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

摘要

Density functional theory (DFT) predicts a detailed mechanism for the reported potentialnphotocatalytic system for solar hydrogen production from water, (P-da-PNN)RuH(CO) (1, P-da )ndearomatized at the phosphorus side arm, PNN ) (2-(di-tert-butylphosphinomethyl)-6-diethylaminomethyl)npyridine) (Science 2009, 324, 74). In the initial thermal reaction, the coordination of a water molecule isnfollowed by cleavage of an O-H bond and aromatization of the PNN ligand to form (PNN)RuH(CO)(OH)n(3′), the most stable complex in the reaction. This low-barrier step is followed by the rate-determiningndearomatization and formation of H2. Next, a second water molecule is activated, resulting in the formationnof the cis-dihydroxo complex (PNN)Ru(CO)(OH)2 (7), which photolytically eliminates H2O2. Time-dependentnDFT calculations predict that the breaking of the two strong Ru-O bonds and the formation of the O-Onbond in this photolytic reaction involve low-energy triplet states and singlet–triplet crossings. Rather thannregeneration of initial complex 1 after the light-induced H2O2 evolution in the catalytic cycle, the DFTncalculations predict a new route with a lower energy barrier via the regeneration of 1′, an isomer of 1 withnthe unsaturated carbon at the nitrogen side arm of the PNN ligand. This new route involves hydride transfernfrom the methylene group at the nitrogen side, rather than the previously proposed regeneration of 1 throughnhydride transfer from the phosphorus side arm of the PNN ligand.