Competent Electrocatalytic and Photocatalytic Proton Reduction by a Dechelated [Co(tpy)2]2+ Scaffold

The complexes [Co-II(QCl-tpy) 2]Cl-2 (1) and [Co-III-( QCl-tpy)(2)](PF6)(3) [where QCl-tpy = 2-chloro-3-(2,6-di(pyridin-2-yl) pyridine-4-yl) quinoline] have been synthesised and characterised by various spectroscopic techniques. Complex 1 acts as a hydrogen-evolving catalyst under electrochemical and photochemical conditions. It exhibits photochemical proton reduction by irradiation with visible light in the presence of [Ru(bpy)(3)]Cl-2 as a photosensitiser and ascorbic acid as a sacrificial electron donor. The formation of [Co-II(QCl-tpy)((2)QCl-tpyH)-DMF](3+) was detected after 24 h, due to the dechelation of one of the QCl-tpy ligands in DMF/H2O (95:5 v/v, where (2)QCl-tpy is a bidentate ligand). The introduction of 2-chloroquinoline into the terpyridine framework reduces the overpotential by 300 mV for proton reduction, as compared with the parent [Co(tpy)(2)](2+) complex. Considering the spectroscopic studies, the possible involvement of the active species are depicted and mechanistic paths are proposed.