CO2 functionalization to commodity chemicals and molecular energy provide an powerful mechanism to leverage one of the world most abundant alternative sustainable carbon resources. This work focuses on the development of inexpensive earth abundant iron and molybdenum catalysts for the reduction CO2 toward two key commodity chemical targets: acrylate and formate. NaEt3BH reduction of (Triphos)MoCl3 afforded novel molybdenum tetrahydride species (Triphos)MoH4PPh3, which was found to promote CO2 functionalization to afford acrylate, propionate and formate species. Mechanistic study permitted control of the selective to the CO2-ethylene coupling product. Methods of acrylate elimination from the molybdenum center were also explored including the use of external bases and exogenous ligands. Strong bases and strong ligands such as CO or tBuNC, were found to induce acrylate elimination along with concomitant generation of acrylates and zero-valent molybdenum species. The reductive acrylate elimination studies may open pathways for catalytic acrylate generation. The nature of ancillary ligand flexibility on CO2-ethylene coupling at molybdenum complexes was probed by comparative reactivity of two PNP pincer ligands. The first isolable zero-valent (PhPNpyP)Mo(C2H4)2(CO2) exhibited no acrylate formation despite having the requisite components coordinate to the metal. Alternative and more flexible complexes, (PhPNagostic-CH3P)Mo0(C2H4)2 and (PhPNCH2P)Mo(H)(C2H4)2, were also studied. CO2 insertion into the Mo-H bond of (PhPNCH2P)Mo(H)(C2H4)2 produced (PhPNCH2P)Mo(C2H4)(κ2-CHO2), which proved a modest and rare catalyst for CO2 hydrogenation to formate with assistance of Lewis acid (LA). Further development of catalysts for CO2 reduction to formate pursued iron(II) carbonyl hydride complexes supported by either a bifunctional PNP ligand containing a secondary amine, or a PNP ligand with a tertiary amine that prevents metal-ligand cooperativity. Both of these species were found to promote the catalytic hydrogenation of CO2 to formate in the presence of Brønsted base. A remarkable enhancement in catalytic activity was observed upon addition of LA co-catalyst in both cases. For the secondary amine supported system, TON of approximately 9,000 for formate production was achieved, while for catalysts supported by the tertiary amine ligand, nearly 60,000 turnovers were observed; the highest activity reported for an earth abundant catalyst to date.
Zhang, Yuanyuan,
"Carbon Dioxide Functionalization to Acrylate and Formate by Homogenous Mo and Fe Complexes"
(2015).
Chemistry Theses and Dissertations.
Brown Digital Repository. Brown University Library.
https://doi.org/10.7301/Z04M92XV
