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

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 ((MLCT)-M-3*) of 1 is firstly formed upon photoirradiation and the (MLCT)-M-3* state is reductively quenched with an electron donor to afford [Ru-II(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 H-2 evolution and the hydrogenation of organic substrates. In the reactions, complex 2 is required to be excited to form its (MLCT)-M-3* 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.