Understanding catalytic systems for water oxidation is the most challenging aspect of artificial photosynthesis, the fundamental chemical process that extracts reducing equivalents from water to generate fuels. Half-sandwich Cp*IrIII complexes (Cp* = pentamethyl-cyclopentadienyl) were developed, serving as efficient catalysts for water oxidation. However, the actual species formed after oxidative transformation of the Cp*IrIII precatalysts have remained elusive, in spite of repeated efforts to crystallize the catalyst resting state species. Here, we unveil the structures of the catalysts combining density functional theory (DFT) with High-Energy X-Ray Scattering (HEXS) and extended X-ray absorption fine structure (EXAFS) Spectroscopy. Our results indicate that the catalysts are Ir dimers with mono-μ-oxo and terminal anionic ligands, generated in situ through oxidation of the catalyst precursor. The model structures are consistent with extended X-ray fine structure (EXAFS) and high-energy X-ray scattering (HEXS) spectroscopy, 1H and 17O NMR spectroscopy, electrophoresis, resonance Raman spectra, UV-Vis spectra and matrix-assisted laser desorption ionization-time of flight mass spectrometry data. These results provide new information on the solution structure of the catalytically active species that is vital to understand the mechanism of water oxidation by these highly active Ir catalysts.

Solution structures of highly active molecular Ir water-oxidation catalysts from density functional theory combined with high-energy x-ray scattering and EXAFS spectroscopy

251st National Meeting (2016)

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