Mechanochemical oxidative complexation of copper(I) chloride with acetic acid, benzoic acid, their chloro derivatives and l-histidine affording anhydrous copper(II) complexes

This work reports solvent-free synthesis of copper(II)-carboxylates by grinding the reactants in solid state (the mechanochemical method). Here we used copper(I) salt to synthesize copper(II) complexes to demonstrate the potential of oxidative synthesis of the mechanochemical technique. The complexes were synthesized by grinding copper(I) chloride with a carboxylic acid together in 1:2 molar ratio in mortar and pestle. The carboxylic acids used were acetic acid, benzoic acid, 2-chloroacetic acid, 2-chlorobenzoic acid and l-histidine. The oxidative complexation occurred in the solid state producing anhydrous dimeric copper(II)-carboxylates (except that of l- histidine) in quantitative yield (>96 %) having 1:2 metal to ligand ratio. The first evidence to oxidation of copper (I) to copper(II) ion was a change in color (pale to greenish blue) and a change in magnetic susceptibility from diamagnetic to paramagnetic value. The complexes were also characterized by elemental analysis, electronic absorption spectroscopy, FT-IR spectra, mass spectrometry, powder X-ray diffraction and thermal analysis. The electronic absorption, magnetic susceptibility, ESR and X-ray diffraction data supported a distorted square-planer geometry. This study provided for a distinct test in the form of magnetic susceptibility for the formation of copper (II)-complexes from a copper(I) salt. The dimeric nature of the complexes, except that of bis(histidinato)copper (II), was confirmed by the lower magnetic-moments (1.40-1.50 BM) than the spin only value (1.73 BM).