Monitoring the dissolution process of metals in the gas phase: reactions of nanoscale Al and Ga metal atom clusters and their relationship to similar metalloid clusters

Formation and dissolution of metals are two of the oldest technical chemical processes. On the atomic scale, these processes are based on the formation and cleavage of metal-metal bonds. During the past 15 years we have studied intensively the intermediates during the formation process of metals, i.e. the formation of compounds containing many metal-metal bonds between naked metal atoms in the center and ligand-bearing metal atoms at the surface. We have called the clusters metalloid or, more generally, elementoid clusters. Via a retrosynthetic route, the many different Al and Ga metalloid clusters which have been structurally characterized allow us to understand also the dissolution process; i.e. the cleavage of metal-metal (M-M) bonds. However, this process can be detected much more directly by the reaction of single metal atom clusters in the gas phase under high vacuum conditions. A suitable tool to monitor the dissolution process of a metal cluster in the gas phase is FT-ICR (Fourier transform ion cyclotron resonance) mass spectrometry. Snapshots during these cleavage processes are possible because only every 1-10 s is there a contact between a cluster molecule and an oxidizing molecule (e.g. Cl2). This period is long, i.e. the formation of the primary product (a smaller metal atom cluster) is finished before the next collision happens. We have studied three different types of reaction:(1) Step-by-step fragmentation of a structurally known metalloid cluster allows us to understand the bonding principle of these clusters because in every step only the weakest bond is broken.(2) There are three oxidation reactions of an Al13(-) cluster molecule with Cl2, HCl and O2 central to this review. These three reactions represent three different reaction types, (a) an exothermic reaction (Cl2), (b) an endothermic reaction (HCl), and (c) a kinetically limited reaction based on spin conservation rules (O2).(3) Finally, we present the reaction of a metalloid cluster with Cl2 in order to show that in this cluster only the central naked metal atoms are oxidized, and a smaller metalloid cluster results containing the entire protecting shell as the primary cluster.All the experimental results, supported by quantum chemical calculations, give a rough idea about the complex reaction cascades which occur during the dissolution and formation of metals. Furthermore, these results cast a critical light on many simplifying and generalizing rules in order to understand the bonding and structure of metal clusters. Finally, the experiments and some recent results provided by physical measurements on a crystalline Ga(84) compound build a bridge to nanoscience; i.e. they may be a challenge for chemistry in the next decades, since it has been shown that only with a perfect orientation of nanoscale metal clusters, e.g. in a crystal, can novel, unexpected properties (e.g. superconducting nanoscale materials) be obtained.     

Promoting of non-transition metal alkylation with organyl halides in the presence of binary systems based on an organometallic compound and a transition metal compound: I. Hypothetical stepwise scheme and verification of the system efficiency

A hypothetical scheme has been proposed for the alkylation of non-transition metals with organyl halides in the presence of binary systems consisting of an organometallic compound and a transition metal compound. The scheme implies catalysis by transition metal atoms, small clusters, and subhalides adsorbed on the surface of metal to be alkylated. These particles are formed during the process as a result of interaction between the binary system components. The alkylation of commercial zinc powder with ethyl bromide has been used as a model reaction to demonstrate that the binary system ethylzinc bromide-copper(I) iodide is superior in its efficiency and experimental simplicity to all other examined methods for stimulation of the alkylation of elements with organyl halides yielding organometallic compounds.     

Molecular Organometallic Chemistry on Surfaces: Reactivity of Metal Carbonyls on Metal Oxides

Metal carbonyls react on metal oxide surfaces to give a wide range of structures analogous to those of known compounds. The reactions leading to formation of surface-bound metal carbonyls are explained by known molecular organometallic chemistry and the functional group chemistry of the surfaces. The reaction classes include formation of acid-base adducts as the oxygen of a carbonyl group donates an electron pair to a Lewis acidic center; nucleophilic attack at CO ligands by basic surface hydroxyl groups or O^2? ions; ion-pair formation by deprotonation of hydrido carbonyls to give carbonylate ions; interaction of bifunctional complexes with surface acid-base pair sites such as [Mg^2⊕O^2?]; and oxidative addition of surface hydroxyl groups to metal clusters. The reactions of surface-bound organometallic species include redox condensation and cluster formation on basic surfaces (paralleling the reactions in basic solution) as well as oxidation of mononuclear metal complexes and oxidative fragmentation of metal clusters by reaction with surface hydroxyl groups. Most supported metal carbonyls are unstable at high temperatures, but some, including osmium carbonyl cluster anions on the basic MgO surface, are strongly stabilized in the presence of CO and are precursors of catalysts for CO hydrogenation at 550 K.     

