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 Ce(4+) 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) [Ru(IV)=O](2+) → [Ru(V)=O](3+); (ii) [Ru(V)=O](3+) → [Ru(III)-(OOH)](2+); and (iii) [Ru(III)-(OOH)](2+) → [Ru(II)-OH2](2+). UV-Visible spectrophotometric experiments on the 1-4 and 2-4 mixtures, also carried out with four equivalents of Ce(4+), 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 Ru(II)/Ru(III) redox process of mediators 3 and 4 and the predicted [Ru(IV)=O](2+)/[Ru(V)=O](3+) potential of catalyst 2. In contrast, the [Ru(IV)=O](2+)/[Ru(V)=O](3+) process for catalyst 1 was found to occur at a higher potential than the Ru(II)/Ru(III) 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.