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Issue 5, 2015
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Unexpected effect of catalyst concentration on photochemical CO2 reduction by trans(Cl)–Ru(bpy)(CO)2Cl2: new mechanistic insight into the CO/HCOO selectivity

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Abstract

Photochemical CO2 reduction catalysed by trans(Cl)–Ru(bpy)(CO)2Cl2 (bpy = 2,2′-bipyridine) efficiently produces carbon monoxide (CO) and formate (HCOO) in N,N-dimethylacetamide (DMA)/water containing [Ru(bpy)3]2+ as a photosensitizer and 1-benzyl-1,4-dihydronicotinamide (BNAH) as an electron donor. We have unexpectedly found catalyst concentration dependence of the product ratio (CO/HCOO) in the photochemical CO2 reduction: the ratio of CO/HCOO decreases with increasing catalyst concentration. The result has led us to propose a new mechanism in which HCOO is selectively produced by the formation of a Ru(I)–Ru(I) dimer as the catalyst intermediate. This reaction mechanism predicts that the Ru–Ru bond dissociates in the reaction of the dimer with CO2, and that the insufficient electron supply to the catalyst results in the dominant formation of HCOO. The proposed mechanism is supported by the result that the time-course profiles of CO and HCOO in the photochemical CO2 reduction catalysed by [Ru(bpy)(CO)2Cl]2 (0.05 mM) are very similar to those of the reduction catalysed by trans(Cl)–Ru(bpy)(CO)2Cl2 (0.10 mM), and that HCOO formation becomes dominant under low-intensity light. The kinetic analyses based on the proposed mechanism could excellently reproduce the unusual catalyst concentration effect on the product ratio. The catalyst concentration effect observed in the photochemical CO2 reduction using [Ru(4dmbpy)3]2+ (4dmbpy = 4,4′-dimethyl-2,2′-bipyridine) instead of [Ru(bpy)3]2+ as the photosensitizer is also explained with the kinetic analyses, reflecting the smaller quenching rate constant of excited [Ru(4dmbpy)3]2+ by BNAH than that of excited [Ru(bpy)3]2+. We have further synthesized trans(Cl)–Ru(6Mes-bpy)(CO)2Cl2 (6Mes-bpy = 6,6′-dimesityl-2,2′-bipyridine), which bears bulky substituents at the 6,6′-positions in the 2,2′-bipyridyl ligand, so that the ruthenium complex cannot form the dimer due to the steric hindrance. We have found that this ruthenium complex selectively produces CO, which strongly supports the catalytic mechanism proposed in this work.

Graphical abstract: Unexpected effect of catalyst concentration on photochemical CO2 reduction by trans(Cl)–Ru(bpy)(CO)2Cl2: new mechanistic insight into the CO/HCOO− selectivity

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Supplementary files

Article information


Submitted
19 Jan 2015
Accepted
03 Mar 2015
First published
12 Mar 2015

This article is Open Access
All publication charges for this article have been paid for by the Royal Society of Chemistry

Chem. Sci., 2015,6, 3063-3074
Article type
Edge Article
Author version available

Unexpected effect of catalyst concentration on photochemical CO2 reduction by trans(Cl)–Ru(bpy)(CO)2Cl2: new mechanistic insight into the CO/HCOO selectivity

Y. Kuramochi, J. Itabashi, K. Fukaya, A. Enomoto, M. Yoshida and H. Ishida, Chem. Sci., 2015, 6, 3063
DOI: 10.1039/C5SC00199D

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