B) -1545 6942 1331 33 H(19C) -204 6865 1906 33 H(21A) 2122 7356 1667 23 H(21B) 3695 7155 2024 23 70 Table 3-C2-1. Crystal data and structure refinement for [2b]PF6. _____________________________________________________________________ Identification code [2b]PF6 Empirical formula C42 H60 F12 O6 P2 Re2 Formula weight 1323.24 Temperature 296(2) K Wavelength 0.71073 A Crystal system, space group ?, ? Unit cell dimensions a = 19.922(5) A alpha = 90 deg. b = 23.118(5) A beta = 130.666(5) deg. c = 13.913(5) A gamma = 90 deg. Volume 4860(2) A^3 Z, Calculated density 4, 1.808 Mg/m^3 Absorption coefficient 5.132 mm^-1 F(000) 2592 Crystal size 0.20 x 0.20 x 0.10 mm Theta range for data collection 1.61 to 30.32 deg. Limiting indices -28<=h<=27, -32<=k<=32, -19<=l<=15 Reflections collected / unique 45814 / 6864 [R(int) = 0.0290] Completeness to theta = 30.32 94.1 % Max. and min. transmission 0.6279 and 0.4268 Refinement method Full-matrix least-squares on F^2 Data / restraints / parameters 6864 / 42 / 295 Goodness-of-fit on F^2 1.165 Final R indices [I>2sigma(I)] R1 = 0.0633, wR2 = 0.1875 R indices (all data) R1 = 0.0768, wR2 = 0.1970 Largest diff. peak and hole 5.841 and -1.671 e.A^-3 71 Table 3-C2-2. Atomic coordinates ( x 10^4) and equivalent isotropic displacement parameters (A^2 x 10^3) for [2b]PF6. U(eq) is defined as one third of the trace of the orthogonalized Uij tensor. ________________________________________________________________ x y z U(eq) ________________________________________________________________ Re(1) 2439(1) 569(1) 7362(1) 37(1) O(1) 1611(7) -196(4) 5041(9) 78(3) O(2) 4081(6) -204(4) 8917(13) 118(5) O(3) 1522(8) -206(4) 7999(10) 89(3) C(1) 1928(8) 58(4) 5912(10) 52(2) C(2) 3461(8) 63(5) 8361(14) 77(4) C(3) 1880(8) 65(4) 7779(10) 53(2) C(4) 1962(6) 1366(4) 6012(8) 40(2) C(5) 2912(6) 1374(4) 6904(9) 44(2) C(6) 3367(6) 1383(4) 8230(9) 44(2) C(7) 2902(6) 1381(4) 8688(9) 48(2) C(8) 1956(6) 1377(4) 7787(9) 42(2) C(9) 1484(5) 1375(4) 6468(8) 39(2) C(10) 1473(7) 1375(4) 4610(9) 51(2) C(11) 3426(8) 1409(5) 6441(12) 61(3) C(12) 4377(7) 1422(5) 9182(11) 60(3) C(13) 3405(8) 1442(5) 10102(10) 60(3) C(14) 1445(8) 1403(5) 8258(11) 56(2) C(15) 481(6) 1407(4) 5511(10) 50(2) C(16) 1297(9) 1990(5) 4111(12) 69(3) C(17) 3674(9) 822(6) 6263(14) 73(3) C(18) 4666(9) 2063(6) 9425(15) 85(4) C(19) 3681(10) 844(7) 10798(12) 79(4) C(20) 1261(11) 2001(6) 8392(15) 77(4) C(21) 12(7) 831(6) 5078(15) 81(4) P(1) 2404(1) 3296(1) 7510(2) 29(1) F(1) 3282(10) 3301(8) 7720(20) 293(11) F(2) 2934(17) 3039(14) 8827(17) 360(14) F(3) 1578(9) 3311(8) 7342(18) 187(6) F(4) 2594(12) 3904(7) 8000(30) 303(11) F(5) 2157(12) 2725(7) 6850(20) 267(9) F(6) 1924(14) 3600(12) 6295(18) 292(10) ________________________________________________________________ 72 73 Table 3-C2-3. Bond lengths [A] and angles [deg] for [2b]PF6. _____________________________________________________________ Re(1)-C(2) 1.939(11) Re(1)-C(3) 1.944(10) Re(1)-C(1) 1.957(10) Re(1)-C(4) 2.345(8) Re(1)-C(8) 2.346(9) Re(1)-C(6) 2.346(9) Re(1)-C(5) 2.354(9) Re(1)-C(9) 2.355(8) Re(1)-C(7) 2.358(10) O(1)-C(1) 1.102(13) O(2)-C(2) 1.123(14) O(3)-C(3) 1.133(13) C(4)-C(5) 1.436(12) C(4)-C(9) 1.447(12) C(4)-C(10) 1.510(13) C(5)-C(6) 1.431(13) C(5)-C(11) 1.524(14) C(6)-C(7) 1.427(14) C(6)-C(12) 1.528(13) C(7)-C(8) 1.430(13) C(7)-C(13) 1.524(14) C(8)-C(9) 1.416(13) C(8)-C(14) 1.529(13) C(9)-C(15) 1.518(12) C(10)-C(16) 1.519(15) C(10)-H(10A) 0.9700 C(10)-H(10B) 0.9700 C(11)-C(17) 1.518(17) C(11)-H(11A) 0.9700 C(11)-H(11B) 0.9700 C(12)-C(18) 1.546(16) C(12)-H(12A) 0.9700 C(12)-H(12B) 0.9700 C(13)-C(19) 1.567(17) C(13)-H(13A) 0.9700 C(13)-H(13B) 0.9700 C(14)-C(20) 1.475(16) C(14)-H(14A) 0.9700 C(14)-H(14B) 0.9700 C(15)-C(21) 1.509(15) 74 C(15)-H(15A) 0.9700 C(15)-H(15B) 0.9700 C(16)-H(16A) 0.9600 C(16)-H(16B) 0.9600 C(16)-H(16C) 0.9600 C(17)-H(17A) 0.9600 C(17)-H(17B) 0.9600 C(17)-H(17C) 0.9600 C(18)-H(18A) 0.9600 C(18)-H(18B) 0.9600 C(18)-H(18C) 0.9600 C(19)-H(19A) 0.9600 C(19)-H(19B) 0.9600 C(19)-H(19C) 0.9600 C(20)-H(20A) 0.9600 C(20)-H(20B) 0.9600 C(20)-H(20C) 0.9600 C(21)-H(21A) 0.9600 C(21)-H(21B) 0.9600 C(21)-H(21C) 0.9600 P(1)-F(6) 1.470(11) P(1)-F(5) 1.495(10) P(1)-F(4) 1.501(11) P(1)-F(3) 1.505(10) P(1)-F(2) 1.517(11) P(1)-F(1) 1.577(10) 75 C(2)-Re(1)-C(3) 87.8(6) C(2)-Re(1)-C(1) 88.4(5) C(3)-Re(1)-C(1) 86.1(4) C(2)-Re(1)-C(4) 137.5(5) C(3)-Re(1)-C(4) 134.3(4) C(1)-Re(1)-C(4) 89.0(4) C(2)-Re(1)-C(8) 135.4(5) C(3)-Re(1)-C(8) 89.6(4) C(1)-Re(1)-C(8) 135.9(4) C(4)-Re(1)-C(8) 63.8(3) C(2)-Re(1)-C(6) 90.5(4) C(3)-Re(1)-C(6) 137.7(4) C(1)-Re(1)-C(6) 136.1(4) C(4)-Re(1)-C(6) 63.6(3) C(8)-Re(1)-C(6) 63.1(3) C(2)-Re(1)-C(5) 104.7(5) C(3)-Re(1)-C(5) 164.5(4) C(1)-Re(1)-C(5) 103.3(4) C(4)-Re(1)-C(5) 35.6(3) C(8)-Re(1)-C(5) 75.0(3) C(6)-Re(1)-C(5) 35.5(3) C(2)-Re(1)-C(9) 164.9(4) C(3)-Re(1)-C(9) 102.1(4) C(1)-Re(1)-C(9) 103.6(4) C(4)-Re(1)-C(9) 35.9(3) C(8)-Re(1)-C(9) 35.1(3) C(6)-Re(1)-C(9) 74.5(3) C(5)-Re(1)-C(9) 63.9(3) C(2)-Re(1)-C(7) 103.2(5) C(3)-Re(1)-C(7) 104.6(4) C(1)-Re(1)-C(7) 164.4(4) C(4)-Re(1)-C(7) 75.5(3) C(8)-Re(1)-C(7) 35.4(3) C(6)-Re(1)-C(7) 35.3(3) C(5)-Re(1)-C(7) 64.0(3) C(9)-Re(1)-C(7) 63.5(3) O(1)-C(1)-Re(1) 174.9(11) O(2)-C(2)-Re(1) 174.8(15) O(3)-C(3)-Re(1) 176.4(11) C(5)-C(4)-C(9) 119.6(8) C(5)-C(4)-C(10) 119.6(8) C(9)-C(4)-C(10) 120.8(8) C(5)-C(4)-Re(1) 72.5(5) C(9)-C(4)-Re(1) 72.5(5) 76 C(10)-C(4)-Re(1) 129.0(6) C(6)-C(5)-C(4) 119.0(8) C(6)-C(5)-C(11) 120.5(9) C(4)-C(5)-C(11) 120.4(9) C(6)-C(5)-Re(1) 72.0(5) C(4)-C(5)-Re(1) 71.9(5) C(11)-C(5)-Re(1) 130.8(7) C(7)-C(6)-C(5) 121.7(8) C(7)-C(6)-C(12) 119.0(9) C(5)-C(6)-C(12) 119.2(9) C(7)-C(6)-Re(1) 72.8(5) C(5)-C(6)-Re(1) 72.6(5) C(12)-C(6)-Re(1) 130.1(7) C(6)-C(7)-C(8) 118.5(9) C(6)-C(7)-C(13) 120.3(9) C(8)-C(7)-C(13) 121.0(10) C(6)-C(7)-Re(1) 71.9(6) C(8)-C(7)-Re(1) 71.8(5) C(13)-C(7)-Re(1) 132.5(7) C(9)-C(8)-C(7) 121.3(9) C(9)-C(8)-C(14) 119.4(8) C(7)-C(8)-C(14) 119.3(9) C(9)-C(8)-Re(1) 72.8(5) C(7)-C(8)-Re(1) 72.8(5) C(14)-C(8)-Re(1) 129.5(7) C(8)-C(9)-C(4) 119.9(8) C(8)-C(9)-C(15) 121.3(8) C(4)-C(9)-C(15) 118.8(8) C(8)-C(9)-Re(1) 72.1(5) C(4)-C(9)-Re(1) 71.7(5) C(15)-C(9)-Re(1) 130.6(6) C(4)-C(10)-C(16) 111.4(9) C(4)-C(10)-H(10A) 109.3 C(16)-C(10)-H(10A) 109.3 C(4)-C(10)-H(10B) 109.3 C(16)-C(10)-H(10B) 109.3 H(10A)-C(10)-H(10B) 108.0 C(17)-C(11)-C(5) 113.7(10) C(17)-C(11)-H(11A) 108.8 C(5)-C(11)-H(11A) 108.8 C(17)-C(11)-H(11B) 108.8 C(5)-C(11)-H(11B) 108.8 H(11A)-C(11)-H(11B) 107.7 C(6)-C(12)-C(18) 109.8(9) 77 C(6)-C(12)-H(12A) 109.7 C(18)-C(12)-H(12A) 109.7 C(6)-C(12)-H(12B) 109.7 C(18)-C(12)-H(12B) 109.7 H(12A)-C(12)-H(12B) 108.2 C(7)-C(13)-C(19) 112.8(9) C(7)-C(13)-H(13A) 109.0 C(19)-C(13)-H(13A) 109.0 C(7)-C(13)-H(13B) 109.0 C(19)-C(13)-H(13B) 109.0 H(13A)-C(13)-H(13B) 107.8 C(20)-C(14)-C(8) 112.4(9) C(20)-C(14)-H(14A) 109.1 C(8)-C(14)-H(14A) 109.1 C(20)-C(14)-H(14B) 109.1 C(8)-C(14)-H(14B) 109.1 H(14A)-C(14)-H(14B) 107.9 C(21)-C(15)-C(9) 115.2(9) C(21)-C(15)-H(15A) 108.5 C(9)-C(15)-H(15A) 108.5 C(21)-C(15)-H(15B) 108.5 C(9)-C(15)-H(15B) 108.5 H(15A)-C(15)-H(15B) 107.5 C(10)-C(16)-H(16A) 109.5 C(10)-C(16)-H(16B) 109.5 H(16A)-C(16)-H(16B) 109.5 C(10)-C(16)-H(16C) 109.5 H(16A)-C(16)-H(16C) 109.5 H(16B)-C(16)-H(16C) 109.5 C(11)-C(17)-H(17A) 109.5 C(11)-C(17)-H(17B) 109.5 H(17A)-C(17)-H(17B) 109.5 C(11)-C(17)-H(17C) 109.5 H(17A)-C(17)-H(17C) 109.5 H(17B)-C(17)-H(17C) 109.5 C(12)-C(18)-H(18A) 109.5 C(12)-C(18)-H(18B) 109.5 H(18A)-C(18)-H(18B) 109.5 C(12)-C(18)-H(18C) 109.5 H(18A)-C(18)-H(18C) 109.5 H(18B)-C(18)-H(18C) 109.5 C(13)-C(19)-H(19A) 109.5 C(13)-C(19)-H(19B) 109.5 H(19A)-C(19)-H(19B) 109.5 78 C(13)-C(19)-H(19C) 109.5 H(19A)-C(19)-H(19C) 109.5 H(19B)-C(19)-H(19C) 109.5 C(14)-C(20)-H(20A) 109.5 C(14)-C(20)-H(20B) 109.5 H(20A)-C(20)-H(20B) 109.5 C(14)-C(20)-H(20C) 109.5 H(20A)-C(20)-H(20C) 109.5 H(20B)-C(20)-H(20C) 109.5 C(15)-C(21)-H(21A) 109.5 C(15)-C(21)-H(21B) 109.5 H(21A)-C(21)-H(21B) 109.5 C(15)-C(21)-H(21C) 109.5 H(21A)-C(21)-H(21C) 109.5 H(21B)-C(21)-H(21C) 109.5 F(6)-P(1)-F(5) 90.5(16) F(6)-P(1)-F(4) 81.9(16) F(5)-P(1)-F(4) 172.4(17) F(6)-P(1)-F(3) 89.1(11) F(5)-P(1)-F(3) 92.1(10) F(4)-P(1)-F(3) 88.5(10) F(6)-P(1)-F(2) 174.3(17) F(5)-P(1)-F(2) 95.1(16) F(4)-P(1)-F(2) 92.5(17) F(3)-P(1)-F(2) 91.5(11) F(6)-P(1)-F(1) 91.1(8) F(5)-P(1)-F(1) 90.1(7) F(4)-P(1)-F(1) 89.4(7) F(3)-P(1)-F(1) 177.9(10) F(2)-P(1)-F(1) 88.1(8) _____________________________________________________________ 79 Symmetry transformations used to generate equivalent atoms: Table 3-C2-4. Anisotropic displacement parameters (A^2 x 10^3) for [2b]PF6. The anisotropic displacement factor exponent takes the form: -2 pi^2 [ h^2 a*^2 U11 + ... + 2 h k a* b* U12 ] _______________________________________________________________________ U11 U22 U33 U23 U13 U12 _______________________________________________________________________ Re(1) 41(1) 25(1) 42(1) 0(1) 25(1) -1(1) O(1) 120(8) 57(5) 83(6) -22(4) 77(6) -19(5) O(2) 58(5) 55(5) 152(11) 5(6) 30(6) 13(4) O(3) 151(10) 61(5) 97(7) -2(5) 100(8) -20(6) C(1) 70(6) 38(5) 61(6) -8(4) 48(5) -10(4) C(2) 47(6) 45(6) 85(9) -5(6) 20(6) 8(5) C(3) 75(7) 42(5) 57(6) -9(4) 49(6) -13(5) C(4) 45(4) 29(4) 43(4) 1(3) 28(4) -2(3) C(5) 41(4) 38(4) 52(5) 2(4) 31(4) 0(3) C(6) 39(4) 36(4) 45(5) -2(3) 22(4) -1(3) C(7) 44(4) 43(5) 45(5) 0(4) 23(4) -5(4) C(8) 46(4) 30(4) 48(5) -3(3) 31(4) 0(3) C(9) 37(4) 33(4) 44(4) -1(3) 24(4) 1(3) C(10) 57(5) 48(5) 45(5) 2(4) 31(5) 0(4) C(11) 58(6) 65(7) 72(7) 6(5) 47(6) 0(5) C(12) 39(5) 57(6) 62(6) -2(5) 23(5) -6(4) C(13) 70(7) 55(6) 42(5) -3(4) 30(5) -3(5) C(14) 64(6) 61(6) 63(6) 6(5) 49(6) 4(5) C(15) 37(4) 46(5) 54(5) 5(4) 25(4) 3(4) C(16) 80(8) 67(7) 57(6) 17(5) 43(6) 15(6) C(17) 77(8) 76(8) 92(10) 10(7) 66(8) 14(7) C(18) 56(7) 70(8) 91(10) -4(7) 32(7) -29(6) C(19) 81(9) 85(9) 50(7) 9(6) 33(6) 9(7) C(20) 94(10) 76(8) 90(10) -1(7) 73(9) 14(7) C(21) 42(5) 62(7) 104(10) -2(7) 33(6) -13(5) P(1) 48(1) 16(1) 37(1) -6(1) 34(1) -8(1) F(1) 166(12) 390(30) 430(30) -110(20) 244(18) -35(13) F(2) 370(20) 580(30) 131(10) 179(14) 160(14) 300(30) F(3) 131(9) 266(17) 237(16) -42(13) 152(11) -38(10) F(4) 183(16) 162(12) 520(30) -206(16) 207(18) -81(10) F(5) 213(16) 133(10) 357(19) -139(12) 142(17) -18(10) 80 F(6) 360(20) 348(19) 230(14) 203(14) 216(18) 150(20) _______________________________________________________________________ 81 Table 3-C2-5. Hydrogen coordinates ( x 10^4) and isotropic displacement parameters (A^2 x 10^3) for [2b]PF6. ________________________________________________________________ x y z U(eq) ________________________________________________________________ H(10A) 915 1172 4159 61 H(10B) 1821 1174 4452 61 H(11A) 3071 1615 5641 73 H(11B) 3962 1631 7047 73 H(12A) 4617 1220 8852 72 H(12B) 4606 1239 9971 72 H(13A) 3036 1646 10217 72 H(13B) 3932 1673 10485 72 H(14A) 890 1196 7667 68 H(14B) 1786 1209 9072 68 H(15A) 284 1628 5880 60 H(15B) 304 1617 4773 60 H(16A) 869 2167 4127 104 H(16B) 1074 1983 3257 104 H(16C) 1838 2208 4634 104 H(17A) 4021 613 7049 110 H(17B) 4013 875 6002 110 H(17C) 3146 608 5627 110 H(18A) 4383 2254 8631 127 H(18B) 5296 2085 9939 127 H(18C) 4496 2249 9857 127 H(19A) 3169 601 10375 119 H(19B) 3934 904 11659 119 H(19C) 4108 663 10786 119 H(20A) 1807 2210 8962 116 H(20B) 962 1997 8722 116 H(20C) 892 2187 7578 116 H(21A) 292 575 4890 121 H(21B) -596 885 4332 121 H(21C) 40 666 5738 121 ________________________________________________________________ 82 3.7 References (1) Geiger, W. 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(11) DigiSim is a registered trademark of Bioanalytical Systems, Inc., West Lafayette, IN. 84 (12) It has been reported5d that reduction of (benzene)Mn(CO)3+ in CH3CN at -15oC is a 2-electron chemically reversible and electrochemically quasi-reversible process at a hanging mercury drop electrode. We have been unable to verify this result. 85 Chapter 4 Proton reduction catalyzed by aromatic manganese carbonyl complexes 2e + L + OC CO 2 H H2 86 4.1 Introduction Hydrogen is considered as a promising fuel alternative. Solar, wind power can be indirectly stored as hydrogen gas. The production of hydrogen from proton reduction is the simplest fuel generation reaction. Platinum is a great catalyst for this reaction and its reverse; however the high price and low abundance limits its usage in industry. Cheap metal catalyst is always the primary alternative. In the nature, the well known hydrogenase iron sulfur clusters, ferredoxin does a gread job on hydrogen production and oxidation. These biological catalysts can accept or discharge electrons. The core structure is iron atoms with the oxidation states switched between +2 and +3.1 One proposed structure in which azadithiolate promotes the bonding and heterolytic cleavage of hydrogen is illustrated in figure 2.2,3 Transition metal hydride complexes are attracting a lot of attention due to their intimate relationship to the dihydrogen oxidation and production. Early transition metal complexes mimic of these enzymes have been investigated extensively by a number of chemists. Figure 4-1. Reaction equilibrium of proton reduction and hydrogen oxidation. 