Cooperative effects in homogenous water oxidation catalysis by mononuclear ruthenium complexes

摘要

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, Me_2bipy = 4,4'-dimethyl-2,2'-bipyridine) under the influence of two redox mediators [Ru(bipy)_3]~(2+) (3) and [Ru(phen)_2(Me_2bipy)]~(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)-OH_2]~(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(Me_2bipy)]~(3+) → [Ru(phen)_2(Me_2bipy)]~(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.