The development of cost effective renewable energy sources is necessary to meet increasing demand and transition from fossil fuels as a primary energy source. Catalysis by transition metal molecular complexes for fuel forming and utilization reactions show promise due to their abundance and low cost. Additionally, molecular systems can be studied using common techniques. Systematic ligand design and structure-function relationships can be utilized to learn fundamental properties of these important fuel storage and generation transformations, which in turn is essential to the design of new electrocatalysts.
This dissertation describes the design of transition metal complexes that incorporate pendant bases in the secondary coordination sphere to mimic the functionally of proton relays in biological systems. The complexes were investigated for their reactivity towards the efficient transformation of small molecules to chemical fuels or use in fuel cells. Investigating these complexes electrocatalytically allows for the elucidation of bond strengths, mechanistic information, thermochemical cycles and free energy of the overall target reactions. A multidisciplinary approach is invoked, founded on the design and synthesis of organic compounds, development of metal complexes, and characterization by organic, inorganic and analytical methods to determine structure-activity relationships of rationally designed ligand frameworks.
Chapter 1 describes the design, synthesis and characterization of neutral tetradentate diamino–dipyridal ligands (N2Py2) containing hydrogen bond donor (LMMA) and hydrogen bond acceptor (LDMA) functionalities.
Chapter 2 describes the synthesis and characterization of divalent copper complexes utilizing LMMA and LDMA. The structural, electronic, and electrochemical properties of these complexes are described. The protonation of the monovalent complexes exhibited the unexpected site of protonation on the ethylenediamine backbone. The reactivity of the copper complexes with dioxygen was also investigated.
Chapter 3 describes the synthesis, characterization, structural, electronic, and electrochemical properties of cobalt complexes containing the LMMA and LDMA ligands. The reactivity of the divalent cobalt complexes with protons, water, dioxygen, and carbon dioxide is presented. The most interesting finding with these complexes is the improved water oxidation reactivity of [CoLDMA(CH3CN)2][BF4]2 compared to the analogous complex with no pendant base present. Additionally, solid state structures of stable aquo complexes were obtained that demonstrated intramolecular hydrogen-bonding between water and pendant bases.