Prediction of pKa values of nido-carboranes by density functional theory methods

A great parallel exists between metal complexes of cyclopentadienyl and arene ligands on one side and metal complexes of the nido derivatives of the icosahedral o-carborane clusters. With few exceptions, the metal complexation in the cluster can be viewed as the substitution of one or more bridging hydrogen atoms by the metal. Therefore, a necessary requirement for the complexation is the deprotonation of the nido cluster to generate a coordination site for that metal. The reaction to remove these protons, which most probably is one of the most commonly done processes in boron and metallaborane chemistry, is barely known, and no quantitative data are available on the magnitude of their pKa values. With the purpose of determining the acidity of nido-carboranes, a procedure to calculate the pKa values of nido boron clusters is presented in this paper for the first time. To this objective, some nido clusters have been selected and their geometry and NMR-spectroscopic properties have been studied, giving a good correlation between the theoretical and experimental data in both geometry distances and 11B NMR spectroscopy. Of notice is the result that proves that the singular carbon atom in the thermodynamic isomer of [C2B10H13]- is definitely part of the cluster and that its connection with the C2B3 face would be better defined by adding additional interactions with the two boron atoms nearest to the second cluster carbon. The pKa values of the nido species have been calculated by correlating experimental pK(a) values and calculated reaction Gibbs energies DeltaG(s). Some pKa values of importance are -4.6 and +13.5 for 7,8-[C2B9H13] (1) and 7,8-[C2B9H12]- (2), respectively.     

Atomic view of cluster diffusion on metal surfaces

Field ion microscope measurements show a strong correlation between the mobility and shape of small clusters on fcc(100) metal surfaces. For self-diffusion on Rh(100) this correlation leads to an oscillatory behavior in the activation energy of surface diffusion as a function of cluster size. Comparison of the measured activation energies to previous theoretical calculations indicate that the mechanism of cluster diffusion involves individual displacements of edge atoms (i.e., perimeter diffusion). The rate-determining step in the migration of clusters is the partial detachment of one of the perimeter atoms. The relative ease of adatom motion along straight edges of stationary clusters also permits measurements of diffusion barriers at steps — measurements useful in the interpretation of fractal vs. compact island growth on fcc metal surfaces.     

Reductiveversus coupling pathways in the reactions of nickel and copper vapours with the mono-halobenzenes

Summary The reactions of nickel and copper vapours with mono-halobenzenes to form benzene and biphenyl are reported. Benzene formation apparently proceeds through the intermediacy of the phenyl radical, which is produced when metal clusters abstract halogen from the halobenzene. Biphenyl formation proceedsvia an oxidative insertion/disproportionation/reductive elimination sequence, and requires that metal atoms, rather than metal clusters, initiate the reaction sequence. The concentration of the metal in the matrix determines the pathway of the reaction. Cluster formation and benzene production are favouted by high metal concentrations, while low metal concentrations in the matrix favour biphenyl production. Copper vapours are anomalously less reactive than nickel vapours, even though copper reagents are used in the Ullmann reaction to produce biaryls through the coupling of haloarenes. This result appears to be due to the preferential formation of atoms or very small clusters over large clusters or crystallites under the conditions of the metal vapour synthesis technique.     

