The influence of the pH on the reaction mechanism of water oxidation by a Ru(bda) catalyst

Abstract

Recent results of Concepcion’s group (Chem. Com51 (2015) 4105) on water oxidation catalysis (WOC) by a ruthenium complex suggest that, at pH = 8, O2 release takes place after formation of a rhomboid bis(μ-oxo)-RuV2 species and not after generation of the typical μ-η1:η1-peroxo-RuIV2 intermediate, coming from the coupling of two RuV=O moieties (I2M mechanism), which is widely accepted to be formed at pH = 1. To analyze the differences between the reaction mechanisms of this WOC at different pHs, we performed DFT calculations of the full mechanism at pH = 1 and 8 of the WOC process catalyzed by the 2,2′-bipyridine-6,6′-dicarboxylate Ru complex. At pH = 8, we found that barriers leading to the hypothetic formation of rhombic RuV2O2 species are higher than those involved in the canonical I2M mechanism. The rate determining step at the latter pH is found to be the dimer formation while the bond cleavage for the O2 liberation process is barrierless. The computational results confirm that the most common I2M mechanism is preferred at both pHs, as the new proposal comprising formation of bis(μ-oxo)-RuV2 species involves higher energy barriers.