Correlation and Prediction of Redox Potentials of Hydrogen Evolution Mononuclear Cobalt Catalysts via Molecular Electrostatic Potential: A DFT Study.

Abstract

Reduction potentials (E(0)) of six mononuclear cobalt catalysts (1-6) for hydrogen evolution reaction and electron donating/withdrawing effect of nine X-substituents on their macrocyclic ligand are reported at solvation effect-included B3P86/6-311+G** level of density functional theory. The electrostatic potential at the Co nucleus (V(Co)) is found to be a powerful descriptor of the electronic effect experienced by Co from the ligand environment. The V(Co) values vary substantially with respect to the nature of macrocycle, type of apical ligands, nature of substituent and oxidation state of the metal center. Most importantly, V(Co) values of both the oxidized and reduced states of all the six complexes show strong linear correlation with E(0). The correlation plots between V(Co) and E(0) provide an easy-to-interpret graphical interpretation and quantification of the effect of ligand environment on the reduction potential. Further, on the basis of a correlation between the relative V(Co) and relative E(0) values of a catalyst with respect to the CF3-substituted reference system, the E(0) of any X-substituted 1-6 complexes is predicted.