Towards an artificial formate dehydrogenase: mechanistic studies of formate oxidation and carbon dioxide reduction by metal P2N2 complexes

The efficient electrochemical production and use of CO2-based solar fuels is a problem of precisely coordinating the associated proton and electron transfers. One strategy for controlling these proton-coupled electron transfers is to use catalysts that contain proton relays in their secondary coordination spheres. The work described in this thesis explores the function of 1,5-diaza-3,7-diphosphacyclooctane (P2N2) ligands in molecular electrocatalysts for HCOOH/CO2 conversion. By focusing on a mechanistic understanding of the catalysis that occurs with these ligands, we seek to develop the chemistry of these systems and to guide the design of better CO2 catalysts.;A variety of NMR and electrochemical experiments were used to explore the likelihoods of several different proton or hydride transfer pathways for the oxidation of formate by [Ni(P2N2)2] 2+ complexes. The experiments suggest that oxidation occurs via a ratedetermining proton transfer from the Ni–O2CH β-H to the pendant base, coupled with a 2e– transfer to Ni(II). The measurement of electrocatalytic kH/kD KIEs between 3–7 suggests that this unexpected non-hydride process may be an unusual example of multi-site concerted proton-coupled electron transfer, which has been rarely observed in well-defined catalyst systems.;We attempted to develop a catalyst for the reduction of CO2 to formic acid by using metals with increased electron donating ability, as predicted by their hydride donating ability (hydricity). [Co(P2N 2)2]1– complexes react with CO 2 even in the absence of extra protons, but are unstable under the high potentials necessary to generate these species. [Pd(P2N2) 2]2+ complexes crystallize in square planar or minimally tetrahedrally distorted geometries and exhibit a single quasi-reversible 2e – Pd(II/0) redox couple in voltammetric studies. [Pd(P Ph2NBn2)2] 2+ and [Pd(PMe2NPh2 )2]2+ were tested for electrochemical CO 2 reduction in the presence of excess protons and found to preferentially produce H2. Comparative analysis of the intermediates involved in proton reduction by analogous [Pd(P2N2)2] 2+ and [Ni(P2N2)2]2+ complexes suggests that large reorganizational energy barriers render the Pd catalysts much less efficient than their Ni counterparts. The ability of the Ni-P2N2 metal-ligand combination to access multiple redox and protonation states with a minimum of reorganization appears to be essential to both proton reduction and formate oxidation.