87 Figure 4-2. Proposed hydrogen bonding and hyterolytic cleavage via an Fe2S2 ferredoxin Hydrogenase mimics, nickel [Ni(PR2NR’2)2]2+ (R = Ph, Cy; R’= Ph, Bz) 3and cobalt phosphine systems 4 designed by Dubois group were proved to catalyze proton reduction at relative low overpotential5 based on the following mechanism shown in scheme 4-1 . Scheme 4-1. The mechanism of hydrogen oxidation and production catalyzed by [Ni(PR2NR’2)2]2+. Mechanism for proton reduction is shown anticlockwisely; Mechanism for hydrogen is shown clockwisely. 88 Dubois concluded that two positioned pendant bases are superior to complexes having two bases that are not positioned near the metal center. This pendant bases priorities were also supported by computational studies for the nickel complexes.6 Also Peters group found similar result using their aryl-substituted 7,8 tetraimine cobalt complexes (a). Similar research was conducted on [(η5-C5H5)Fe(CO)2]2 9, which is a procatalyst of the proton reduction catalyzed by (η5-C5H5)Fe(CO)2- anion. Organomanganese compound wasn’t recognized as a good candidate for catalysis, the most wildly used manganese complex in the industry is methylcyclopentadienyl manganese tricarbonyl (MMT, which is used to increase the fuel's octane rating). Thus, there are very few examples of its catalytic properties. 10 Ustynyuk reported the unprecedented observation of a catalytic electrochemical proton reduction based on metallocumulene complexes: manganese diphenylallenylidene (η5-C5H5) (CO)2Mn=C=C=CPh2 is shown to catalyze the reduction of protons from HBF4 to H2 at -0.84 V (in acetonitrile) vs. Fc/Fc+. The scheme is shown as following. 89 Scheme 4-2. Proposed mechanism of proton reduction catalyzed by (η5-C5H5) (CO)2Mn=C=C=CPh2 10 Ustynyuk proposed an uncommon mechanism for dihydrogen production, which is the formation of hydrogen radicals followed by their recombination into H2 gas. He tentatively proposed that the driving force might be the reformation of the stable 18 electrons configuration. The real intermediate hydrogen species remains unclear. No matter what metal was used, the general reaction pathway of dihydrogen production was suggested to go through two parallel routes11: (1) bimolecular reductive elimination of dihydrogen from two metal hydrides; (2) dihydrogen generation from protonation of metal hydride complex 90 Scheme 4-3. Homolytic (left) and heterolytic (right) mechanism for proton reduction catalyzed by a organometallic compound.12 In chapter 3, the mechanistic insight of the reductive electrochemistry of (η6-HMB)Mn(CO)3+ was introduced13 (scheme 4-4). There are several intermediates generated from the reduction of the starting material, one of them is (η6-HMB)Mn(CO)2- anion which was chemically synthesized by Eyman14,15 This dicarbonyl anion is 18 electrons, but very reactive because of the negative charge on the manganese. It is “extremely air and moisture sensity”16 And it reacts with the starting material stoichiometrically to afford the known green color [(η6-HMB)Mn(CO)2]2 dimer16, which perfectly matches the cyclic voltammetry of the (η6-HMB)Mn(CO)3+. Most importantly, compound 4 reacts with strong acid to liberate dihydrogen. Compound 1 is a great procatalyst for this proton reduction reaction; it can survive in strong acid. The molecular structure was modified, another Mn (I) analog, (η5-C5H5)Mn(CO)2NO+ which can be reduced at -0.8V vs Fc/Fc+ caught our attention. Not surprisingly, the potential of proton reduction shifted around 1V towards the positive side, which was a big improvement. Herein, we want to report the first example of proton reduction catalyzed by 91 aromatic manganese carbonyl complex based on metal hydride mechanism. These aromatic manganese derivatives are promising catalysts for proton reduction. The proton catalytic reduction can be achieved at -1.7V vs Fc/Fc+ when [(η6-HMB)Mn(CO)3]PF6 is used as procatalyst while -0.8V vs Fc/Fc+ when [(η5-C5H5)Mn(CO)2NO]BF4 is used. The efficiency of both is not high. We are working on optimizing the molecular structure to improve the efficiency of this class of catalyst. Scheme 4-4． Mechanism of (η6-HMB)Mn(CO)3+ reduction electrochemistry Moreover, (η5-hydroarene) manganese tricarbonyl complex, such as polyarene (η5- hydronaphthalene) Mn(CO)3 or monoarene (η5- hydrobenzene) Mn(CO)3, could react with proton gradually to liberate dihydrogen gas and re-generate (η6-arene) Mn(CO)3+ cation. Especially, (η6-naphthalene) Mn(CO)3+ can 92 be electrochemically reduced at -1V vs Fc/Fc+ and form (η5-hydronaphthalene) Mn(CO)3 complex in the presence of proton. Since the protonation of (η5-hydronaphthalene) Mn(CO)3 is slow process (takes overnight), (eta6- polyarene) manganese tricarbonyl cations have potential usage to transfer and store electric energy to stable “metal-hydride” complex and releases the hydrogen gas when needed, and the resulting manganese complex can be reused. 4.2 Experiments 1. Synthesis Solvents were purchased from commercial sources as HPLC grade. Methylene chloride and acetonitrile solvents were stored and opened under nitrogen. [(η6-hexamethylbenzene)Mn(CO)3]BF4. Acenaphthene manganese tricarbonyl tetrafluoroborate (0.760 g, 2.00 mmol) and hexamethylbenzene (0.504 g, 4.00 mmol) were combined with 17 ml methylene chloride (Fisher Scientific Co.) in a 20 ml pressure tube under nitrogen. The tube was sealed, wrapped in aluminum foil, and placed in a 75 oC oil bath for 2 hrs. The solvent was then removed and the yellow solid residue was washed with diethyl ether to afford the product in 75% yield (0.528 g). A crystal suitable for X-ray analysis was obtained by diethyl ether diffusion into a methylene chloride solution at room temperature. IR (CH2Cl2, cm-1): 2060, 2000. (η6-hexamethylbenzene)Mn(CO)2I, (η6-hexamethylbenzene)Mn(CO)2H and [(η6-hexamethylbenzene)Mn(CO)2]Li. All above three compounds were synthesized via published literature method.16 (η6-hexamethylbenzene)Mn(CO)2I was 93 synthesized from [(η6-hexamethylbenzene)Mn(CO)3]+ with 1 equivalent of Me3NO and TBAI, and was purified by running through a silica gel column, dichloromethane was used as eluent. Then (η6-hexamethylbenzene)Mn(CO)2I was treated with 1 equivalent of TBABH4 in THF to affoard (η6-hexamethylbenzene)Mn(CO)2H, which was purified via washing with hexane. [(η6-hexamethylbenzene)Mn(CO)2]Li was synthesized from deprotonation of (η6-hexamethylbenzene)Mn(CO)2H using t-butyl lithium. Since [(η6-hexamethylbenzene)Mn(CO)2]Li is very reactive, it was used as made. [(η5-methylcyclopentadienyl)Mn(CO)2NO]BF4. (η5-Methylcyclopentadienyl)Mn(CO)3 was purchased from Aldrich and used as it was. 2ml (2.760g, 12.67mmol) of (η5-Methylcyclopentadienyl)Mn(CO)3 and nitrosyl tetrafluoroborate NOBF4 (1.776g, 15.20mmol) were combined with 15ml acetone in 50ml round bottom flask. The flask was cooled in the ice bath. There was a lot of gas bubbling inside the flask while acetone was added. The color of the solution changed from light yellow to dark brown. After 10mins, IR indicated that all starting material was gone and the reaction completed. Then acetone was removed by blowing nitrogen into the solution. Diethyl ether was added to wash the solid several times. The product was recrystallized by diffusing ether into its acetone solution, and the yield is 90% (3.450g, 11.40mmol). IR (CH2Cl2, cm-1): 2118, 2076 for CO; 1840 for NO. [(η6-naphthalene)Mn(CO)3]BF4. Manganese pentacarbonyl bromide (1.280g, 4.65mmol) and silver tetrafluoroborate (1.000g, 5.14mmol) were combined 94 with 50ml dichloromethane in a 100ml round bottom flask under nitrogen. The mixture was stirred and heated to reflux for 2 hours. 2equivlent of naphthalene (1.317g, 10.28mmol) was added to the solution and heated to reflux for 8 hours. The product was recrystallized by diffusing ether into its dichloromethane solution, and the yield is 70% (1.153g, 3.26mmol) IR in dichloromethane 2077, 2019 cm-1 (η5-hydronaphthalene)Mn(CO)3 [(η6-naphthalene)Mn(CO)3]BF4 (0.354g, 1mmol) and tetrabutylammonium borohydride (0.283g, 1.1mmol) were combined with 20ml dichloromethane in a 50ml round bottom flask in ice bath. The reaction mixture was stirred for 30mins in ice bath and then 30mins at room temperature. The product was purified by running through the silica gel column using dichloromethane as eluent. IR in dichloromethane 2009, 1929 cm-1. IR in hexane 2017, 1943 and 1931cm-1 2. Protonation of (η5-hydronaphthalene)Mn(CO)3 (η5-hydronaphthalene)- Mn(CO)3 was dissolved in diethyl ether, excess HBF4·Et2O was added. The solution was stirred and after 5mins, yellow solid precipitated out, which turned out to be [(η6-naphthalene)Mn(CO)3]BF4. 3. Electrochemistry Electrochemical instrumentation and the source and treatment of solvents and supporting electrolytes have been reported earlier, in chapter 3. All potentials are reported vs the potential of the silver chloride reference electrode. The voltammetric experiments were conducted at room temperature, 298 K. 4. Further Experiments 95 This research project is still undergoing, only preliminary data is demonstrated in chapter 4. There are further experiments to be done. (1) Bulk electrolysis and in-situ IR will be incorporated to a dichloromethane solution of [(η6-HMB)Mn(CO)3]BF4 and excess HBF4 · Et2O to detect proposed reduction intermediate (η6-HMB)Mn(CO)2H and (η6-HMB)Mn(CO)2Cl or [(η6-HMB)Mn(CO)2Et2O]+. (2) Bulk electrolysis and in-situ IR will be incorporated to a dichloromethane solution of [(η5-cyclopentadienyl)Mn(CO)2NO] BF4 to investigate its electrochemical reduction mechanism, which was proposed to mimic the one of [(η6-HMB)Mn(CO)3]BF4. (3) After understanding [(η5-cyclopenta -dienyl)Mn(CO)2NO]BF4, the reductive electrochemistry of [(η5-cyclopentadienyl) Mn(CO)2NO]BF4, bulk electrolysis will be applied to CH2Cl2 solution of [(η5-cyclopentadienyl)Mn(CO)2 NO]BF4 and excess HBF4· Et2O as proton source. GC will be used to confirm the existence of dihydrogen. (4) Further optimization was proposed in the result and discussion section. 4.3 Result and discussion 1. Aromatic manganese dicarbonyl Compound 4, the reduction intermediate of procatalyst 1, can be easily oxidized and protonated16 when proton source presents, to afford a neutral manganese hydride complex (η6-HMB)Mn(CO)2H, which can be further protonated to liberate dihydrogen and [(η6-HMB)Mn(CO)2L+] (L is solvent). [(η6-HMB)Mn(CO)2+] is a 96 16-electron species, which was never isolated by itself. [(η6-HMB)Mn(CO)2+] is readily accessible by numbers of non or less coordination ligand, such as ether. It can even grab a chloride from dichloromethane to form a neutral (η6-HMB)Mn(CO)2Cl. + Mn + Mn OC NMe3 OC Cl CO CO 10 11 Figure 4-3. Synthesis of (η6-hexamethylbenzene) manganese dicarbonyl complex using trimethylamine N-oxide Figure 4-3 shows typical for synthesizing η6-arene manganese dicarbonyl complex using trimethylamine N-oxide as an oxidant. When dichloromethane is the solvent, a mixture of compound 10 and 11 is the only product. While in the presence of halide (figure 4), compound 12 is the only product instead of 10 and 11 (figure 3). Eyman reported the very first method of synthesizing (η6-hexamethylbenzene) manganese dicarbonyl hydride (compound 13) chemically by using tetrabutylammonium borohydride as a hydride source. Figure 4-4. Synthesis of (η6-hexamethylbenzene) manganese dicarbonyl anion. Further deprotonation of 13 with tetrabutyllithium results in 4 (figure 4-4), which is a very strong nucleophile. It reacts with 1 stoichiometrically to form a 97 heterodinuclear complex with core structure of Mn-Mn metal-metal bond (compound 5),16 with the characteristic bridging CO ligand. Eyman16 reported hydrogen formation and compound 14 from reaction between 13 and HBF4·Et2O. There isn’t enough information about the reactivity of 4, however it is reasonable to believe that 4 would be oxidized and protonated easily by HBF4·Et2O ( or even weaker acid) to form 13 or 14 (if excess HBF4·Et2O presents) (figure 4-5). HBF4 . Et2O HBF4 . Et2O Mn _ Mn + + H2 Mn OC OC H OC OEt2 CO CO CO 4 13 14 Figure 4-5. Protonation of (η6-hexamethylbenzene) manganese dicarbonyl anion. 2. Hydrogen formation Through the above information, we conclude that the reduction product (from compound 1) compound 4 would react with proton and liberate dihydrogen to affoard a very reactive 16-electron species compound 9, which will react with solvent or any coordination ligand to sustain its strong desire to become stable 18-electron structure, such as compound 10, 11, 14 and 15 (if using trifluoroacetic acid as proton source) As discussed in previous chapter (chapter 3), the reduction electrochemistry of compound 10, 14 (in the case of its analog [(η6-HMB)Mn(CO)2THF]+ cation) is sharing the same reaction pathway with compound 117. However it might not be the case that when under continuous reduction potential with proton present, unreacted compound 9 might partially be reduced back to compound 4 (the reduction potential 98 of reducing compound 9 to 4 must be much more positive than the one of compound 1), and then oxidized and protonated by proton and liberate the dihydrogen gas and …… compound 9, This is a full cycle of catalyzing proton reduction via (η6-hexamethylbenzene) manganese dicarbonyl anion, and the initial starting material 1 is inert to strong acidic environment. 