Reactivity and thermal stability of oxidized copper clusters on the tantalum(V) oxide surface

Oxidized copper clusters 2–5 nm in size have been obtained by RF discharge sputtering of a copper wire in an oxygen atmosphere. Isolated CuO clusters or, at long deposition times, their agglomerates form on the support. The thermal stability of the oxidized nanoparticles in a vacuum and their reactivity toward CO in relation to the deposition time have been investigated by X-ray photoelectron spectroscopy. The asprepared clusters show low reactivity (10^?7?10^?9), but their activation by reduction and subsequent reoxidation in an oxygen medium raises their reactivity to ～10^?5. This is due to the appearance of weakly charged oxygen species on the surface. The reactivity of the CuO clusters has been compared to the reactivity of earlier studied nanosized copper oxide model objects.     

Reactivities and biological functions of iron-sulfur clusters

Iron-sulfur clusters are prevalent in biological systems. Through studies of iron-sulfur proteins and synthetic model clusters, it was realized early on that these clusters functioned as facile electron transfer agents. Until recently it was widely thought that they served exclusively in that capacity. However, in the last decade, it has become clear that their reactivities and biological functions are much more diverse. It is now apparent that these clusters can serve as the active sites of enzymes, as well as in the regulation of enzymatic activity. Synthetic clusters, which have been shown to undergo a variety of core rearrangements or structural changes, have provided insight into possible mechanisms of cluster formation or activity regulation in enzymes. Rigid tripodal ligands have been constructed which capture synthetic iron-sulfur clusters in a cavity which permits controlled reactivity studies. In this article, we review these recent developments and suggest some future directions the field may take.     

Metal–Carbonyl Clusters with Alkynes Bound in “Parallel” or “Perpendicular” Fashion

Alkyne-substituted tri- and polynuclear clusters have been known for a long time. Among these, the derivatives with acetylene substituents coordinated parallel to one edge of a trimetallic cluster represent the majority. Clusters with alkynes perpendicular to one edge of a trimetallic cluster are not very common. The reactivity of these clusters is discussed in this paper; particular attention is devoted to the chemical transformations which could occur during catalytic reactions. Some examples of the behavior of these clusters in homogeneous and solid–gas hydrogenation catalysis are given.     

Platinum Group Metal Clusters: From Gas‐Phase Structures and Reactivities towards Model Catalysts

Transition‐metal clusters have long been proposed as model systems to study heterogeneous catalysts. In this Concept article we show how advanced spectroscopic techniques can be used to determine the structures of gas‐phase transition‐metal clusters and their complexes with small molecules. Combined with computational studies, this can help to develop an understanding of the reactivity of these catalytic models.     

The N-heterocyclic carbene chemistry of transition-metal carbonyl clusters

In the last decade, chemists have dedicated many efforts to investigate the coordination chemistry of N-heterocyclic carbenes (NHCs). Although most of that research activity has been devoted to mononuclear complexes, transition-metal carbonyl clusters have not escaped from these investigations. This critical review, which is focussed on the reactivity of NHCs (or their precursors) with transition-metal carbonyl clusters (mostly are of ruthenium and osmium) and on the transformations underwent by the NHC-containing species initially formed in those reactions, shows that the polynuclear character of these metallic compounds or, more precisely, the close proximity of one or more metal atoms to that which is or can be attached to the NHC ligand, is responsible for reactivity patterns that have no parallel in the NHC chemistry of mononuclear complexes (74 references).     

Do hydroxyl radical-water clusters, OH(H2O)n, n = 1-5, exist in the atmosphere?

It has been speculated that the presence of OH(H2O)n clusters in the troposphere could have significant effects on the solar absorption balance and the reactivity of the hydroxyl radical. We have used the G3 and G3B3 model chemistries to model the structures and predict the frequencies of hydroxyl radical/water clusters containing one to five water molecules. The reaction between hydroxyl radical clusters and methane was examined as a function of water cluster size to gain an understanding of how cluster size affects the hydroxyl radical reactivity.     

STUDIES ON SYNTHESIS, STRUCTURE AND REACTIVITY OF ACTIVATED Os_6 RAFT-LIKE CLUSTERS

In this paper we report the synthesis and activation of OS-6 raft-llke planar clusters. The activated OS-66 raft-like clusters [OS-66(CO)_(20)(MeCN)] and [OS-66(CO)_(19) (MeCN)_2] have been prepared by involving PdCl_2 as catalyst. Trimethylamine oxide (Me_3NO) has been used as an activation reagent to oxidize several carbonyl ligands coordinated in the activated clusters and to get a series of activated OS-66 raft-like clusters [OS-66(CO)_(21-x)(MeCN)_x] (n = 1--5) and corresponding substituted clusters [OS-66(CO)_(21-x)(P(OMe)_3)_x] (n=1--5). The structures of these OS-66 raft-like clusters have been characterized by the infrared spectra (IR), nuclear magnetic resonance spectra (NMR) and mass spectra (MS), and the characters of the raft-like planar structure and reactivity above have also been discussed in this paper.     