Clean coupling of unfunctionalized porphyrins at surfaces to give highly oriented organometallic oligomers

The direct coupling of complex, functional organic molecules at a surface is one of the outstanding challenges in the road map to future molecular devices. Equally demanding is to meet this challenge without recourse to additional functionalization of the molecular building blocks and via clean surface reactions that leave no surface contamination. Here, we demonstrate the directional coupling of unfunctionalized porphyrin molecules--large aromatic multifunctional building blocks--on a single crystal copper surface, which generates highly oriented one-dimensional organometallic macromolecular nanostructures (wires) in a reaction which generates gaseous hydrogen as the only byproduct. In situ scanning tunneling microscopy and temperature programmed desorption, supported by theoretical modeling, reveal that the process is driven by C-H bond scission and the incorporation of copper atoms in between the organic components to form a very stable organocopper oligomer comprising organometallic edge-to-edge porphyrin-Cu-porphyrin connections on the surface that are unprecedented in solution chemistry. The hydrogen generated during the reaction leaves the surface and, therefore, produces no surface contamination. A remarkable feature of the wires is their stability at high temperatures (up to 670 K) and their preference for 1D growth along a prescribed crystallographic direction of the surface. The on-surface formation of directional organometallic wires that link highly functional porphyrin cores via direct C-Cu-C bonds in a single-step synthesis is a new development in surface-based molecular systems and provides a versatile approach to create functional organic nanostructures at surfaces.     

Sinter‐Resistant Platinum Catalyst Supported by Metal–Organic Framework

Single atoms and few‐atom clusters of platinum are uniformly installed on the zirconia nodes of a metal‐organic framework (MOF) NU‐1000 via targeted vapor‐phase synthesis. The catalytic Pt clusters, site‐isolated by organic linkers, are shown to exhibit high catalytic activity for ethylene hydrogenation while exhibiting resistance to sintering up to 200 °C. In situ IR spectroscopy reveals the presence of both single atoms and few‐atom clusters that depend upon synthesis conditions. Operando X‐ray absorption spectroscopy and X‐ray pair distribution analyses reveal unique changes in chemical bonding environment and cluster size stability while on stream. Density functional theory calculations elucidate a favorable reaction pathway for ethylene hydrogenation with the novel catalyst. These results provide evidence that atomic layer deposition (ALD) in MOFs is a versatile approach to the rational synthesis of size‐selected clusters, including noble metals, on a high surface area support.     

Application of solid-state early-transition metal clusters as catalysts

Metal cluster compounds are expected to be catalysts for new reactions because of synergistic effect of the metal atoms. In solid-state halide clusters and sulfide clusters, metal cluster frameworks are linked in two- or three-dimensions to form a cluster network. Halogen- or sulfur-deficient metal sites in an octahedral metal cluster framework are retained intact and act as catalytically active sites even at high temperatures of 400–700?°C. This review reports recent advances in the development of coordinatively unsaturated metal atoms on solid-state clusters with an octahedral metal framework and their application to organic catalytic reactions.     

铁、钴、镍/磷二元团簇离子的形成与光解

利用激光直接溅射法产生了铁、钴、镍／磷二元团簇正负离子,并用串级飞行时间质谱仪研究了团族离子的组份和激光光解规律,质谱研究表明,铁、钴、镍易与磷结合成簇,而且样品中磷含量的增加有助于大尺寸团簇离子的生成,当形成的团族簇离子中含金属原子数目较少时,磷原子数目可在较大范围内变动,其中ＭＰ２＾＋、ＭＰ４＾＋、Ｍ２Ｐ４＾＋（Ｍ＝Ｆｅ、Ｃｏ、Ｎｉ,ｎ＝２、３、４）团簇离子均具有较高的丰度;随着金属原子数目的     

The formation and kinetics of Ionized Cluster Beams

The formation and kinetics of large vapourized-material cluster beams (large size metal clusters) are discussed. The clusters are formed by injecting the vapour of solid state materials into a high vacuum region through a nozzle of a heated crucible. The conditions under which metal clusters form are analysed using nucleation theory. Computer simulation by combining the nucleation and flow equations has also been made. The results show that the theory can be useful in predicting qualitative dependences of metal cluster formation on operation conditions. Several experimental results are also presented, which support the finding that a large size metal cluster is formed by homogeneous nucleation and growth. The advantageous characteristics of ionized cluster beam for thin film formation are also discussed.     

