Experimental and Mechanistic Understanding of Aldehyde Hydrogenation Using Au25 Nanoclusters with Lewis Acids: Unique Sites for Catalytic Reactions

The catalytic activity of Au-25(SR)(18) nanoclusters (R = C2H4Ph) for the aldehyde hydrogenation reaction in the presence of a base, e.g., ammonia or pyridine, and transition-metal ions Mz+, such as Cu+, Cu2+, Ni2+ and Co2+, as a Lewis acid is studied. The addition of a Lewis acid is found to significantly promote the catalytic activity of Au-25(SR)(18)/CeO2 in the hydrogenation of benzaldehyde and a number of its derivatives. Matrix-assisted laser desorption ionization (MALDI) and electrospray ionization (ESI) mass spectrometry in conjunction with UV-vis spectroscopy confirm the generation of new species, Au25-n(SR)(18-n) (n = 1-4), in the presence of a Lewis acid. The pathways for the speciation of Au-24(SR)(17) from its parent Au-25(SR)(18) nanocluster as well as its structure are investigated via the density functional theory (DFT) method. The adsorption of M(z+) onto a thiolate ligand -SR- of Au-25(SR)(18), followed by a stepwise detachment of -SR- and a gold atom bonded to -SR- (thus an Au-SR unit) is found to be the most likely mechanism for the Au-24(SR)(17) generation. This in turn exposes the Au-13-core of Au-24(SR)(17) to reactants, providing an active site for the catalytic hydrogenation. DFT calculations indicate that Mz+ is also capable of adsorbing onto the Au13-core surface, producing a possible active metal site of a different kind to catalyze the aldehyde hydrogenation reaction. This study suggests, for the first time, that species with an open metal site like adducts [nanoparticle-M]((z-1)+) or fragments Au25-n(SR)(18-n) function as the catalysts rather than the intact Au-25(SR)(18).