The role of an oxometallic complex in OH dissociation during water oxidation: a microscopic insight from DFT study

The uncatalyzed atomic dissociation of water requires breaking of a strong O–H bond with an enthalpy of 494 kJ mol−1, which necessitates the understanding and designing of appropriate catalysts. Here we employ transition state theory within quantum chemical density functional theory to understand the role of metal-oxide inorganic complexes in the OH → O + H process, the most important reaction in water oxidation. We study the effect of (a) chemical bonding in different M4O4 (M = Mn, Co) cubane complexes, (b) heterocubane geometry containing Ca, in addition to a transition metal ion, (c) dimensionality by considering both three-dimensional and two-dimensional geometry of the oxometallic unit, and (d) connectivity between two oxometallic cubane units, corner shared versus edge shared geometry. Analysis of our density functional theory based calculations singles out a robust microscopic quantity among various plausible and competing factors, which elucidates the important role of metal–oxygen covalency at the oxidized site. The M–O bonding strength inversely determines the strength of the O–H bond, and thus the energy required for OH dissociation. This provides one with an important microscopic design principle for a metal-oxide complex catalyst responsible for water oxidation.