Gas phase ion chemistry of titanium-oxofullerene with ligated solvents.

We investigated the gas phase fragmentation events of highly symmetric fullerene-like (FN-like) titanium oxo-cluster anions, [H12Ti42O60(OCH3)42(HOCH3)10(H2O)2]2- (1) and [H7Ti42O60(OCH3)42(HOCH3)10(H2O)3]1- (2). These oxo-clusters contain a closed cage Ti42O60 core, protected by a specific number of methoxy, methanol, and water molecules acting as ligands. These dianionic and monoanionic species were generated in the gas phase by electrospray ionization of the H6[Ti42(µ3-O)60(OiPr)42(OH)12] (TOF) cluster in methanol. Collision induced dissociation studies of 1 revealed that upon increasing the collision energy, the protecting ligands were stripped off first, and [Ti41O58]2- was formed as the first fragment from the Ti42O60 core. Thereafter, systematic TiO2 losses were observed giving rise to subsequent fragments like [Ti40O56]2-, [Ti39O54]2-, [Ti38O52]2-, etc. Similar fragments were also observed for monoanionic species 2 as well. Systematic 23 TiO2 losses were observed, which were followed by complete shattering of the cage. We also carried out computational studies using density functional theory (DFT) to investigate the structures and fragmentation mechanism. The fragmentation of TOF was comparable to the fragmentation of C60 ions, where systematic C2 losses were observed. We believe that this is a consequence of topological similarity. The present study provides valuable insights into the structural constitution of TOF clusters and stability of the parent as well as the resulting cage-fragments in the gas phase.

Metal-Ligand Interface in the Chemical Reactions of Ligand-Protected Noble Metal Clusters.

We discuss the role of the metal-ligand (M-L) interfaces in the chemistry of ligand-protected, atomically precise noble metal clusters, a new and expanding family of nanosystems, in solution as well as in the gas phase. A few possible mechanisms by which the structure and dynamics of M-L interfaces could trigger intercluster exchange reactions are presented first. How interparticle chemistry can be a potential mechanism of Ostwald ripening, a well-known particle coarsening process, is also discussed. The reaction of Ag59(2,5-DCBT)32 (DCBT = dichlorobenzenethiol) with 2,4-DCBT leading to the formation of Ag44(2,4-DCBT)30 is presented, demonstrating the influence of the ligand structure in ligand-induced chemical transformations of clusters. We also discuss the structural isomerism of clusters such as Ag44(SR)30 (-SR = alkyl/aryl thiolate) in the gas phase wherein the occurrence of isomerism is attributed to the structural rearrangements in the M-L bonding network. Interfacial bonding between Au25(SR)18 clusters leading to the formation of cluster dimers and trimers is also discussed. Finally, we show that the desorption of phosphine and hydride ligands on a silver cluster, [Ag18(TPP)10H16]2+ (TPP = triphenylphosphine) in the gas phase, leads to the formation of a naked silver cluster of precise nuclearity, such as Ag17+. We demonstrate that the nature of the M-L interfaces, i.e., the oxidation state of metal atoms, structure of the ligand, M-L bonding network, and so forth, plays a key role in the chemical reactivity of clusters. The structure, dynamics, and chemical reactivity of nanosystems in general are to be explored together to obtain new insights into their emerging science.

Camouflaging Structural Diversity: Co-crystallization of Two Different Nanoparticles Having Different Cores But the Same Shell.

Two ligand-protected nanoscale silver moieties, [Ag46 (SPhMe2 )24 (PPh3 )8 ](NO3 )2 and [Ag40 (SPhMe2 )24 (PPh3 )8 ](NO3 )2 (abbreviated as Ag46 and Ag40 , respectively) with almost the same shell but different cores were synthesized simultaneously. As their external structures are identical, the clusters were not distinguishable and become co-crystallized. The occupancy of each cluster was 50 %. The outer shell of both is composed of Ag32 S24 P8 , which is reminiscent of fullerenes, and it encapsulates a well-studied core, Ag14 and a completely new core, Ag8 , which correspond to a face-centered cube and a simple cube, respectively, resulting in the Ag46 and Ag40 clusters. The presence of two entities (Ag40 and Ag46 clusters) in a single crystal and their molecular formulae were confirmed by detailed electrospray ionization mass spectrometry. The optical spectrum of the mixture showed unique features which were in good agreement with the results from time-dependent density functional theory (TD-DFT).

