Synthesis and Characterization of a Tetradentate, <i>N</i>-Heterocyclic Carbene Copper(II) Complex and Its Use as a Chan–Evans–Lam Coupling Catalyst

Abstract

Copper N-heterocyclic carbenes (NHCs) are an emerging area of focus for catalysis and other applications. Using a straightforward methodology, a new and highly modifiable tetradentate copper(II) NHC complex was generated and characterized using X-ray crystallography, UV–vis and EPR spectroscopy, cyclic voltammetry, and ESI-MS. This copper(II) NHC complex adopted a distorted 4-coordinate coordination mode and demonstrates a unique absorption spectrum for a copper(II) species, but more interestingly, its redox properties indicate that it can readily access all three common copper oxidation states under atmospheric conditions. The tetradentate copper(II) NHC complex was used to catalytically generate new C–N bonds from a series of phenylboronic acids and amines. Once this CEL methodology was refined, moderate to high yields were achieved using catalytic amounts of the copper(II) complex to couple phenylboronic acids to a series of aniline derivatives. Substituted phenylboronic acids and anilines had minimal impact on the catalytic capabilities of this copper complex; however, there is some indication that steric interactions between catalyst and substrates may have an impact on efficient catalysis. The straightforward synthesis of this framework and the utilization of an inexpensive, first-row transition metal center in this system highlight the usefulness of copper(II) NHCs as catalyst for cross-coupling reactions.