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

• Published in: Dalton transactions : an international journal of inorganic chemistry 2020-12, Vol.49 (47), p.1723-17242
• Main Authors: Sawaki, Takuya, Ishizuka, Tomoya, Namura, Nanase, Hong, Dachao, Miyanishi, Mayuko, Shiota, Yoshihito, Kotani, Hiroaki, Yoshizawa, Kazunari, Jung, Jieun, Fukuzumi, Shunichi, Kojima, Takahiko
• Format: Article
• Language: English
• Subjects: Aldehydes; Bonding; Charge transfer; Density functional theory; Electrons; Excitation; Flash photolysis; Hydrogen bonds; Hydrogen evolution; Hydrogen storage; Inert atmospheres; Isotope effect; Ligands; Photocatalysis; Protons; Reaction mechanisms; Solvents; Substrates; White light
• ISSN: 1477-9226; 1477-9234
• DOI: 10.1039/d0dt03546g

A Ru II complex, [Ru II (tpphz)(bpy) 2 ] 2+ ( 1 ) (tpphz = tetrapyridophenazine, bpy = 2,2′-bipyridine), whose tpphz ligand has a pyrazine moiety, is converted efficiently to [Ru II (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 ( 3 MLCT*) of 1 is firstly formed upon photoirradiation and the 3 MLCT* state is reductively quenched with an electron donor to afford [Ru II (tpphz&z.rad; − )(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 H 2 evolution and the hydrogenation of organic substrates. In the reactions, complex 2 is required to be excited to form its 3 MLCT* state to react with a proton and aldehydes. In photocatalytic H 2 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 H 2 evolving rates. Density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations on the reaction mechanism of H 2 evolution from the ground and photo-excited states of 2 were performed to have a better understanding of the photocatalytic processes. A Ru II complex with a π-expanded diimine ligand performs photocatalytic H 2 evolution and hydrogenation of organic substrates under visible-light irradiation, involving an intermediate with the dihydrogenated ligand; a mechanistic insight into the H 2 evolution is also gained.