Electrocatalytic reduction of CO2 to CO by a mononuclear ruthenium(ii) complex

Abstract

A new mononuclear ruthenium(II) complex, [Ru(dmbpy)(tptz)(Cl)](PF₆) (where dmbpy = 4,4′-dimethyl-2,2′-bipyridine and tptz = 2,4,6-tris(2-pyridyl)-1,3,5-triazine), has been prepared and characterized by spectroscopic methods, cyclic voltammetry (CV), and single-crystal X-ray crystallography. The electrocatalytic activity of the Ru(II) complex was investigated for the reduction of CO₂ to CO in acetonitrile by cyclic voltammetry under different reaction conditions including temperature variation, CO₂ concentration, catalyst concentration, and the scan rate. A detailed mechanism for the electrocatalytic reduction of CO₂ to CO by the Ru(II) complex has been investigated in the gas and solution phases using DFT calculations. These calculations indicate that the first catalytic step involves two sequential one-electron ligand-based reductions, followed by a CO₂ coordination reaction and a concerted two-electron transfer from tptz²⁻ to the coordinated CO₂ ligand to give a metallocarboxylate intermediate (Ru–CO₂²⁻). The HOMO and LUMO of this intermediate are still polypyridyl-based and the next two electrons enter molecular orbitals with tptz- and dmb-character. These two one-electron reductions result in the formation of an intermediate which can be targeted by free CO₂ to release a CO molecule. The calculations provide detailed mechanistic information in agreement with the experimental results.