Gold Nanocluster-Catalyzed Semihydrogenation: A Unique Activation Pathway for Terminal Alkynes

We report high catalytic activity of ultrasmall spherical Au₂₅(SC₂H₄Ph)₁₈ and rod-shaped Au₂₅(PPh₃)₁₀(CCPh)₅X₂ (X = Br, Cl) nanoclusters supported on oxides for the semihydrogenation of terminal alkynes into alkenes with >99% conversion of alkynes and ∼100% selectivity for alkenes. In contrast, internal alkynes cannot be catalyzed by such “ligand-on” Au₂₅ catalysts; however, with “ligand-off” Au₂₅ catalysts the internal alkynes can undergo semihydrogenation to yield <i>Z</i>-alkenes, similar to conventional gold nanoparticle catalysts. On the basis of the results, a unique activation pathway of terminal alkynes by “ligand-on” gold nanoclusters is identified, which should follow a deprotonation activation pathway via a R′CC[Au_<i>n</i>L_<i>m</i>] (where L represents the protecting ligands on the cluster), in contrast with the activation mechanism on conventional gold nanocatalysts. This new activation mode is supported by observing the incorporation of deprotonated CCPh as ligands on rod-shaped Au₂₅(PPh₃)₁₀(CCPh)₅X₂ nanoclusters under conditions similar to the catalytic reaction and by detecting the R′CC[Au_<i>n</i>(SC₂H₄Ph)_<i>m</i>] via FT-IR spectroscopy

Thermally Robust Au₉₉(SPh)₄₂ Nanoclusters for Chemoselective Hydrogenation of Nitrobenzaldehyde Derivatives in Water

We report the synthesis and catalytic application of thermally robust gold nanoclusters formulated as Au₉₉(SPh)₄₂. The formula was determined by electrospray ionization and matrix-assisted laser desorption ionization mass spectrometry in conjunction with thermogravimetric analysis. The optical spectrum of Au₉₉(SPh)₄₂ nanoclusters shows absorption peaks at ∼920 nm (1.35 eV), 730 nm (1.70 eV), 600 nm (2.07 eV), 490 nm (2.53 eV), and 400 nm (3.1 eV) in contrast to conventional gold nanoparticles, which exhibit a plasmon resonance band at 520 nm (for spherical particles). The ceria-supported Au₉₉(SPh)₄₂ nanoclusters were utilized as a catalyst for chemoselective hydrogenation of nitrobenzaldehyde to nitrobenzyl alcohol in water using H₂ gas as the hydrogen source. The selective hydrogenation of the aldehyde group catalyzed by nanoclusters is a surprise because conventional nanogold catalysts instead give rise to the product resulting from reduction of the nitro group. The Au₉₉(SPh)₄₂/CeO₂ catalyst gives high catalytic activity for a range of nitrobenzaldehyde derivatives and also shows excellent recyclability due to its thermal robustness. We further tested the size-dependent catalytic performance of Au₂₅(SPh)₁₈ and Au₃₆(SPh)₂₄ nanoclusters, and on the basis of their crystal structures we propose a molecular adsorption site for nitrobenzaldehyde. The nanocluster material is expected to find wide application in catalytic reactions

Thiolate-Protected Au₂₄(SC₂H₄Ph)₂₀ Nanoclusters: Superatoms or Not?

We report a new gold thiolate cluster with molecular purity. Electrospray ionization (ESI) mass spectrometry in conjunction with thermogravimetric analysis (TGA), elemental analysis (EA), and ¹H NMR, unambiguously determined the composition of the as-prepared Au nanocluster to be Au₂₄(SC₂H₄Ph)₂₀. The optical absorption spectrum of this cluster shows a highest occupied molecular orbital to lowest unoccupied molecular orbital (HOMO−LUMO) transition at 765 nm, indicating quantum confinement of electrons in the particle. The HOMO−LUMO gap (∼1.5 eV) of Au₂₄(SR)₂₀ is much smaller than that of Au₂₀(SR)₁₆ (∼2.1 eV) but slightly larger than that of Au₂₅(SR)₁₈ (∼1.3 eV). The number of valence electrons in Au₂₄(SC₂H₄Ph)₂₀ is 4e, which is not predicted by the superatom model

