Cooperative effects in homogenous water oxidation catalysis by mononuclear ruthenium complexes

Abstract

The homogenous water oxidation catalysis by [Ru(terpy)(bipy)Cl]+ (1) and [Ru(terpy)(Me2bipy)Cl]+ (2) (terpy = 2,2′:6′,2′′-terpyridine, bipy = 2,2′-bipyridine, Me2bipy = 4,4′-dimethyl-2,2′-bipyridine) under the influence of two redox mediators [Ru(bipy)3]2+ (3) and [Ru(phen)2(Me2bipy)]2+ (4) (phen = 1,10-phenanthroline) was investigated using Ce4+ as sacrificial oxidant. Oxygen evolution experiments revealed that mixtures of both 2-4 and 2-3 produced more molecular oxygen than catalyst 2 alone. In contrast, the combination of mediator 4 and catalyst 1 resulted in a lower catalytic performance of 1. Measurements of the temporal change in the intensity of a UV transition at 261 nm caused by the addition of four equivalents of Ce 4+ to 2 revealed three distinctive regions-suggested to correspond to the stepwise processes: (i) [RuIVO]2+ → [Ru VO]3+; (ii) [RuVO]3+ → [Ru III-(OOH)]2+; and (iii) [RuIII-(OOH)] 2+ → [RuII-OH2]2+. UV-Visible spectrophotometric experiments on the 1-4 and 2-4 mixtures, also carried out with four equivalents of Ce4+, demonstrated a faster [Ru(phen) 2(Me2bipy)]3+ → [Ru(phen) 2(Me2bipy)]2+ reduction rate in 2-4 than that observed for the 1-4 combination. Cyclic voltammetry data measured for the catalysts and the mixtures revealed a coincidence in the potentials of the RuII/RuIII redox process of mediators 3 and 4 and the predicted [RuIVO]2+/[RuVO]3+ potential of catalyst 2. In contrast, the [RuIVO]2+/ [RuVO]3+ process for catalyst 1 was found to occur at a higher potential than the RuII/RuIII redox process for 4. Both the spectroscopic and electrochemical experiments provide evidence that the interplay between the mediator and the catalyst is an important determinant of the catalytic activity. This journal is © the Partner Organisations 2014.