Photo-And Electrocatalytic Oxidation Of Water And Organic Substrates By Manganese And Cobalt Complexes

Light-driven water splitting, a novel method to convert solar energy into chemical energy, is one way to realize artificial photosynthesis. This process requires the coupling of two half-reactions, oxidation of water and reduction of protons. Water oxidation involving the transferring of four electrons and four protons, which is an uphill energy reaction, is the bottleneck of artificial photosynthesis. Thus, the key in the research of water oxidation is to develop the effective catalyst and the stable reaction system. Meanwhile, if water oxidation could be replaced by oxidation of organic substrates with water as an oxygen source, which involves two electrons transferring, the efficiency of water splitting would increase.In the research of photocatalytic oxidation of organic substrates, a nonheme manganese-based complex Mn(II)(BQCN)(OTf)2 (Mnl, BQCN=N, N’-dimethyl-N, N’-bis(8-quinolyl)cyclohexanediamine) was synthesized and characterized. Under illumination, the catalytic activity of Mnl was investigated in solution containing photosensitizer [Ru(II)(bpy)3]Cl2 (bpy= bipyridine), electron acceptor [Co(III)(NH3)5Cl]Cl2, catalyst Mn1 and organic substrates (benzyl alcohol and thioanisole), both in acidic and neutral conditions. Mnl displays the highest reactivity in acidic condition, and the TONs of 75 and 210 were obtained with benzyl alcohol and thioanisole, respectively. The highest quantum yield(φ) of 0.26 ((φmax= 0.5) with thioanisole as the organic substrate was calculated. These results indicate that the practical application of earth-abundant metal-based catalysts in photocatalytic oxidation of organic substrates is possible. Finally the high-valent manganese-oxo intermediate was characterized by various spectroscopic methods, and the plausible mechanism was proposed based on the experimental measurements.In order to develop the practical (photo)anode, realizing electrochemical catalytic water oxidation, we chose manganese complexes bearing different kinds of N4 and N5 ligands and Co-TAML (TAML=tetraamido macrocyclic ligand) as the earth-abundant metal-based catalysts. The homogeneous electrocatalytic activity of the complexes at different pH buffers was investigated.