Synchrotron XPS was used to investigate a series of chemically synthesised, atomically precise gold clusters Au(n)(PPh3)y (n = 8, 9 and 101, y depending on the cluster size) immobilized on anatase (titania) nanoparticles. Effects of post-deposition treatments were investigated by comparison of untreated samples with analogues that have been heat treated at 200 °C in O2, or in O2 followed by H2 atmosphere. XPS data shows that the phosphine ligands are oxidised upon heat treatment in O2. From the position of the Au 4f(7/2) peak it can be concluded that the clusters partially agglomerate immediately upon deposition. Heating in oxygen, and subsequently in hydrogen, leads to further agglomeration of the gold clusters. It is found that the pre-treatment plays a crucial role in the removal of ligands and agglomeration of the clusters.

Chemically made, atomically precise phosphine-stabilized clusters Au9(PPh3)8(NO3)3 were deposited on titania and silica from solutions at various concentrations and the samples heated under vacuum to remove the ligands. Metastable induced electron spectroscopy was used to determine the density of states at the surface, and X-ray photoelectron spectroscopy for analysing the composition of the surface. It was found for the Au9 cluster deposited on titania that the ligands react with the titania substrate. Based on analysis using the singular value decomposition algorithm, the series of MIE spectra can be described as a linear combination of 3 base spectra that are assigned to the spectra of the substrate, the phosphine ligands on the substrate, and the Au clusters anchored to titania after removal of the ligands. On silica, the Au clusters show significant agglomeration after heat treatment and no interaction of the ligands with the substrate can be identified.

Two phosphine-stabilised gold clusters, Au 101 (PPh 3 ) 21 Cl 5 and Au 9 (PPh 3 ) 8 (NO 3 ) 3 , were deposited and activated on anatase TiO 2 and fumed SiO 2 . These catalysts showed almost a complete oxidation of benzyl alcohol (>90 %) within 3 hours at 80 °C and 3 bar O 2 in methanol with a high substrate-to-metal molar ratio of 5800 and turn-over frequency of 0.65 s -1 . Factors influencing catalytic activity were investigated, 10 including metal-support interaction, effects of heat treatments, chemical composition of gold clusters, the size of gold nanoparticles and catalytic conditions. It was found that the anions present in gold clusters play a role in determining the catalytic activity in this reaction, with NO 3 -diminishing the catalytic activity. High catalytic activity was attributed to the formation of large gold nanoparticles (> 2 nm) that coincides with partial removal of ligands which occurs during heat treatment and catalysis. Selectivity 15 towards the formation of methyl benzoate can be tuned by selection of the reaction temperature. The catalysts were characterised using transmission electron microscopy, UV-vis diffuse reflectance spectroscopy and X-ray photoelectron spectroscopy.

Atomically precise gold clusters are highly desirable due to their well-defined structure which allows the study of structure-property relationships. In addition, they have potential in technological applications such as nanoscale catalysis. The structural, chemical, electronic, and optical properties of ligated gold clusters are strongly defined by the metal-ligand interaction and type of ligands. This critical feature renders gold-phosphine clusters unique and distinct from other ligand-protected gold clusters. The use of multidentate phosphines enables preparation of varying core sizes and exotic structures beyond regular polyhedrons. Weak gold-phosphorous (Au-P) bonding is advantageous for ligand exchange and removal for specific applications, such as catalysis, without agglomeration. The aim of this review is to provide a unified view of gold-phosphine clusters and to present an in-depth discussion on recent advances and key developments for these clusters. This review features the unique chemistry, structural, electronic, and optical properties of gold-phosphine clusters. Advanced characterization techniques, including synchrotron-based spectroscopy, have unraveled substantial effects of Au-P interaction on the composition-, structure-, and size-dependent properties. State-of-the-art theoretical calculations that reveal insights into experimental findings are also discussed. Finally, a discussion of the application of gold-phosphine clusters in catalysis is presented.

The far infra-red absorption spectra of a series of chemically synthesised, atomically precise phosphinestabilised gold cluster compounds have been recorded using synchrotron light for the first time. Far-IR spectra of the Au 6 (Ph 2 P(CH 2 ) 3 PPh 2 ) 4 (NO 3 ) 2 , Au 8 (PPh 3 ) 8 (NO 3 ) 2 , Au 9 (PPh 3 ) 8 (NO 3 ) 3 , and Pd(PPh 3 ) Au 6 (PPh 3 ) 6 (NO 3 ) 2 clusters reveal a complex series of peaks between 80 and 475 cm À1 , for which all significant peaks can be unambiguously assigned by comparison with Density Functional Theory (DFT) geometry optimisations and frequency calculation. Strong absorptions in all spectra near 420 cm À1 are assigned to the P-Ph 3 stretching vibrations. Distinct peaks within the spectrum of each specific cluster are assigned to the cluster core vibrations: 80.4 and 84.1 cm À1 (Au 6 ) 165.1 and 166.4 cm À1 (Au 8 ), 170.1 and 185.2 cm À1 (Au 9 ), and 158.9, 195.2, and 206.7 cm À1 (Au 6 Pd). The positions of these peaks are similar to those observed to occur for the neutral Au 7 cluster in the gas phase (Science, 2008, 321, 674-676). Au-P stretching vibrations only occur for Au 6 near 420 cm À1 , although they appear near 180 cm À1 for Au 6 Pd and involve gold core vibrations.

Well-definedAu−TiO 2 materials were synthesized by deposition of triphenylphosphine-protected Au 9 clusters on TiO 2 (Aeroxide P-25), pre-treated in eight different ways and subsequently exposed to two post-treatments. X-ray photoelectron spectroscopy and UV-vis diffuse reflectance spectroscopy studies showed that in most cases the PPh 3 ligand shell was removed upon deposition even before post-treatment, coinciding with some cluster aggregation. However, clusters deposited on TiO 2 treated using H 2 SO 4 and H 2 O 2 showed remarkable resistance to aggregation, even after high-temperature calcination, while clusters on H 2 -treated TiO 2 showed the greatest resistance to aggregation under ozonolysis.

High-quality far-IR absorption spectra for a series of ligated atomically precise clusters containing Ru3, Ru4, and AuRu3 metal cores have been observed using synchrotron radiation, the latter two for the first time. The experimental spectra are compared with predicted IR spectra obtained following complete geometric optimization of the full cluster, including all ligands, using DFT. We find strong correlations between the experimental and predicted transitions for the low-frequency, low-intensity metal core vibrations as well as the higher frequency and intensity metal-ligand vibrations. The metal core vibrational bands appear at 150 cm(-1) for Ru3(CO)12, and 153 and 170 cm(-1) for H4Ru4(CO)12, while for the bimetallic Ru3(μ-AuPPh3)(μ-Cl)(CO)10 cluster these are shifted to 177 and 299 cm(-1) as a result of significant restructuring of the metal core and changes in chemical composition. The computationally predicted IR spectra also reveal the expected atomic motions giving rise to the intense peaks of metal-ligand vibrations at ca. 590 cm(-1) for Ru3, 580 cm(-1) for Ru4, and 560 cm(-1) for AuRu3. The obtained correlations allow an unambiguous identification of the key vibrational modes in the experimental far-IR spectra of these clusters for the first time.

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