3. Ion analog The aromatic ion carbonyl complex (η5-C5H5)Fe(CO)2H shares a lot of similarities with aromatic manganese complexes in my study.9 According to the calculation and scale established for transition metal hydride by Norton and co-workers18, the pKa of the (η6-C6H6)Mn(CO)2H is 26.8, and the pKa of methylated complex (η6-C6Me6)Mn(CO)2H is in the range between 33 and 35. This makes compound 13 the least acidic and most hydridic compound ever characterized. In comparision, the pKa of (η5-C5H5)Fe(CO)2H is reported to be 19.4, which is much lower than the Manganese hydride analog. This explains why (η6-C6Me6)Mn(CO)2H can react with acid easily while (η5-C5H5)Fe(CO)2H is stable upon acid contacts. The catalytic cycle is shown below: 99 Scheme 4-5. [(η5-C5H5)Fe(CO)2]2 (Fp2) reduction followed by catalytic reduction of proton to hydrogen by (η5-C5H5)Fe(CO)2- (Fp-) In scheme 4-5, (η5-C5H5)Fe(CO)2- (Fp-) anion is formed upon the electrochemical reduction of [(η5-C5H5)Fe(CO)2]2. Fp- is then protonated by various kinds of acid to afford a metal hydride complex, FpH, which undergoes further reduction at more negative potentials to afford FpH-. To finish a full catalytic cycle, FpH- will be protonated and liberate hydrogen gas and Fp, and then Fp will be reduced by one electron to yield Fp-. Protonation of Fp- is found to be the rate determined step. The potential for catalytic reduction of 4-tertbutylphenol is -2.6V vs Fc/Fc+, with 0.8V overpotential. 4. Electrochemical formation and protonation of Arene-Mn-hydride complex The direct energetically downhill step of metal hydride protonation to produce molecular hydrogen may be common for sufficiently electron rich metal hydrides and/or sufficiently strong acids among many of the hydrogenase mimics reported thus far. Aromatic ion carbonyls system gives us a good example of how our 100 aromatic manganese catalyst works. Thus, we proposed a reaction mechanism for the proton reduction catalysis. Scheme 4-6. Proposed mechanism of proton reduction catalyzed by (η6-hexamethylbenzene) manganese dicarbonyl anion. The cyclic voltammetry (figure 4-7) also support our proposed mechanism. In dichloromethane, when there was acid present (for example, HBF4·Et2O), as the acid concentration increasing, the cyclic voltammetry of [(η6-HMB)Mn(CO)3]+ changed: The reduction peak current kept increasing, the oxidation peak current corresponding to the [(η6-HMB)Mn(CO)2]2 dimer kept decreasing until fully vanished. We propose that the increase of reduction peak current is the result of two facts: (1) With the presence of acid, the reduction of compound 1 consumes 2-electron instead of 101 1-electron. (2) Plausible proton catalytic reduction. When the concentration of acid is strong enough, most compound 4 would react with proton instead of compound 1 and liberate dihydrogen. This explains the diminishing of oxidation peak current of compound 5. Figure 4-6. Reduction electrochemistry of compound 1 changes from 1-electron without acid to 2-electron with the presence of acid. With the completely suppression of dimer formation, we expect to see peak current change from 1-electron to 2-electron. The reaction pathway is illustrated in figure 4-6. Note that for a control experiment, when only altering the number of electron transferred from 1e- to 2e-, the peak current should increase by 2.85 times. i p = (2.69×105 )n3 / 2 AD1/ 2v1/ 2C* Formula 4-1. The peak current in a cyclic voltammogram containing only one species is described by above fomula at 25 °C where ip is the peak current, n is the number of electrons transferred, A is the electrode area, D is the diffusion coefficient of the species, v is the scan rate and C* is the bulk concentration of the species. n is the number of electrons transferred. 102 Enhanced 4. proton Yes Mn Yes acid reduction 3. Yes Mn no acid 2. no Mn Yes acid 1. no Mn no acid Figure 4-7. CVs of [(η6-HMB)Mn(CO)3]PF6 with and without the presence of HBF4 in CH2Cl2. Cyclic voltammetry (1). In red, blank solution of dichloromethane with only 0.1M TBAPF6 present. (2). In blue, 50mM HBF4 with 0.1M TBAPF6 in dichloromethane solution. (3). In green, 1mM [(η6-HMB)Mn(CO)3]PF6 in the presence of no acid. (4). In light blue, 1mM [(η6-HMB)Mn(CO)3]PF6 in the presence of 6.6mM HBF4. Glassy carbon electrode was used and scan rate was 50mV/s, 0.1M TBAPF6 was used as electrolyte. HBF4 acid is in the form of HBF4 · O(CH2CH3)2. Nitrogen was used to bubble through the solution before experiment and bubble above the solution during the experiment. From the above CVs in figure 4-7, we did see a big current jump when there is enough acid to suppress the formation of dimer. A reasonable explanation is that, in the CV time scale, the increase of peak current mostly comes from the change of electrons transferred (1e- to 2e-), and (η6-HMB)Mn(CO)2H would be the primary product. Of course, we can not exclude the formation of dihydrogen from protonation 103 of (η6-HMB)Mn(CO)2H, and as a result, partially contributing to the peak current increase. From the extent of current jump, we would point out that, the contribution of peak current increase from proton catalytic reduction is limited. Furthermore, the above CVs also imply that the protonation of (η6-HMB)Mn(CO)2H is rate determined step, proton catalytic reduction may not complete a cycle in the CV time scale. Even though small portion of proton already has been reduced without the presence of our manganese procatalyst (η6-HMB)Mn(CO)3+, the generation (η6-HMB)Mn(CO)2H would certainly speed up the process of proton reduction. 2 1 Figure 4-8. CVs of [(η6-HMB)Mn(CO)3]PF6 with and without the presence of HBF4 in acetonitrile. Cyclic voltammetry (1). In red, 1mM [(η6-HMB)Mn(CO)3]PF6 in the presence of no acid. (2). In blue, 6.6mM HBF4 with 0.1M TBAPF6 in dichloromethane solution. Glassy carbon electrode was used and scan rate was 50mV/s, 0.1M TBAPF6 was used as electrolyte. HBF4 acid is in the form of HBF4 · O(CH2CH3)2. Nitrogen was used to bubble through the solution before 104 experiment and bubble above the solution during the experiment. 5. Solvent effect Different solvent was used. Even the formation of dimer was completely suppressed, there was no peak current increase for reduction of [(η6-HMB)Mn(CO)3]+ in acetonitrile with the same amount of acid present, of which the reaction mechanism is illustrated as following. CV in red (Figure 4-8(1)) implies partially reversibility, which we think is because of surface and solvent effect. Thus, the peak current is a mix of 2e- ( ring slippage (η4-HMB)Mn(CO)3- as product, reversibility was limited because of fast ring coupled reaction with (η6-HMB)Mn(CO)3+ ) and 1e- reduction, it is possible that 2e- reduction might contribute a lot in figure 8 (1). When 1e- reduction is completely suppressed by acid, 2e- reduction primarily contributes to the reduction current but with different reduction product (η6-HMB)Mn(CO)2H. That might be the reason we didn’t see big peak current jump. Dichloromethane could be the only solvent suitable for this catalytic reaction. Strong acid such as trifluoroacetic acid could be used as acid source. Weak acid such as benzoic acid didn’t support the catalytic reduction. The efficiency of our catalytic reduction is low because of the slow protonation step. Our catalytic reduction potential is around -1.7V vs Fc/Fc+, which is quite low compared to -0.9V of the same cobalt catalyst Co(PtBu2NPh2)(CH3CN)3](BF4)2 from Dubios group2-4. 6. Cp-Mn-NO system The reduction potential of proton is equal or more negative than the reduction potential of the procatalyst. Since manganese (-1) anion is the initiator of proton 105 reduction catalytic cycle, we want to confine the structure of this class of catalyst to find a specific procatalyst which could be reduced at a more positive potential, thus getting the proton to be reduced at more positive potential. The nitrosyl substituted analog of cyclopentendienyl manganese tricarbonyl [(η5-MeCp)Mn(CO)2NO]BF4 caught our attention. (η5-methylcyclopentadienyl)Mn(CO)3 can be electrochemically oxidized by one electron to generate [(η5-methylcyclopentadienyl)Mn(CO)3]+ cation, and reversibly reduced back. We can use the IR bands of CO ligand as a good reference to demonstrate the electron density of the manganese center. Usually, higher wavenumbers correspond to lower electron density, and less positive reduction potential. By introducing NO into the (η5-methylcyclopentadienyl)Mn(CO)3 complex, first of all, it will change the oxidation state of the Mn center from 0 to +1; secondly, the electrochemical respondence will be reversed (the electrochemistry of Mn will be swiched from oxidation active to reduction active.). The electrochemistry of (η5-MeCp)Mn(CO)3 and its analoge with alkyl substitution on the Cp ring was studied. It is chemically reversible oxidation at around +0.6V vs Ag/AgCl. Even though the interpretation of [(MeCp)Mn(CO)2NO]+ wasn’t well established, it was believed that, its electrochemical pathway should mimic the one of [(η6-HMB)Mn(CO)3]+. Not surprisingly, from the reduction of [(η5-MeCp)Mn(CO)2NO]+, we did observe similar reductive electrochemistry at much more positive potential when strong acid was present. 106 Figure 4-9. Synthesis method of [(η5-methylcyclopentadienyl)Mn(CO)2NO]BF4 NO can be chemically introduced into the molecule by reacting (η5-methylcyclopentadienyl)Mn(CO)3 with NOBF4 (Figure 4-9). One carbonyl will be replaced by NO, and Mn (0) will be oxidized to Mn (I), which will change its electrochemistry from oxidation active to reduction active. Figure 4-10. Cyclic voltammetry of 1.29mM [(η5-methylcyclopentadienyl)Mn(CO)2- NO]BF4 with (blue line) and without the presence (red line) of 100mM TFA. Glassy carbon electrode is used and scan rate was 50mV/s, 0.1M TBAPF6 was used as electrolyte. 0.1M trifluoroacetic acid was used as acid source. Nitrogen was used to bubble through the solution before experiment and bubble above the solution during 107 the experiment. Based on the similarity between [(η5-methylcyclopentadienyl)Mn(CO)2NO] BF4 and [(η5-hexamethylbenzene)Mn(CO)3]BF4, the proton reduction mechanism was proposed as following. Scheme 4-7. Proposed mechanism of proton reduction catalyzed by [(η5-methylcyclopentadienyl)Mn(CO)2NO]BF4 The protonation of compound 13 and 24 is the rate determined step, which is the primary limitation of the low efficiency of aromatic manganese catalyst. Optimization was considered. 7. Optimazation 108 R R + 1e- R - CO R'2HN + R'2HN R'2HN Mn Mn Mn OC OC NO OC NO OC NO 22' OC 20' 21' + 1e- R R + H+ R'2N _ 24' H H Mn + R'2HN Mn NO OC NO OC base Hbase+ + H+ Hbase+ 23' Proton - H2 base + H2 - 2e- R + 2e- Hydrogen R'2HN R + oxidation Mn OC NO R'2N H Mn NO 25' OC Scheme 4-8. Optimization of (η5-Cp)Mn(CO)2NO+ procatalyst The efficiency of our Cp-Mn catalyst is expected to be enhanced by incorporating an amine group onto Cp ring (scheme 4-8). It is preferable that the amine group is sigma bonded through the linker R group to the Cp ligand, thus the amine group could move freely. The existence of the amine group will facilitate the uptake of proton from weaker acid and accelerate the rate determined step. 8. Electronic energy stored as chemical bonding mediated by polyarene-Mn(CO)3+ In the case of (η6-polyarene) manganese tricarbonyl cations, they undergo chemically reversible 2-electron reduction and generate ring slippage (η4-polyarene) manganese tricarbonyl anions, which were reported to react with proton and afford (η5-cyclohexadienyl) manganese tricarbonyl.19,20 The reaction is shown in scheme 8. Low temperature NMR was used to capture the transient formation of the ring 109 slippage metal hydride complex 28’. R R + 2e- Mn + _ OC CO - Mn CO - 2e OC CO CO 26 27 + H+ + H+ - H2 R R SLOW Mn Mn H OC CO CO CO OC CO 28 28' Scheme 4-9. Proposed mechanism of slow hydrogen release, “hydride” generation and storage mediated by (η6- polyarene) manganese tricarbonyl cation. Specifically, (η6- naphthalene) manganese tricarbonyl (R = H), of which the reduction potential is -1V vs Fc/Fc+, thus, the hydride complex (η5- hydronaphthalene ) manganese tricarbonyl could be produced at -1V with acid presence. The cyclic voltammetry of (η6- naphthalene) manganese tricarbonyl cation is shown below: it shows nicely reversible 2-electron reduction in dichloromethane at room temperature, however, the reversibility decreases when the concentration of acid increases. The reduction peak potential won’t increase much even the reversibility vanished completely. This is because of fast protonation of reduction product 27 and slow protonation of 28. The reduction product of 26 in the presence of acid is 28 in 110 the CV time scale (0.1-a few seconds). Figure 4-11. CVs of 1mM [(η6- naphthalene)Mn(CO)3]PF6 and 1mM ferrocene with the presence of HBF4·Et2O in different concentrations: (a). Red line, no HBF4 (b). Blue line, 1.1mM HBF4 (c). Green line, 2.2mM HBF4 (d). Black line, 3.3mM HBF4. No further change was observed when up to 6.6 mM HBF4 was treated. Chemically synthesized 28 (R = H) was treated with HBF4·Et2O in diethyl ether. After a few minutes, precipitate formed and was characterized as compound 26. 