Racemization of Chiral Pd2Au36(SC2H4Ph)24: Doping Increases the Flexibility of the Cluster Surface

On the mechanism of rapid metal exchange between thiolate-protected gold and gold/silver clusters: a time-resolved in situ XAFS study

The fast metal exchange reaction between Au₃₈ and Ag_xAu_38-x nanoclusters in solution at -20 °C has been studied by in situ X-ray absorption spectroscopy (time resolved quick XAFS) in transmission mode. A cell was designed for this purpose consisting of a cooling system, remote injection and mixing devices. The capability of the set-up is demonstrated for second and minute time scale measurements of the metal exchange reaction upon mixing Au₃₈/toluene and Ag_xAu_38-x/toluene solutions at both Ag K-edge and Au L₃-edge. It has been proposed that the exchange of gold and silver atoms between the clusters occurs via the SR(-M-SR)_n (n = 1, 2; M = Au, Ag) staple units in the surface of the reacting clusters during their collision. However, at no point during the reaction (before, during, after) evidence is found for cationic silver atoms within the staples. This means that either the exchange occurs directly between the cores of the involved clusters or the residence time of the silver atoms in the staples is very short in a mechanism involving the metal exchange within the staples.

Au38Cu1(2-PET)24 nanocluster: synthesis, enantioseparation and luminescence

Two different species of the chiral Au₃₈Cu₁(2-PET)₂₄ adduct, showing enhanced fluorescence, were successfully separated using chiral HPLC.

Isolation of atomically precise mixed ligand shell PdAu24 clusters

Exposure of PdAu24(2-PET)18 (2-PET: 2-phenylethylthiolate) to BINAS (1,1-binaphthyl-2,2-dithiol) leads to species of composition PdAu24(2-PET)18-2x(BINAS)x due to ligand exchange reactions. The BINAS adsorbs in a specific mode that bridges the apex and one core site of two adjacent S(R)-Au-S(R)-Au-S(R) units. Species with different compositions of the ligand shell can be separated by HPLC. Furthermore, site isomers can be separated. For the cluster with exactly one BINAS in its ligand shell only one isomer is expected due to the symmetry of the cluster, which is confirmed by High-Performance Liquid Chromatography (HPLC). Addition of a second BINAS to the ligand shell leads to several isomers. In total six distinguishable isomers are possible for PdAu24(2-PET)14(BINAS)2 including two pairs of enantiomers concerning the adsorption pattern. At least four distinctive isomers are separated by HPLC. Calculations indicate that one of the six possibilities is energetically disfavoured. Interestingly, diastereomers, which have an enantiomeric relationship concerning the adsorption pattern of chiral BINAS, have significantly different stabilities. The relative intensity of the observed peaks in the HPLC does not reflect the statistical weight of the different isomers. This shows, as supported by the calculations, that the first adsorbed BINAS molecule influences the adsorption of the second incoming BINAS ligand. In addition, experiments with the corresponding Pt doped gold cluster reveal qualitatively the same behaviour, however with slightly different relative abundances of the corresponding isomers. This finding points towards the influence of electronic effects on the isomer distribution. Even for clusters containing more than two BINAS ligands a limited number of isomers were found, which is in contrast to the corresponding situation for monothiols, where the number of possible isomers is much larger.

High-resolution separation of thiolate-protected gold clusters by reversed-phase high-performance liquid chromatography

This perspective summarizes our work on high-resolution separation of thiolate-protected gold clusters using reversed-phase high-performance liquid chromatography, new findings obtained by those separation, and future prospects for this field.

Structures and magnetism of mono-palladium and mono-platinum doped Au25(PET)18nanoclusters

Herein we report the crystal structures of [Au₂₄Pt(PET)₁₈]⁰and [Au₂₄Pd(PET)₁₈]⁰as well as the main source of magnetism in [Au₂₅(PET)₁₈]⁰.

Silver migration between Au38(SC2H4Ph)24 and doped AgxAu38−x(SC2H4Ph)24 nanoclusters

A fast redistribution of metal atoms occurs upon collision between Ag_xAu_38−x and Au₃₈ nanoclusters in solution.

Insights into the effect of surface ligands on the optical properties of thiolated Au25nanoclusters

Ligand shell engineering of Au nanoclusters could induce their structural distortions for generating interesting optical properties.

X-Ray crystal structure, and optical and electrochemical properties of the Au15Ag3(SC6H11)14 nanocluster with a core-shell structure

We report the X-ray crystallographic structure of an 18-metal atom Au-Ag bimetallic nanocluster (NC) formulated as [Au15Ag3(SC6H11)14]. This NC consists of a Au6Ag3 bi-octahedral kernel, which is built up by two octahedral Au3Ag3 units through sharing one Ag3 triangular face. The [Au15Ag3(SC6H11)14] can be viewed as a core-shell structure with the doped Ag atoms as the core and Au atoms as the shell. Detailed analyses by UV-vis spectroscopy, X-ray photoelectron spectroscopy (XPS), and electrochemical measurements clearly show distinct differences in the electronic structure between [Au15Ag3(SC6H11)14] and the homometal [Au18(SC6H11)14] NC. This study contributes to the deep understanding on bimetallic nanoclusters.

