Carbon Dioxide as a C1 Source: The Activation of Carbon Dioxide with Molybdenum and Iron Complexes

MacIntosh, Alex Dale

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Year:: 2015
Contributor:: MacIntosh, Alex Dale (creator); Bernskoetter, Wesley (Director); Kim, Eunsuk (Reader); Williard, Paul (Reader); Brown University. Chemistry (sponsor)
Genre:: theses
Subject:: Triphos; Turnover Number; Organometallo Ligand; Quinone Methide; Carbon dioxide; Molybdenum; Acrylic acid; Iron; Lewis acids; Formic acid; Catalysis
Extent:: 17, 155 p.
DOI: https://doi.org/10.7301/Z03B5XJP

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Abstract:: The current reliance on finite fossil fuels for the production of commodity chemicals has been recognized as unsustainable, and therefore has led to increasing pressure for scientists to curb carbon dioxide emissions and develop efficient CO2 capture and utilization systems. One such system aimed toward the utilization of CO2, as well as decreasing dependence on fossil fuels, is using CO2 as a feedstock for fuels and commodity chemicals. CO2 has enormous potential as an abundant, cheap, and safe C1 source, but it is underutilized due to the harsh and difficult conditions normally required to activate it. To circumvent this, high-energy starting materials can be used, which exhibit high reactivity under mild conditions. Specifically, we are interested in the coupling of CO2 with ethylene to produce acrylic acid and the hydrogenation of CO2 to produce formic acid. A series of zerovalent molybdenum complexes bearing triphosphine ligands, [Ar2PCH2CH2]2PPh, were synthesized and found to couple CO2 and ethylene, forming isolable molybdenum(II) acrylate hydride species. Kinetic analysis of acrylate formation revealed a first-order dependence on molybdenum. Comparison of steric and electronic features of the ligands showed a strong influence of ligand size on rate, with [(3,5-tBu-C6H3)2PCH2CH2]2PPh, coupling slower than [(3,5-Me-C6H3)2PCH2CH2]2PPh. Additionally, a milder electronic effect was observed for the complexes, with [(4-F-C6H4)2PCH2CH2]2PPh reducing CO2 at half the rate of [Ph2PCH2CH2]2PPh. A pair of pincer-supported iron(II) carbonyl hydride complexes were found to catalyze the hydrogenation of CO2 to formate in the presence of a Brønsted base. A remarkable enhancement in catalytic activity was observed upon addition of a Lewis acid co-catalyst, where mechanistic investigation indicate that it destabilizes the resting state of catalysis, aiding in formate extrusion. From these mechanistic insights, a new class of iron catalysts based on an N-methylated PNMeP ligand were synthesized, and their activity for CO2 hydrogenation was examined. Using this new class of iron catalysts, turnover numbers near 60,000 were achieved for formate production, the highest activity reported for an earth abundant system to date.
Notes:: Thesis (Ph.D. -- Brown University (2015)

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MacIntosh, Alex Dale, "Carbon Dioxide as a C1 Source: The Activation of Carbon Dioxide with Molybdenum and Iron Complexes" (2015). Chemistry Theses and Dissertations. Brown Digital Repository. Brown University Library. https://doi.org/10.7301/Z03B5XJP

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Chemistry Theses and Dissertations

Theses and Dissertations for the Chemistry department.
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