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

Size Growth of Au₄Cu₄: From Increased Nucleation to Surface Capping

The size conversion of atomically precise metal nanoclusters is fundamental for elucidating structure-property correlations. In this study, copper salt (CuCl)-induced size growth from [Au4Cu4(Dppm)2(SAdm)5]+ (abbreviated as [Au4Cu4S5]+) to [Au4Cu6(Dppm)2(SAdm)4Cl3]+ (abbreviated as [Au4Cu6S4Cl3]+) (SAdmH = 1-adamantane mercaptan, Dppm = bis-(diphenylphosphino)methane) was investigated via experiments and density functional theory calculations. The [Au4Cu4S5]+ adopts a defective pentagonal bipyramid core structure with surface cavities, which could be easily filled with the sterically less hindered CuCl and CuSCy (i.e., core growth) (HSCy = cyclohexanethiol) but not the bulky CuSAdm. As long as the Au4Cu5 framework is formed, ligand exchange or size growth occurs easily. However, owing to the compact pentagonal bipyramid core structure, the latter growth mode occurs only for the surface-capped [Au4Cu6(Dppm)2(SAdm)4Cl3]+ structure (i.e., surface-capped size growth). A preliminary mechanistic study with density functional theory (DFT) calculations indicated that the overall conversion occurred via CuCl addition, core tautomerization, Cl migration, the second [CuCl] addition, and [CuCl]-[CuSR] exchange steps. And the [Au4Cu6(Dppm)2(SAdm)4Cl3]+ alloy nanocluster exhibits aggregation-induced emission (AIE) with an absolute luminescence quantum yield of 18.01% in the solid state. This work sheds light on the structural transformation of Au–Cu alloy nanoclusters induced by Cu(I) and contributes to the knowledge base of metal-ion-induced size conversion of metal nanoclusters

Combining the Single-Atom Engineering and Ligand-Exchange Strategies: Obtaining the Single-Heteroatom-Doped Au₁₆Ag₁(S-Adm)₁₃ Nanocluster with Atomically Precise Structure

Obtaining cognate single-heteroatom doping is highly desirable but least feasible in the research of nanoclusters (NCs). In this work, we reported a new Au₁₆Ag₁(S-Adm)₁₃ NC, which is synthesized by the combination of single-atom engineering and ligand-exchange strategies. This new NC is so far the smallest crystallographically characterized Au-based NC protected by thiolate. The Au₁₆Ag₁(S-Adm)₁₃ exhibited a tristratified Au₃–Au₂Ag₁–Au₁ kernel capped by staple-like motifs including one dimer and two tetramers. In stark contrast to the size-growth from Au₁₈(S–C₆H₁₁)₁₄ to Au₂₁(S-Adm)₁₅ via just the ligand-exchange method, combining single Ag doping on Au₁₈(S–C₆H₁₁)₁₄ resulted in the size-decrease from Au₁₇Ag₁(S–C₆H₁₁)₁₄ to Au₁₆Ag₁(S-Adm)₁₃. DFT calculations were performed to both homogold Au₁₈ and single-heteroatom-doped Au₁₇Ag₁ to explain the opposite results under the same ligand-exchange reaction. Our work is expected to inspire the synthesis of new cognate single-atom-doped NCs by combining single-atom engineering and ligand-exchange strategies and also shed light on extensive understanding of the metal synergism effect in the NC range

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

Synthesis of [2.2]Paracyclophane-Fused Heterocycles via Palladium-Catalyzed C–H Activation/Annulation of [2.2]Paracyclophanecarboxamides with Arynes

[2.2]Paracyclophane-fused heterocycles represent an important scaffold. Traditional approaches often suffer from tedious synthetic routes, and the development of catalytic synthesis of them remains in its infancy. Herein, by employing highly strained aryne intermediates as partners, we have developed a concise protocol by palladium-catalyzed C–H activation/annulation from [2.2]paracyclophanecarboxamide substrates. [2.2]Paracyclophane-fused quinolinone products are obtained in good yields (up to 84%). Furthermore, the utility of the process has been shown through the synthesis of [2.2]paracyclophane-fused heterocyclic catalysts

