N-heterocyclic carbene-mediated oxidation of copper(I) in an imidazolium ionic liquid

Abstract The aerobic oxidation of copper(I) to copper(II) was studied in the ionic liquid (IL) 1- n -butyl-3-methylimidazolium acetate [BMIm][OAc]. Temperatures above 100 °C promote the deprotonation of the C 2 atom of the imidazolium ring and the dissolution of CuCl. 1 H and 13 C NMR spectra indicate the formation of the N-heterocyclic carbene (NHC) complex [NHC] Cu I Cl under inert conditions. Upon aerobic oxidation, air-stable blue-green crystals of [BMIm] 2 [Cu II 2 (OAc) 4 Cl 2 ] precipitate in high yield and the NHC is recovered. X-ray diffraction on a single-crystal of the complex salt revealed a monoclinic structure with space group P 2 1 / n . The centrosymmetric dinuclear acetate complex [Cu 2 (OAc) 4 Cl 2 ] 2– has the paddle-wheel motif and is weakly paramagnetic.