Sulfur-stabilised copper nanoparticles for the aerobic oxidation of amines to imines under ambient conditions

Abstract

The stabilisation of metal nanoparticles and control of their oxidation state are crucial factors in nanocatalysis. Elemental sulfur has been found to be a cheap and effective stabilising agent for copper nanoparticles in the form of copper(I) oxide. The Cu₂ONPs/S₈ system has been characterised by ICP-OES, EDX, XRD, XPS, FE-SEM, SEM, TEM and Cryo-EM. Astonishingly, in organic medium, the copper nanoparticles are organised as concentric rings within nanodroplets of sulfur of ca. 20–70 nm. In synthetic organic chemistry, imines can be directly obtained by the less studied and practiced oxidation of primary amines; however, the reaction conditions utilised are usually harsh and far from meeting the principles of Green Chemistry. Cu₂ONPs/S₈ has been successfully applied to the solvent-free aerobic oxidation of primary amines to imines under ambient conditions, using air as a terminal oxidant. The catalyst is effective in the homo- and heterocoupling of benzylic amines at very low copper loading (0.3 mol%), being catalytically superior to a range of commercial copper catalysts. A reaction mechanism has been proposed based on experimental evidence, which clarifies the major uncertainty regarding the key intermediate. The results of this study suggest a number of new avenues for research in nanocatalysis.