This work demonstrates the synthesis of an efficient photocatalyst, Au25(PPh3)10Cl2(SC3H6SiO3)5/TiO2, for selective oxidation of amines to imines. The photocatalyst is prepared via hydrolysis of Au25(PPh3)10Cl2[(SC3H6Si(OC2H5)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 (O2). The turnover frequency (TOF) of 4-methylbenzylamine 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.Keywords: Au25; benzylamine oxidation; gold nanoclusters; photocatalysis; selective organic transformation;

We here report a protocol for the synthesis of Au38S2(SAdm)20 nanoclusters (−SAdm = 1-adamantanethiolate) with a higher production yield (10%) in comparison to previous reported methods. The photosensitizing properties of the gold nanoclusters are investigated for the formations of singlet oxygen (1O2) using visible light wavelengths of 532 and 650 nm. The formation of 1O2 was detected by 1,3-diphenylisobenzofuran as the chemical trapping probe as well as direct observation of the characteristic 1O2 emission (ca. 1276 nm). The efficiency of the 1O2 formation using the Au38S2(SAdm)20 nanoclusters is found to be notably higher than that of Au25(SR)18 nanoclusters. Finally, selective aerobic oxidations of sulfide to sulfoxide and benzylamine to imine in the presence of oxygen (3O2) and photoexcited Au38S2(SAdm)20 are well studied. This work demonstrates the promise of Au38S2(SAdm)20 nanoclusters in the generation of activated singlet oxygen for selective catalytic reactions.Keywords: aerobic oxidation; Au nanocluster; Au38; photocatalysis; singlet oxygen; sulfoxidation;

An atomically dispersed Waugh type [CoMo9O32]6− cluster is obtained, employing the most flexible structure unit Anderson type [Co(OH)6Mo6O18]3− as a precursor. The structure of the [CoMo9O32]6− cluster is identified by single crystal X-ray diffraction and also well characterized by FT-IR, ESI-MS, UV-Vis, EA, and TGA spectroscopy. Its 3D framework forms a quasi 2D material and possesses curved edge triangle shape nanopores with a diameter of 8.9 Å. The CoIV and MoVI oxidation states and the related valence band and electronic state of Co are definitely confirmed by X-ray photoelectron spectroscopy (XPS), ultraviolet photoemission spectroscopy (UPS), and bond valence sum (BVS). The [CoMo9O32]6− cluster is a typical n-type inorganic semiconductor with a HOMO–LOMO gap of ca. 1.67 eV and exhibits reversible two-electron redox properties, evidenced by UPS, cyclic voltammetric (CV), and Mott–Schottky plot analyses. Furthermore, [CoMo9O32]6− can effectively generate 1O2 under laser (365 and 532 nm) and sunlight irradiation, detected using a water-soluble DAB probe. Such an n-type multielectron reservoir semiconductor anionic [CoMo9O32]6− cluster with thermal and electrochemical stability as an effective photosensitizer serves as a promising material in solar energy scavenging.

Size-dependence is an important factor in gold nanocatalysis. In this study, we explored the catalytic performance of atomically precise Aun(PET)m nanocluster catalysts (where, PET = phenylethanethiolate) of different gold atoms and sizes, including Au25(PET)18 (∼1.2 nm), Au38(PET)24 (∼1.5 nm), and Au144(PET)60 (∼1.9 nm) nanoclusters. These Aun(PET)m gold clusters, immobilized on activated carbon (AC) and used as heterogeneous catalysts, were characterized by transmission electron microscope (TEM), BET as well as X-ray photoelectron spectroscopy (XPS). They showed good catalytic activity in the aerobic oxidation of D-glucose into gluconic acid (or gluconates) with ∼98% selectivity. We observed a distinct size dependence of the gold nanocluster in the oxidation reactions, which follows as Au144(PET)60/AC > Au38(PET)24/AC > Au25(PET)18/AC. It was primarily determined by the surface area of the nanoscopic Au nanocluster. Further, the turnover frequency (TOF) for the Au144(PET)60/AC catalyst was found to be 2.3 s−1, which is comparable with that for Au/EC300 and much higher than those for the commercial Pd/AC and Pd-Bi/AC catalysts under the identical reaction conditions. On the whole, the core size of the gold nanoclusters played an important role in the catalytic process.

