Towards an artificial formate dehydrogenase : : mechanistic studies of formate oxidation and CO₂ 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 rate- determining proton transfer from the Ni-O₂CH [beta]-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(P₂N₂)₂]¹⁻ complexes react with CO₂ 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(PPh₂NBn₂)₂]²⁺ and [Pd(PMe₂NPh₂)₂]²⁺ were tested for electrochemical CO₂ 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 oxidatio