Chiral Ag-23 nanocluster with open shell electronic structure and helical face-centered cubic framework

We report the synthesis and crystal structure of a nanocluster composed of 23 silver atoms capped by 8 phosphine and 18 phenylethanethiolate ligands. X-ray crystallographic analysis reveals that the kernel of the Ag nanocluster adopts a helical face-centered cubic structure with C-2 symmetry. The thiolate ligands show two binding patterns with the surface Ag atoms: tri- and tetra-podal types. The tetra-coordination mode of thiolate has not been found in previous Ag nanoclusters. No counter ion (e.g., Na+ and NO3-) is found in the single-crystal and the absence of such ions is also confirmed by X-ray photoelectron spectroscopy analysis, indicating electrical neutrality of the nanocluster. Interestingly, the nanocluster has an open shell electronic structure (i.e., 23(Ag 5s(1))-18(SR) = 5e), as confirmed by electron paramagnetic resonance spectroscopy. Time-dependent density functional theory calculations are performed to correlate the structure and optical absorption/emission spectra of the Ag nanocluster

Experimental and Mechanistic Understanding of Aldehyde Hydrogenation Using Au₂₅ Nanoclusters with Lewis Acids: Unique Sites for Catalytic Reactions

The catalytic activity of Au₂₅(SR)₁₈ nanoclusters (R = C₂H₄Ph) for the aldehyde hydrogenation reaction in the presence of a base, e.g., ammonia or pyridine, and transition-metal ions M^z+, such as Cu⁺, Cu²⁺, Ni²⁺ and Co²⁺, as a Lewis acid is studied. The addition of a Lewis acid is found to significantly promote the catalytic activity of Au₂₅(SR)₁₈/CeO₂ in the hydrogenation of benzaldehyde and a number of its derivatives. Matrix-assisted laser desorption ionization (MALDI) and electrospray ionization (ESI) mass spectrometry in conjunction with UV–vis spectroscopy confirm the generation of new species, Au_{25‑<i>n</i>}(SR)_{18‑<i>n</i>} (<i>n</i> = 1–4), in the presence of a Lewis acid. The pathways for the speciation of Au₂₄(SR)₁₇ from its parent Au₂₅(SR)₁₈ nanocluster as well as its structure are investigated via the density functional theory (DFT) method. The adsorption of M^<i>z</i>+ onto a thiolate ligand “SR” of Au₂₅(SR)₁₈, followed by a stepwise detachment of “SR” and a gold atom bonded to “SR” (thus an “Au-SR” unit) is found to be the most likely mechanism for the Au₂₄(SR)₁₇ generation. This in turn exposes the Au₁₃-core of Au₂₄(SR)₁₇ to reactants, providing an active site for the catalytic hydrogenation. DFT calculations indicate that M^z+ is also capable of adsorbing onto the Au₁₃-core surface, producing a possible active metal site of a different kind to catalyze the aldehyde hydrogenation reaction. This study suggests, for the first time, that species with an open metal site like adducts [nanoparticle-M]^{(<i>z</i>‑1)+} or fragments Au_{25‑<i>n</i>}(SR)_{18‑<i>n</i>} function as the catalysts rather than the intact Au₂₅(SR)₁₈

Atom-by-Atom Evolution of the Same Ligand-Protected Au21, Au22, Au22Cd1, and Au24 Nanocluster Series

Atom-by-atom manipulation on metal nanoclusters (NCs) has long been desired, as the resulting series of NCs can provide insightful understanding of how a single atom affects the structure and properties as well as the evolution with size. Here, we report crystallizations of Au22(SAdm)16 and Au22Cd1(SAdm)16 (SAdm = adamantanethiolate) which link up with Au21(SAdm)15 and Au24(SAdm)16 NCs and form an atom-by-atom evolving series protected by the same ligand. Structurally, Au22(SAdm)16 has an Au3(SAdm)4 surface motif which is longer than the Au2(SAdm)3 on Au21(SAdm)15, whereas Au22Cd1(SAdm)16 lacks one staple Au atom compared to Au24(SAdm)16 and thus the surface structure is reconstructed. A single Cd atom triggers the structural transition from Au22 with a 10-atom bioctahedral kernel to Au22Cd1 with a 13-atom cuboctahedral kernel, and correspondingly, the optical properties are dramatically changed. The photoexcited carrier lifetime demonstrates that the optical properties and excited state relaxation are highly sensitive at the single atom level. By contrast, little change in both ionization potential and electron affinity is found in this series of NCs by theoretical calculations, indicating the electronic properties are independent of adding a single atom in this series. The work provides a paradigm that the NCs with continuous metal atom numbers are accessible and crystallizable when meticulously designed, and the optical properties are more affected at the single atom level than the electronic properties

Sharp Transition from Nonmetallic Au₂₄₆ to Metallic Au₂₇₉ with Nascent Surface Plasmon Resonance

The optical properties of metal nanoparticles have attracted wide interest. Recent progress in controlling nanoparticles with atomic precision (often called nanoclusters) provide new opportunities for investigating many fundamental questions, such as the transition from excitonic to plasmonic state, which is a central question in metal nanoparticle research because it provides insights into the origin of surface plasmon resonance (SPR) as well as the formation of metallic bond. However, this question still remains elusive because of the extreme difficulty in preparing atomically precise nanoparticles larger than 2 nm. Here we report the synthesis and optical properties of an atomically precise Au₂₇₉(SR)₈₄ nanocluster. Femtosecond transient absorption spectroscopic analysis reveals that the Au₂₇₉ nanocluster shows a laser power dependence in its excited state lifetime, indicating metallic state of the particle, in contrast with the nonmetallic electronic structure of the Au₂₄₆(SR)₈₀ nanocluster. Steady-state absorption spectra reveal that the nascent plasmon band of Au₂₇₉ at 506 nm shows no peak shift even down to 60 K, consistent with plasmon behavior. The sharp transition from nonmetallic Au₂₄₆ to metallic Au₂₇₉ is surprising and will stimulate future theoretical work on the transition and many other relevant issues