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)(CH3CN)Cl-2](CF3SO3) (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 mu M of Ru, the Rh-tpy catalyst produces H-2 with turnover number versus catalyst (TONCat*) of 300 at a Rh concentration of 10 mu M, and up to 1000 at a concentration of 1 mu M. 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 pi-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 pi-acceptor property of terpyridine also confers to the Rh-tpy catalyst a moderately negative reduction potential (Ep(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(center dot-))](+) (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.

Automated summary

In this study, a new mono-terpyridyl rhodium complex was isolated and shown to act as a catalyst for the light-induced proton reduction into H-2 in the presence of a photosensitizer and sacrificial electron donor. The catalyst showed efficient photocatalytic production of molecular hydrogen.