Base Metal Bifunctional Catalysis

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More than 80% of commercial chemical processes involve catalysts that decrease energy use, increase reaction selectivity and contribute to process intensification, e.g., by combining multiple steps into one. While commodity chemical manufacturing typically employs heterogeneous solid catalysts, production of higher value chemicals often involves molecular catalysts that can be rationally tuned' to provide high yields of the desired product. Although precious metal (Ru, Rh, Ir, Pd, Pt) complex catalysts have been broadly developed over the last 50 years, there has been an intensive drive to develop more effective catalysts that include base-metals (Mn, Fe, Co, Ni, Cu) that are less toxic and more earth abundant. Some of the greatest reported successes in base metal catalysis include complexes in which the metal-coordinated ligand is able to provide an additional mechanistic role to those offered by the metal center. In several prominent examples a primary amine or amido ligand is able to function as a hydrogen bond donor or internal base that contributes to the activation of E-H bonds (E = H, B, C, N, O, Si). In catalysts modeled after Nature, the thiolate ligand can also participate in bifunctional E-H bond activation although the coordinated thiol S-H partner is typically more acidic than its N-H counterpart. Finally, a redox-active ligand can serve as an electron source or acceptor, working in tandem with the base-metal center to achieve multi-electron transformations. In this proposal we outline a program that will challenge 11 graduate and undergraduate students, using simple SNS ligands with five different base-metals to probe the fundamental properties of metal-ligand cooperativity in catalytic reactions. Over the last funding period we developed easily prepared SNS amido and thiolate ligands and investigated their coordination chemistry to compare the efficiency of hard' N vs. soft' S internal bases for a number of catalyzed reactions. Highlights include demonstration of bifunctional Fe catalysts for amine-borane dehydrogenation, Co catalysts for alcohol dehydrogenation and Mn catalysts for alkene hydrosilylation. An internal redox reaction on Fe, Co, and Ni SNS thiolate complexes afforded a redox non-innocent N2S2 ligand via imine C-C bond formation. Initial studies suggest that this reversible process may be involved in the M(N2S2) catalytic cycles and the presence of multiple stable redox states (3 for Fe, 4 for Co) opens up the exciting possibility of redox-dependent catalytic activity and selectivity. Objectives for the next funding period include tuning donor basicity by changing the metal and nature of the ancillary ligands and probing the cooperativity of ligand donors and redox activity. These fundamental investigations of bifunctional catalysis by a combination of experimental and computational approaches will eventually allow us to develop and optimize new hydrogen-borrowing' tandem catalysis processes.