Visible Light Gold Nanocluster Photocatalyst: Selective Aerobic Oxidation of Amines to Imines

This work demonstrates the synthesis of an efficient photocatalyst, Au-25(PPh3)(10)Cl-2(SC3H6SiO3)(5)/TiO2, for selective oxidation of amines to imines. The photocatalyst is prepared via hydrolysis of Au-25(PPh3)(10)Cl-2[(SC3H6Si(OC2H3)(3)](5) nanoclusters in the presence of TiO2 support. The gold nanoclusters exhibit good photocatalytic activity using visible light and under mild thermal conditions for the selective oxidation with molecular oxygen (O-2). The turnover frequency (TOF) of 4-methylbenzyla-mine oxidation is found to be 1522 h(-1), which is considerably higher than that conventional gold catalysts. The gold nanoclusters present good recyclability and stability for the oxidation of a wide range of amines. The superior activity of the photocatalyst is associated with its unique electronic structure and framework. The catalytically active sites are deemed to be the exposed gold atoms upon detaching protecting ligands: i.e., PPh3. The Hammett parameter suggests that the photocatalytic process involves the formation of carbocation intermediate species. Further, Au-H species were confirmed by TEMPO (2,2,6,6-tetramethylpiperidinyloxy) as a trapping agent.