Thiol Ligand-Induced Transformation of Au₃₈(SC₂H₄Ph)₂₄ to Au₃₆(SPh‑<i>t</i>‑Bu)₂₄

We report a disproportionation mechanism identified in the transformation of rod-like biicosahedral Au₃₈(SCH₂CH₂Ph)₂₄ to tetrahedral Au₃₆(TBBT)₂₄ nanoclusters. Time-dependent mass spectrometry and optical spectroscopy analyses unambiguously map out the detailed size-conversion pathway. The ligand exchange of Au₃₈(SCH₂CH₂Ph)₂₄ with bulkier 4-<i>tert</i>-butylbenzenethiol (TBBT) until a certain extent starts to trigger structural distortion of the initial biicosahedral Au₃₈(SCH₂CH₂Ph)₂₄ structure, leading to the release of two Au atoms and eventually the Au₃₆(TBBT)₂₄ nanocluster with a tetrahedral structure, in which process the number of ligands is interestingly preserved. The other product of the disproportionation process, <i>i</i>.<i>e</i>., Au₄₀(TBBT)_<i>m</i>+2(SCH₂CH₂Ph)_{24–<i>m</i>}, was concurrently observed as an intermediate, which was the result of addition of two Au atoms and two TBBT ligands to Au₃₈(TBBT)_<i>m</i>(SCH₂CH₂Ph)_{24–<i>m</i>}. The reaction kinetics on the Au₃₈(SCH₂CH₂Ph)₂₄ to Au₃₆(TBBT)₂₄ conversion process was also performed, and the activation energies of the structural distortion and disproportionation steps were estimated to be 76 and 94 kJ/mol, respectively. The optical absorption features of Au₃₆(TBBT)₂₄ are interpreted on the basis of density functional theory simulations

Magic Size Au₆₄(S‑<i>c</i>‑C₆H₁₁)₃₂ Nanocluster Protected by Cyclohexanethiolate

We report a new magic-sized gold nanocluster of atomic precision formulated as Au₆₄(S-<i>c</i>-C₆H₁₁)₃₂. The Au₆₄ nanocluster was obtained in relatively high yield (∼15%, Au atom basis) by a two-step size-focusing methodology. Obtaining this new magic size through the previously established “size focusing” method relies on the introduction of a new synthetic “parameter”the type of protecting thiolate ligand. It was found that Au₆₄(S-<i>c</i>-C₆H₁₁)₃₂ was the most thermodynamically stable specie of the cyclohexanethiolate-protected gold nanoclusters in the size range from ~5k to 20k (where, k = 1000 dalton); hence, it can be selectively synthesized through a careful control of the size-focusing kinetics. The Au₆₄ nanocluster is the first gold nanocluster achieved through direct synthesis (i.e., without postsynthetic size separation) in the medium size range (i.e., ∼40 to ∼100 gold atoms). This medium-sized Au₆₄(S-<i>c</i>-C₆H₁₁)₃₂ exhibits a highly structured optical absorption spectrum, reflecting its discrete electronic states. The discovery of this new Au₆₄(S-<i>c</i>-C₆H₁₁)₃₂ nanocluster bridges the gap of the gold nanoclusters in the medium size range and will facilitate the understanding of the structure and property evolution of magic-size gold nanoclusters

Peeling the Core–Shell Au₂₅ Nanocluster by Reverse Ligand-Exchange

Peeling the Core–Shell Au₂₅ Nanocluster by Reverse Ligand-Exchang

Tuning the Magic Size of Atomically Precise Gold Nanoclusters via Isomeric Methylbenzenethiols