Size-dependence is an important factor in gold nanocatalysis. In this study, we explored the catalytic performance of atomically precise Aun(PET)m nanocluster catalysts (where, PET = phenylethanethiolate) of different gold atoms and sizes, including Au25(PET)18 (∼1.2 nm), Au38(PET)24 (∼1.5 nm), and Au144(PET)60 (∼1.9 nm) nanoclusters. These Aun(PET)m gold clusters, immobilized on activated carbon (AC) and used as heterogeneous catalysts, were characterized by transmission electron microscope (TEM), BET as well as X-ray photoelectron spectroscopy (XPS). They showed good catalytic activity in the aerobic oxidation of D-glucose into gluconic acid (or gluconates) with ∼98% selectivity. We observed a distinct size dependence of the gold nanocluster in the oxidation reactions, which follows as Au144(PET)60/AC > Au38(PET)24/AC > Au25(PET)18/AC. It was primarily determined by the surface area of the nanoscopic Au nanocluster. Further, the turnover frequency (TOF) for the Au144(PET)60/AC catalyst was found to be 2.3 s−1, which is comparable with that for Au/EC300 and much higher than those for the commercial Pd/AC and Pd-Bi/AC catalysts under the identical reaction conditions. On the whole, the core size of the gold nanoclusters played an important role in the catalytic process.

Herein, a novel 3D open bimetallic Cu3[H6W12O42] cluster was designed as an effective solid acid catalyst for the hydrolysis of renewable biomass inulin by heterogeneous catalytic reaction towards fructose production with a high inulin conversion (∼100%) and fructose selectivity (∼90%) at low temperatures under aqueous condition with economic feasibility.

In this study, the gold clusters Au11(PPh3)7Cl3 and Au11(PPh2Py)7Br3 (PPh2Py = diphenyl-2-pyridylphosphine) are synthesized via a one-pot procedure based on the wet chemical reduction method. The Au11(PPh3)7Cl3 cluster is found to be active in the chemoselective hydrogenation of 4-nitrobenzaldehyde in the presence of hydrogen (H2) and a base (e.g., pyridine). Interestingly, the cluster with the functional ligand PPh2Py shows similar activity without losing catalytic efficiency in the absence of the base. The structure of the gold clusters and reaction pathway of the catalytic hydrogenation are investigated at the atomic/molecular level via UV–vis spectroscopy, electrospray ionization (ESI) mass spectrometry, and density functional theory (DFT) calculations. It is found that one ligand (PPh3 or PPh2Py) removal is the first step to expose the core of the gold clusters to reactants, providing an active site for the catalytic reaction. Then, the H–H bond of the H2 molecule becomes activated with the aid of either free amine (base) or ligand PPh2Py which is attached to the gold clusters. This work demonstrates the promise of the functional ligand PPh2Py in the catalytic hydrogenation to reduce the amount of materials (free base: e.g., pyridine) that ultimately enter the waste stream, thereby providing a more environmentally friendly reaction medium.Keywords: amine; Au nanocluster; chemoselective; DFT; hydrogenation

We here explore a kinetically controlled synthetic protocol for preparing solvent-solvable Au102(SPh)44 nanoclusters which are isolated from polydispersed gold nanoclusters by solvent extraction and size exclusion chromatography (SEC). The as-obtained Au102(SPh)44 nanoclusters are determined by matrix-assisted laser desorption ionization (MALDI) and electrospray ionization (ESI) mass spectrometry, in conjunction with UV-vis spectroscopy and thermogravimetric analysis (TGA). However, Au99(SPh)42, instead of Au102(SPh)44, is yielded when the polydispersed gold nanoclusters are etched in the presence of excess thiophenol under thermal conditions (e.g., 80 °C). Interestingly, the Au102(SPh)44 nanoclusters also can convert to Au99(SPh)42 with equivalent thiophenol ligands, evidenced by the analyses of UV-vis and MALDI mass spectrometry. Finally, the TiO2-supported Au102(SPh)44 nanocluster catalyst is investigated in the selective oxidation of sulfides into sulfoxides by the PhIO oxidant and gives rise to high catalytic activity (e.g., 80–99% conversion of R–S–R′ sulfides with 96–99% selectivity for R–S(O)–R′ sulfoxides). The Au102(SPh)44/TiO2 catalyst also shows excellent recyclability in the sulfoxidation process.

