Phosphine/phenylacetylide-ligated Au clusters for multicomponent coupling reactions

Metal nanocatalysts with well-defined surface structures are fundamentally important for mechanistic investigations of chemical reactions and symmetric development of catalytic systems for higher specificity and efficiency. We investigate the catalytic activity of atomically precise phosphine/phenylacetylide-liga ted Au-25(PPh3)(10)(C equivalent to CPh)(5)X-2(X = Cl/Br) clusters in the multicomponent coupling reaction of alkynes, aldehydes, and amines. The clusters are well characterized using UV-vis spectroscopy, scanning transmission electron microscopy, and matrix-assisted laser desorption ionization mass spectrometry. The TiO2-supported cluster in polar solvents is found to be an efficient and recyclable catalyst for remarkable conversion of the substrate. The Au-25(PPh3)(10)(C equivalent to CPh)(5)X-2/TiO2 catalyst is tested for a range of alkyne, aldehyde and ketone, and amine derivatives. Aromatic aldehydes give rise to higher conversion than aliphatic counterparts. Interestingly, ketones are found to suppress the reaction completely, in contrast with that on conventional gold nanocatalysts. The reaction mechanism is investigated via kinetic studies and UV-vis spectroscopy as well as density functional theory calculations. Results show that ligand removal under reaction conditions is the first step in exposing surface gold atoms to the reactants, thereby providing catalytic sites for the activation of bonds of terminal alkynes. This work demonstrates the promise of Au-25(PPh3)(10)(C equivalent to CPh)(5)X-2 cluster in the multicomponent coupling reaction, and a combined approach of experiment and theory leads to mechanistic understanding at the atomic level. (C) 2016 Elsevier Inc. All rights reserved.