Toward controlling the magic sizes of atomically precise gold nanoclusters, herein we have devised a new strategy by exploring the para<i>-</i>, meta<i>-</i>, ortho-methylbenzenethiol (MBT) for successful preparation of pure Au₁₃₀(<i>p</i>-MBT)₅₀, Au₁₀₄(<i>m</i>-MBT)₄₁ and Au₄₀(<i>o</i>-MBT)₂₄ nanoclusters. The decreasing size sequence is in line with the increasing hindrance of the methyl group to the interfacial Au–S bond. That the subtle change of ligand structure can result in drastically different magic sizes under otherwise similar reaction conditions is indeed for the first time observed in the synthesis of thiolate-protected gold nanoclusters. These nanoclusters are highly stable as they are synthesized under harsh size-focusing conditions at 80–90 °C in the presence of excess thiol and air (i.e., without exclusion of oxygen)

Site-Specific and Size-Dependent Bonding of Compositionally Precise Gold−Thiolate Nanoparticles from X-ray Spectroscopy

Understanding the site-specific and size-dependent structure and bonding properties of gold−thiolate nanoparticles is one of the most fundamental questions in nanoscience. The recent synthesis of compositionally precise Au_<i>m</i>(SR)_<i>n</i> nanoparticles offers a promising opportunity to explicitly elucidate the structure−property correlation of nanomaterials from site-specific and size-dependent perspectives. Herein, we report an X-ray absorption (gold L₃-, sulfur K-, and L_3,2-edge) and photoemission (gold 4f, sulfur 2p, and valence band) study of compositionally precise Au_<i>m</i>(SR)_<i>n</i> nanoparticles. An X-ray spectroscopy approach to deduce the atomic structure of Au₁₄₄(SR)₆₀ is first demonstrated. In conjunction with ab initio calculations, a high-precision, atomic-site-specific illustration of the bonding in Au₁₄₄(SR)₆₀ is achieved. By comparing size-varied samples [Au₁₄₄(SR)₆₀, Au₃₈(SR)₂₄, and Au₂₅(SR)₁₈], the size-dependent nature of bonding in gold−thiolate nanoparticles is revealed from both the sulfur and gold perspective

Electron Transfer between [Au₂₅(SC₂H₄Ph)₁₈]⁻TOA⁺ and Oxoammonium Cations

We report intermolecular electron transfer between 2,2,6,6-tetramethylpiperidin-1-oxoammonium tetrafluoroborate (TEMPO⁺BF₄^–) and thiol-stabilized [Au₂₅(SC₂H₄Ph)₁₈]⁻TOA⁺ (abbreviated as Au₂₅^–) nanoclusters. The TEMPO⁺ cations are generated by single electron oxidation of piperidine aminoxyl radical TEMPO (2,2,6,6-tetramethylpiperidinyl-oxy). Cyclic voltammetry (CV) and electron spin resonance (ESR) explicitly indicate that two consecutive single-electron transfer reactions occur between TEMPO⁺ cations and Au₂₅^– nanoclusters. Nuclear magnetic resonance (¹H NMR) analysis demonstrates that the methylene proton resonances of the thiolate ligands can also be used to monitor the redox process. UV–vis spectroscopic analysis reveals the changes in the absorption peaks of Au₂₅ nanoclusters upon consecutive single-electron transfers between the nanoclusters and TEMPO⁺ cations. The ease of control over the redox process involving TEMPO⁺ allows the preparation of pure Au₂₅(SC₂H₄Ph)₁₈⁺ nanoclusters. The interesting electron-donating properties of Au₂₅(SR)₁₈ clusters may find some promising applications in future studies

Facile Synthesis and Properties of Hierarchical Double-Walled Copper Silicate Hollow Nanofibers Assembled by Nanotubes

The hierarchical assembly of multilevel, nonspherical hollow structures remains a considerable challenge. Here, we report a facile approach for synthesizing copper silicate hollow nanofibers with an ultrasmall nanotube-assembled, double-walled structure. The as-prepared hollow fibers possess a tailored complex wall structure, high length-to-diameter ratio, good structural stability, and a high surface area, and they exhibit excellent performance as an easily recycled adsorbent for wastewater treatment and as an ideal support for noble metal catalysts. In addition, this strategy can be extended as a general approach to prepare other double-walled, hollow, fibrous silica-templated materials