| Solar fuels hold the promise of being low-cost energy stores for intermittent renewable energy sources (e.g., solar and wind) and end-use fuels (e.g., for the transportation sector). Hydrogen produced from water that is driven by the energy of solar photons is of interest and the first step, water oxidation, is not well understood. Water oxidation occurs at the anode of an electrochemical cell or within photosystem II in photosynthesis.;Development of a water oxidation catalyst has been approached from a transition metal complex perspective with a focus on non-innocent ligands rather than metal centers for homogeneous or heterogeneous catalysis. Evaluation of tris(tetrachlorosemiquinonate)iron(III) neutral complex, FeIII(Cl 4SQ)3, as a water oxidation catalyst comprises this work, which builds on the Pierpont Lab's experience in synthesis and characterization of metal complexes with quinone-based ligands. Quinones occur in biological systems including photosynthesis and have distinct redox characteristics.;Physical characterization shows crystal structure anomalous carbon-oxygen bond lengths for the trianion complex, [FeIII(Cl4Cat) 3]3-, and solvatochromic effects for the neutral and trianion complexes. Electrochemical features exhibit slow heterogeneous electron transfer kinetics (especially for cathodic current), and apparent EECE mechanism for oxidation of the trianion complex in acetonitrile and EEE mechanism in methanol, and no deterioration of current as solution acidity is varied from 3 to 12 pH. Spectroelectrochemical analysis demonstrates a disproportionation reaction for the monoanion complex, [FeIII(Cl4SQ)2(Cl 4Cat)]1-. Controlled potential electrolysis for electrolytically generated neutral complex in buffered ACN showed three to five one electron equivalent redox cycles with water present. End products for the neutral complex reaction in water include ∼0.5 molar equivalents of trianion complex or dianion complex, [FeIII(Cl4SQ)(Cl4Cat) 2]2-, ∼0.5 molar equivalents of chloride and ∼3 molar equivalents of protons. No catalytic oxidation waves were observed for the neutral complex in the presence of water in any system evaluated. The reaction with water may occur through a hydrolysis of electrophilic carbon positions on the semiquinonate ligands or another mechanism. The work has formed a foundation for study of transition metal-quinone species as water oxidation catalysts. |