9. Hydrogen slow release mediated via (η5-H monoarene)-Mn(CO)3 The same slow hydrogen release reaction was also discovered to be mediated via monoarene manganese tricarbonyl complex. For example, (η5-H-C6H6)Mn(CO)3 was synthesized and treated with strong acid TFA overnight. The hapticity was changed from eta5 back to eta6, the product was confirmed to be (η6-C6H6)Mn(CO)3+. Similar ring slippage form of (η4-C6H6)MnH(CO)3 was proposed by Brookhart. 21 Based on their proposed mechanism and our experimental data, (η4-C6H6)MnH(CO)3 111 does exsit as an high energy transient state, thus the protonation is possible but slow and is the rate determined step. In the above case, it took 8 hours for the protonation to be finished. Scheme 4-10. Proposed mechanism of slow hydrogen release mediated via eta6 monoarene manganese tricarbonyl cation. (R = H) 4.4 Conclusions The reduction potential of proton is equal or more negative than the reduction potential of compound 1 ( -1.7 vs Fc/Fc+ ). HBF4·Et2O is the proton source, the overpotential of this catalytic proton reduction is around 1.4V, and the efficiency is not high. Nevertheless, aromatic manganese (I) carbonyl complex could be a new class of catalyst for proton reduction. Based on the prototype of compound 1, optimization of the molecular structure might lead us to more efficient catalyst with smaller overpotential. Different from (η5-C5H5)Mn(CO)3 (which can only be oxidized ), (η5-C5H5)Mn(CO)2NO+ can be reduced at -0.8 V vs Fc/Fc+. Not surprisingly, in the presence of HBF4· Et2O or trifluoroacetic acid, catalytic property of proton reduction was observed at -0.8V vs Fc/Fc+ with around 0.5V overpotential. 112 However, the efficiency was not improved. The electrochemistry of (η5-C5H5)Mn(CO)2NO+ has never been studied. We propose that it shares the similar reduction mechanism as of [(η6-C6Me6)Mn(CO)3+], thus, similar pathway of catalytic proton reduction. Acetonitrile can not be applied to the above study since its strong nucleophilic property. The efficiency of our Cp-Mn catalyst is expected to be enhanced by incorporating an amine group onto Cp ring. It is preferable that the amine group is sigma bonded through the linker R group to the Cp ligand. The existence of the amine group will facilitate the uptake of proton from weaker acid and accelerate the rate determined step. Interestingly, in the case of (η6-naphthalene) Mn(CO)3+, the cyclic voltammetry lost reversibility when there was proton present, which we proposed is because of the formation of η5-hydronaphthalene complex (η5- C10H9) Mn(CO)3 from the reaction between the reduced (η4-naphthalene) Mn(CO)3- and H+. Thereafter, hydrogen gas can be liberated gradually when (η5-C10H9) Mn(CO)3 is treated with excess acid in dichloromethane. 113 4.5 References 1. Fontecilla-Camps, J. C.;Volbeda, A.; Cavazza, C.; Nicolet, Y. Chem. Rev. 2007, 107 2. Aaron D. Wilson,† Rachel H. Newell,† Michael J. McNevin,† James T. Muckerman, M. Rakowski DuBois,*,† and Daniel L. DuBois*, J. Am. Chem. Soc. 2006, 128, 358-366 3. M. Rakowski Dubois and Daniel L. Dubois* Accounts of Chemical Research 2009 Vol. 42, No. 12. 1974–1982 4. Eric S. Wiedner, Jenny Y. Yang, William G. Dougherty, W. Scott Kassel, R. Morris Bullock, M. Rakowski DuBois, and Daniel L. DuBois*, Organometallics 2010, 29, 5390–5401 5. Greg A. N. Felton, Richard S. Glass, Dennis L. Lichtenberger, and Dennis H. Evans* Inorganic Chemistry, Vol. 45, No. 23, 2006 9181-9184 6. Wilson, A. D.; Shoemaker, R. K.; Meidaner, A.; Muckerman, J. T.; DuBois, D. L.; Rakowski DuBois, Proc. Natl. Acad. Sci. U.S.A. 2007, 104, 6951–6956 7. Louise A. Berben and Jonas C. Peters* Chem. Commun., 2010, 46, 398–400 8. Xile Hu, Bruce S. Brunschwig, and Jonas C. Peters* J. am. Chem. Soc. 2007, 129, 8988-8998 9. Greg A. N. Felton, Aaron K. Vannucci, Noriko Okumura, L. Tori Lockett, Dennis H. Evans,* Richard S. Glass,* and Dennis L. Lichtenberger* Organometallics 2008, 27, 4671–4679 114 10. Dmitry A. Valyaev, Mikhail G. Peterleitner, Oleg V. Semeikin , Kamil I. Utegenov, Nikolai A. Ustynyuk*, Alix Sournia-Saquet , Noe, Lugan, Guy Lavigne Journal of Organometallic Chemistry 692 (2007) 3207–3211 11. James P. Collman,* Paul S. Wagenknecht, and Nathan S. Lewist J. Am. Chem. Soc. 1992, 114, 5665-5673 12. Vincent Artero , Marc Fontecave Coordination Chemistry Reviews 249 (2005) 1518–1535 13. Catherine C. Neto, Carl D. Baer, Young K. Chung and Dwight A. Sweigart" J. Chem. Soc., Chem. Comun., 1993 816-818 14. Bernhardt, R. J.; Eyman, D. P. Organometallics 1984,3,1445. 15. Bernhardt,R. J.;Wilmoth,M.A.; Weers, J. J.;LaBmh, D.M.;Eyman, D. P.; Huffman, J. C. Organometallics 1986,5, 883 16. Peter J. Schlom, Ann M. Morken, Darrell P. Eyman,’ Norman C. Baenziger, and Steven J. Schauer Organometallics 1993,12, 3461-3467 17. Chapter 3. Unpublished result of reductive electrochemistry of (η6-HMB)Mn(CO)2Cl and [(η6-HMB)Mn(CO)2THF]+ 18. (a) Kristjansdottir, S. S.; Moody, A. E.; Weberg, R. T.; Norton, J. R. Organometallics 1988, 7,1983. (b) Jordan, R. F.; Norton, J. R. J.Am. Chem. SOC 1982,104, 1255. (c) Jordan, R. F.; Norton, J. R. ACS Symp. Ser. 1982, No. 198,403. (d) Moore, E. J.; Sullivan, J. M.; Norton, J. R. J. Am. Chem. SOC. 1986,108,2267. (e) Edidm, R. T.; Sullivan, J. M.; Norton, J. R. J. Am. Chem. SOC. 1987,109,3945. 19. Jacqueline M. Veauthier, Albert Chow, Gideon Fraenkel, Steven J. Geib and N. 115 John Cooper* Organometallics 2000, 19, 3942-3947 20. Jacqueline M. Veauthier, Albert Chow, Gideon Fraenkel Steven J. Geib and N. John Cooper* Organometallics 2000, 19, 661-671 21. W. Lamanna and M. Brookhart* Journal of the American Chemical Society 1980 102 3490-3494 116 Chapter 5 Direct synthesis of (η6-aniline) manganese tricarbonyl cation and its analogs 3 (R = H) 3 (R = m-NH2) 117 5.1 Introduction π-bonded arene manganese carbonyl complexes have applications in interesting areas such as: (1) Organic synthesis of natural and pharmaceutical products1-2 (2) electrophilic activation and functionalization of arene3a-g (3) Supramolecular metal-organometallic coordination framwork based on manganese quinonoid complexes4-10 (4) surface modification of magnetic nanostructures11-12 (5) Proton reduction catalysis and hydrogen storage and slow release13. The reactivity of arene is greatly enhanced by the coordination of manganese tricarbonyl moiety. Thus, the manganese functionalized arenes are readily accessible by varieties of nucleophiles, which provide us a unique synthetic method of functionalizing arenes with desired functional groups3a-g. Right now there are four kinds of method14 been applied to the synthesis of (η6-arene) manganese tricarbonyl cation: (1). Fisher-Hafner method15; (2). TFA-anhydride method16-17; (3). Silver (AgBF4) method18-20; (4). Manganese Tricarbonyl Transfer (MTT) method.21 In order to improve the yield, all the above methods share one similarity: excess mount of desired arene ligand were used, usually over 2 equivalents. However, there are limitations: (1) Electron deficient arene could not coordinate to manganese tricarbonyl moiety. For example, (η6-o-/m-/p-dichlorobenzene) manganese tricarbonyl cation could not be synthesized via none of the above method since the two chloride atoms would make the benzene ring too electron deficient to sustain the coordination of manganese tricarbonyl moiety.14 When a benzene ring bearing an adjacent carbonyl, such as benzylic acid, 118 benzaldehyde, etc, or more than one chloride, bromide or iodide, cyanide, it is difficult to have manganese tricarbonyl π-bonded onto the arene. (2) Electron rich arene could not π-bonded to the manganese moiety directly. Benzene ring with one or two adjacent primary amine group, such as aniline, phenylenediamine or its analog, could not be introduced to manganese tricarbonyl system.14 Previously the yield of (η6-bromobenzene) manganese tricarbonyl was too low to be calculated, and the (η6-iodidebenzene) manganese tricarbonyl could not even be synthesized. Rose22 reported the direct synthesis of (η6-bromo/iodide-benzene) manganese tricarbonyl cation analogs in high yield. They simply introduced an electron donating methoxy group to the benzene ring to make the system more electron rich, and then followed the “silver method”, they improved the yield of the anticipating product from none or 8% to around 70%. η6-bonded aniline manganese tricarbonyl cation was first introduced by Pauson23 in 1975. He reported the indirect synthesis method: nucleophilic substitution of the cationic (η6-chloroarene) manganese tricarbonyl complex by NH3. However, neither the yield of pure product was reported, nor the crystal structure was obtained. Pike14 summarized the synthetic scope of eta6-aromatic manganese tricarbonyl in 1994, and found that “no direct route has yet provided the coordinated aniline”. Since then, there is no report about the characterization or synthesis of (η6-aniline) manganese tricarbonyl complex. Recently, we found a unique direct synthesis method for (η6-aniline) manganese tricarbonyl complex and its alkyl substituent analogs with high yield. We 119 are using the MTT method, however, we altered the ligand ratio to be 2.8:1 (MTT: aniline). Trianiline manganese tricarbonyl was formed initially; excess MTT would π-bond to the benzene ring of the aniline and liberate the amine group from the coordination of the first manganese. The yield of aniline analog is 65%. We even got the crystal structure of the (η6-m-phenylenediamine) manganese tricarbonyl complex which was impossible to be synthesized via indirect method (nucleophilic substitution from halogenoarene manganese tricarbonyl). The scope and reaction mechanism of this direct method was studied and will be discussed in this chapter. 5.2 Experiments 1. Synthesis [(η6-aniline) Mn(CO)3]BF4 Acenaphthene manganese tricaronyl tetrafluoroborate ( 0.500g, 1.32mmol) and aniline (0.0429ml, 0.47mmol) were combined with 15ml dichloromethane in 20ml pressure tube. The pressure tube was bubbled through nitrogen for 2mins and sealed. It was placed in a 75℃ oil bath for 6hours. The product is a yellow precipitate from the solution. The solution was dried and washed with diethyl ether and dichloromethane once and the compound was recrystallized by diffusing ether into its acetone solution. IR: 2065, 2001 cm-1 in acetonitrile. The yield is 65% based on the purified product. Crystal structure was analyzed by single crystal X-ray crystallographic machine. [(η6-3,4-dimethylaniline) Mn(CO)3]BF4 was synthesized following the same procedure as [(η6-aniline) Mn(CO)3]BF4 above. IR of it in acetonitrile is 2061, 1996 cm-1. 120 [(η6-4-methylaniline) Mn(CO)3]BF4 was synthesized following the same method listed above; however, it has moderate solubility in dichloromethane, and can only be washed by ether. IR: 2063, 1998 cm-1 in acetone. [(η6-phenylenediamine) Mn(CO)3]BF4 was synthesized following the same procedure of [(η6-aniline) Mn(CO)3]BF4 above. IR of it in acetonitrile is 2050, 1978 cm-1. 2. Polymerization reaction and diazotization reaction Polymerization of the aniline functionalized by manganese tricarbonyl moiety failed. [(η6-aniline) Mn(CO)3]+ could neither be protonated nor diazotized by nitrite. 3 (R = H) 3 (R = m-NH2) Figure 5-1. Crystal structure of compound 3 3. Infrared Spectroscopy 121 Figure 5-2. IR of the following complexes in acetonitrile (a). compound 3 (R = H) 122 (b). compound 3 (R = 3,4-dimethyl) (c). compound 3 (R = m-NH2) From above infrared spectroscopy, as the electron donating group gets stronger, the wavenumbers of the carbonyl group get lower. 4. NMR C A NMR in AcCN-D3 B D BF4 6.51 6.51 6.50 6.18 5.67 5.66 5.64 5.49 5.48 3.45 3.43 3.41 3.39 2.15 2.09 1.95 1.95 1.94 1.93 1.93 1.14 1.12 1.10 9000 impurity from 8000 AcCN-D3 7000 D C B 6.51 6.51 6.50 6.18 5.67 5.66 5.64 5.49 5.48 2500 AcCN 6000 2000 1500 5000 1000 A 4000 500 0 3000 2.02 1.83 1.00 2.04 6.5 6.4 6.3 6. 2 6.1 6. 0 5.9 5.8 5.7 5.6 5.5 f1 (ppm) 2000 1000 Et2O Et2O 0 2.02 1.83 1.00 2.04 6.6 6.4 6.2 6.0 5.8 5.6 5.4 5.2 5. 0 4.8 4.6 4. 4 4.2 4.0 3.8 3.6 3.4 3.2 3.0 2.8 2.6 2.4 2.2 2.0 1.8 1.6 1.4 1.2 1.0 f1 (ppm) 123 MeCN-D3 7.09 6.46 6.44 5.94 5.42 5.42 5.36 5.36 5.35 5.34 2.59 2.35 2.06 1.98 1.95 1.93 1.91 1.90 1.77 2300 2200 F C 2100 6.46 6.44 5.94 5.42 5.42 5.36 5.36 5.35 5.34 E A 2000 300 D B NH2 1900 D 1800 C B Mn + 200 1700 A OC CO 1600 100 CO 1500 0 1.00 1.67 0.97 0.97 1400 6. 4 6.2 6. 0 5.8 5.6 5.4 1300 f1 (pp m) 1200 2.38 2.35 2.32 2.32 2.22 2.09 2.06 2.05 1.95 1.93 1.92 1.91 1.89 1.77 3 000 1100 1000 E 2 000 F 900 800 1 000 700 600 0 3.28 3.46 500 2.4 2.3 2.2 2.1 2. 0 1. 9 1.8 1.7 f1 (ppm) 400 300 200 100 0 1.00 1.67 0.97 0.97 3.28 3.46 -10 0 8.0 7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3. 5 3.0 2 .5 2.0 1. 