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

Takuya Sawaki, Tomoya Ishizuka, Nanase Namura, Dachao Hong, Mayuko Miyanishi, Yoshihito Shiota, Hiroaki Kotani, Kazunari Yoshizawa, Jieun Jung, Shunichi Fukuzumi, Takahiko Kojima

Research output: Contribution to journalArticlepeer-review

11 Scopus citations

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.

Original languageEnglish
Pages (from-to)17230-17242
Number of pages13
JournalDalton Transactions
Volume49
Issue number47
DOIs
StatePublished - 21 Dec 2020

Bibliographical note

Funding Information:
This work was partially supported by Grants-in-Aid (No. 24245011, 15H00861 and 15H00915 (AnApple) to T. K., and no. 16H02268 to S. F.) and 24-2397 (T. S.) from the Japan Society of Promotion of Science of Japan. T. K. also appreciates financial support from The Mitsubishi Foundation and Yazaki Memorial Foundation for Science and Technology. K. Y. thanks the MEXT projects Integrated Research Consortium on Chemical Sciences, Cooperative Research Program of Network Joint Research Centre for Materials and Devices, Elements Strategy Initiative to Form Core Research Centre, JST-CREST JPMJCR15P5 and JST-Mirai JPMJMI18A2 for their financial support. The computations were mainly carried out by using the computer facilities at the Research Institute for Information Technology, Kyushu University.

Publisher Copyright:
© 2020 The Royal Society of Chemistry.

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