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

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; L1 = bph = bi-phenyl; L2 = phpy = 2-phenylpyridine; L3 = bpy = 2,2′-bipyridyl; and L4 = 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 (L1–L4) 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 (IrV) 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···O coupling.