Revisiting O-O Bond Formation through Outer-Sphere Water Molecules versus Bimolecular Mechanisms in Water-Oxidation Catalysis (WOC) by Cp*Ir Based Complexes

摘要

Herein, we present a detailed computational investigation of the mechanistic aspects of the water-oxidation catalysis (WOC) for iridium-based catalysts, Cp*Ir-Lx = 1-4, (where Cp* = pentamethylcyclopentadiene; L-1 = bph = bi-phenyl; L-2 = phpy = 2-phenylpyridine; L-3 = bpy = 2,2 '-bipyridyl; and L-4 = bnql = benzo[n]quinoline). Our density functional theory (DFT) calculations not only confirm that the O-O coupling step is the rate-limiting step, as expected, but also provide useful insights about the number of water molecules involved in the catalytic cycle, which is under immense debate from a kinetic stand point. To test the effect of the metal environment, we tune the ligands, choosing four ligands (L-1-L-4) holding four kinds of chelation: C-C, N-C, N-N, and C-N ', respectively. A screening analysis of the potential-energy surface reveals the water-oxidation mechanism, together with the optimum number of water molecules, concluding that three water molecules are optimal, and that a highly positive iridium oxo center with a predicted high oxidation state (Ir-V) pulls the electron density from the lone pair of the oxo oxygen and the O center shows positive density. Moreover, the bimolecular mechanism for the O-O bond step is also calculated, for comparison. This study reveals that high cationic character of the metal is helpful for O center dot center dot center dot O coupling.