Isomerism in Supramolecular Adducts of Atomically Precise Nanoparticles.

We present isomerism in a few supramolecular adducts of atomically precise nanoparticles, [Ag29(BDT)12∩(CD) n]3- ( n = 1-6), abbreviated as I where BDT and CD are 1,3-benzenedithiol and cyclodextrins (α, ß and γ), respectively; ∩ symbolizes an inclusion complex. The different host-guest complexes of I were characterized in the solution state as well as in the gas phase. The CDs (α, ß and γ) encapsulate a pair of BDT ligands protecting the Ag29 core. This unique geometry of the supramolecular adducts makes the system similar to octahedral complexes of transition metals, which manifest various isomers. These isomers of I ( n = 2-4) were separated by ion mobility mass spectrometry (IM MS). We proposed structures of all the inclusion complexes with the help of IM MS measurements and molecular docking, density functional theory (DFT), and collision cross section (CCS) calculations.

Bent Keto Form of Curcumin, Preferential Stabilization of Enol by Piperine, and Isomers of Curcumin∩Cyclodextrin Complexes: Insights from Ion Mobility Mass Spectrometry.

A detailed examination of collision cross sections (CCSs) coupled with computational methods has revealed new insights into some of the key questions centered around curcumin, one of the most intensively studied natural therapeutic agents. In this study, we have distinguished the structures and conformers of the well-known enol and the far more elusive keto form of curcumin by using ion mobility mass spectrometry (IM MS). The values of the theoretically predicted isomers were compared with the experimental CCS values to confirm their structures. We have identified a bent structure for the keto form and the degree of bending was estimated. Using IM MS, we have also shown that ESI MS reflects the solution phase structures and their relative populations, in this case. Piperine, a naturally occurring heterocyclic compound, is known to increase the bioavailability of curcumin. However, it is still not clearly understood which tautomeric form of curcumin is better stabilized by it. We have identified preferential stabilization of the enol form in the presence of piperine using IM MS. Cyclodextrins (CDs) are used as well-known carriers in the pharmaceutical industry for increasing the stability, solubility, bioavailability, and tolerability of curcumin. However, the crystal structures of supramolecular complexes of curcumin∩CD are unknown. We have determined the structures of different isomers of curcumin∩CD (α- and ß-CD) complexes by comparing the CCSs of theoretically predicted structures with the experimentally obtained CCSs, which will further help in understanding the specific role of the structures involved in different biological activities.

Interparticle Reactions: An Emerging Direction in Nanomaterials Chemistry.