How a Single Electron Affects the Properties of the “Non-Superatom” Au₂₅ Nanoclusters

In this study, we successfully synthesized the rod-like [Au₂₅(PPh₃)₁₀(SePh)₅Cl₂]^<i>q</i> (<i>q</i> = +1 or +2) nanoclusters through kinetic control. The single crystal X-ray crystallography determined their formulas to be [Au₂₅(PPh₃)₁₀(SePh)₅Cl₂]­(SbF₆) and [Au₂₅(PPh₃)₁₀(SePh)₅Cl₂]­(SbF₆)­(BPh₄), respectively. Compared to the previously reported Au₂₅ coprotected by phosphine and thiolate ligands (i.e., [Au₂₅(PPh₃)₁₀(SR)₅Cl₂]²⁺), the two new rod-like Au₂₅ nanoclusters show some interesting structural differences. Nonetheless, each of these three nanoclusters possesses two icosahedral Au₁₃ units (sharing a vertex gold atom) and the bridging “Au–Se­(S)–Au” motifs. The compositions of the two new nanoclusters were characterized with ESI-MS and TGA. The optical properties, electrochemistry, and magnetism were studied by EPR, NMR, and SQUID. All these results demonstrate that the valence character significantly affects the properties of the “non-superatom” Au₂₅ nanoclusters, and the changes are different from the previously reported “superatom” Au₂₅ nanoclusters. Theoretical calculations indicate that the extra electron results in the half occupation of the highest occupied molecular orbitals in the rod-like Au₂₅⁺ nanoclusters and, thus, significantly affects the electronic structure of the “non-superatom” Au₂₅ nanoclusters. This work offers new insights into the relationship between the properties and the valence of the “non-superatom” gold nanoclusters

The Key Gold: Enhanced Platinum Catalysis for the Selective Hydrogenation of α,β-Unsaturated Ketone

AuPt alloy nanoparticles (NPs) were facilely synthesized with oleylamine as the stabilizing ligand and characterized by high-resolution transmission electron microscopy, powder X-ray diffraction, inductively coupled plasma-atomic emission spectrometer analysis, and so on. In addition, the AuPt alloys supported by the nano CeO₂ exhibit high selectivity and efficiency in hydrogenation of benzylidene acetone under ambient temperature and pressure. By analyzing the catalytic performance over the NPs with different Au:Pt compositions, we found that the TON_Pt values (based on the number of Pt atoms) vary in the same trend with the change of conversion. Despite that gold itself shows no catalytic activity, the improved conversion and TON_Pt with the alloy catalysts clearly show the promotion effect of gold on the catalytic activity of the platinum. The inactive metal significantly improves the catalytic activity of active metal, which shows that the AuPt alloy exhibits an interesting synergistic effect

Crystal Structures of Two New Gold–Copper Bimetallic Nanoclusters: Cu_<i>x</i>Au_{25–<i>x</i>}(PPh₃)₁₀(PhC₂H₄S)₅Cl₂²⁺ and Cu₃Au₃₄(PPh₃)₁₃(^tBuPhCH₂S)₆S₂³⁺

Herein, we report the synthesis and atomic structures of the cluster-assembled Cu_<i>x</i>­Au_{25–<i>x</i>}­(PPh₃)₁₀­(PhCH₂CH₂S)₅­Cl₂²⁺ and Cu₃­Au₃₄­(PPh₃)₁₃­(^tBuPhCH₂S)₆­S₂³⁺ nanoclusters (NCs). The atomic structures of both NCs were precisely determined by single-crystal X-ray crystallography. The Cu_<i>x</i>­Au_{25–<i>x</i>}­(PPh₃)₁₀­(PhC₂H₄S)₅­Cl₂²⁺ NC was assembled by two icosahedral M₁₃ via a vertex-sharing mode. The Cu atom partially occupies the top and waist sites and is monocoordinated with chlorine or thiol ligands. Meanwhile, the Cu₃­Au₃₄­(PPh₃)₁₃­(^tBuPhCH₂S)₆­S₂³⁺ NC can be described as three 13-atom icosahedra sharing three vertexes in a cyclic fashion. The three Cu atoms all occupy the internal positions of the cluster core. What is more important is that all three Cu atoms in Cu₃Au₃₄ are monocoordinated by the bare S atoms. The absorption spectra of the as-synthesized bimetallic NCs reveal that the additional metal doping and different cluster assemblies affect the electronic structure of the monometallic NCs

