Investigations of a Copper(II) Bipyridyl-N-Heterocyclic Carbene Macrocycle for CO2 Reduction: Apparent Formation of an Imidazolium Carboxylate Intermediate Leading to Demetalation

A copper complex supported by a redox-active bipyridyl-N-heterocyclic carbene based ligand framework is reported. From X-ray crystallography, the tetradentate macrocycle provides a distorted square planar geometry around the copper metal center. The complex was investigated for the electrocatalytic CO2 reduction reaction (CO2RR) in acetonitrile solutions. Electronic structure calculations were performed on the complex and associated intermediates to provide a fundamental understanding of the metal–ligand redox chemistry and are compared to the previously reported nickel and cobalt analogues. Unlike its predecessors, which are active catalysts for the CO2RR, the copper complex decomposes under reducing conditions in the presence of CO2. A novel decomposition route involving coordination of CO2 to an N-heterocyclic carbene (NHC) donor of the macrocyclic ligand is proposed based on density functional theory (DFT) calculations, which is supported by isolation of a putative ligand-CO2 adduct from the electrolyzed solution and its characterization by 1H NMR spectroscopy and mass spectrometry. The noninnocent behavior of the NHC donors presented here may have important implications for the stability and reactivity of other complexes supported by N-heterocyclic carbenes, and further suggests that cooperative and productive pathways involving metal-bound NHCs could be exploited for CO2 reduction.