Investigations of a Copper(II) Bipyridyl- N -Heterocyclic Carbene Macrocycle for CO 2 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 CO 2 reduction reaction (CO 2 RR) 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 CO 2 RR, the copper complex decomposes under reducing conditions in the presence of CO 2 . A novel decomposition route involving coordination of CO 2 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-CO 2 adduct from the electrolyzed solution and its characterization by 1 H 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 CO 2 reduction.