Pd2Au36(SR)24 cluster: structure studies

The location of the Pd atoms in Pd2Au36(SC2H4Ph)24, is studied both experimentally and theoretically. X-ray photoelectron spectroscopy (XPS) indicates oxidized Pd atoms. Palladium K-edge extended X-ray absorption fine-structure (EXAFS) data clearly show Pd-S bonds, which is supported by far infrared spectroscopy and by comparing theoretical EXAFS spectra in R space and circular dichroism spectra of the staple, surface and core doped structures with experimental spectra.

Properties of the gold-sulphur interface: from self-assembled monolayers to clusters

The gold-sulphur interface of self-assembled monolayers (SAMs) was extensively studied some time ago. More recently tremendous progress has been made in the preparation and characterization of thiolate-protected gold clusters. In this feature article we address different properties of the two systems such as their structure, the mobility of the thiolates on the surface and other dynamical aspects, the chirality of the structures and characteristics related to it and their vibrational properties. SAMs and clusters are in the focus of different communities that typically use different experimental approaches to study the respective systems. However, it seems that the nature of the Au-S interfaces in the two cases is quite similar. Recent single crystal X-ray structures of thiolate-protected gold clusters reveal staple motifs characterized by gold ad-atoms sandwiched between two sulphur atoms. This finding contradicts older work on SAMs. However, newer studies on SAMs also reveal ad-atoms. Whether this finding can be generalized remains to be shown. In any case, more and more studies highlight the dynamic nature of the Au-S interface, both on flat surfaces and in clusters. At temperatures slightly above ambient thiolates migrate on the gold surface and on clusters. Evidence for desorption of thiolates at room temperature, at least under certain conditions, has been demonstrated for both systems. The adsorbed thiolate can lead to chirality at different lengths scales, which has been shown both on surfaces and for clusters. Chirality emerges from the organization of the thiolates as well as locally at the molecular level. Chirality can also be transferred from a chiral surface to an adsorbate, as evidenced by vibrational spectroscopy.

Gold tetrahedra coil up: Kekulé-like and double helical superstructures

Magic-sized clusters, as the intermediate state between molecules and nanoparticles, exhibit critical transitions of structures and material properties. We report two unique structures of gold clusters solved by x-ray crystallography, including Au40 and Au52 protected by thiolates. The Au40 and Au52 clusters exhibit a high level of complexity, with the gold atoms in the cluster first segregated into four-atom tetrahedral units-which then coil up into a Kekulé-like ring in the Au40 cluster and a DNA-like double helix in Au52. The solved structures imply a new "supermolecule" origin for revealing the stability of certain magic-sized gold clusters. The formation of supermolecular structures originates in the surface ligand bonding-induced stress and its propagation through the face-centered cubic (FCC) lattice. Moreover, the two structures reveal anisotropic growth of the FCC lattice in the cluster regime, which provides implications for the important roles of ligands at the atomic level. The rich structural information encoded in the Au40 and Au52 clusters provides atomic-scale insight into some important issues in cluster, nanoscale, and surface sciences.

Ion-precursor and ion-dose dependent anti-galvanic reduction

Controlling alloy nanoparticles with atomic monodispersity is challenging, and the recently revealed anti-galvanic reduction (AGR) provides a unique solution to this challenge. Herein we demonstrate that AGR is ion-precursor and ion-dose dependent, which offers novel strategies to tune the composition, structure and properties of nanoparticles by varying the ion-precursor and ion-dose in the AGR reaction.

X-ray Crystal Structure of Au38-xAgx(SCH2CH2Ph)24 Alloy Nanomolecules

Herein, we report the X-ray crystallographic structure of a 38-metal atom Au-Ag alloy nanomolecule. The structure of monometallic Au38(SR)24 consists of 2 central Au atoms and 21 Au atoms forming a bi-icosahedral core protected by 6 dimeric and 3 monomeric units. In Au38-xAgx(SR)24,where x ranges from 1 to 5, the silver atoms are selectively incorporated into the Au21 bi-icosahedral core. Within the Au21 core, the silver atoms preferentially occupy nine selected locations: (a) the two vertex edges, three atoms on each edge and six atoms total, and (b) the middle face-shared three-atom ring, adding to a total of nine locations. X-ray crystallography yielded a composition of Au34.04Ag3.96(SCH2CH2Ph)24. The crystal structure of the alloy nanomolecule can be described in terms of shells as Au2@Au17.04Ag3.96@ 6×[-SR-Au-SR-Au-SR] 3×[-SR-Au-SR-].

Optical properties of nanoalloys

Optical absorption spectra of bare (left) and monolayer-protected (right) metal nanoalloys.

Self-assembly of two high-nuclearity manganese calixarene-phosphonate clusters: diamond-like Mn16 and drum-like Mn14

Variation of the phosphonic acid “converts” a tetradecanuclear drum-like MnII14 cluster (1) into a hexadecanuclear diamond-like MnII16 cluster (2).