5 1.0 f1 (ppm) A B NMR in AcCN-D3 C A D C 6.37 6.35 6.33 5.73 5.11 5.10 5.04 2.76 2.74 2.69 2.16 2.13 2.09 1.94 2400 2300 impurity 2200 6.37 6.35 6.33 5.73 5.11 5.10 5.04 2100 900 from 2000 800 1900 A C 700 1800 600 AcCN 1700 B 500 1600 1500 D 400 1400 300 1300 200 1200 100 1100 0 1000 1.12 3.87 2.23 1.00 900 6. 5 6. 4 6. 3 6. 2 6. 1 6. 0 5 .9 5 .8 5 .7 5 .6 5 .5 5 .4 5 .3 5 .2 5 .1 5 .0 800 f 1 (p p m) 700 600 500 400 300 200 100 0 -1 0 0 1.12 3.87 2.23 1.00 -2 0 0 6.6 6 .4 6. 2 6.0 5 .8 5. 6 5.4 5 .2 5. 0 4.8 4 .6 4 .4 4.2 4 .0 3 .8 3.6 3 .4 3 .2 3.0 2 .8 2 .6 2.4 2 .2 2 .0 1.8 f1 (p p m) Figure 5-3. NMR of the following complexes in deuterated acetonitrile 124 (a). compound 3 (R = H) (b). compound 3 (R = 3,4-dimethyl) (c). compound 3 (R = m-NH2) 5. Mass spectrometry: electron spray ionization and fragmentation. Mass spec. NH2 04202009RXN20_2inacetonerepeat #33-36 RT: 0.90-0.98 AV: 4 NL: 7.38E7 Mn + T: + c ESI Full ms [150.00-2000.00] 232.45 OC CO 100 0 4 2 0 2 0 0 9 R XN2 0 _ 2 in a c e to n e r e p e a t # 3 3 - 3 6 R T : 0 . 9 0 - 0 .9 8 AV: 4 NL : 7 . 3 8 E 7 CO T : + c E S I F u ll m s [1 5 0 . 0 0 - 2 0 0 0 . 0 0 ] 2 3 2 .4 5 90 100 95 90 80 85 80 Relative Abundance 75 70 70 65 Relative Abundance 60 60 55 50 50 45 40 35 40 30 25 2 3 4 .0 5 20 30 15 10 20 5 0 2 2 1 . 3 7 2 2 3 .3 2 2 2 4 . 9 6 2 3 1 .5 7 2 3 5 .3 9 2 3 9 .5 4 2 4 5 .3 4 2 5 0 .3 4 220 225 230 235 240 245 250 10 551.72 637.63 m /z 262.15 455.67 794.53 897.40 1068.57 1246.11 1388.85 1555.92 1740.08 1816.96 1947.78 0 200 400 600 800 1000 1200 1400 1600 1800 2000 m/z 04202009RXN20_090420142423 #294-340 RT: 1.29-1.46 AV: 47 NL: 5.27E7 T: + c ESI Full ms2 232.00@cid35.00 [60.00-300.00] 188.75 100 90 80 Relative Abundance 70 60 50 40 30 203.56 20 165.90 220.70 10 180.01 244.64 261.44 93.10 109.07 122.70 148.12 211.56 229.33 278.50 293.48 0 60 80 100 120 140 160 180 200 220 240 260 280 300 m/z 125 Mass spec after reprecipitation(acetone/ether) 03262009MeCNwashed11_3productrepeat #17-23 RT: 0.47-0.63 AV: 7 NL: 2.13E8 T: + c ESI Full ms [150.00-2000.00] 260.23 100 95 90 85 80 75 70 65 NH2 Relative Abundance 60 55 50 Mn + 45 40 OC CO 35 CO 30 25 20 15 607.04 289.98 823.45 10 5 895.49 307.00 723.48 1114.46 1447.53 1554.07 1779.94 1993.07 0 200 400 600 800 1000 1200 1400 1600 1800 2000 m/z 03262009MeCNwashed11_3productrepeat #217-256 RT: 3.17-3.32 AV: 40 NL: 2.45E7 F: + c ESI Full ms2 260.00@cid35.00 [70.00-300.00] 216.68 100 95 90 85 Fragments of compound 2 Mass spec fragments analysis 80 75 70 65 Relative Abundance 60 Mn + 55 OC 50 CO 45 40 m/z = 216 35 207.86 231.49 176.02 30 25 248.65 20 193.75 272.63 121.12 15 10 289.39 5 131.58 169.01 257.29 290.47 0 100 150 200 250 300 m/z H2N NH2 m-phenylenediamine Mn tricarbonyl Mn + F.W. of cation=247 OC CO CO 0 4 2 7 2 0 0 9 R X N 2 3 y e llo w c r y s t a l_ a # 2 5 1 - 2 8 3 R T : 1 . 8 8 - 2 . 0 2 AV: 33 N L : 6 .1 8 E 7 04272009RXN23yellowcrystal #42-44 RT: 1.18-1.23 AV: 3 NL: 1.14E8 T : + c E S I F u ll m s 2 2 4 7 . 0 0 @ c id 3 5 . 0 0 [ 6 5 . 0 0 - 5 0 0 . 0 0 ] T: + c ESI Full ms [150.00-2000.00] 2 0 3 .6 5 247.42 100 100 95 95 90 90 85 85 80 80 75 75 70 70 65 65 Relative Abundance Relative Abundance 60 60 55 55 50 50 45 45 40 40 35 35 30 30 1 6 3 .0 4 25 870.62 25 2 3 5 .6 4 20 20 1 9 4 .9 0 15 783.56 15 323.46 683.45 10 886.38 10 1 0 8 .1 3 2 5 9 .6 2 236.22 277.22 748.62 5 513.65 854.53 937.57 5 611.19 391.72 455.91 2 8 5 .1 8 0 9 3 .0 2 1 4 9 .0 4 3 2 5 .4 4 4 1 9 .0 1 0 200 300 400 500 600 700 800 900 1000 100 150 200 250 300 350 400 450 500 m/z m /z Mass spec shows that the F.W. of the cation is Figure 5-4. Mass spectrometry (electron spray ionization) of the following complexes in 50% acetonitrile/50% H2O solution 126 (a). compound 3 (R = H) (b). compound 3 (R = 3,4-dimethyl) (c). compound 3 (R = m-NH2) The above mass spec. information gives information about ionization and fragmentation of (η6-aniline) Mn(CO)3+ and its analogs (compound 3a,b,c). And it is for the first time that the fragmentation of the arene manganese tricarbonyl cation being analyzed. 5.3. Result and discussion Figure 5-5. Nucleophilic reactions pathways of (η6-chlorobenzene) Mn(CO)3+. Nucleophiles can react with (η6-chlorobenzene) Mn(CO)3+ cation by several alternative pathways which depend on the nature of the nucleophile, solvent, temperature. 127 Previously, Pauson reported the indirect synthesis of (η6-aniline) Mn(CO)3+ by nucleophilic substitution of the chloride from the (η6-chlorobenzene) Mn(CO)3+ using ammonia. The yield of the raw product was reported to be 60%.23 Figure 5-6. How Aniline Manganese Tricarbonyl was made 1. Reaction mechanism We modified the “MTT” method by altering the MTT : aniline ratio to 2.8 : 1, then after 6 hours of reaction, we got yellow precipitates, which is the [(η6-aniline) Mn(CO)3]BF4. Since the aromatic amine is basic, it prefers bonding to the manganese in a η2 manner and form trianiline manganese tricarbonyl cation instead of (η6-aniline) manganese tricarbonyl. Initially, we sought ways that would protect the aromatic amine group from coordinating to the manganese moiety. Di-tert-butyl dicarbonate is a wildly used amine protection group in organic synthesis, after Boc-protection, the Boc-amine group still prefers bonding to the manganese in a η2 manner. Instead of being bothered, why don’t we accept the fact and consider manganese tricarbonyl as an amine protection group by itself? In fact, three equivalents of anilines are protected by one equivalent of manganese tricarbonyl, and there are three equivalents of arene left for further coordination. The mechanism of the direct synthesis method is 128 proposed as following: Scheme 5-1. Mechanism of direct synthesis of (η6-aniline) Mn(CO)3+ and its analogs (R = alkyl, or m-NH2 group) We did one control experiment: trianiline manganese tricarbonyl cation was treated with over one equivalent of MTT, and lead to the formation of [(η6-aniline) Mn(CO)3]BF4. Later on, we found out that this reaction could be finished in one step. Simply mixed excess MTT with aniline in the pressure tube, and heated it up to 75℃ for 6 hours, and then we ended up with the η6 bonded product as precipitate. MTT NH 2 NH 2NHH22N Mn + OC Mn + OC CO CO CO CO 3 1 ratio 1 : 3 Yield = 65% R=H based on aniline Figure 5-7. Control experiment between compound 1 and MTT The basicity of the aromatic amine is strongly decreased via coordination of manganese tricarbonyl moiety. The long pair electrons are greatly localized by the 129 benzene ring. Compound 3 (R = H) could neither be protonated by strong acid, nor bond to electrophile such as MTT, which is in support of the “spontaneous cleavage” mechanism. Figure 5-8. Chemical property of aniline after coordination of manganese moiety Figure 5-9. Decrease of the basicity of (η6-aniline) Mn(CO)3+ 130 NH Mn + OC CO CO 4 Figure 5-10. Reversible deprotonation Not being able to be protonated by strong acid, instead, aniline could be deprotonated by strong base after π-bonding to the manganese moiety through the benzene ring. After deprotonation, the product compound 4 is an active nucleophile, and could react with different electrophiles to afford new species, which could be one of its potential applications in organice synthesis. 2. Scope Other than alkyl-substituted aniline analogs, the scope of our direct synthesis method was investigated. The following aniline analogs bearing heteroatoms were tried. 131 a d MeO NH2 NH2 NH2 b e HO NH2 NH2 H2N c f H2N NH2 Cl NH2 Chart 5-1. Aniline bearing heteroatom: electron donating (a-e) group and electron withdrawing group (f). Aniline analogs shown above were tested via our direct synthesis method. Unfortunately, only m-phenylenediamine gave positive result. In the case of p/m/o- phenylenediamine, we think that, the basicity of the amine group plays an important role in the crucial step (spontaneous cleavage) of forming eta6-bonding species. When two amine groups are in o/p position, the basicity increases and as a result two amines function as a linkage between two manganese tricarbonyl centers. In the contrary, when the two amines are in meta-position of each other, the basicity doesn’t change much. So we may able to have success in a serial of m-functionalized aniline. As we have limited source of meta-functionalized aniline, we haven’t been able to test the following ligand, which seem to be promising: m-methoxylaniline, m-hydroxylaniline, m-chloroaniline (even chloride is not a electron donoting group, but it is still worth trying). 132 3. Aromatic amine protection — remote protection Aniline by itself could easily be protonated by strong acid, such as TFA, HBF4. In another aspect, the most common synthesis of polyaniline is by oxidative polymerization with ammonium peroxodisulfate as an oxidant; also, aniline could be oxidized by nitrite to form diazonium salt. However, after coordination of manganese tricarbonyl moiety, aniline loses its basicity. It could neither be protonated nor oxidized. The best way to understand (η6-aniline) Mn(CO)3+ is to consider it as (η6-toluene) Mn(CO)3+ since they share very similar chemical property. The amine group is totally neutralized by manganese moiety. Based upon the above discussion, manganese tricarbonyl could be treated as an alternative for protection of aniline and its analogs. The advantage of our method is we can protect the amine group without any modification to the amine. The amine group remains intact. We call it “remote protection”. 5.4 Conclusions To summarize, we reported the first direct synthetic method of (η6-aniline) Mn(CO)3+ and its analogs. Primary aromatic amine functionalized by alkyl group could be applied to this one-step method. Secondary and tertiary aromatic amines haven’t been tried. However, we are confident that our direct synthesis method could 133 be applied. Compared with indirect method, our one-step method is much convenient. More importantly, aniline with different functional group could be activated directly. Sometimes it is difficult to find a chlorobenzene analog to make a detour. For example, it is not possible to synthesize (η6-dichlorobenzene) manganese tricarbonyl catoin, thus, we could not synthesize the phenylenediamine manganese tricarbonyl by nucleophilic substitution. It is encouraging that (η6-m-phenylenediamine) manganese tricarbonyl catoin could be synthesized. We will work on the possibility of loading manganese moiety onto other meta-functionalized aniline analogs bearing heteroatoms. By the coordination of the manganese tricarbonyl moiety to the benzene ring functionalized by amine group, the basicity of the amine group is greatly diminished. It might be an alternative for aromatic amine protection method. To remove the manganese moiety, we can simply leave the compound in acetonitrile for days or heat for a few hours. Furthermore, (η6-aniline) Mn(CO)3+ is an important precursor for the study in another interesting area: heterogeneous electrochemistry, which will be discussed in the next chapter. 134 5.5 X-ray crystal structure data 3 (R = H) 3 (R = m-NH2) Crystal structure of compound 3 135 Table 5-C1-1. Crystal data and structure refinement for 3 (R = m-NH2) ____________________________________________________________________________ Identification code 3 (R = m-NH2) Empirical formula C9 H8 B F4 Mn N2 O3 Formula weight 333.92 Temperature 100(2) K Wavelength 1.54178 Crystal system Triclinic Space group P-1 Unit cell dimensions a = 6.70060(10) Å α= 90.9980(10)° b = 9.00050(10) Å β=106.5300(10)º c = 10.32920(10) Å γ = 90.0710(10)° Volume 597.092(13) Å3 Z 2 Density (calculated) 1.857 Mg/m3 Absorption coefficient 9.615 mm-1 F(000) 332 Crystal size 0.41 x 0.26 x 0.21 mm3 Theta range for data collection 4.47 to 67.00? Index ranges -7<=h<=7, -10<=k<=10, -12<=l<=12 Reflections collected 10061 Independent reflections 2066 [R(int) = 0.0357] Completeness to theta = 67.00° 96.9 % Absorption correction Numercial Max. and min. transmission 0.2350 and 0.1110 Refinement method Full-matrix least-squares on F2 Data / restraints / parameters 2066 / 0 / 213 Goodness-of-fit on F2 1.100 Final R indices [I>2sigma(I)] R1 = 0.0293, wR2 = 0.0744 R indices (all data) R1 = 0.0295, wR2 = 0.0746 Largest diff. peak and hole 0.342 and -0.651 e. Å-3 136 Table 5-C1-2. Atomic coordinates ( x 104) and equivalent isotropic displacement parameters (Å 2x 103) U(eq) is defined as one third of the trace of the orthogonalized Uij tensor. ________________________________________________________________________________ x y z U(eq) ________________________________________________________________________________ Mn(1) 1410(1) 2331(1) 8498(1) 8(1) N(1) 2963(3) 3154(2) 5727(2) 16(1) N(2) -1096(3) -717(2) 6868(2) 15(1) O(1) 2686(2) -452(2) 9979(2) 19(1) O(2) 5647(2) 3642(2) 9296(2) 19(1) O(3) 518(2) 3603(2) 10931(1) 18(1) C(1) 2225(3) 641(2) 9428(2) 13(1) C(2) 4027(3) 3119(2) 9008(2) 13(1) C(3) 865(3) 3148(2) 9982(2) 13(1) C(4) 1481(3) 2740(2) 6287(2) 12(1) C(5) 1161(3) 1217(2) 6504(2) 11(1) C(6) -632(3) 738(2) 6869(2) 11(1) C(7) -1801(3) 1825(2) 7337(2) 12(1) C(8) -1364(3) 3346(2) 7243(2) 13(1) C(9) 306(3) 3818(2) 