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

A Ru complex, [Ru(tpphz)(bpy)] (1) (tpphz = tetrapyridophenazine, bpy = 2,2'-bipyridine), whose tpphz ligand has a pyrazine moiety, is converted efficiently to [Ru(tpphz-HH)(bpy)] (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*) of 1 is firstly formed upon photoirradiation and the MLCT* state is reductively quenched with an electron donor to afford [Ru(tpphz˙)(bpy)], 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 evolution and the hydrogenation of organic substrates. In the reactions, complex 2 is required to be excited to form its MLCT* state to react with a proton and aldehydes. In photocatalytic H 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 evolving rates. Density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations on the reaction mechanism of H evolution from the ground and photo-excited states of 2 were performed to have a better understanding of the photocatalytic processes.