How a Single Electron Affects the Properties of the “Non-Superatom” Au₂₅ Nanoclusters

In this study, we successfully synthesized the rod-like [Au₂₅(PPh₃)₁₀(SePh)₅Cl₂]^<i>q</i> (<i>q</i> = +1 or +2) nanoclusters through kinetic control. The single crystal X-ray crystallography determined their formulas to be [Au₂₅(PPh₃)₁₀(SePh)₅Cl₂]­(SbF₆) and [Au₂₅(PPh₃)₁₀(SePh)₅Cl₂]­(SbF₆)­(BPh₄), respectively. Compared to the previously reported Au₂₅ coprotected by phosphine and thiolate ligands (i.e., [Au₂₅(PPh₃)₁₀(SR)₅Cl₂]²⁺), the two new rod-like Au₂₅ nanoclusters show some interesting structural differences. Nonetheless, each of these three nanoclusters possesses two icosahedral Au₁₃ units (sharing a vertex gold atom) and the bridging “Au–Se­(S)–Au” motifs. The compositions of the two new nanoclusters were characterized with ESI-MS and TGA. The optical properties, electrochemistry, and magnetism were studied by EPR, NMR, and SQUID. All these results demonstrate that the valence character significantly affects the properties of the “non-superatom” Au₂₅ nanoclusters, and the changes are different from the previously reported “superatom” Au₂₅ nanoclusters. Theoretical calculations indicate that the extra electron results in the half occupation of the highest occupied molecular orbitals in the rod-like Au₂₅⁺ nanoclusters and, thus, significantly affects the electronic structure of the “non-superatom” Au₂₅ nanoclusters. This work offers new insights into the relationship between the properties and the valence of the “non-superatom” gold nanoclusters

Crystal Structures of Two New Gold–Copper Bimetallic Nanoclusters: Cu_<i>x</i>Au_{25–<i>x</i>}(PPh₃)₁₀(PhC₂H₄S)₅Cl₂²⁺ and Cu₃Au₃₄(PPh₃)₁₃(^tBuPhCH₂S)₆S₂³⁺

Herein, we report the synthesis and atomic structures of the cluster-assembled Cu_<i>x</i>­Au_{25–<i>x</i>}­(PPh₃)₁₀­(PhCH₂CH₂S)₅­Cl₂²⁺ and Cu₃­Au₃₄­(PPh₃)₁₃­(^tBuPhCH₂S)₆­S₂³⁺ nanoclusters (NCs). The atomic structures of both NCs were precisely determined by single-crystal X-ray crystallography. The Cu_<i>x</i>­Au_{25–<i>x</i>}­(PPh₃)₁₀­(PhC₂H₄S)₅­Cl₂²⁺ NC was assembled by two icosahedral M₁₃ via a vertex-sharing mode. The Cu atom partially occupies the top and waist sites and is monocoordinated with chlorine or thiol ligands. Meanwhile, the Cu₃­Au₃₄­(PPh₃)₁₃­(^tBuPhCH₂S)₆­S₂³⁺ NC can be described as three 13-atom icosahedra sharing three vertexes in a cyclic fashion. The three Cu atoms all occupy the internal positions of the cluster core. What is more important is that all three Cu atoms in Cu₃Au₃₄ are monocoordinated by the bare S atoms. The absorption spectra of the as-synthesized bimetallic NCs reveal that the additional metal doping and different cluster assemblies affect the electronic structure of the monometallic NCs