New tricks for an old dog: Visible light-driven hydrogen production from water catalyzed by fac- and mer- geometrical isomers of tris(thiosemicarbazide) cobalt(III)

Increasing interest has been paid to the development of earth-abundant metal complexes as promising surrogates of platinum for the electrocatalytically and photocatalytically driven hydrogen evolution reaction. In this work, we report on molecular H2-evolving catalysts based on two octahedral complexes of cobalt thiosemicarbazide, fac-[Co(Htsc)3]Cl3·3H2O (C1, Htsc = thiosemicarbazide) and mer-[Co(Htsc)3]Cl3·4H2O (C2), which have facial (fac) and meridional (mer) geometry, respectively. Electrochemical studies confirmed that both C1 and C2 are active electrocatalysts in MeOH solution using acetic acid as the proton source, with the same overpotential of ∼ 640 mV and TOF of ∼ 210 s−1. The complex C1 also exhibits electrocatalytic activity for hydrogen evolution reaction in aqueous media free of organic solvent with a moderate overpotential (560 mV). Visible light-driven hydrogen evolution experiments were carried out in combination with fluorescein as photosensitizer and triethylamine as sacrificial reductant in homogeneous environments. Our studies showed that both C1 and C2 can be used as efficient proton-reduction catalysts in purely aqueous solution and have the same photocatalytic activities. A TOF of 125 h−1 with a TON of 900 for photocatalytic H2 generation using C1 and C2 in water were achieved for the noble-metal-free homogeneous system. It should be noted that this is the first reported study investigating the effect on the catalytic hydrogen production performance of using fac- and mer-isomers of molecular catalysts.