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

• Published in: Chemical science (Cambridge) Vol. 6; no. 5; pp. 3063 - 3074
• Main Authors: Kuramochi, Yusuke, Itabashi, Jun, Fukaya, Kyohei, Enomoto, Akito, Yoshida, Makoto, Ishida, Hitoshi
• Format: Journal Article
• Language: English
• Published: 01.04.2015
• Subjects: Carbon dioxide; Carbon monoxide; Catalysts; Dimers; Excitation; Formations; Photochemical; Reduction
• ISSN: 2041-6520; 2041-6539
• DOI: 10.1039/c5sc00199d

Photochemical CO sub(2) reduction catalysed by trans(Cl)-Ru(bpy)(CO) sub(2)Cl sub(2) (bpy = 2,2'-bipyridine) efficiently produces carbon monoxide (CO) and formate (HCOO super(-)) in N,N-dimethylacetamide (DMA)/water containing [Ru(bpy) sub(3)] super(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 super(-)) in the photochemical CO sub(2) reduction: the ratio of CO/HCOO super(-) decreases with increasing catalyst concentration. The result has led us to propose a new mechanism in which HCOO super(-) 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 CO sub(2), and that the insufficient electron supply to the catalyst results in the dominant formation of HCOO super(-). The proposed mechanism is supported by the result that the time-course profiles of CO and HCOO super(-) in the photochemical CO sub(2) reduction catalysed by [Ru(bpy)(CO) sub(2)Cl] sub(2) (0.05 mM) are very similar to those of the reduction catalysed by trans(Cl)-Ru(bpy)(CO) sub(2)Cl sub(2) (0.10 mM), and that HCOO super(-) 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 CO sub(2) reduction using [Ru(4dmbpy) sub(3)] super(2+) (4dmbpy = 4,4'-dimethyl-2,2'-bipyridine) instead of [Ru(bpy) sub(3)] super(2+) as the photosensitizer is also explained with the kinetic analyses, reflecting the smaller quenching rate constant of excited [Ru(4dmbpy) sub(3)] super(2+) by BNAH than that of excited [Ru(bpy) sub(3)] super(2+). We have further synthesized trans(Cl)-Ru(6Mes-bpy)(CO) sub(2)Cl sub(2) (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.