Hydrogen photogeneration catalyzed by a cobalt complex of a pentadentate aminopyridine-based ligand

Abstract

A pentadentate aminopyridine ligand [(9-methyl-1,10-phenanthrolin-2-yl)methyl]bis-(pyridin-2-ylmethyl)amine (DPA-Dmphen) has been prepared and characterized. This ligand readily accommodates Co(II) or Ni(II) bearing a coordinated apical water ligand, and the resulting complexes of general formula [M(Dmphen-DPA)(H₂O)](ClO₄)₂ (MCo (1) and MNi (2)) have been investigated for photo- and electrocatalytic proton reduction in CH₃CN–H₂O (1/3, v/v) mixed solvent and CH₃CN, respectively. Under visible-light irradiation (λ > 400 nm), complex 1 shows hydrogen evolution activity in the presence of [Ir(ppy)₂(bpy)]PF₆ as a photosensitizer and TEA as an electron donor, whereas 2 displays much lower catalytic activity under the same conditions. The highest turnover number (TON) of 210 is achieved for H₂ evolution from an optimized system containing 1 (0.1 mM), [Ir(ppy)₂(bpy)](PF₆) (0.5 mM), and 10 vol% TEA in CH₃CN–H₂O (1/3, v/v) mixed solvents at pH 10. Under those conditions catalytic hydrogen production is mainly limited by photosensitizer and catalyst stability. Furthermore, electrochemical studies reveal that both complexes are active for electrocatalytic proton reduction in acetonitrile, when using acetic acid as a proton source with overpotentials of (0.48 V vs. Fc⁺/Fc) for 1 and (0.46 V vs. Fc⁺/Fc) for 2.