Electrochemical Properties of a Rhodium(III) Mono-Terpyridyl Complex and Use as a Catalyst for Light-Driven Hydrogen Evolution in Water.

Molecular hydrogen (H) is considered one of the most promising fuels to decarbonize the industrial and transportation sectors, and its photocatalytic production from molecular catalysts is a research field that is still abounding. The search for new molecular catalysts for H production with simple and easily synthesized ligands is still ongoing, and the terpyridine ligand with its particular electronic and coordination properties, is a good candidate to design new catalysts meeting these requirements. Herein, we have isolated the new mono-terpyridyl rhodium complex, Rh(tpy)(CHCN)Cl (), and shown that it can act as a catalyst for the light-induced proton reduction into H in water in the presence of the [Ru(bpy)]Cl () photosensitizer and ascorbate as sacrificial electron donor. Under photocatalytic conditions, in acetate buffer at pH 4.5 with 0.1 M of ascorbate and 530 μM of , the catalyst produces H with turnover number versus catalyst (TON*) of 300 at a Rh concentration of 10 μM, and up to 1000 at a concentration of 1 μM. The photocatalytic performance of //HA/HA has been also compared with that obtained with the bis-dimethyl-bipyridyl complex [Rh(dmbpy)Cl] () as a catalyst in the same experimental conditions. The investigation of the electrochemical properties of in DMF solvent reveals that the two-electrons reduced state of the complex, the square-planar [Rh(tpy)Cl] (), is quantitatively electrogenerated by bulk electrolysis. This complex is stable for hours under an inert atmosphere owing to the π-acceptor property of the terpyridine ligand that stabilizes the low oxidation states of the rhodium, making this catalyst less prone to degrade during photocatalysis. The π-acceptor property of terpyridine also confers to the catalyst a moderately negative reduction potential (p(Rh/Rh) = -0.83 V vs. SCE in DMF), making possible its reduction by the reduced state of , [Ru(bpy)(bpy)] () ((Ru/Ru) = -1.50 V vs. SCE) generated by a reductive quenching of the excited state () by ascorbate during photocatalysis. A Stern-Volmer plot and transient absorption spectroscopy confirmed that the first step of the photocatalytic process is the reductive quenching of by ascorbate. The resulting reduced species () were then able to activate the H-evolving catalyst by reduction generating , which can react with a proton on a sub-nanosecond time scale to form a hydride, the key intermediate for H evolution.