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The synthesis of atomically precise noble metal clusters using various protocols often results in a mixture of clusters with different cores. Hence, it is important to isolate such clusters in their pure form in terms of composition especially for crystallization. High-performance liquid chromatography (HPLC) is a powerful tool to achieve this. The interaction of ligands with column functionalities determine the extent of separation and their stability under conditions used. We demonstrate a systematic flow rate dependent study of three different aliphatic ligand protected Au25 clusters, with three commercially available alkyl and aryl functionalized reversed-phase HPLC columns, as they represent the variations encountered commonly. Molecular docking simulations were carried out to understand the interactions between the stationary phase and the cluster surface. These investigations enabled the selection of an appropriate column for better separation of structurally different ligand protected clusters. High-resolution separation of anionic and neutral Au25 clusters was acheived with a selectivity (α) of 1.2 by tuning the chromatographic conditions. This study would provide new insights in developing better methods for the efficient separation of monolayer protected clusters.

A detailed mass-spectrometric study of atomically precise monolayer-protected clusters revealed the potential application of such materials as mass-spectrometric standards, mostly in negative-ion mode and in the high-mass range. To date, very few molecules are known that can be efficiently ionized and detected at lower concentrations as negative ions with high signal intensities beyond m/ z 3000. Noble-metal clusters are molecules with definite masses, sizes, and shapes, which makes them excellent candidates to choose as standards over conventional low-molecular-weight polymers or clusters of ionic salts. They may be used as calibrants in all possible modes, including tandem mass spectrometry and ion mobility. With the advancement in materials science, more and more molecules are being added to the list that are inherently negatively charged in solution and can be examined by mass spectrometry. In this report, we demonstrate the use of three such model cluster systems for their potential to calibrate mass spectrometers in negative-ion mode. This idea can be extended to many other clusters known so far to achieve calibration in extended mass ranges.

[Au25(SR)18]− nanoclusters have been tested as a model catalyst in multiple oxidation reactions involving CO, alkenes, cyclohexane, and alcohols. Oxygen is used as an oxidizing agent in most of the...

Reactivity of monolayer protected atomically precise clusters of noble metals is of significant research interest. To date very few experimental data are available on the reaction thermodynamics of such clusters. Here we report a calorimetric study of the reaction of glutathione (GSH) protected silver clusters in the presence of excess ligand, GSH, using isothermal titration calorimetry (ITC). We have studied Ag11(SG)7 and Ag32(SG)19 clusters and compared their reactivity with GSH protected silver nanoparticles (AgNPs) and silver ions. Clusters show intermediate reactivity toward excess ligand compared to nanoparticles and silver ions. Several control experiments were performed to understand the degradation mechanism of these silver clusters and nanoparticles. The effect of dissolved oxygen in the degradation process was studied in detail, and it was found that it did not have a significant role, although alternate pathways of degradation with the involvement of oxygen cannot be ruled out. Direct confirma...

Experimental evidence for the existence of gas phase isomers in monolayer protected noble metal clusters is presented, taking Ag44(SR)30 (SR = 4-fluorothiophenol, p-mercaptobenzoic acid) and Ag29(BDT)12 (BDT: benzene dithiol) clusters as examples which do not show any isomeric structures in their crystals. Electrospray ionization coupled with ion mobility separation allowed for the identification of multiple isomers of Ag44SR30 cluster in its 3– and 4– charge states, their most abundant gas phase ions. Ag29(BDT)12 showed isomerism in its common 3– charge state. Isomerism is likely to be due to different types of ligand orientations in the staples leading to changes in the overall size and shape of the cluster ions, which was further confirmed by density functional theory calculations on Ag44(FTP)304–. No isomers were seen in the ions of the well-known cluster, Au25SR18 (SR = phenylethanethiol, dodecanethiol, and butanethiol).

An intercluster reaction between Au25(PET)18 and Ir9(PET)6 producing the alloy cluster, Au22Ir3(PET)18 exclusively, is demonstrated where the ligand PET is 2-phenylethanethiol. Typical reactions of this kind between Au25(PET)18 and Ag25(SR)18, and other clusters reported previously, produce mixed cluster products. The cluster composition was confirmed by detailed high-resolution electrospray ionization mass spectrometry (ESI MS) and other spectroscopic techniques. This is the first example of Ir metal incorporation in a monolayer-protected noble metal cluster. The formation of a single product was confirmed by thin layer chromatography (TLC). Density functional theory (DFT) calculations suggest that the most favorable geometry of the Au22Ir3(PET)18 cluster is one wherein the three Ir atoms are arranged triangularly with one Ir atom at the icosahedral core and the other two on the icosahedral shell. Significant contraction of the metal core was observed due to strong Ir–Ir interactions.