Structural, electronic, and chemical properties of multiply iodized aluminum clusters

The electronic structure, stability, and reactivity of iodized aluminum clusters, which have been investigated via reactivity studies, are examined by first-principles gradient corrected density functional calculations. The observed behavior of Al13I(x)- and Al14I(x)- clusters is shown to indicate that for x < or = 8, they consist of compact Al13- and Al14++ cores, respectively, demonstrating that they behave as halogen- or alkaline earth-like superatoms. For x > 8, the Al cores assume a cagelike structure associated with the charging of the cores. The observed mass spectra of the reacted clusters reveal that Al13I(x)- species are more stable for even x while Al14I(x)- exhibit enhanced stability for odd x(x > or = 3). It is shown that these observations are linked to the formation and filling of "active sites," demonstrating a novel chemistry of superatoms.     

Ion-molecule reactions and fragmentation patterns in helium nanodroplets

A study has been made of the ion chemistry of a series of small molecules that have been embedded in helium nanodroplets. In most instances, the molecules H2O, SO2, CO2, CH3OH, C2H5OH, C3H7OH, CH3F, and CH3Cl have been allowed to form clusters, and reactivity within these has been initiated through electron impact ionization. For two of the molecules studied, CF2Cl2 and CF3I, reactivity is believed to originate from single molecules embedded in the droplets. Electron impact on the droplets is thought to first create a helium ion, and formation of molecular ions is then assumed to proceed via a charge hopping mechanism that propagates though the droplet and terminates with charge-transfer to a molecule or cluster. The chemistry exhibited by many of the cluster ions and at least one of the single molecular ions is very different from that observed for the same species in isolation. In most cases, reactivity appears to be dominated by high-energy bond breaking processes as opposed to, in the case of the clusters, ion-molecule reactions. Overall, charge-transfer from He+ does not appear to be a "soft" ionization mechanism.     

Quantum chemical study of IrFn (n = 1–7) clusters: An investigation of superhalogen properties

In present investigation, the interactions of iridium (Ir) atom with fluorine (F) atoms have been studied using the density functional theory. Up to seven F atoms were able to bind to a single Ir atom which resulted in increase of electron affinities successively, reaching a peak value of 7.85 eV for IrF₇. The stability and reactivity of these clusters were analyzed by calculating highest occupied molecular orbital (HOMO)–LUMO gaps, molecular orbitals and binding energies of these clusters. The unusual properties of these clusters are due to the involvement of inner shell 5d‐electrons, which not only allows IrF_n clusters to belong to the class of superhalogens but also shows that its valence can exceed the nominal value of 2. © 2012 Wiley Periodicals, Inc.     

Coadsorption of CO and O(2) on selected gold clusters: evidence for efficient room-temperature CO(2) generation

Spurred by the recent demonstrations of the size- and support-dependent reactivity of supported gold clusters, here we present results on the coadsorption of CO and O(2) on selected anionic gold clusters, Au(N)(-), in the gas phase. O(2) adsorbs in a binary (0,1) fashion as a one-electron acceptor on the Au(N)()(-) clusters, with even-N clusters showing varying reactivity toward O(2) adsorption, while odd-N clusters show no evidence of reactivity. CO shows a highly size-dependent reactivity for Au(N)(-) sizes from N = 4 to 19, but no adsorption on the gold dimer or trimer. When the gold clusters are exposed to both reactants, either simultaneously or sequentially, interesting effects have been observed. While the same rules pertaining to individual O(2) or CO adsorption continue to apply, the preadsorption of one reactant on a cluster may lead to the increased reactivity of the cluster to the other reactant. Thus, the adsorbates are not competing for bonding sites (competitive coadsorption), but, instead, aid in the adsorption of one another (cooperative coadsorption). New peaks also arise in the mass spectrum of Au(6)(-) under CO and O(2) coadsorption conditions, which can be attributed to the loss of a CO(2) molecule (or molecules). By studying the relative amount of reaction, and relating it to the reaction time, it is found that the gas-phase Au(6) anion is capable of oxidizing CO at a rate 100 times that reported for commercial or model gold catalysts.     

Dramatic size effects and evidence of structural isomers in the reactions of rhodium clusters, Rhn+/-, with nitrous oxide

The reactions of gas phase rhodium clusters, Rhn+/- (n<30), with nitrous oxide, N2O, have been investigated under single collision conditions by Fourier transform ion cyclotron resonance mass spectrometry. The only significant reaction observed is the sequential generation of oxides. Absolute rate constants for the reactions of all clusters have been determined and, in the case of the cationic clusters especially, they exhibit large fluctuations as a function of cluster size with local minima observed for n=5, 19, 28. Striking similarities are observed with the variation in rate constants for these clusters in reactions with small hydrocarbons (C. Adlhart and E. Uggerud, J. Chem. Phys., 2005, 123, 214709). Corresponding size effects are also observed but are less marked in the reactions of the anionic clusters. The reactions of several clusters exhibit marked deviations from simple pseudo-first-order kinetics suggesting the presence of multiple isomeric forms: Rh11+, Rh12+ and Rh8- exhibit characteristic biexponential decays which are interpreted in terms of the existence of different structural forms of the cluster which have markedly different reactivity. By contrast, Rh6+, Rh7+ and Rh8+ show rates which apparently increase with time, probably due to collisional activation. Thermalisation of the clusters prior to reaction by exposure to pulses of argon results in changes to the kinetics of these anomalous systems which can be explained in terms of collision induced isomerisation.     