Nanoparticles exhibit a rich variety in terms of structure, composition, and properties. However, reactions between them remain largely unexplored. In this Account, we discuss an emerging aspect of nanomaterials chemistry, namely, interparticle reactions in solution phase, similar to reactions between molecules, involving atomically precise noble metal clusters. A brief historical account of the developments, starting from the bare, gas phase clusters, which led to the synthesis of atomically precise monolayer protected clusters in solution, is presented first. Then a reaction between two thiolate-protected, atomically precise noble metal clusters, [Au25(PET)18]- and [Ag44(FTP)30]4- (PET = 2-phenylethanethiol, FTP = 4-fluorothiophenol), is presented wherein these clusters spontaneously exchange metal atoms, ligands, and metal-ligand fragments between them under ambient conditions. The number of exchanged species could be controlled by varying the initial compositions of the reactant clusters. Next, a reaction of [Au25(PET)18]- with its structural analogue [Ag25(DMBT)18]- (DMBT = 2,4-dimethylbenzenethiol) is presented, which shows that atom-exchange reactions happen with structures conserved. We detected a transient dianionic adduct, [Ag25Au25(DMBT)18(PET)18]2-, formed between the two clusters indicating that this adduct could be a possible intermediate of the reaction. A reaction involving a dithiolate-protected cluster, [Ag29(BDT)12]3- (BDT = 1,3-benzenedithiol), is also presented wherein metal atom exchange alone occurs, but with no ligand and fragment exchanges. These examples demonstrate that the nature of the metal-thiolate interface, that is, its bonding network and dynamics, play crucial roles in dictating the type of exchange processes and overall rates. We also discuss a recently proposed structural model of these clusters, namely, the Borromean ring model, to understand the dynamics of the metal-ligand interfaces and to address the site specificity and selectivity in these reactions. In the subsequent sections, reactions involving atomically precise noble metal clusters and one- and two-dimensional nanosystems are presented. We show that highly protected, stable clusters such as [Au25(PET)18]- undergo chemical transformation on graphenic surfaces to form a bigger cluster, Au135(PET)57. Finally, we present the transformation of tellurium nanowires (Te NWs) to Ag-Te-Ag dumbbell nanostructures through a reaction with an atomically precise silver cluster, Ag32(SG)19 (SG = glutathione thiolate). The starting materials and the products were characterized using high resolution electrospray ionization mass spectrometry, matrix assisted laser desorption ionization mass spectrometry, UV/vis absorption, luminescence spectroscopies, etc. We have analyzed principally mass spectrometric data to understand these reactions. In summary, we present the emergence of a new branch of chemistry involving the reactions of atomically precise cluster systems, which are prototypical nanoparticles. We demonstrate that such interparticle chemistry is not limited to metal clusters; it occurs across zero-, one-, and two-dimensional nanosystems leading to specific transformations. We conclude this Account with a discussion of the limitations in understanding of these reactions and future directions in this area of nanomaterials chemistry.

Atomically Precise Nanocluster Assemblies Encapsulating Plasmonic Gold Nanorods.

The self-assembled structures of atomically precise, ligand-protected noble metal nanoclusters leading to encapsulation of plasmonic gold nanorods (GNRs) is presented. Unlike highly sophisticated DNA nanotechnology, this strategically simple hydrogen bonding-directed self-assembly of nanoclusters leads to octahedral nanocrystals encapsulating GNRs. Specifically, the p-mercaptobenzoic acid (pMBA)-protected atomically precise silver nanocluster, Na4 [Ag44 (pMBA)30 ], and pMBA-functionalized GNRs were used. High-resolution transmission and scanning transmission electron tomographic reconstructions suggest that the geometry of the GNR surface is responsible for directing the assembly of silver nanoclusters via H-bonding, leading to octahedral symmetry. The use of water-dispersible gold nanoclusters, Au≈250 (pMBA)n and Au102 (pMBA)44 , also formed layered shells encapsulating GNRs. Such cluster assemblies on colloidal particles are a new category of precision hybrids with diverse possibilities.

Intercluster Reactions between Au25(SR)18 and Ag44(SR)30.

We present the first example of intercluster reactions between atomically precise, monolayer protected noble metal clusters using Au25(SR)18 and Ag44(SR)30 (RS- = alkyl/aryl thiolate) as model compounds. These clusters undergo spontaneous reaction in solution at ambient conditions. Mass spectrometric measurements both by electrospray ionization and matrix assisted laser desorption ionization show that the reaction occurs through the exchange of metal atoms and protecting ligands of the clusters. Intercluster alloying is demonstrated to be a much more facile method for heteroatom doping into Au25(SR)18, as observed by doping up to 20 Ag atoms. We investigated the thermodynamic feasibility of the reaction using DFT calculations and a tentative mechanism has been presented. Metal core-thiolate interfaces in these clusters play a crucial role in inducing these reactions and also affect rates of these reactions. We hope that our work will help accelerate activities in this area to establish chemistry of monolayer protected clusters.

