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

Molecular hydrogen (H 2 ) 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 2 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 III (tpy)(CH 3 CN)Cl 2 ](CF 3 SO 3 ) ( Rh-tpy ), and shown that it can act as a catalyst for the light-induced proton reduction into H 2 in water in the presence of the [Ru(bpy) 3 ]Cl 2 ( Ru ) 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 Ru , the Rh-tpy catalyst produces H 2 with turnover number versus catalyst (TON Cat *) of 300 at a Rh concentration of 10 μM, and up to 1000 at a concentration of 1 μM. The photocatalytic performance of Ru / Rh-tpy /HA - /H 2 A has been also compared with that obtained with the bis-dimethyl-bipyridyl complex [Rh III (dmbpy) 2 Cl 2 ] + ( Rh2 ) as a catalyst in the same experimental conditions. The investigation of the electrochemical properties of Rh-tpy in DMF solvent reveals that the two-electrons reduced state of the complex, the square-planar [Rh I (tpy)Cl] ( Rh I -tpy ), 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 Rh-tpy catalyst a moderately negative reduction potential ( E p c (Rh III /Rh I ) = -0.83 V vs. SCE in DMF), making possible its reduction by the reduced state of Ru , [Ru II (bpy)(bpy •- )] + ( Ru - ) ( E 1/2 (Ru II /Ru - ) = -1.50 V vs. SCE) generated by a reductive quenching of the Ru excited state ( *Ru ) 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 *Ru by ascorbate. The resulting reduced Ru species ( Ru - ) were then able to activate the Rh III -tpy H 2 -evolving catalyst by reduction generating Rh I -tpy , which can react with a proton on a sub-nanosecond time scale to form a Rh III (H)-tpy hydride, the key intermediate for H 2 evolution.