Issue 47, 2020

Photocatalytic hydrogen evolution using a Ru(ii)-bound heteroaromatic ligand as a reactive site

Abstract

A RuII complex, [RuII(tpphz)(bpy)2]2+ (1) (tpphz = tetrapyridophenazine, bpy = 2,2′-bipyridine), whose tpphz ligand has a pyrazine moiety, is converted efficiently to [RuII(tpphz-HH)(bpy)2]2+ (2) having a dihydropyrazine moiety upon photoirradiation of a water–methanol mixed solvent solution of 1 in the presence of an electron donor. In this reaction, the triplet metal-to-ligand charge-transfer excited state (3MLCT*) of 1 is firstly formed upon photoirradiation and the 3MLCT* state is reductively quenched with an electron donor to afford [RuII(tpphz˙)(bpy)2]+, which is converted to 2 without the observation of detectable reduced intermediates by nano-second laser flash photolysis. The inverse kinetic isotope effect (KIE) was observed to be 0.63 in the N–H bond formation of 2 at the dihydropyrazine moiety. White-light (380–670 nm) irradiation of a solution of 1 in a protic solvent, in the presence of an electron donor under an inert atmosphere, led to photocatalytic H2 evolution and the hydrogenation of organic substrates. In the reactions, complex 2 is required to be excited to form its 3MLCT* state to react with a proton and aldehydes. In photocatalytic H2 evolution, the H–H bond formation between photoexcited 2 and a proton is involved in the rate-determining step with normal KIE being 5.2 on H2 evolving rates. Density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations on the reaction mechanism of H2 evolution from the ground and photo-excited states of 2 were performed to have a better understanding of the photocatalytic processes.

Graphical abstract: Photocatalytic hydrogen evolution using a Ru(ii)-bound heteroaromatic ligand as a reactive site

Supplementary files

Article information

Article type
Paper
Submitted
13 oct. 2020
Accepted
10 nov. 2020
First published
10 nov. 2020

Dalton Trans., 2020,49, 17230-17242

Author version available

Photocatalytic hydrogen evolution using a Ru(II)-bound heteroaromatic ligand as a reactive site

T. Sawaki, T. Ishizuka, N. Namura, D. Hong, M. Miyanishi, Y. Shiota, H. Kotani, K. Yoshizawa, J. Jung, S. Fukuzumi and T. Kojima, Dalton Trans., 2020, 49, 17230 DOI: 10.1039/D0DT03546G

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