Remarkable stability of copper(II)-N-heterocyclic carbene complexes void of an anionic tether

A library of pyridyl- and picolyl-substituted imidazolium salts have been synthesized and coordinated to copper, via transmetalation from silver(I)-N-heterocyclic carbenes (NHCs), to prepare several copper(I)- and copper(II)-NHC complexes. The copper(I)-NHCs are complexes of the type Cu(NHC)Br, with the solid-state structures revealing a variety of coordination environments around the copper centers. The stability of the copper(II) complexes is particularly unusual, given the absence of a "hard" anionic tethering group appended to the ligands. The stability has been attributed to the pyridyl substituent, with the complexes being extremely stable, while those with an appended anionic group tend to be more sensitive to air/moisture. The ligands and complexes have been examined in an Ullmann-type etherification reaction and exhibit improved activity in comparison to copper in the absence of a ligand or the common Cu(I)-NHC complexes Cu(IMes)Cl and [Cu(IMes)2]PF6, indicating stabilization of higher oxidation state species by the ligands during the catalytic cycle.