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

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

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)

Chiral Structure of Thiolate-Protected 28-Gold-Atom Nanocluster Determined by X‑ray Crystallography

We report the crystal structure of a new nanocluster formulated as Au₂₈(TBBT)₂₀, where TBBT = 4<i>-tert-</i>butylbenzenethiolate. It exhibits a rod-like Au₂₀ kernel consisting of two interpenetrating cuboctahedra. The kernel is protected by four dimeric “staples” (-SR-Au-SR-Au-SR-) and eight bridging thiolates (-SR-). The unit cell of Au₂₈(TBBT)₂₀ single crystals contains a pair of enantiomers. The origin of chirality is primarily rooted in the rotating arrangement of the four dimeric staples as well as the arrangement of the bridging thiolates (quasi-<i>D</i>₂ symmetry). The enantiomers were separated by chiral HPLC and characterized by circular dichroism spectroscopy

Unique Bonding Properties of the Au₃₆(SR)₂₄ Nanocluster with FCC-Like Core

The recent discovery on the total structure of Au₃₆(SR)₂₄, which was converted from biicosahedral Au₃₈(SR)₂₄, represents a surprising finding of a face-centered cubic (FCC)-like core structure in small gold–thiolate nanoclusters. Prior to this finding, the FCC feature was only expected for larger (nano)­crystalline gold. Herein, we report results on the unique bonding properties of Au₃₆(SR)₂₄ that are associated with its FCC-like core structure. Temperature-dependent X-ray absorption spectroscopy (XAS) measurements at the Au L₃-edge, in association with <i>ab initio</i> calculations, show that the local structure and electronic behavior of Au₃₆(SR)₂₄ are of more molecule-like nature, whereas its icosahedral counterparts such as Au₃₈(SR)₂₄ and Au₂₅(SR)₁₈ are more metal-like. Moreover, site-specific S K-edge XAS studies indicate that the bridging motif for Au₃₆(SR)₂₄ has different bonding behavior from the staple motif from Au₃₈(SR)₂₄. Our findings highlight the important role of “pseudo”-Au₄ units within the FCC-like Au₂₈ core in interpreting the bonding properties of Au₃₆(SR)₂₄ and suggest that FCC-like structure in gold thiolate nanoclusters should be treated differently from its bulk counterpart

Gold–Thiolate Ring as a Protecting Motif in the Au₂₀(SR)₁₆ Nanocluster and Implications

Understanding how gold nanoclusters nucleate from Au^ISR complexes necessitates the structural elucidation of nanoclusters with decreasing size. Toward this effort, we herein report the crystal structure of an ultrasmall nanocluster formulated as Au₂₀(TBBT)₁₆ (TBBT = SPh-<i>t</i>-Bu). The structure features a vertex-sharing bitetrahedral Au₇ kernel and an unprecedented “ring” motifAu₈(SR)₈. This large ring protects the Au₇ kernel through strong Au_ring–Au_kernel bonding but does not involve S–Au_kernel bonding, in contrast to the common “staple” motifs in which the S–Au_kernel bonding is dominant but the Au_staple–Au_kernel interaction is weak (i.e., aurophilic). As the smallest member in the TBBT “magic series”, Au₂₀(TBBT)₁₆, together with Au₂₈(TBBT)₂₀, Au₃₆(TBBT)₂₄, and Au₄₄(TBBT)₂₈, reveals remarkable size-growth patterns in both geometric structure and electronic nature. Moreover, Au₂₀(TBBT)₁₆, together with the Au₂₄(SR)₂₀ and Au₁₈(SR)₁₄ nanoclusters, forms a “4e” nanocluster family, which illustrates a trend of shrinkage of bitetrahedral kernels from Au₈⁴⁺ to Au₇³⁺ and possibly to Au₆²⁺ with decreasing size

Chiral Structure of Thiolate-Protected 28-Gold-Atom Nanocluster Determined by X‑ray Crystallography

We report the crystal structure of a new nanocluster formulated as Au₂₈(TBBT)₂₀, where TBBT = 4<i>-tert-</i>butylbenzenethiolate. It exhibits a rod-like Au₂₀ kernel consisting of two interpenetrating cuboctahedra. The kernel is protected by four dimeric “staples” (-SR-Au-SR-Au-SR-) and eight bridging thiolates (-SR-). The unit cell of Au₂₈(TBBT)₂₀ single crystals contains a pair of enantiomers. The origin of chirality is primarily rooted in the rotating arrangement of the four dimeric staples as well as the arrangement of the bridging thiolates (quasi-<i>D</i>₂ symmetry). The enantiomers were separated by chiral HPLC and characterized by circular dichroism spectroscopy

Atomic Structure of Self-Assembled Monolayer of Thiolates on a Tetragonal Au₉₂ Nanocrystal

Unveiling the ligand binding mode on the crystalline surfaces is important for deciphering the long-standing structural enigma in self-assembled monolayers (SAMs). Here, the binding and patterning structures of thiolates (SR) on the Au(100) crystalline facet are revealed on the basis of the atomic structure of a highly regular, single crystalline Au₉₂(SR)₄₄ nanocrystal. The six exposed facets of this tetragonal nanocrystal give rise to six pieces of “nanoSAMs”. We found that thiolates bind to the planar (100) facets of the nanocrystal via a simple bridge-like mode and are assembled into an overlayer with c(2 × 2) symmetry. The Au–S binding mode and translational symmetry in the kernel and on the surface of the Au₉₂ nanocrystal can be generalized infinitely to construct the bulk two-dimensional SAMs and various tetragonal nanocrystals

Nonsuperatomic [Au₂₃(SC₆H₁₁)₁₆]⁻ Nanocluster Featuring Bipyramidal Au₁₅ Kernel and Trimeric Au₃(SR)₄ Motif

We report the X-ray structure of a cyclohexanethiolate-capped [Au₂₃(SR)₁₆]⁻ nanocluster (counterion: tetraoctylammonium, TOA⁺). The structure comprises a cuboctahedron-based bipyramidal Au₁₅ kernel, which is protected by two staple-like trimeric Au₃(SR)₄ motifs, two monomeric Au­(SR)₂ and four plain bridging SR ligands. Electronic structure analysis reveals nonsuperatomic feature of [Au₂₃(SR)₁₆]⁻ and confirms the Au₁₅ kernel and surface motifs. The Au₁₅ kernel and trimeric staple motif are unprecedented and offer new insight in understanding the structure evolution of gold nanoclusters