Issue 18, 2014

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 24 and 23 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) [RuIV[double bond, length as m-dash]O]2+ → [RuV[double bond, length as m-dash]O]3+; (ii) [RuV[double bond, length as m-dash]O]3+ → [RuIII–(OOH)]2+; and (iii) [RuIII–(OOH)]2+ → [RuII–OH2]2+. UV-Visible spectrophotometric experiments on the 14 and 24 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 24 than that observed for the 14 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 [RuIV[double bond, length as m-dash]O]2+/[RuV[double bond, length as m-dash]O]3+ potential of catalyst 2. In contrast, the [RuIV[double bond, length as m-dash]O]2+/[RuV[double bond, length as m-dash]O]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.

Graphical abstract: Cooperative effects in homogenous water oxidation catalysis by mononuclear ruthenium complexes

Supplementary files

Article information

Article type
Paper
Submitted
28 Feb 2014
Accepted
07 Mar 2014
First published
10 Mar 2014
This article is Open Access
Creative Commons BY license

Dalton Trans., 2014,43, 6819-6827

Cooperative effects in homogenous water oxidation catalysis by mononuclear ruthenium complexes

Y. Mulyana, F. R. Keene and L. Spiccia, Dalton Trans., 2014, 43, 6819 DOI: 10.1039/C4DT00629A

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