Mechanism of Water Splitting and Oxygen-Oxygen Bond Formation by a Mononuclear Ruthenium Complex

Density functional theory (DFT) predicts a detailed mechanism for the reported potential photocatalytic system for solar hydrogen production from water, (P-da-PNN)RuH(CO) (1, P-da = dearomatized at the phosphorus side arm, PNN = (2-(di-tert-butylphosphinomethyl)-6-diethylaminomethyl)pyridine) (Science 2009, 324, 74). In the initial thermal reaction, the coordination of a water molecule is followed by cleavage of an O-H bond and aromatization of the PNN ligand to form (PNN)RuH(CO)(OH) (3'), the most stable complex in the reaction. This low-barrier step is followed by the rate-determining dearomatization and formation of H-2. Next, a second water molecule is activated, resulting in the formation of the cis-dihydroxo complex (PNN)Ru(CO)(OH)(2) (7), which photolytically eliminates H2O2. Time-dependent DFT calculations predict that the breaking of the two strong Ru-O bonds and the formation of the O-O bond in this photolytic reaction involve low-energy triplet states and singlet-triplet crossings. Rather than regeneration of initial complex 1 after the light-induced H2O2 evolution in the catalytic cycle, the DFT calculations predict a new route with a lower energy barrier via the regeneration of 1', an isomer of 1 with the unsaturated carbon at the nitrogen side arm of the PNN ligand. This new route involves hydride transfer from the methylene group at the nitrogen side, rather than the previously proposed regeneration of 1 through hydride transfer from the phosphorus side arm of the PNN ligand.