Rapid isotopic exchange in nanoparticles.

Rapid solution-state exchange dynamics in nanoscale pieces of matter is revealed, taking isotopically pure atomically precise clusters as examples. As two isotopically pure silver clusters made of 107Ag and 109Ag are mixed, an isotopically mixed cluster of the same entity results, similar to the formation of HDO, from H2O and D2O. This spontaneous process is driven by the entropy of mixing and involves events at multiple time scales.

A covalently linked dimer of [Ag25(DMBT)18].

We report the first example of a covalently bound dimer of monolayer protected atomically precise silver nanocluster [Ag25(DMBT)18]- (DMBT stands for 2,4-dimethylbenzenethiol). Covalently linked dimers could be important to design new cluster assembled materials with composite properties.

Fullerene-Functionalized Monolayer-Protected Silver Clusters: [Ag29(BDT)12(C60) n]3- ( n = 1-9).

We report the formation of supramolecular adducts between monolayer-protected noble metal nanoclusters and fullerenes, specifically focusing on a well-known silver cluster, [Ag29(BDT)12]3-, where BDT is 1,3-benzenedithiol. We demonstrate that C60 molecules link with the cluster at specific locations and protect the fragile cluster core, enhancing the stability of the cluster. A combination of studies including UV-vis, high-resolution electrospray ionization mass spectrometry, collision-induced dissociation, and nuclear magnetic resonance spectroscopy revealed structural details of the fullerene-functionalized clusters, [Ag29(BDT)12(C60) n]3- ( n = 1-9). Density functional theory (DFT) calculations and molecular docking simulations affirm compatibility between the cluster and C60, resulting in its attachment at specific positions on the surface of the cluster, stabilized mainly by π-π and van der Waals interactions. The structures have also been confirmed from ion mobility mass spectrometry by comparing the experimental collision cross sections (CCSs) with the theoretical CCSs of the DFT-optimized structures. The gradual evolution of the structures with an increase in the number of fullerene attachments to the cluster has been investigated. Whereas the structure for n = 4 is tetrahedral, that of n = 8 is a distorted cube with a cluster at the center and fullerenes at the vertices. Another fullerene, C70, also exhibited similar behavior. Modified clusters are expected to show interesting properties.

Manifestation of Geometric and Electronic Shell Structures of Metal Clusters in Intercluster Reactions.

Monolayer protected clusters exhibit rich diversity in geometric and electronic structures. However, structure-reactivity relationships in these clusters are rarely explored. In this context, [Ag44(SR)30]4-, where -SR is an alkyl/aryl thiolate, is an interesting system due to its geometrically and electronically closed-shell structures and distinct charge states. We demonstrate that these structural features of [Ag44(SR)30]4- are distinctly manifested in its solution-state reaction with another cluster, [Au25(SR)18]-. Through this reaction, an alloy cluster anion, [Au12Ag32(SR)30]4-, evolves spontaneously as revealed by high-resolution electrospray ionization mass spectrometry. Ultraviolet-visible absorption spectroscopy and density functional theory calculations indicate that [Au12Ag32(SR)30]4- is formed by the substitution of all of the Ag atoms in the innermost icosahedral shell of [Ag44(SR)30]4- and the abundance is attributed to its higher stability due to closed geometric as well as electronic shell structure, similar to the reactant clusters. We further demonstrate that the substitution of metal atoms in the middle dodecahedral shell and the outermost mount sites are also possible, however such substitutions produce AuxAg44-x(SR)30 alloy clusters with geometrically and electronically open shells. Depending on specific sites of substitution, an unexpected superatom-nonsuperatom transition occurs in the distribution of AuxAg44-x(SR)30 alloy clusters formed in this reaction. Our results present a unique example of a structure-reactivity relationship in the metal atom substitution chemistry of monolayer protected clusters, wherein a systematic trend, reflecting the geometric and the electronic shell structures of the reactant as well as the product clusters, was observed.