Growth Ability and Stability Indices of Clusters

Electronegativity and hardness are well known electronic concepts that help to interpret the electronic rearrangements of atoms in molecules. Parallel atomic concepts can be established that help to understand the growth processes occurring in cluster formation experiments. Those, which are called here the growth ability and the stability index, are essentially the first and second derivatives of the cluster binding energy with respect to the number of atoms, respectively. The first one has not been used before, and by application to transition metal clusters we show that a consideration of both indices helps to interpret the observed cluster mass spectra.     

Formation of bimetallic Ag-Pd nanoclusters via the reaction between Ag nanoclusters and Pd2+ ions

We have investigated systematically the mechanistic aspects of the Ag-Pd bimetallic cluster formation within sodium bis(2-ethylhexyl)sulfosuccinate (AOT) reverse micelles by using in-situ X-ray absorption spectroscopy (XAS). A two-step sequential reduction method is employed for the synthesis of Ag-Pd bimetallic clusters. The first step involves preparation of Ag nanoclusters, by mixing the Ag+ ions containing the AOT microemulsion system with a reducing agent hydrazine (N2H4) containing the AOT microemulsion system. In the second step, the addition of Pd2+ ions to Ag nanoclusters led to the formation of Ag-Pd bimetallic clusters via the reaction between Ag nanoclusters and Pd2+ ions in AOT reverse micelles. The reduction of silver ions and the formation of corresponding Ag nanoclusters are monitored as a function of the dosage of the reducing agent, hydrazine. In-situ XAS allowed probing of the reaction between Ag nanoclusters and Pd2+ ions during the formation of Ag-Pd bimetallic clusters. Analysis of Ag and Pd K-edge XAS spectra reveals that in the final stage Ag-Pd clusters, in which "Ag" atoms prefer to be surrounded by "Pd" and "Pd" atoms prefer to be surrounded by "Pd", were formed. On the basis of XAS results presented here, we are able to propose a structural model for each step so that this work provides a detailed insight into the mechanism of nucleation and growth of Ag-Pd bimetallic clusters. We also discussed the atomic distribution of Ag and Pd atoms in Ag-Pd bimetallic clusters based on the calculated XAS structural parameters.     

Cluster Chemistry

Metal cluster chemistry is one of the most rapidly developing areas of inorganic and organometallic chemistry. Prior to 1960 only a few metal clusters were well characterized. However, shortly after the early development of boron cluster chemistry, the field of metal cluster chemistry began to grow at a very rapid rate and a structural and a qualitative theoretical understanding of clusters came quickly. Analyzed here is the chemistry and the general significance of clusters with particular emphasis on the cluster research within my group. The importance of coordinately unsaturated, very reactive metal clusters is the major subject of discussion.     

Synergistic Behavior of Bimetallic Rhenium Cluster Catalysts: Spectroscopic Investigation into the Nature of the Active Site

Single‐site Re nanoparticles were produced by anchoring dirhenium organometallic clusters on to the inner walls of mesoporous silica. The presence of oxophilic atoms (Sb or Bi) is essential to obtain well dispersed Re⁰ centers. The interaction between the organometallic cluster and the silica support is critical for the generation of well‐defined and isolated Re⁰ single sites. FTIR spectroscopy was used to track the decomposition of the organometallic precursors and the adsorption of probe molecules such as CO on the metal sites sheds valuable information on the catalytic potential of this new class of bimetallic nanocatalysts.     

Understanding the Metal-Directed Growth of Single-Crystal M-TCNQF(4) Organic Nanowires with Time-Resolved, in Situ X-ray Diffraction and First-Principles Theoretical Studies

