Ligand structure and charge state-dependent separation of monolayer protected Au25 clusters using non-aqueous reversed-phase HPLC.

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.

Monolayer-Protected Noble-Metal Clusters as Potential Standards for Negative-Ion Mass Spectrometry.

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.

Au22Ir3(PET)18: An Unusual Alloy Cluster through Intercluster Reaction.

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.

[Au25(SR)18]2(2-): a noble metal cluster dimer in the gas phase.

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.

Cluster-Mediated Crossed Bilayer Precision Assemblies of 1D Nanowires.

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.

Blue emitting undecaplatinum clusters.

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.