The activation of hydrogen is an essential process in many organometallic reactions, especially in the hydrogenation of small molecules. Activation of H2 is often promoted by a base embedded within the secondary coordination sphere of a metal complex, which deprotonates the metal-bound H2, generating a nucleophilic M–H species. This presentation describes a distinct strategy for achieving H2 activation through metal-ligand cooperativity. In this design, both H2 activation and substrate reduction are facilitated by cooperative action of the metal and a reactive, NADH-like organic group, termed a “hydride relay.” The relay is incorporated into a transition metal complex by tethering it to a phosphine ligand. This ligand is combined with a reduced, basic metal center to yield a bifunctional complex with distinct character from previous systems: the reaction with H2 reduces the ligand, forming a hydridic C–H bond, and protonates the metal, generating an acidic M–H species. Cooperative substrate reduction can then occur through hydride transfer from the ligand and proton transfer from the metal. A series of several ligands have been synthesized based on this general design, in which the organic hydride donor and the linking group that connects it to the phosphorus donor are varied. Bifunctional complexes have been prepared by combining these ligands with metal-phosphine and metal-cyclopentadienyl precursors of late transition metals, including Ru, Rh, and Pd. The characterization of these complexes, in various catalytically-relevant redox and protonation states, will be presented, including studies of their reactions with H2 and CO2. The conformational and thermodynamic considerations required for efficient hydrogenation of CO2 and other substrates, and the optimization of these factors using the modular ligand design, will also be discussed.

Synthesis of metal-phosphine complexes containing redox-active “hydride relays” for efficient hydrogenation catalysis

2019 Spring National Meeting

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