Cooperative effects in homogenous water oxidation catalysis by mononuclear ruthenium complexes

Mulyana, Yanyan, Keene, Richard, and Spiccia, Leone (2014) Cooperative effects in homogenous water oxidation catalysis by mononuclear ruthenium complexes. Dalton Transactions, 43 (18). pp. 6819-6827.

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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^IV=O]²⁺ → [Ru^V=O]³⁺; (ii) [RuO]³⁺ → [Ruᴵᴵᴵ–(OOH)]²⁺; and (iii) [Ruᴵᴵ–(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 [RuIVvO]⁺/[RuVvO]⁺ potential of catalyst 2. In contrast, the [Ru^IV=O]²⁺/[Ru^=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.

Item ID: 32171
Item Type: Article (Research - C1)
ISSN: 1477-9234
Keywords: water oxidation; photochemistry; homogeneous catalysis; ruthenium polypyridyl
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This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.

Funders: Australian Research Council (ARC)
Date Deposited: 07 Apr 2014 22:58
FoR Codes: 03 CHEMICAL SCIENCES > 0302 Inorganic Chemistry > 030207 Transition Metal Chemistry @ 100%
SEO Codes: 97 EXPANDING KNOWLEDGE > 970103 Expanding Knowledge in the Chemical Sciences @ 100%
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