The surface functionality of Au38S2(SAdm)20 nanoclusters (−SAdm = adamantanethiolate) in the presence of α-, β-, and γ-cyclodextrins (CDs) is studied. The supramolecular chemistry and host–guest interactions of CDs and the protecting ligands of nanoclusters are investigated using UV-vis and NMR spectroscopies, MALDI mass spectrometry, and molecular dynamics simulations. In contrast to α- and γ-CDs, the results show that β-CDs are capable of efficiently chemisorbing onto the Au38S2(SAdm)20 nanoclusters to yield Au38S2(SAdm)20–(β-CD)2 conjugates. MD simulations revealed that two –SAdm ligands of the nanoparticle with the least steric hindrance are capable to selectively be accommodated into hydrophobic cavity of β-CDs, as furthermore confirmed by NMR spectroscopy. The conjugates largely improve the stability of the nanoclusters in the presence of strong oxidants (e.g., TBHP). Further, the electrochemical properties of Au38S2(SAdm)20 nanoclusters and Au38S2(SAdm)20–(β-CD)2 conjugates are compared. The charge transfer to the redox probe molecules (e.g., K3Fe(CN)6) in solution was monitored by cyclic voltammetry. It is found that β-CDs act as an umbrella to cover the fragile metal cores of the nanoclusters, thereby blocking direct interaction with destabilizing agents and hence quenching the charge transfer process.

Heteroatom-doped gold clusters have been of great interest to modify the catalytic performance of the clusters, although the structure–activity correlation is rarely discussed. Herein, we explore the catalytic activity of bimetallic gold-based MxAu25–x(SR)18 (M = Cu and Ag, and SR = SC2H4Ph) clusters and compare with that of homogold Au25(SR)18 in the CO oxidation reaction. It is found that the CeO2-supported clusters show catalytic activity for the reaction in the order CuxAu25–x(SR)18 > Au25(SR)18 > AgxAu25–x(SR)18. The supported clusters exhibit excellent catalytic activity and durability for prolonged conversion of CO into CO2 (98% in the case of CuxAu25–x(SR)18 at 120 °C for 110 h). Fourier-transform infrared (FT–IR) analyses as well as the density functional theory (DFT) calculations suggest that the thiolate ligands are partially removed under reaction conditions (T > 120 °C). The metal atoms thus exposed (Au, Ag, and Cu) are deemed as the catalytic active sites. DFT calculations suggest that metal exchange between the icosahedral core and the staple motif of the clusters at elevated temperature plays an important role on the conversion rate of CO oxidation. The adsorption of CO on the clusters would be more preferable to occur in the order Cu2Au23(SCH3)15 > Au25(SCH3)15 > Ag2Au23(SCH3)15, which is in line with the experimental results on the CO oxidation catalyzed by the bimetallic clusters.

To explore the electronic and catalytic properties of nanoclusters, here we report an aromatic-thiolate-protected gold nanocluster, [Au25(SNap)18]− [TOA]+, where SNap = 1-naphthalenethiolate and TOA = tetraoctylammonium. It exhibits distinct differences in electronic and catalytic properties in comparison with the previously reported [Au25(SCH2CH2Ph)18]−, albeit their skeletons (i.e., Au25S18 framework) are similar. A red shift by ∼10 nm in the HOMO–LUMO electronic absorption peak wavelength is observed for the aromatic-thiolate-protected nanocluster, which is attributed to its dilated Au13 kernel. The unsupported [Au25(SNap)18]− nanoclusters show high thermal and antioxidation stabilities (e.g., at 80 °C in the present of O2, excess H2O2, or TBHP) due to the effects of aromatic ligands on stabilization of the nanocluster’s frontier orbitals (HOMO and LUMO). Furthermore, the catalytic activity of the supported Au25(SR)18/CeO2 (R = Nap, Ph, CH2CH2Ph, and n-C6H13) is examined in the Ullmann heterocoupling reaction between 4-methyl-iodobenzene and 4-nitro-iodobenzene. Results show that the activity and selectivity of the catalysts are largely influenced by the chemical nature of the protecting thiolate ligands. This study highlights that the aromatic ligands not only lead to a higher conversion in catalytic reaction but also markedly increase the yield of the heterocoupling product (4-methyl-4′-nitro-1,1′-biphenyl). Through a combined approach of experiment and theory, this study sheds light on the structure–activity relationships of the Au25 nanoclusters and also offers guidelines for tailoring nanocluster properties by ligand engineering for specific applications.Keywords: Au25; gold; ligand effects; nanoclusters; Ullmann coupling