Chalcogen-Containing Manganese Carbonyl Clusters: Synthesis and Structural Transformations

Recently, we have developed several synthetic routes to the new classes of chalcogen-containing manganese carbonyl clusters, and the interesting structural transformations and reactivity of the resultant clusters have been investigated as well. In this short review, the syntheses and bonding modes of the sulfur, selenium, and tellurium-containing manganese carbonyl complexes reported by our laboratory will be presented, and the cluster growth and transformation will be systematically compared and discussed.     

Borohydride, azide, and chloride anions as terminal ligands on Fe/Mo/S clusters. Synthesis, structure and characterization of [(Cl4-cat)(PPr3) MoFe3S4(X)2]2(Bu4N)4 and [(Cl4-cat)(PPr3)MoFe3S4 (PPr3)(X)]2(Bu4N)2 (X = N3-, BH4-, Cl-) double-fused cubanes. NMR reactivity studies of [(Cl4-cat)(PPr3) MoFe3S4(BH4)2]2(Bu4N)4

Our explorations of the reactivity of Fe/Mo/S clusters of some relevance to the FeMoco nitrogenase have led to new double-fused cubane clusters with the Mo2Fe6S8 core as derivatives of the known (Cl4-cat)2Mo2Fe6S8(PPr3)6 (I) fused double cubane. The new clusters have been obtained by substitution reactions of the PPr3 ligands with Cl-, BH4-, and N3-. By careful control of the conditions of these reactions, the clusters [(Cl4-cat)(PPr3)MoFe3S4(BH4)2]2(Bu4N)4 (II), [(Cl4-cat)(PPr3)MoFe3S4(PPr3)(BH4)]2(Bu4N)2 (III), [(Cl4-cat)(PPr3)MoFe3S4(N3)2]2(Bu4N)4 (IV), [(Cl4-cat)(PPr3)MoFe3S4(PPr3)(N3)]2(Bu4N)2 (V), and [(Cl4-cat)(PPr3)MoFe3S4Cl2]2(Et4N)4 (VI) have been obtained and structurally characterized. A study of their electrochemistry shows that the reduction potentials for the derivatives of I are shifted to more positive values than those of I, suggesting a stabilization of the reduced clusters by the anionic ligands BH4- and N3-. Using 1H NMR spectroscopy, we have explored the lability of the BH4- ligand in II in coordinating solvents and its hydridic character, which is apparent in its reactivity toward proton sources such as MeOH or PhOH.     

Oxygen adsorption on hydrated gold cluster anions: experiment and theory

The discovery that supported gold clusters act as highly efficient catalysts for low-temperature oxidation reactions has led to a great deal of work aimed at understanding the origins of the catalytic activity. Several studies have shown that the presence of trace moisture is required for the catalysts to function. Using near-atmospheric pressure flow reactor techniques, we have studied humidity and temperature effects on the reactivity of gas-phase gold cluster anions with O2. Near room temperature, the humid source produces abundant gold-hydroxy cluster anions, Au(N)OH(-), and these have a reversed O2 adsorption activity: Nonreactive bare gold clusters become active when in the form Au(N)OH(-), while active bare clusters are inactive when -OH is bound. The binding energies for the stable structures obtained from density functional calculations confirm fully these findings. Moreover, the theory provides evidence that electron-transfer induced by the binding of a OH group enhances the reactivity toward molecular oxygen for odd anionic gold clusters and suppresses the reactivity for the even ones. The temperature dependence of O2 addition to Au(3)OH(-) and Au(4)(-) indicates deviations from equilibrium control at temperatures below room temperature. The effects of humidity on gold cluster adsorption activity support the conclusion drawn for the mechanism of O2 adsorption on "dry" gold cluster anions and provides insight into the possible role of water in the enhanced activity of supported gold cluster catalysts.     

Silicon clusters: chemistry and structure

The chemical reactions of size selected silicon cluster ions (containing up to 70 atoms) have been studied with a number of different reagents using injected ion drift tube techniques. Both kinetic and equilibrium measurements have been performed as a function of temperature, and the influence of cluster annealing on chemical reactivity explored. Unlike metal clusters, where bulk behavior appears to be approached with around 30 atoms, large silicon clusters (n up to 70) are much less reactive than bulk silicon surfaces. These results suggest that the clusters in the size range examined here are not small crystals of bulk silicon, but have compact, high coordination number structures with few dangling bonds.