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

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 & DEG;C promote the deprotonation of the C-2 atom of the imidazolium ring and the dissolution of CuCl. H-1 and C-13 NMR spectra indicate the formation of the N-heterocyclic carbene (NHC) complex [NHC] (CuCl)-Cl-I 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 P2(1)/n. The centrosymmetric dinuclear acetate complex [Cu-2(OAc)(4)Cl-2](2-) has the paddle-wheel motif and is weakly paramagnetic.