Electrocatalytic reduction of protons to hydrogen by a copper complex of the pentadentate ligand Dmphen-DPA in a nonaqueous electrolyte

Abstract

A copper complex based on the pentadentate aminopyridine ligand [(9-methyl-1,10-phenanthrolin-2-yl)methyl]bis-(pyridin-2-ylmethyl)amine (Dmphen-DPA), namely [Cu(Dmphen-DPA)](ClO₄)₂ (1), was synthesized and characterized by elemental analysis, HR-MS spectroscopy and X-ray single crystal diffraction. The complex has five-coordinated solid-state structures, but has an open coordination site in acetonitrile. The electrochemical studies reveal that complex 1 has an electrocatalytic proton reduction activity in acetonitrile, when using acetic acid as a proton source with i_cat/i_p ∼ 2.5. DFT calculations suggest that the parent copper complex undergoes two successive reductions to generate the radical species [Cu(I)(Dmphen-DPA˙⁻)]⁰, which is protonated at the low-valence copper center to afford a reactive Cu^II–H intermediate. The Cu^II–H species combines bimolecularly and generates H₂via reductive elimination.