Electro- and photocatalytic hydrogen evolution by a cobalt complex based on a tripodal iminopyridine ligand

A cobalt complex based on the tripodal iminopyridine type ligand (2E)-2-N-(pyridin-2-ylmethylidene)-1-N,1-N-bis({2-[(E)-(pyridin-2-ylmethylidene)amino]phenyl})benzene-1,2-diamine (L1), namely, [CoL1](BF4)2 (1a), was synthesized and characterized by FT-IR, UV–Vis spectroscopy, magnetic moment and elemental analysis. X-ray structural study of 1a shows that the complex is coordinatively saturated and contains the uncoordinated bridgehead nitrogen atom. Electrochemical studies reveal that the complex 1a has an electrocatalytic proton reduction activity in acetonitrile, when using acetic acid as a proton source with icat/ip ∼ 3.3. Therefore we estimate that the complex 1a could give the highest TOF of 2.3 s−1 for the reduction of proton to hydrogen. Furthermore, under visible-light irradiation, an optimized H2 evolution system containing 1a (0.1 mM), [Ir(ppy)2(bpy)](PF6) (0.1 mM), and 10 vol% TEA in CH3CN–H2O (1/1, v/v) mixed solvents at pH 10 gives a highest turnover number of 34. For comparison, an electron-withdrawing ester group (CO2CH2CH3) is introduced into the pyridine moiety of the ligand as in L2, however, which leads to its corresponding cobalt complex 2a less active than the complex 1a for electro- and photocatalytic H2 evolution. Additionally, the control experiment establishes that the isostructural zinc complex [ZnL1](BF4)2 (1b) is ineffective for H2 generation.

A cobalt complex 1a of tripodal iminopyridine type ligand was synthesized and characterized. The complex is capable of electro-catalyzing proton reduction in acetonitrile using acetic acid as a proton source and photo-catalyzing hydrogen generation in combination with the photosensitizer [Ir(ppy)2(bpy)]PF6.Figure optionsDownload as PowerPoint slide