6785(2) 13(1) B(1) 5866(3) 2559(2) 3343(2) 14(1) F(1) 5048(2) 3920(1) 3597(1) 27(1) F(2) 4685(2) 1951(2) 2133(1) 28(1) F(3) 7919(2) 2770(1) 3328(1) 26(1) F(4) 5822(2) 1576(1) 4368(1) 22(1) ________________________________________________________________________________ 137 Table 5-C1-3. Bond lengths [Å] and angles [°] _____________________________________________________ Mn(1)-C(1) 1.813(2) O(2)-C(2) 1.139(2) Mn(1)-C(3) 1.8179(19) O(3)-C(3) 1.139(2) Mn(1)-C(2) 1.8201(19) C(4)-C(5) 1.419(3) Mn(1)-C(8) 2.1552(18) C(4)-C(9) 1.428(3) Mn(1)-C(7) 2.1878(18) C(5)-C(6) 1.426(3) Mn(1)-C(9) 2.1932(18) C(5)-H(5) 0.92(3) Mn(1)-C(5) 2.2380(17) C(6)-C(7) 1.416(3) Mn(1)-C(6) 2.3151(17) C(7)-C(8) 1.411(3) Mn(1)-C(4) 2.3337(18) C(7)-H(7) 0.93(2) N(1)-C(4) 1.338(3) C(8)-C(9) 1.401(3) N(1)-H(1) 0.86(3) C(8)-H(8) 0.97(3) N(1)-H(2) 0.80(3) C(9)-H(9) 0.92(3) N(2)-C(6) 1.346(2) B(1)-F(2) 1.378(2) N(2)-H(3) 0.80(3) B(1)-F(3) 1.393(2) N(2)-H(4) 0.85(3) B(1)-F(1) 1.394(2) O(1)-C(1) 1.143(3) B(1)-F(4) 1.398(3) C(1)-Mn(1)-C(3) 89.23(8) C(7)-Mn(1)-C(5) 66.80(7) C(1)-Mn(1)-C(2) 92.99(8) C(9)-Mn(1)-C(5) 66.90(7) C(3)-Mn(1)-C(2) 90.82(8) C(1)-Mn(1)-C(6) 84.14(7) C(1)-Mn(1)-C(8) 140.01(8) C(3)-Mn(1)-C(6) 129.07(7) C(3)-Mn(1)-C(8) 89.51(8) C(2)-Mn(1)-C(6) 139.83(7) C(2)-Mn(1)-C(8) 126.99(8) C(8)-Mn(1)-C(6) 66.20(6) C(1)-Mn(1)-C(7) 102.93(8) C(7)-Mn(1)-C(6) 36.52(7) C(3)-Mn(1)-C(7) 97.87(7) C(9)-Mn(1)-C(6) 78.35(7) C(2)-Mn(1)-C(7) 161.89(8) C(5)-Mn(1)-C(6) 36.44(6) C(8)-Mn(1)-C(7) 37.90(7) C(1)-Mn(1)-C(4) 125.46(7) C(1)-Mn(1)-C(9) 159.87(8) C(3)-Mn(1)-C(4) 145.26(7) C(3)-Mn(1)-C(9) 109.51(8) C(2)-Mn(1)-C(4) 85.97(7) C(2)-Mn(1)-C(9) 94.00(8) C(8)-Mn(1)-C(4) 65.76(7) C(8)-Mn(1)-C(9) 37.58(7) C(7)-Mn(1)-C(4) 77.82(7) C(7)-Mn(1)-C(9) 68.15(7) C(9)-Mn(1)-C(4) 36.62(7) C(1)-Mn(1)-C(5) 93.08(7) C(5)-Mn(1)-C(4) 36.09(6) C(3)-Mn(1)-C(5) 164.63(8) C(6)-Mn(1)-C(4) 64.36(6) C(2)-Mn(1)-C(5) 104.22(7) C(4)-N(1)-H(1) 121.2(19) C(8)-Mn(1)-C(5) 78.97(7) C(4)-N(1)-H(2) 118.3(18) 138 H(1)-N(1)-H(2) 120(3) C(8)-C(7)-C(6) 119.75(17) C(6)-N(2)-H(3) 120.0(18) C(8)-C(7)-Mn(1) 69.80(10) C(6)-N(2)-H(4) 117.6(19) C(6)-C(7)-Mn(1) 76.64(10) H(3)-N(2)-H(4) 121(3) C(8)-C(7)-H(7) 120.7(14) O(1)-C(1)-Mn(1) 177.51(16) C(6)-C(7)-H(7) 119.4(14) O(2)-C(2)-Mn(1) 177.95(17) Mn(1)-C(7)-H(7) 128.5(15) O(3)-C(3)-Mn(1) 177.18(17) C(9)-C(8)-C(7) 121.60(16) N(1)-C(4)-C(5) 120.56(17) C(9)-C(8)-Mn(1) 72.68(10) N(1)-C(4)-C(9) 121.08(17) C(7)-C(8)-Mn(1) 72.30(10) C(5)-C(4)-C(9) 118.14(16) C(9)-C(8)-H(8) 119.8(15) N(1)-C(4)-Mn(1) 134.15(13) C(7)-C(8)-H(8) 118.4(15) C(5)-C(4)-Mn(1) 68.28(10) Mn(1)-C(8)-H(8) 123.3(15) C(9)-C(4)-Mn(1) 66.34(10) C(8)-C(9)-C(4) 119.30(17) C(4)-C(5)-C(6) 120.98(17) C(8)-C(9)-Mn(1) 69.74(10) C(4)-C(5)-Mn(1) 75.63(10) C(4)-C(9)-Mn(1) 77.05(11) C(6)-C(5)-Mn(1) 74.72(10) C(8)-C(9)-H(9) 118.0(16) C(4)-C(5)-H(5) 117.8(16) C(4)-C(9)-H(9) 122.6(16) C(6)-C(5)-H(5) 120.7(15) Mn(1)-C(9)-H(9) 125.6(17) Mn(1)-C(5)-H(5) 128.9(16) F(2)-B(1)-F(3) 110.81(17) N(2)-C(6)-C(7) 121.13(17) F(2)-B(1)-F(1) 110.42(18) N(2)-C(6)-C(5) 120.60(17) F(3)-B(1)-F(1) 109.03(16) C(7)-C(6)-C(5) 118.07(16) F(2)-B(1)-F(4) 108.09(16) N(2)-C(6)-Mn(1) 132.73(13) F(3)-B(1)-F(4) 109.24(17) C(7)-C(6)-Mn(1) 66.84(9) F(1)-B(1)-F(4) 109.23(16) C(5)-C(6)-Mn(1) 68.83(10) _____________________________________________________________ 139 Symmetry transformations used to generate equivalent atoms: Table 5-C1-4. Anisotropic displacement parameters (Å 2x 103)for p1bar. The anisotropic displacement factor exponent takes the form: -2　2[ h2a*2U11 + ... + 2 h k a* b* U12 ] ______________________________________________________________________________ U11 U22 U33 U23 U13 U12 ______________________________________________________________________________ Mn(1) 8(1) 8(1) 8(1) -2(1) 2(1) -1(1) N(1) 19(1) 15(1) 18(1) -2(1) 10(1) -5(1) N(2) 14(1) 11(1) 19(1) -3(1) 5(1) -4(1) O(1) 21(1) 15(1) 21(1) 5(1) 4(1) 1(1) O(2) 12(1) 20(1) 25(1) -5(1) 5(1) -5(1) O(3) 21(1) 19(1) 15(1) -5(1) 6(1) 3(1) C(1) 11(1) 18(1) 11(1) -5(1) 3(1) -3(1) C(2) 16(1) 11(1) 12(1) -3(1) 5(1) 2(1) C(3) 10(1) 13(1) 14(1) 0(1) 1(1) -1(1) C(4) 13(1) 16(1) 7(1) -2(1) 1(1) -3(1) C(5) 13(1) 11(1) 9(1) -4(1) 3(1) -1(1) C(6) 10(1) 13(1) 7(1) -3(1) -1(1) -2(1) C(7) 7(1) 17(1) 9(1) -2(1) 0(1) -2(1) C(8) 10(1) 16(1) 10(1) -2(1) -1(1) 2(1) C(9) 17(1) 10(1) 12(1) -1(1) 0(1) -2(1) B(1) 12(1) 12(1) 16(1) -1(1) 3(1) -1(1) F(1) 28(1) 14(1) 43(1) -2(1) 18(1) 2(1) F(2) 27(1) 39(1) 16(1) -5(1) 1(1) -6(1) F(3) 16(1) 21(1) 44(1) -1(1) 14(1) -3(1) F(4) 24(1) 22(1) 19(1) 5(1) 4(1) 0(1) ______________________________________________________________________________ 140 Table 5-C1-5. Hydrogen coordinates ( x 104) and isotropic displacement parameters (A2x 103) ________________________________________________________________________________ x y z U(eq) ________________________________________________________________________________ H(1) 3650(40) 2510(30) 5410(30) 27(7) H(2) 3290(40) 4010(30) 5770(30) 15(6) H(3) -270(40) -1330(30) 6790(30) 17(6) H(4) -2140(50) -960(30) 7130(30) 25(7) H(5) 2040(40) 540(30) 6280(30) 15(6) H(7) -2910(40) 1530(30) 7650(20) 12(5) H(8) -2180(40) 4070(30) 7580(30) 16(6) H(9) 580(40) 4820(30) 6800(30) 21(6) ________________________________________________________________________________ 141 Table 5-C1-6. Hydrogen bonds ___________________________________________________________________________ D-H...A d(D-H) d(H...A) d(D...A) <(DHA) ____________________________________________________________________________ N(1)-H(1)...F(4) 0.86(3) 2.20(3) 3.021(2) 160(3) N(1)-H(2)...F(1)#1 0.80(3) 2.16(3) 2.932(2) 162(2) N(2)-H(3)...F(3)#2 0.80(3) 2.07(3) 2.868(2) 175(3) N(2)-H(4)...F(2)#3 0.85(3) 2.25(3) 3.087(2) 171(3) N(2)-H(4)...F(4)#3 0.85(3) 2.57(3) 3.149(2) 127(2) ____________________________________________________________________________ Symmetry transformations used to generate equivalent atoms: #1 -x+1,-y+1,-z+1 #2 -x+1,-y,-z+1 #3 -x,-y,-z+1 142 Table 5-C2-1. Crystal data and structure refinement for 3 (R = H). ________________________________________________________________ Identification code 3 (R = H) Empirical formula C9 H7 B F4 Mn N O3 Formula weight 318.91 Temperature 100(2) K Wavelength 1.54178 Crystal system Monoclinic Space group P2(1)/c Unit cell dimensions a = 10.1479(2) α= 90 º b = 9.8810(2) β= 113.0580(10) º c = 12.7269(2) γ= 90 º Volume 1174.19(4) A3 Z 4 Density (calculated) 1.804 Mg/m3 Absorption coefficient 9.720 mm-1 F(000) 632 Crystal size 0.28 x 0.26 x 0.19 mm3 Theta range for data collection 4.74 to 66.96? Index ranges -12<=h<=12, -11<=k<=10, -14<=l<=15 Reflections collected 12393 Independent reflections 2047 [R(int) = 0.0295] Completeness to theta = 66.96º 98.0 % Absorption correction Numerical Max. and min. transmission 0.2651 and 0.1752 Refinement method Full-matrix least-squares on F2 Data / restraints / parameters 2047 / 0 / 200 Goodness-of-fit on F2 1.095 Final R indices [I>2sigma(I)] R1 = 0.0236, wR2 = 0.0617 R indices (all data) R1 = 0.0242, wR2 = 0.0620 Largest diff. peak and hole 0.218 and -0.603 e.A-3 143 Table 5-C2-2. Atomic coordinates ( x 104) and equivalent isotropic displacement parameters (A2x 103) for3 (R = H). U(eq) is defined as one third of the trace of the orthogonalized Uij tensor. ________________________________________________________________________________ x y z U(eq) ________________________________________________________________________________ Mn(1) 2450(1) 4302(1) 2386(1) 12(1) O(1) 174(1) 6124(1) 897(1) 23(1) O(2) 4118(2) 4587(2) 939(1) 33(1) O(3) 4049(1) 6603(1) 3803(1) 21(1) N(1) -915(2) 2918(2) 1576(1) 22(1) C(1) 1073(2) 5429(2) 1468(1) 16(1) C(2) 3482(2) 4489(2) 1496(2) 20(1) C(3) 3431(2) 5719(2) 3251(2) 16(1) C(4) 491(2) 2881(2) 2186(1) 16(1) C(5) 1452(2) 2303(2) 1746(2) 17(1) C(6) 2916(2) 2152(2) 2446(2) 18(1) C(7) 3489(2) 2667(2) 3563(2) 19(1) C(8) 2573(2) 3375(2) 3961(1) 18(1) C(9) 1117(2) 3540(2) 3272(1) 17(1) B(1) 2643(2) 8831(2) 1164(2) 16(1) F(1) 2423(1) 9009(1) 2170(1) 31(1) F(2) 3712(1) 9706(1) 1167(1) 20(1) F(3) 1376(1) 9141(1) 238(1) 31(1) F(4) 3021(1) 7506(1) 1065(1) 30(1) ________________________________________________________________________________ 144 Table 5-C2-3. Bond lengths [A] and angles [º] for 3 (R = H). ______________________________________________________________________________ Mn(1)-C(1) 1.8113(17) C(4)-C(5) 1.420(2) Mn(1)-C(3) 1.8174(17) C(4)-C(9) 1.432(2) Mn(1)-C(2) 1.8292(18) C(5)-C(6) 1.410(2) Mn(1)-C(8) 2.1622(17) C(5)-H(5) 0.963(19) Mn(1)-C(6) 2.1723(17) C(6)-C(7) 1.404(3) Mn(1)-C(7) 2.1750(17) C(6)-H(6) 0.93(2) Mn(1)-C(9) 2.2052(16) C(7)-C(8) 1.407(3) Mn(1)-C(5) 2.2238(16) C(7)-H(7) 0.93(2) Mn(1)-C(4) 2.3640(16) C(8)-C(9) 1.401(2) O(1)-C(1) 1.145(2) C(8)-H(8) 0.95(2) O(2)-C(2) 1.134(2) C(9)-H(9) 0.91(2) O(3)-C(3) 1.142(2) B(1)-F(4) 1.384(2) N(1)-C(4) 1.332(2) B(1)-F(2) 1.386(2) N(1)-H(1) 0.84(2) B(1)-F(3) 1.396(2) N(1)-H(2) 0.89(2) B(1)-F(1) 1.394(2) C(1)-Mn(1)-C(3) 91.09(7) C(7)-Mn(1)-C(9) 67.86(7) C(1)-Mn(1)-C(2) 91.27(8) C(1)-Mn(1)-C(5) 100.61(7) C(3)-Mn(1)-C(2) 89.23(8) C(3)-Mn(1)-C(5) 165.64(7) C(1)-Mn(1)-C(8) 127.51(7) C(2)-Mn(1)-C(5) 98.73(7) C(3)-Mn(1)-C(8) 86.92(7) C(8)-Mn(1)-C(5) 79.37(6) C(2)-Mn(1)-C(8) 141.07(7) C(6)-Mn(1)-C(5) 37.39(6) C(1)-Mn(1)-C(6) 136.60(7) C(7)-Mn(1)-C(5) 67.70(6) C(3)-Mn(1)-C(6) 132.21(7) C(9)-Mn(1)-C(5) 66.49(6) C(2)-Mn(1)-C(6) 87.17(7) C(1)-Mn(1)-C(4) 83.81(7) C(8)-Mn(1)-C(6) 67.67(7) C(3)-Mn(1)-C(4) 139.54(7) C(1)-Mn(1)-C(7) 160.90(7) C(2)-Mn(1)-C(4) 130.85(7) C(3)-Mn(1)-C(7) 98.75(7) C(8)-Mn(1)-C(4) 65.74(6) C(2)-Mn(1)-C(7) 105.12(7) C(6)-Mn(1)-C(4) 65.48(6) C(8)-Mn(1)-C(7) 37.85(7) C(7)-Mn(1)-C(4) 78.07(6) C(6)-Mn(1)-C(7) 37.68(7) C(9)-Mn(1)-C(4) 36.32(6) C(1)-Mn(1)-C(9) 93.87(7) C(5)-Mn(1)-C(4) 35.89(6) C(3)-Mn(1)-C(9) 104.71(7) C(4)-N(1)-H(1) 119.7(14) C(2)-Mn(1)-C(9) 165.02(7) C(4)-N(1)-H(2) 119.6(14) C(8)-Mn(1)-C(9) 37.40(6) H(1)-N(1)-H(2) 121(2) C(6)-Mn(1)-C(9) 79.43(7) O(1)-C(1)-Mn(1) 178.01(15) 145 O(2)-C(2)-Mn(1) 179.07(16) F(4)-B(1)-F(2) 110.19(14) O(3)-C(3)-Mn(1) 179.43(15) F(4)-B(1)-F(3) 109.13(14) N(1)-C(4)-C(5) 121.70(16) F(2)-B(1)-F(3) 109.05(14) N(1)-C(4)-C(9) 121.27(16) F(4)-B(1)-F(1) 110.67(15) C(5)-C(4)-C(9) 116.77(15) F(2)-B(1)-F(1) 108.99(14) N(1)-C(4)-Mn(1) 135.26(12) F(3)-B(1)-F(1) 108.78(14) C(5)-C(4)-Mn(1) 66.67(9) C(9)-C(4)-Mn(1) 65.79(9) C(6)-C(5)-C(4) 120.68(15) C(6)-C(5)-Mn(1) 69.32(9) C(4)-C(5)-Mn(1) 77.45(10) C(6)-C(5)-H(5) 120.3(10) C(4)-C(5)-H(5) 118.8(10) Mn(1)-C(5)-H(5) 129.9(11) C(7)-C(6)-C(5) 121.14(16) C(7)-C(6)-Mn(1) 71.27(10) C(5)-C(6)-Mn(1) 73.29(9) C(7)-C(6)-H(6) 118.7(13) C(5)-C(6)-H(6) 120.0(13) Mn(1)-C(6)-H(6) 124.6(12) C(6)-C(7)-C(8) 118.35(16) C(6)-C(7)-Mn(1) 71.06(10) C(8)-C(7)-Mn(1) 70.58(9) C(6)-C(7)-H(7) 119.1(12) C(8)-C(7)-H(7) 122.4(12) Mn(1)-C(7)-H(7) 125.1(12) C(9)-C(8)-C(7) 121.10(16) C(9)-C(8)-Mn(1) 72.96(10) C(7)-C(8)-Mn(1) 71.56(10) C(9)-C(8)-H(8) 118.9(12) C(7)-C(8)-H(8) 119.5(12) Mn(1)-C(8)-H(8) 120.9(12) C(8)-C(9)-C(4) 120.73(16) C(8)-C(9)-Mn(1) 69.63(9) C(4)-C(9)-Mn(1) 77.89(10) C(8)-C(9)-H(9) 119.6(13) C(4)-C(9)-H(9) 119.6(13) Mn(1)-C(9)-H(9) 127.0(13) 146 Symmetry transformations used to generate equivalent atoms: Table 5-C2-4. Anisotropic displacement parameters (A2x 103)for p21onc. The anisotropic displacement factor exponent takes the form: -2　2[ h2a*2U11 + ... + 2 h k a* b* U12 ] ______________________________________________________________________________ U11 U22 U33 U23 U13 U12 ______________________________________________________________________________ Mn(1) 11(1) 12(1) 12(1) -2(1) 4(1) -1(1) O(1) 21(1) 22(1) 20(1) -2(1) 1(1) 5(1) O(2) 40(1) 33(1) 40(1) 2(1) 31(1) 3(1) O(3) 18(1) 17(1) 23(1) -6(1) 2(1) -2(1) N(1) 14(1) 32(1) 19(1) -9(1) 6(1) -7(1) C(1) 17(1) 17(1) 15(1) -6(1) 6(1) -4(1) C(2) 23(1) 15(1) 23(1) -2(1) 9(1) 2(1) C(3) 12(1) 19(1) 16(1) 3(1) 4(1) 5(1) C(4) 18(1) 16(1) 16(1) -1(1) 8(1) -5(1) C(5) 20(1) 13(1) 18(1) -5(1) 7(1) -5(1) C(6) 19(1) 11(1) 25(1) -1(1) 9(1) 0(1) C(7) 16(1) 14(1) 21(1) 4(1) 3(1) 0(1) C(8) 23(1) 16(1) 13(1) 0(1) 5(1) -5(1) C(9) 19(1) 16(1) 18(1) -2(1) 11(1) -4(1) B(1) 16(1) 14(1) 18(1) -1(1) 7(1) -2(1) F(1) 41(1) 35(1) 27(1) 0(1) 23(1) -4(1) F(2) 17(1) 19(1) 25(1) 1(1) 9(1) -4(1) F(3) 19(1) 32(1) 29(1) -2(1) -4(1) -2(1) F(4) 33(1) 15(1) 48(1) 0(1) 22(1) 1(1) ______________________________________________________________________________ 147 5.6 References 1. Anthony J. Pearson* and Paul R. Bruhn J. Org. Chem. 1991. 56, 70927097 2. Pearson, A. J and Shin, H. Tetrahedron 1992 48, 7527-7538 3. (a) Rose-Munch, F.; Rose, E. Eur. J. Inorg. Chem. 2002, 1269. (b) Auffrant, A.; Prim, D.; Rose-Munch, F.; Rose, E.; Schouteeten, S.; Vaissermann, J. Organometallics 2003, 22, 1898. (c) Prim, D.; Andrioletti, B.; Rose-Munch, F.; Rose, E.; Couty, F. Tetrahedron 2004, 60, 3325. (d) Ku¨ndig, E. P.; Pape, A. Top. Organomet. Chem. 2004, 7, 71. (e) Sweigart, D. A.; Reingold, J. A.; Son, S. U. Comprehensive Organometallic Chemistry III; Crabtree, R. H., Mingos, D. M. P., Eds.; Elsevier: Oxford, U.K., 2006; Vol. 5, Chapter 10, pp 761-814. (f) Jacques, B.; Chavarot, M.; Rose-Munch, F.; Rose, E. Angew. Chem., Int. Ed. 2006, 45, 3481. (g) Jacques, B.; Chanaewa, A.; Chavarot-Kerlidou, M.; Rose-Munch, F.; Rose, E.; Ge´rard, H. Organometallics 2008, 27, 626. 