Toward Rational Design of Mononuclear Nickel Complexes as Water Oxidation Catalysts Exploring the Ligand Effects on the Rate-Determining Step.

The major impediment in realizing a carbon-neutral hydrogen fuel economy is the cost and inadequacy of contemporary electrochemical water splitting approaches towards the energy intensive oxygen evolution reaction (OER). The O-O bond formation in the water oxidation half-cell reaction is both kinetically and thermodynamically challenging and amplifies the overpotential requirement in most of the active water oxidation catalysts. Herein, density functional theory is employed to interrogate 20 Ni(II) complexes, out of which 17 are in silico designed molecular water oxidation catalysts, coordinated to electron-rich tetra-anionic redox non-innocent phenylenebis(oxamidate) and dibenzo-1,4,7,10-tetraazacyclododecane-2,3,8,9-tetraone parent ligands and their structural analogues, and identify the role of substituent changes or ligand effects in the order of their reactivity. Importantly, our computational mechanistic analyses predict that the activation free energy of the rate-determining O-O bond formation step obeys an inverse scaling relationship with the global electrophilicity index of the intermediate generated on two-electron oxidation of the starting complex. Additionally, the driving force is directly correlated with this OER descriptor which enables two-dimensional volcano representation and thereby extrapolation towards the ideal substitution with the chosen ligand. Our study, therefore, establish fundamental insights to overcome the imperative overpotential issue with simple and precise computational rationalization preceding experimental validation.