The deterministic growth of oriented crystalline organic nanowires (CONs) from the vapor-solid chemical reaction (VSCR) between small-molecule reactants and metal nanoparticles has been demonstrated in several studies to date; however, the growth mechanism has not yet been conclusively understood. Here, the VSCR growth of M-TCNQF(4) (where M is Cu- or Ag-) nanowires is investigated both experimentally and theoretically with time-resolved, in situ X-ray diffraction (XRD) and first-principles atomistic calculations, respectively, to understand how metals (M) direct the assembly of small molecules into CONs, and what determines the selectivity of a metal for an organic vapor reactant in the growth process. Analysis of the real-time growth kinetics data using a modified Avrami model indicates that the formation of CONs from VSCR follows a one-dimensional ion diffusion-controlled tip growth mechanism wherein metal ions diffuse from a metal film through the nanowire to its tip where they react with small molecules to continue growth. The experimental data and theoretical calculations indicate that the selectivity of different metals to induce nanowire growth depends strongly upon effective charge transfer between the organic molecules and the metal. Specifically, the experimental finding that Cu ions can exchange and replace Ag ions in Ag-TCNQF(4) to form Cu-TCNQF(4) nanowires is explained by the significantly stronger chemical bond between Cu and TCNQF(4) molecules than for Ag, due to the strong electronic contribution of Cu d-orbitals near the Fermi level. Understanding how to control the VSCR growth process may enable the synthesis of novel organic nanowires with axial or coaxial p/n junctions for organic nanoelectronics and solar energy harvesting.     

Insights into the early stages of metal nanoparticle formation via first-principle calculations: the roles of citrate and water

The early stages of silver nanoparticle formation in the presence of citrate and water have been investigated via first-principle theoretical calculations. Our study revealed that the charge density of the clusters is a key factor determining the selectivity among various growth pathways. An optimal charge density appears to control the selection between neutral and charged species in cluster growth; partially positively charged clusters are thermodynamically preferred and can serve as seeds for further growth. They interact favorably with both the solvent, leading to their solubility, and the citrate. The solvent (water) plays an important role in cluster growth both on the energetics of reactions including highly charged clusters and on the geometry of the resulting silver structures by preventing the formation of asymmetric ones (a structure directing action). Contrary to the common belief we found, from an energetic viewpoint, that growth of small clusters is not blocked by the citrate. Citrate, by acting as a reducing agent, opens up new channels for cluster growth involving highly charged species. By regulating the cluster charge, cluster-cluster associations may be promoted by the citrate, providing a new mechanistic interpretation for the effect of citrate concentration on nanoparticle size substantially different from the classic nucleation theory. From the citrate-silver and water-silver cluster interactions, linear free energy relationships have been retrieved that provide insights into metal nanoparticle growth mechanisms.     

Melting of Lennard-Jones clusters in confined geometries

The melting of Lennard-Jones (argon) clusters of various size (N = 500 to 10000 atoms), confined in a rigid matrix, is studied by molecular dynamics simulations. For spherical clusters we show the existence of a cluster size below which the dependence of the melting temperature cannot be described by the classical Gibbs-Thompson equation. We also provide evidence of the formation of a quasi-liquid layer at the surface of mesoscopic clusters. A good agreement is found between the theoretical model due to Celestini et al. and the simulation results obtained in this work. The melting of an ellipsoidal cluster is also investigated. We observe, in agreement with recent experimental and theoretical work, that the thickness of the molten layer is larger in the region of higher local curvature.     

The Coordination Chemistry of Multidentate Isocyanide Ligands

In spite of the excellent ligation properties of isocyanides, until a few years ago there was only a small number of known multidentate ligands of this type. One of the reasons for this lack of interest, when compared to monodentate isocyanides, was the linear arrangement of the M? C?N? R group, which usually inhibits the formation of mononuclear chelate complexes and leads to the formation of multinuclear or polymeric metal complexes. In these, the multidentate ligand acts in a monodentate fashion towards each metal atom. Only recently has a series of polyisocyanides with large ligand backbones been synthesized successfully. Bidentate isocyanides can bridge two metal atoms or react to give chelates with only one metal center. Tripodal ligands form mono- or binuclear complexes, in which the largest organometallic rings observed to date occur (up to 36 atoms). This class of ligands promises to be interesting for the synthesis of stable, diagnostically important technetium complexes of the type [Tc(CNR) ₆ ]⁺. There also appear to be applications for tripodal isocyanides in catalysis. A facial, chiral Cr(CNR^*)₃ unit might be able to catalyze the hydrogenation or isomerization of prochiral double bonds. It is even possible to bind triisocyanides with suitable backbones to carbonyl trimetal clusters, thereby stabilizing them, or making selective cluster formation possible. Coordinated isocyanides can be transformed readily into carbene ligands, which, in the future, could lead to complexes with polycarbene ligation.