Explore the vast range of titles available.

The effective design of an artificial photosynthetic system entails the optimization of several important interactions. Herein we report stopped-flow UV-visible (UV-vis) spectroscopy, X-ray crystallographic, density functional theory (DFT), and electrochemical kinetic studies of the Re(bipy- t Bu)(CO) ₃(L) catalyst for the reduction of CO ₂ to CO. A remarkable selectivity for CO ₂ over H ⁺ was observed by stopped-flow UV-vis spectroscopy of [Re(bipy- t Bu)(CO) ₃] ⁻¹. The reaction with CO ₂ is about 25 times faster than the reaction with water or methanol at the same concentrations. X-ray crystallography and DFT studies of the doubly reduced anionic species suggest that the highest occupied molecular orbital (HOMO) has mixed metal-ligand character rather than being purely doubly occupied [Formula], which is believed to determine selectivity by favoring CO ₂ (σ + π) over H ⁺ (σ only) binding. Electrocatalytic studies performed with the addition of Brönsted acids reveal a primary H/D kinetic isotope effect, indicating that transfer of protons to Re -CO ₂ is involved in the rate limiting step. Lastly, the effects of electrode surface modification on interfacial electron transfer between a semiconductor and catalyst were investigated and found to affect the observed current densities for catalysis more than threefold, indicating that the properties of the electrode surface need to be addressed when developing a homogeneous artificial photosynthetic system.

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.