We here report the catalytic effects of foreign atoms (Cu, Ag, and Pt) doped into well-defined 25-gold-atom nanoclusters. Using the carbon-carbon coupling reaction of p-iodoanisole and phenylacetylene as a model reaction, the gold-based bimetallic MxAu25−x(SR)18 (–SR=–SCH2CH2Ph) nanoclusters (supported on titania) were found to exhibit distinct effects on the conversion of p-iodoanisole as well as the selectivity for the Sonogashira cross-coupling product, 1-methoxy-4-(2-phenylethynyl)benzene). Compared to Au25(SR)18, the centrally doped Pt1Au24(SR)18 causes a drop in catalytic activity but with the selectivity retained, while the AgxAu25−x(SR)18 nanoclusters gave an overall performance comparable to Au25(SR)18. Interestingly, CuxAu25−x(SR)18 nanoclusters prefer the Ullmann homo-coupling pathway and give rise to product 4,4′-dimethoxy-1,1′-biphenyl, which is in opposite to the other three nanocluster catalysts. Our overall conclusion is that the conversion of p-iodoanisole is largely affected by the electronic effect in the bimetallic nanoclusters’ 13-atom core (i.e., Pt1Au12, CuxAu13−x, and Au13, with the exception of Ag doping), and that the selectivity is primarily determined by the type of atoms on the MxAu12−x shell (M=Ag, Cu, and Au) in the nanocluster catalysts.Figure optionsDownload full-size imageDownload as PowerPoint slide

We report a new synthetic protocol of Au99(SPh)42 nanoclusters with moderate efficiency (∼15% yield based on HAuCl4), via a combination of the ligand-exchange and “size-focusing” processes. The purity of the as-prepared gold nanoclusters is characterized by matrix-assisted laser desorption ionization mass spectrometry and size exclusion chromatography.

We report the controlled synthesis of [Au25(PPh3)10(SR1)5X2]2+ nanorods (H-SR1: alkyl thiol, H-SC2H4Ph and H-S(n-C6H13)) and Au25(SR2)18 nanospheres (H-SR2: aromatic thiol, H-SPh and H-SNap) under the one-phase thiol etching reaction of the polydisperse Aun(PPh3)m parent-particles (core diameter: 1.3 ± 0.4 nm, 20 < n < 50). These as-obtained gold nanoclusters are identified by UV-vis spectroscopy and matrix-assisted laser desorption ionization mass spectrometry. Furthermore, the conversion process, from Aun(PPh3)m nanoparticles to Au25(SNap)18 nanospheres, is monitored by UV-vis spectroscopy. It is observed that the Au25(PPh3)10(SR1)5X2 nanorods cannot convert to Au25(SR)18 nanospheres in the presence of excess thiol (both the alkyl and aromatic thiol) even under thermal conditions (e.g., 55 and 80 °C), indicating that both the Au25 nanorods and nanospheres are in a stable state during the alkyl and aromatic thiol etching reactions, respectively. The two different conversion pathways (i.e., to Au25(PPh3)10(SR1)5X2 nanorods and Au25(SR2)18 nanospheres) mainly are attributed to the different electronic properties and the steric effects of the alkyl and aromatic thiol ligands. The significant ligand effect also is observed in catalytic CO oxidation. The Au25(SC2H4Ph)18/CeO2 catalyst shows catalytic activity at 80 °C and reaches up to 80.7% and 98.5% (based on CO conversion) at 100 and 150 °C, while Au25(SNap)18/CeO2 and Au25(PPh3)10(SC2H4Ph)5X2/CeO2 give rise to a low activity at 100 °C with only 3.3% and 10.2% CO conversion and 98.0% and 94.6% at 150 °C.