4. J. A. Reingold, S. U. Son, G. B. Carpenter and D. A. Sweigart, J. Inorg. Organomet. Polym., 2006 16, 1, 1-13. 5. J. A. Reingold, M. Jin and D. A. Sweigart, Inorg. Chim. Acta, 2006 359, 1983. 6. M. Oh, J. A. Reingold and D. A. Sweigart, Macromolecules Containing Metal and Metal-Like Elements , 2005 John Wiley, Vol. 5, Chapter 10, 259. 7. M. Oh, G. B. Carpenter and D. A. Sweigart, Acc. Chem. Res., 2004 37, 1. 8. M. Oh, G. B. Carpenter and D. A. Sweigart Angew. Chem. Int. Ed., 2003 42, 2025. 9. M. Oh, G. B. Carpenter and D. A. Sweigart, Organometallics, 2003 22, 1437. 148 10. M. Oh, G. B. Carpenter and D. A. Sweigart, Angew. 2002 Chem. Int. Ed., 41, 3650. 11. Sang Bok Kim, Chen Cai, Shouheng Sun, and Dwight A. Sweigart, Angew. Chem. Int. Ed., 2009 48, 2907 –2910. 12. Sang Bok Kim, Chen Cai, Jaemin Kim, Shouheng Sun, and Dwight A. Sweigart, Organometallics, 2009 28, 5341–5348. 13. Chapter 4. Proton reduction catalyzed by aromatic manganese carbonyl complexes Manuscript in process. 14. J. Derek Jackson, Sharon J. Villa, Deborah S. Bacon, and Robert D. Pike* Organometallics 1994, 13, 3972-3980 15. (a) Fischer, E. O. and Hafner, W. Z.Naturforsch., B 1966, 10, 665. (b) Fischer, E. O. and Seus, D. Chem. Ber. 1956,89,1809. (c) Cofield, T. H.; Sandel, V.; Closson, R. D. J. Am. Chem. Soc. 1967, 79, 5826. 16. Rybinskaya, M. I.; Kaganovich, V. S.; Kydinov, A. R. Izu. &ad. Nauk SSR, Ser. A Khim. 1984,885. 17. Pearson, A. J.; Shin, H. Tetrahedron 1992, 48, 7527. 18. Pearson, A. J.; Richards, I. C. J. Organomet. Chem. 1983,258, C41 19. (a) Mews, R. Angew. Chem., Int. Ed. Engl. 1975, 14, 640. (b) Wimmer, F. L.; Snow, M. R. Aust. J. Chem. 1978, 31,267. (c) Uson, R.; Riera, V.; Gimeno, J.; Laguna, M.; Gamasa, M. P. J. Chem. SOC., Dalton Trans. 1974, 966. (d) Cotton, F. A.; Darensbourg, D. J.; Kolthammer, W. S. Inorg. Chem. 1981, 20, 1287. (e) Schmidt, S. P.; Nitschze, J.; Trogler, W. C. Inorg. Synth. 1989,26, 113. 149 20. Basin, K. K.; Balkeen, W. G.; Pauson, P. L. J. Organomet. Chem. 1981, 204, C 25. 21. S. Sun, L. K. Yeung, S. R. Switzer, T.-Y. Lee, S. S. Lee, Y. K. Chung, R. D. Pike, and D. A. Sweigart, Organometallics, 1995 14, 2613. 22. Antoine Eloi, Franc-oise Rose-Munch,* Eric Rose,* and Petra Lennartz Organometallics 2009, 28, 5757–5764 23. Peter L. Pauson and John A. Segal, J.C.S. Dalton 1975 1677-1682 150 Chapter 6 Surface electrochemistry of aromatic manganese tricarbonyl complexes using diazonium attachment and physical surface modification —— organometallic electrode 6.1 Introduction As already being discussed in chapter 3, the electrochemistry of monoarene manganese tricarbonyl in homogeneous solution is investigated: Compared with the + far greater reactivity of the polyarene Mn(CO) 3 complexes,1,2 monoarene [HMB Mn(CO)3]PF6 and it’s analogs exhibit quite stable properties. It is reduced irreversibly by one electron at much more negative potential to give 19e- unstable radical at room temperature, the free radicals will dissociate one carbonyl from the Mn(CO) 3+ moiety followed by dimerization to afford a green color bimetallic compound with a metal-metal bond.3 151 Figure 6-1. Mechanism of electrochemistry of (η6-HMB)Mn(CO)3+ Figure 6-2. CV of 1mM compound 1 at room temperature on Pt electrode. 1mM ferrocene was used as an internal potential standard. Scan rate is 0.50V/s.4 152 The monoarene manganese tricarbonyl loses its chemical reverisibility under applied potential and decomposed to a green color manganese dimer. There is no chemical reversibility observed for monoarene manganese tricarbonyl. Electrochemist prefers reversibility not only because that it indicates the stability under reduction or oxidation, but also the potential application as an electrocatalyst. If the decomposition, in another word, dimerization of the complex 1 could be stopped, the electrochemistry could very possibly be chemically reversible. Enabling the reversibility of complex 1 would widen the scope of our view of electrochemistry of monoarene manganese complex. A redox active complex would be more attractive, might have potential applications in catalysis and sensor. 1. The following methods might stop the dimerization: 1). Excess CO was bubbled into the solution where cyclic voltammetry is conducted. Excess carbon monoxide might prevent complex 1 from losing CO after reduction, hence, stop the dimerizaton (discussed in chapter 3). 2). Physical attachment: powder of complex 1 was loaded onto the surface of the electrode. CV was conducted in aqueous solution since complex 1 has no solubility in aqueous solution. Ideally, the complex 1 on the surface of the electrode would not dissolve into the aqueous solution, so under the applied reductive potential, even losing CO ligand, two of them can not move freely to dimerize. 3). Chemical attachment: diazonium attachment is one of the wildly used chemical attachments. The precursor has already been synthesized (chapter 5). So in this 153 chapter, I will take it from there. For the purpose of applying our research to more interesting and applicable areas, like surface electrocatalyst, bio-sensor, we want to combine the previous work with the chemical modified electrode, that is, we will try to load our Mn, Re or any other promising transition metal moiety to the surface of the commonly used electrodes, like Pt, Glassy Carbon, Au. Furthermore, covalent attachment is the most promising attachment method for our research, since it’s a stronger attachment to the substrate surface, and it might be stronger enough to undergo high activation energy barrier (high temperature, high pressure). What we know mostly is the interaction between the aryl group and the metal moiety on a conductive substrate and what we are most interested is how differently it will behave if it is directly attached to the substrate other than moving freely in the solution. The surface modification of the conductive substrate —— “electrode surface synthesis”, was introduced by Royce. W. Murray5 and his colleagues at early 1970s. For the first time, the conductive substrate tin dioxide electrode was chemically functionalized with amine group through covalent bond, which was a milestone, because amine surface was much reactive and could be further modified by other desired functional groups via linker chemistry. 154 Figure 6-3. Surface modification of Tin dioxide with ruthenium complex There are several methods of strong attachment for modifying electrode surface, in another word, chemical bonding: diazonium salt based attachment, click chemistry, spontaneous oxidation of primary amine to form a C-N bond on glass carbon electrode. Of which, diazonium based attachment is most convenient and straightforward method in our case. Geiger6 reported the first chemically modified electrode by aromatic cobalt and manganese complexes. He successfully loaded cobaltocenium and cyclopentadienyl manganese tricarbonyl7 complex onto the surface of glassy carbon electrode using diazonium based attachment. Scheme 6-1. Mechanism of surface modification using diazonium attachment. Not alike aniline by itself, manganese tricarbonyl functionalized aniline complex (η6-aniline) Mn(CO)3+ could not be diazotized by nitrite. So we change its hapticity from eta6 to eta5 to make the π system more electron rich. (η5- aminocyclohexadienyl) manganese tricarbonyl was synthesized and diazotized to 155 afford (η5-cyclohexadienyldiazonium) manganese tricarbonyl complex. This cyclohexadienyl manganese tricarbonyl diazonium complex can be reduced to liberate nitrogen and form a bond between the cyclohexadienyl group and electrode surface. The substituted hydrogen could be removed by treating with CPh3+ or strong acid thereafter. Herein, we want to report a unique method for surface modification of electrode by (η6-aromatic) manganese tricarbonyl cation. We want to compare its reductive electrochemistry before and after electrode surface modification, and we expect to see that the chemical reversibility shows up. No preliminary data has been acquired yet. We are still working on the purification and characterization of the intermediates. (η6-C6H5NHC6H4NH2)Mn(CO)3+ was synthesized from nucleophilic substitution of (η6-C6H5Cl)Mn(CO)3+ by p-phenylenediamine, then it was diazotized to afford (η6-C6H5NHC6H4N2)Mn(CO)32+. We can see that partial reversibility shows up in the CV time scale (scheme 6-2). In the meanwhile, taking advantage of the solubility of arene manganese tricarbonyl complex, we have done their heterogeneous electrochemistry by physical attachment. (η5-aminocyclohexadienyl) manganese tricarbonyl mediated slow hydrogen release is the side reaction when it is treated with acid. Other (η5-hydrocyclohexadienyl) manganese tricarbonyl analogs show similar properties of hydrogen release. The amine group of (η5-C6H6NH2)Mn(CO)3 facilitates the hydrogen release reaction. 156 Scheme 6-2. Surface modification of electrode by aromatic manganese tricarbonyl cation through diazonium attachment & slow hydrogen release as side reaction. In the meanwhile, taking advantage of the solubility of arene manganese tricarbonyl complex, we have done their heterogeneous electrochemistry by physical attachment. 6.2 Experimental 1. Synthesis (η5-HC6H5NH2)Mn(CO)3 The synthesis method of this (η5-aminocyclohexadienyl) manganese tricarbonyl cation is straightforward. [(η6-aniline)Mn(CO)3]BF4 (0.638g, 2.00mmol) and tetrabutylammonium borohydride 157 (0.566g, 2.20mmol) were combined with 20ml dichloromethane in a 50ml round bottom flask in ice bath. The reaction mixture was stirred for 30mins in ice bath and then 30mins at room temperature. The product was purified by running through the silica gel column using dichloromethane as eluent. IR in dichloromethane: 2005 and 1923 cm-1 ; in ether: 2007, 1930 and 1917 cm-1. [(η5-HC6H5N2)Mn(CO)3]BF4 The diazotization of (η5-HC6H5NH2)Mn(CO)3 was conducted in acetone solution. (η5-HC6H5NH2)Mn(CO)3 (0.233g, 1.00mmol), excess amount of HBF4·Et2O and t-butyl nitrite (0.158ml, 1.20mmol) were combined with 20ml acetone in a 50ml round bottom flask in ice bath. The reaction mixture was stirred for 60mins in ice bath. The color of the solution changed from yellow to orange red. After reaction, solvent was dried to minimum amount and excess amount of ether was added to precipitate the product. Orange red solid were washed several times by ether. Dichloromethane was used to dissolve any soluble species until nothing could be dissolved. Then the dichloromethane solution was combined and dried to minimum amount and was treated with excess ether to precipitate the yellow salt in it. The yellow salt is the desired (η5-cyclohexadienyldiazonium) manganese tricarbonyl complex. IR of it in dichloromethane is 2065 2004 cm-1 for CO; 2291 cm-1 for diazonium. The leftover yellow compound is [(η6-aniline) Mn(CO)3]BF4. It came from a side reaction of the above diazotization. We can reuse the aniline manganese complex. [(η6-chlorobenzene)Mn(CO)3]PF6 Mn(CO)5Br (2.749g, 10.00mmol) and AlCl3 (2.414g, 20.00mmol) were combined with 30ml chlorobenzene in 100ml round 158 bottom flask. The mixture was heated to reflux for 8 hours. After reaction, the mixture was cooled down and washed by 15ml of DI water several times. The water solution was combined and washed by toluene two times and hexane two times. Then, excess amount of 45% HPF6 was added to the water solution to precipitate the product. The mixture was filtered and precipitate was washed by ether three times. The yield of the product is 60%. Recrystallization was taken by slow evaporation of ether into its acetone solution. IR of it in acetone is 2084, 2032 cm-1. [(η6-p-C6H5NHC6H4NH2)Mn(CO)3]PF6 [(η6-chlorobenzen)Mn(CO)3]PF6 (0.397g, 1.00mmol) and p-phenylenediamine (0.225g, 2.08mmol)were dissolved in 4ml and 2ml of acetone separately. The manganese solution was added into the phenylenediamine solution drop by drop while the solution was kept stirring. Precipitate formed instantaneously. After two solution was combined, the precipitate was removed. The left over acetone solution was dried to minimum amount and treated with excess amount of ether. The product precipitated out and recrystallized via slow evaporation of ether into its acetone solution. The yield is 90%. IR of it in acetone is 2063, 1998 cm-1. [(η6-C6H5NHC6H4N2)Mn(CO)3](BF4)2 Similar method was applied to the diazotization of [η6-p-C6H5NHC6H4NH2)Mn(CO)3]PF6. [η6-p-C6H5NHC6H4NH2) Mn(CO)3]PF6 (0.936g, 2.00mmol), excess amount of HBF4·Et2O and t-butyl nitrite (0.316ml, 2.40mmol) were combined with 20ml acetone in a 50ml round bottom flask in ice bath. The reaction mixture was stirred for 60mins in ice bath. The color of the solution changed from yellow to orange red. After reaction, solvent was dried to 159 minimum amount and excess amount of ether was added to precipitate the product. Orange red solid were washed several times by ether and dichloromethane. The orange yellow salt is the desired manganese tricarbonyl diazonium dication complex. IR of it in acetone is 2073, 2014 cm-1 for carbonyls and a small peak at 2258 cm-1 for the diazonium. 