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.

Structure-conserving spontaneous transformations between nanoparticles.

Ambient, structure- and topology-preserving chemical reactions between two archetypal nanoparticles, Ag25(SR)18 and Au25(SR)18, are presented. Despite their geometric robustness and electronic stability, reactions between them in solution produce alloys, AgmAun(SR)18 (m+n=25), keeping their M25(SR)18 composition, structure and topology intact. We demonstrate that a mixture of Ag25(SR)18 and Au25(SR)18 can be transformed to any arbitrary alloy composition, AgmAun(SR)18 (n=1-24), merely by controlling the reactant compositions. We capture one of the earliest events of the process, namely the formation of the dianionic adduct, (Ag25Au25(SR)36)2-, by electrospray ionization mass spectrometry. Molecular docking simulations and density functional theory (DFT) calculations also suggest that metal atom exchanges occur through the formation of an adduct between the two clusters. DFT calculations further confirm that metal atom exchanges are thermodynamically feasible. Such isomorphous transformations between nanoparticles imply that microscopic pieces of matter can be transformed completely to chemically different entities, preserving their structures, at least in the nanometric regime.

Possible isomers in ligand protected Ag11 cluster ions identified by ion mobility mass spectrometry and fragmented by surface induced dissociation.

This communication reports the identification of gas phase isomers in monolayer-protected silver clusters. Two different isomers of Ag11(SG)7(-) (SG-gulathione thiolate) with different drift times have been detected using combined electrospray ionization (ESI) and ion mobility (IM) mass spectrometry (MS). Surface induced dissociation (SID) of the 3(-) charge state of such clusters shows charge stripping to give the 1(-) charged ion with some sodium attachment, in addition to fragmentation. SID and collision induced dissociation (CID) for Ag11(SG)7(-) suggest different pathways being accessed with each method. SID was introduced for the first time for the study of monolayer-protected clusters.

[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.

Supramolecular functionalization and concomitant enhancement in properties of Au(25) clusters.

We present a versatile approach for tuning the surface functionality of an atomically precise 25 atom gold cluster using specific host-guest interactions between ß-cyclodextrin (CD) and the ligand anchored on the cluster. The supramolecular interaction between the Au25 cluster protected by 4-(t-butyl)benzyl mercaptan, labeled Au25SBB18, and CD yielding Au25SBB18∩CDn (n = 1, 2, 3, and 4) has been probed experimentally using various spectroscopic techniques and was further analyzed by density functional theory calculations and molecular modeling. The viability of our method in modifying the properties of differently functionalized Au25 clusters is demonstrated. Besides modifying their optoelectronic properties, the CD moieties present on the cluster surface provide enhanced stability and optical responses which are crucial in view of the potential applications of these systems. Here, the CD molecules act as an umbrella which protects the fragile cluster core from the direct interaction with many destabilizing agents such as metal ions, ligands, and so on. Apart from the inherent biocompatibility of the CD-protected Au clusters, additional capabilities acquired by the supramolecular functionalization make such modified clusters preferred materials for applications, including those in biology.

Probing molecular solids with low-energy ions.

Ion/surface collisions in the ultralow- to low-energy (1-100-eV) window represent an excellent technique for investigation of the properties of condensed molecular solids at low temperatures. For example, this technique has revealed the unique physical and chemical processes that occur on the surface of ice, versus the liquid and vapor phases of water. Such instrument-dependent research, which is usually performed with spectroscopy and mass spectrometry, has led to new directions in studies of molecular materials. In this review, we discuss some interesting results and highlight recent developments in the area. We hope that access to the study of molecular solids with extreme surface specificity, as described here, will encourage investigators to explore new areas of research, some of which are outlined in this review.