Here, we report the one-pot synthesis of atomically precise gold nanoclusters – Au144(SCH2Ph)60 with moderate efficiency (ca. 20% yield based on HAuCl4). The Au144(SCH2Ph)60 nanoclusters are obtained from the polydispersed Aun(SG)m nanoclusters in the presence of excess H–SCH2Ph ligands via a combination of “ligand-exchange” and “size-focusing” processes. The as-obtained Au144(SCH2Ph)60 nanoclusters are well determined by UV-vis spectroscopy and electrospray ionization (ESI) mass spectrometry, and in conjunction with matrix-assisted laser desorption ionization (MALDI) mass spectrometry and thermal gravimetric analysis (TGA). The purity of the Au144(SCH2Ph)60 nanoclusters is further characterized by size exclusion chromatography (SEC) and elemental analysis. The powder X-ray diffraction (PXRD) analysis implies that the Au144(SCH2Ph)60 nanoclusters do not adopt the face-centered cubic (fcc) structure, as the diffraction angle (2θ = 50.5°) is only observed in the Au144(SCH2Ph)60 nanoclusters. Further, the Au144(SCH2Ph)60/TiO2 catalyst exhibits excellent catalytic performance (92% conversion of methyl phenyl sulfide with 99% selectivity for sulfoxide) in the selective sulfoxidation; size-dependence of the gold nanocluster catalyst is observed in the catalytic reactions: Au144(SCH2Ph)60 > Au99(SPh)42 > Au38(SCH2CH2Ph)24 > Au25(SCH2CH2Ph)18.

Here, we report the one-pot synthesis of atomically precise gold nanoclusters – Au144(SCH2Ph)60 with moderate efficiency (ca. 20% yield based on HAuCl4). The Au144(SCH2Ph)60 nanoclusters are obtained from the polydispersed Aun(SG)m nanoclusters in the presence of excess H–SCH2Ph ligands via a combination of “ligand-exchange” and “size-focusing” processes. The as-obtained Au144(SCH2Ph)60 nanoclusters are well determined by UV-vis spectroscopy and electrospray ionization (ESI) mass spectrometry, and in conjunction with matrix-assisted laser desorption ionization (MALDI) mass spectrometry and thermal gravimetric analysis (TGA). The purity of the Au144(SCH2Ph)60 nanoclusters is further characterized by size exclusion chromatography (SEC) and elemental analysis. The powder X-ray diffraction (PXRD) analysis implies that the Au144(SCH2Ph)60 nanoclusters do not adopt the face-centered cubic (fcc) structure, as the diffraction angle (2θ = 50.5°) is only observed in the Au144(SCH2Ph)60 nanoclusters. Further, the Au144(SCH2Ph)60/TiO2 catalyst exhibits excellent catalytic performance (92% conversion of methyl phenyl sulfide with 99% selectivity for sulfoxide) in the selective sulfoxidation; size-dependence of the gold nanocluster catalyst is observed in the catalytic reactions: Au144(SCH2Ph)60 > Au99(SPh)42 > Au38(SCH2CH2Ph)24 > Au25(SCH2CH2Ph)18.

The surface functionality of Au38S2(SAdm)20 nanoclusters (−SAdm = adamantanethiolate) in the presence of α-, β-, and γ-cyclodextrins (CDs) is studied. The supramolecular chemistry and host–guest interactions of CDs and the protecting ligands of nanoclusters are investigated using UV-vis and NMR spectroscopies, MALDI mass spectrometry, and molecular dynamics simulations. In contrast to α- and γ-CDs, the results show that β-CDs are capable of efficiently chemisorbing onto the Au38S2(SAdm)20 nanoclusters to yield Au38S2(SAdm)20–(β-CD)2 conjugates. MD simulations revealed that two –SAdm ligands of the nanoparticle with the least steric hindrance are capable to selectively be accommodated into hydrophobic cavity of β-CDs, as furthermore confirmed by NMR spectroscopy. The conjugates largely improve the stability of the nanoclusters in the presence of strong oxidants (e.g., TBHP). Further, the electrochemical properties of Au38S2(SAdm)20 nanoclusters and Au38S2(SAdm)20–(β-CD)2 conjugates are compared. The charge transfer to the redox probe molecules (e.g., K3Fe(CN)6) in solution was monitored by cyclic voltammetry. It is found that β-CDs act as an umbrella to cover the fragile metal cores of the nanoclusters, thereby blocking direct interaction with destabilizing agents and hence quenching the charge transfer process.