2. Surface modification of glassy carbon electrode via reduction of diazonium salt 1mM of the diazonium salt, 0.1M TBAPF6 electrolyte and 10ml acetonitrile were combined in a cyclic voltammetry cell. Nitrogen was used to bubble through and above the whole system before and after electrochemistry experiment. Fresh polished glassy carbon electrode was used. The reduction potential was kept at -0.6V for 600s with the solution being stirred all the time. After the electrolysis, the electrode was rinsed thoroughly by acetonitrile to remove any chemical that might attach to the surface physically. Sonication was also used to clean the surface of the electrode. After sonication, electrode was kept dry in the nitrogen. 3. To test the electrochemical signal from the modified electrode. A blank dichloromethane solution was prepared with 0.1M TBAPF6 electrolyte. Again, nitrogen was bubbled through and above the solution in the cell before and after cyclic voltammetry experiment. Since (η6-arene) manganese tricarbonyl cation is reductive active electrochemically. Potential was set to start from 0V to -1.5V vs Ag/AgCl. 4. Physical attachment 160 1mM of [(η6-HMB)Mn(CO)3]PF6 solution in acetone was prepared. The glassy carbon electrode was dipped into the solution for 3s, and then was lifted. Nitrogen was used to dry the surface of the electrode. Blank aqueous solution with 0.1M KCl was used to test the electrochemical signal from the modified electrode. 1mM of [(η6-benzene)Mn(CO)3]PF6 solution in acetone was tried also. Instead using aqueous solution, blank dichloromethane solution with 0.1M TBAPF6 was used, since the bad solubility of [(η6-benzene)Mn(CO)3]PF6 in dichloromethane. In the above case, even [(HMB)Mn(CO)3]PF6 dissolves well in dichloromethane, its solution could not be used for unknown reason. Not signal could be detected if pretreating the electrode surface with [HMB Mn(CO)3]PF6 dichloromethane solution. We thought that, dichloromethane solution might result in bad contact between the compound and the electrode surface. 6.3 Result and discussion Surface modification is a method which brings two types of surface together. Organometallic electrode: also called chemically modified electrodes, that is the study of electroactive monolayers and thicker films on the conductive substrates. This aspect of electrochemistry is more and more active in recent years. A lot of people have put their effort into this area and a number of publications related to the preparation, characterization, application and electrochemical behavior of chemically modified electrodes are available 8-13. Modification of electrode surfaces for catalytic or analytical purposes and biotechnological applications currently attracts 161 considerable attention because of potential applications in areas such as electrocatalysis, sensor development and semiconductor productions. There are many ways of modifying the surface of the conductive substrates, such as: irreversible adsorption,12 covalent attachment of a monolayer, and coating the electrode with films of polymers or other materials12. Of which, chemical modification is preferred not only because it provides the strongest attachment but also it fundamentally changed the chemical and physical properties of the conductive substrate, which give versatile surface modified with desired functional group. The purpose of heterogeneous electrochemistry study is to achieve the electrochemical reductive reversibility by covalent attachment of arene manganese tricarbonyl using diazonium-base chemistry or simply physical adsorption attachment. Physical separation of manganese compound may stop the dimerization after one-electron reduction and result in reductive reversibility. Organometallic electrode is a new concept in the area of heterogeneous electrochemistry. One of my projects is focused on covalent attachment of arene manganese tricarbonyl using diazonium-base chemistry and collecting and analyzing e-chem signal from the surface functionalized electrode. As already being discussed in chapter 5, aniline is deactivated after the coordination of manganese moiety. Thus, we need to add a hydride to change the hapticity of (η6-aniline) Mn(CO)3+ from eta6 to eta5 in order to reactivate this manganese functionalized aniline. The strategy is illustrated in scheme 6-2. 1. Side reaction 162 The diazotization of (η5-HC6H5NH2)Mn(CO)3 leads to side reaction with (η6-aniline)Mn(CO)3 as byproduct. The slow hydrogen release reaction was discussed in chapter 4 and the mechanism was discussed. The key intermediate is a formation of ring slippage (η4-R-C6H5)MnH(CO)3. In the above case, aromatic amine is a base with moderate strength, the incorporation of amine will facilitate the uptake of proton and interaction between N-H-Mn, thus accelerate the hydrogen liberation and indeed, Inspired by this side reaction, we are trying to optimize our CpMn(CO)2NO+ pre-catalyst by incorporating an amine group to the aromatic hydrocarbon ring. Diazotization of (η5-HC6H5NH2)Mn(CO)3 leads to the formation of its diazonium analog [(η5-HC6H5N2)Mn(CO)3]BF4. Experiment was held at this step; we are in the process of confirming the existence of this reactive species which contains hydride and diazonium cation. Molecular internal redox reaction between oxidant diazonium salt and reductant hydride would result in explosion. 2. Direct diazotization Compared with the above indirect diazotization method, direct diazotization method was applied. See the following scheme. 163 + + Scheme 6-3. Reaction pathway for indirect diazotization without changing hapticity. By incorporating p-phenylenediamine as a linker, we not only introduce free aniline into our system; but also keep the hapticity unchanged (scheme 6-3). Compared with the indirect method, this strategy is straight forward and more convenience. However, a nitrogen heteroatom was introduced into the system after surface modification. We lack information about good interpretation of homogeneous electrochemistry of N-phenyl-aniline manganese tricarbonyl complex, so their heterogeneous electrochemistry remains unclarified. But it is for the first time that, eta6 aromatic manganese tricarbonyl cation was successfully introduced onto the surface of glassy carbon electrode. The cyclic voltammetry of the surface modified electrode in blank solvent is shown in figure 6-4. 164 Figure 6-4. Continuous five scans of cyclic voltammetry of (η6-p-C6H5NHC6H4-)Mn(CO)3+ modified glassy carbon electrode in blank dichloromethane solution with 0.1M TBAPF6 at room temperature. The scan rate is 5V/s. The electrode was rinsed carefully to remove any physical adsorption. From the above continuous five scans, partially chemical reversibility was observed; however, each scan lost around 80% reversibility, therefore at fifth scan (red line), no peak could be observed any more. We could not give a reasonable explanation about the generation of reversibility and we are not sure about the whether it is two electrons or one electron reduction. 3. Physical attachment One drop of 1mM [(HMB)Mn(CO)3]PF6 acetone solution was dried directly on the surface of the glassy carbon electrode. The adsorbed complex 165 [(HMB)Mn(CO)3]PF6, which is insoluble in water, produced a chemically irreversible reduction at Ep = -1.27 V. The peak current became greatly diminished after the first scan cycle. Since the formation of dimer 5 or ring coupled dimanganese complex 7 should be sterically difficult, our interpretation is that formation of anionic 4 occurs, but is very rapidly followed by protonation by the medium (water) to give the known and electrochemically-inactive (η6-C6Me6)Mn(CO)2H. This explanation is consistent with the diminished reduction current observed on the 2nd scan. Figure 6-5. CVs of solid [(η6-C6Me6)Mn(CO)3]PF6 (1) deposited from a 1 mM acetone solution onto a 3.0 mm diameter glassy carbon working electrode by evaporation at 20 oC. The CV medium was 1.0 M KCl in water under N2. The 166 scan rate was 0.50 V s-1. The first scan is shown in red and the second is shown in black. Similar physical attachment experiment was applied to [(η6-C6H6)Mn(CO)3]PF6. Dichloromethane solution was used since [(η6-benzene)Mn(CO)3]PF6 has limited solubility in CH2Cl2. Figure 6-6. CVs of solid [(η6-C6H6)Mn(CO)3]PF6 deposited from a unknown concentration acetone solution onto a 3.0 mm diameter glassy carbon working electrode by evaporation under nitrogen at 20 oC. The CV medium was 1.0 M KCl in water under N2. The scan rate was 0.05 V s-1. The first scan is shown in red and the second is shown in blue. In the above CVs, the solvent was changed to non-nucleophilic CH2Cl2. The electrochemistry of benzene manganese tricarbonyl cation mimics the one of compound 1.3 Similar anion (η6-C6H6)Mn(CO)2- formed after major reduction at -0.95 V, however it stayed as anion or partially dissolved in the solution. No peak for the dimer was found, and the peak current for the continuous second scan vanished (in 167 blue). Taking advantage of solubility of our interested complexes, physical attachment method could be easily applied into heterogeneous electrochemistry study. Compared with chemical modification, its flexibility and renewability are advantageous, however, it lacks durability and the concentration of the modification could not be well controlled. 6.4 Conclusions. In this chapter, different surface modification methods were applied to heterogeneous electrochemistry study of eta6 aromatic manganese tricarbonyl cation. Interesting intermediates were observed via solid state electrochemistry through physical adsorption. We successfully modified a conductive surface (GC) by aromatic manganese carbonyl moiety using phenylenediamine as a linker; however, the heteroatom makes the electrochemistry of the target complicated. We are still working on permanent attachment using diazonium chemistry indirectly by changing the hapticity from eta 6 to eta 5. 168 6.5 References 1. S. Sun, L.K. Yeung, D. A. Sweigart*, T.-U. Lee, S.S.Lee, Y. K. Chung, S. R. Switzer, and R. D. Pike, Organometallics, 1995, 14, 2613. 2. S. Sun, C. A. Dullaghan, and D. A. Sweigart*, J. Chem. Soc., Dalton Trans., 1996, 4493. 3. Catherine C. Neto, Carl D. Baer, Young K. Chung and Dwight A. Sweigart" J . Chem. Soc., Chem. Comun., 1993 816-818 4. Chapter 3. 5. P. R. Moses, L. Wier, and R. W. Murray, Anal. Chem., 47, 1882 (1975). 6. Jannie C. Swarts, Derek Laws, and William E. Geiger* Organometallics 2005, 24, 341-343 7. Derek R. Laws, John Sheats, Arnold L. Rheingold and William E. Geiger*, Langmuir 2010, 26(18), 15010–15021 8. Bernard, M.-C.; Chausse, A.; Cabet-Deliry, E.; Chehimi, M. M.; Pinson, J.; Podvorica, F.; Vautrin-Ul, C. Chem. Mater. 2003, 15(18), 3450. 9. Michel Delamar, Rachid Hitmi, Jean Pinson, Jean Michel Saveant J. Am. Chem. Soc.; 1992, 114(14), 5883 10. Bahr, J. L.; Yang, J.; Kosynkin, D. V.; Bronikowski, M. J.; Smalley, R. E.; Tour, J. M. J. Am. Chem. Soc., 2001, 123(27), 6536 11. Abbou, J.; Anne, A.; Demaille, C. J. Am. Chem. Soc; 2004, 126(32), 10095 12. Electrochemical Methods Allen J. Bard Larry R. Faulkner 2nd edition Charpter 169 14. 13. Hong, H.-G.; Park, W. Langmuir; 2001, 17(8); 2485 170 Chapter 7 Synthesis and characterization of heterodinuclear complex with core structure of Mn-M metal metal bond 7.1 Introduction The η6-aromatic manganese dicarbonyl anion is strong nucleophile, it was proposed to be the important intermediate which catalyzed the proton reduction reaction (chapter 4). By introducing the dicarbonyl anion with the tricarbonyl cation stoichiometrically, Eyman reported the formation of an aromatic homodinuclear manganese complex with a Mn-Mn bond and bridged carbonyl ligands.1 It would be interesting to check the reaction between the same manganese anion with [(η6-HMB)Re(CO)3]PF6. We want to explore the possibility of forming the heterodinuclear dimmer with a novel core structure of Mn-Re metal-metal bond. There is no report about the study of Mn-Re metal-metal bond; furthermore, heterodinuclear complex has potential applications in molecular catalysis. The importance of this project: The reaction between η6-aromatic manganese dicarbonyl anion and other transition metal cation or neutral species might provide us a unique synthetic template for formation of Mn-M metal metal bond. This project was just initiated and I am still in the process of getting preliminary data of the first Mn-Re heterodinuclear compound. In order to make my thesis to be systematic, I need to include this project into my thesis. Any result will be discussed in my defense and further updates will be put into my thesis. 171 CO CO Mn Mn CO OC Mn + OC CO CO + 1e - + -C O + 1e- _ Mn Mn OC CO OC CO + Re + OC CO CO CO CO Mn Re CO OC Scheme 7-1. Proposed scheme of formation of Mn-Re metal metal bond 7.2 Reference 1. Peter J. Schlom, Ann M. Morken, Darrell P. Eyman,’ Norman C. Baenziger, and Steven J. Schauer Organometallics 1993,12, 3461-3467 172