The synthesis of atomically precise noble metal clusters using various protocols often results in a mixture of clusters with different cores. Hence, it is important to isolate such clusters in their pure form in terms of composition especially for crystallization. High-performance liquid chromatography (HPLC) is a powerful tool to achieve this. The interaction of ligands with column functionalities determine the extent of separation and their stability under conditions used. We demonstrate a systematic flow rate dependent study of three different aliphatic ligand protected Au25 clusters, with three commercially available alkyl and aryl functionalized reversed-phase HPLC columns, as they represent the variations encountered commonly. Molecular docking simulations were carried out to understand the interactions between the stationary phase and the cluster surface. These investigations enabled the selection of an appropriate column for better separation of structurally different ligand protected clusters. High-resolution separation of anionic and neutral Au25 clusters was acheived with a selectivity (α) of 1.2 by tuning the chromatographic conditions. This study would provide new insights in developing better methods for the efficient separation of monolayer protected clusters.

Highly organized crossed bilayer assemblies of nanowires (NWs) are made using directed hydrogen bonding between the protecting ligand shells of atomically precise cluster molecules anchored on NWs. Layers of quantum clusters remain sandwiched between two neighboring NWs at a defined distance, dictated by the core-size of the cluster, while the orientation of the ligands in space dictates the interlayer geometry.

The first atomically precise and monolayer protected iridium cluster in solution, Ir9(PET)6 (PET – 2-phenyethanethiol) was synthesized via a solid state method. The absence of a plasmonic band at ∼350 nm, expected in the UV/Vis spectra for spherical Ir particles of 10 nm size indicated that the synthesized cluster is smaller than this dimension. Small angle X-ray scattering (SAXS) showed that the cluster has a particle size of ∼2 nm in solution which was confirmed by transmission electron microscopy (TEM). The blue emission of the cluster is much weaker than many noble metal clusters investigated so far. X-ray photoelectron spectroscopy (XPS) measurements showed that all Ir atoms of the cluster are close to the zero oxidation state. The characteristic S–H vibrational peak of PET at 2560 cm−1 was absent in the FT-IR spectrum of the cluster indicating RS–Ir bond formation. The molecular formula of the cluster, Ir9(PET)6 was assigned based on the most significant peak at m/z 2553 in the matrix assisted laser desorption ionization mass spectrum (MALDI MS), measured at the threshold laser intensity. Density functional theory calculations on small Ir@SCH3 and Ir@PET clusters and comparison of the predictions with the IR and 1H-NMR spectra of Ir9(PET)6 suggested that the PET ligands have two distinct structural arrangements and are likely to be present as bridging thiolates –(Ir–SR–Ir)– and singly attached thiolates –(Ir–SR).

An intercluster reaction between Au

We present the first example of dimer formation in the monolayer protected atomically precise cluster system, Au25(SR)18, using ion mobility mass spectrometry. These transient species are shown to be important in explaining chemical reactivity between clusters.

Enormous developments in the area of soluble noble metal clusters protected with monolayers are discussed. Mass spectrometry has been the principal tool with which cluster growth has been examined. The composition and chemistry of clusters have been examined extensively by mass spectrometry. Besides gold, silver, platinum, copper and iron clusters have been examined. Clusters have also been examined by tandem mass spectrometry and the importance of ligands in understanding closed shell electronic structure is understood from such studies. Protein protected noble metal clusters belong to a new group in this family of materials. Naked metal clusters bearing the same core composition as that of monolayer protected clusters is another class in this area, which have been discovered by laser desorption ionization from protein templates.

A blue luminescent 11-atom platinum cluster showing step-like optical features and the absence of plasmon absorption was synthesized. The cluster was purified using high performance liquid chromatography (HPLC). Electrospray ionization (ESI) and matrix assisted laser desorption ionization (MALDI) mass spectrometry (MS) suggest a composition, Pt11(BBS)8, which was confirmed by a range of other experimental tools. The cluster is highly stable and compatible with many organic solvents. This journal is � the Partner Organisations 2014.