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 Ce⁴⁺ to 2 revealed three distinctive regions-suggested to correspond to the stepwise processes: (i) [Ru^IVO]²⁺ → [Ru^VO]³⁺; (ii) [Ru^VO]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 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 Ru^II/Ru^III redox process of mediators 3 and 4 and the predicted [Ru^IVO]²⁺/[Ru^VO]³⁺ potential of catalyst 2. In contrast, the [Ru^IVO]²⁺/[Ru^VO]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.