Flash Communication: Copper Complexes Supported by Macrocyclic N-Heterocyclic Carbene Ligands with Ether or Secondary Amine Donors

Abstract

Metal complexes featuring secondary amine donors have been extensively studied in the context of hydrogen-bonding or metal–ligand cooperative interactions. In this regard, pincer-type ligand frameworks have been described, but macrocyclic variants remain rare. Herein, we describe the synthesis of macrocyclic carbene ligand frameworks featuring a carbazolide backbone and an ether or secondary amine donor. While the ether was synthesized through conventional methods, the amine variant required microwave heating. Carbazolide-bis(imidazolium) pro-ligands were isolated as air-stable compounds. Metalation with Cu(acac)2 led to the formation of square-planar Cu(II) complexes characterized by single-crystal X-ray diffraction and cyclic voltammetry. A significantly more cathodic Cu(I)/Cu(II) wave by 330 mV was observed for the amine complex. Combined, the results highlight new macrocyclic carbene platforms with variable coordination environments.