A new class of monolayer-protected Au clusters with Au–C covalent bonds (organogold clusters) was synthesized by ligating phenylacetylene (PhC≡CH) to PVP-stabilized Au clusters. Matrix-assisted laser desorption ionization mass spectrometry revealed for the first time a series of stable compositions of the organogold (Au:C2Ph) clusters.

In this work, we synthesized gold clusters, Aun (n = 10, 18, 25, 39), with atomically controlled sizes on hydroxyapatite (HAP) and studied the catalysis for aerobic oxidation of cyclohexane. These Aun/HAP catalysts could efficiently oxidize cyclohexane to cyclohexanol and cyclohexanone. The turnover frequency monotonically increased with an increase in the size, reaching values as high as 18 500 h−1 Au atom−1 at n = 39, and thereafter decreased with a further increase in n up to n ∼ 85. This finding provides a fundamental insight into size-specific catalysis of gold in the cluster regime (diameter < 2 nm) and a guiding principle for rational design of Au cluster-based catalysts.Keywords (keywords): aerobic oxidation; Au clusters; hydroxiapatite; size effect

This article summarizes our recent research on the size-controlled synthesis of Au clusters stabilized by a polymer or supported by a solid, and related work reported by others. Small Au clusters have excellent, size-specific catalytic activity in the aerobic oxidation of alcohols and alkanes.

Au25 clusters supported on hydroxyapatite oxidized styrene in toluene with 100% conversion and 92% selectivity to the epoxide, under optimum conditions and using anhydrous tert-butyl hydroperoxide (TBHP) as an oxidant.

We previously reported the isolation of octadecanethiolate-protected Au (Au:SC18H37) clusters having a core mass of ∼11 kDa from the crude Au:SC18H37 samples obtained in the reaction between octadecanethiol (C18H37SH) and poly(N-vinyl-2-pyrrolidone) (PVP)-stabilized Au clusters. This stable Au:SC18H37 compound was assigned to be Au55(SCnH2n+1)32 by destructive mass spectrometry and thermogravimetry (TG) performed in the framework of the classical structure model, in which the thiolates are bound to the surface of a magic Au55 core. In the present study, the molecular formula of the Au:SC18H37 (11 kDa) cluster was reassessed by non-destructive matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS), and it was confirmed that the Au:SC18H37 (11 kDa) cluster is a mixture of Au54(SC18H37)30 and Au55(SC18H37)31. On the basis of our present understanding of the structures of other stable Au:SR clusters, we proposed that Au54(SC18H37)30 and Au55(SC18H37)31 have a structural motif comprising a Au37 cluster core that is completely protected by −SR−[Au(I)−SR−]x oligomers (x = 1 and 2). The nonformation of the Au:SR (11 kDa) cluster during the conventional chemical reduction of Au(I)−SR oligomers is ascribed to the kinetics of Au:SR formation.

Au clusters smaller than 1.5 nm and stabilized by poly(N-vinyl-2-pyrrolidone) (PVP) showed higher activity for aerobic oxidation of alcohol than those of larger size or stabilized by poly(allylamine) (PAA). X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy of adsorbed CO, and X-ray absorption near edge structure measurements revealed that the catalytically active Au clusters are negatively charged by electron donation from PVP, and the catalytic activity is enhanced with increasing electron density on the Au core. Based on similar observations of Au cluster anions in the gas phase, we propose that electron transfer from the anionic Au cores of Au:PVP into the LUMO (π*) of O2 generates superoxo- or peroxo-like species, which plays a key role in the oxidation of alcohol. On the basis of these results, a simple principle is presented for the synthesis of Au oxidation catalysts stabilized by organic molecules.

We applied matrix-assisted laser desorption/ionization (MALDI) for mass analysis of Au clusters stabilized by poly(vinyl-pyrrolidone) (PVP) and discovered a series of magic numbers for Au cluster size: 35 ± 1, 43 ± 1, 58 ± 1, 70 ± 3, 107 ± 4, 130 ± 1, and 150 ± 2. Magic numbers smaller than ∼70 agree with those of free Au clusters and can be explained qualitatively by the electronic shell model. In contrast, magic numbers larger than ∼100 are obviously different from those of the free clusters. We suggest that the deviation from the electronic shell model is due to perturbation of the electronic and/or geometric structures caused by interaction with PVP.

A simple, effective method has been demonstrated to immobilize ∼1 nm Au clusters within mesoporous silicas (SBA-15, MCF, HMS) using triphenylphosphine-protected Au11 (Au11:TPP) clusters as precursors, which were deposited on the silica surface in an organic medium. A unique feature of this method is the ability to disperse Au11:TPP homogeneously over a large surface area by optimizing the solvent-mediated interaction. The Au11:TPP−silica composite was then carefully calcined to remove the protecting ligands while suppressing the aggregation of the resulting Au clusters. For SBA-supported Au clusters, the absence of the surface plasmon band in the reflectance spectrum indicated that contamination by AuNPs larger than 2 nm was negligibly small. The Au cluster size supported on SBA was estimated to be 0.8 ± 0.3 nm by HAADF-STEM observations. The SBA-supported Au clusters exhibited catalytic activity for oxidation of various alcohols by H2O2 under microwave irradiation and were found to be reusable.

Small PVP-stabilized gold clusters were successfully prepared by the homogeneous mixing of continuous flows of aqueous AuCl4− and BH4− in a micromixer. Spectroscopic characterization revealed that microfluidic synthesis could yield monodisperse Au:PVP clusters with an average diameter of ∼1 nm, which is smaller than clusters produced by conventional batch methods. These ∼1 nm Au:PVP clusters exhibited higher catalytic activity for the aerobic oxidation of p-hydroxybenzyl alcohol than did Au:PVP clusters prepared by batch methods.

Au25 clusters supported on hydroxyapatite oxidized styrene in toluene with 100% conversion and 92% selectivity to the epoxide, under optimum conditions and using anhydrous tert-butyl hydroperoxide (TBHP) as an oxidant.