Tungsten Clusters Research Articles

Two-dimensional to three-dimensional transition of tungsten clusters anchored on graphite surface

Experimental studies have been performed on unisize tungsten clusters constructed on a graphite surface by means of the scanning tunneling microscopy. It was found that the geometry of the clusters changes instantaneously from a monatomic-layer tungsten disk to a diatomic-layer structure between the cluster size of 10 and 11. We concluded that this transition is driven by a change in the dominant interaction from the attractive electrostatic interaction between the cluster and the surface to intracluster cohesive metallic interaction.

In Vivo Differentiation of Complementary Contrast Media at Dual-Energy CT

To evaluate the feasibility of using a commercially available clinical dual-energy computed tomographic (CT) scanner to differentiate the in vivo enhancement due to two simultaneously administered contrast media with complementary x-ray attenuation ratios. Approval from the institutional animal care and use committee was obtained, and National Institutes of Health guidelines for the care and use of laboratory animals were observed. Dual-energy CT was performed in a set of iodine and tungsten solution phantoms and in a rabbit in which iodinated intravenous and bismuth subsalicylate oral contrast media were administered. In addition, a second rabbit was studied after intravenous administration of iodinated and tungsten cluster contrast media. Images were processed to produce virtual monochromatic images that simulated the appearance of conventional single-energy scans, as well as material decomposition images that separate the attenuation due to each contrast medium. Clear separation of each of the contrast media pairs was seen in the phantom and in both in vivo animal models. Separation of bowel lumen from vascular contrast medium allowed visualization of bowel wall enhancement that was obscured by intraluminal bowel contrast medium on conventional CT scans. Separation of two vascular contrast media in different vascular phases enabled acquisition of a perfectly coregistered CT angiogram and venous phase-enhanced CT scan simultaneously in a single examination. Commercially available clinical dual-energy CT scanners can help differentiate the enhancement of selected pairs of complementary contrast media in vivo.

Reduction of N2 by supported tungsten clusters gives a model of the process by nitrogenase.

Metalloenzymes catalyze difficult chemical reactions under mild conditions. Mimicking their functions is a challenging task and it has been investigated using homogeneous systems containing metal complexes. The nitrogenase that converts N2 to NH3 under mild conditions is one of such enzymes. Efforts to realize the biological function have continued for more than four decades, which has resulted in several reports of reduction of N2, ligated to metal complexes in solutions, to NH3 by protonation under mild conditions. Here, we show that seemingly distinct supported small tungsten clusters in a dry environment reduce N2 under mild conditions like the nitrogenase. N2 is reduced to NH3 via N2H4 by addition of neutral H atoms, which agrees with the mechanism recently proposed for the N2 reduction on the active site of nitrogenase. The process on the supported clusters gives a model of the biological N2 reduction.

W4Br10 Cluster Intermediates in the Solid State Nucleation of W6Br12

AbstractThe new adduct W4Br10·2SbBr3 and the new binary compound W4Br10 were obtained as products in a reaction cascade in which WBr6 was reacted with elemental antimony at successively increased temperatures. The crystal structures of both compounds were refined from X‐ray powder diffraction data and their electronic structures were analyzed by MO calculations. The cluster compounds W4Br10·2SbBr3 and W4Br10 appear as intermediates in the solid state nucleation of W6Br12. The overall reaction cascade involves tungsten clusters having tetrahedral (W4), square pyramidal (W5) and finally octahedral (W6) cluster cores.

Low-temperature preparation of tungsten halide clusters: Crystal structure of the adduct W5Br12 · SbBr3

Single-crystals of a new compound W5Br12 · SbBr3 (I) were isolated as a reaction product from the reduction of WBr6 with elemental antimony at 250°C. The crystal structure was determined by single-crystal X-ray diffraction analysis. The structure contains square-pyramidal tungsten clusters being linked into infinite [(W5Br 8 i )Br 3 a Br 2 2/a-a ] chains through shared W-Bra-a-W contacts of adjacent clusters. The structure of the adduct I can be viewed as an intercalation compound composed of double-layers of cluster chains alternating with mono-layers of SbBr3 molecules.

Gas-Phase Alkylation of Pyridine and Phenol with Alcohols Over Halide Clusters of Group 5–7 Transition Metals as Solid Acid Catalysts

Pyridine is allowed to react with methanol under a hydrogen stream in the presence of (H3O)2[(W6Cl8)Cl6]·6H2O supported on silica gel. When the temperature is raised above 200 °C, the catalytic activity of the cluster appears. Methylation of pyridine proceeds yielding 2-methylpyridine in 61% selectivity at 400 °C. The corresponding hexanuclear chloride clusters of niobium, molybdenum, and tantalum also catalyze the reaction. Ethanol affords the corresponding 2-ethylpyridine. When phenol is allowed to react with methanol in the presence of (H3O)2[(Mo6Cl8)Cl6]·6H2O supported on silica gel in the same manner, selective O-methylation proceeds yielding anisole in 57% selectivity at 150–200 °C. Above 250 °C, C-methylation predominates and provides o-cresol with 67% selectivity at 300 °C. The corresponding clusters of niobium, tantalum, and tungsten also catalyze the reaction. Ethanol and 1-propanol afford the corresponding 2-alkylphenols. Alkyl cations produced over weak Bronsted acid sites (H 0 ≈ +1.3) developed on the clusters are assumed as intermediates for both reactions.

Surface self-diffusion behavior of individual tungsten adatoms on rhombohedral clusters

The diffusion of single tungsten adatoms on the surfaces of rhombohedral clusters is studied by means of molecular dynamics and the embedded atom method. The energy barriers for the adatom diffusing across and along the step edge between a {110} facet and a neighboring {110} facet are calculated using the nudged elastic band method. We notice that the tungsten adatom diffusion across the step edge has a much higher barrier than that for face-centered cubic metal clusters. The result shows that diffusion from the {110} facet to a neighboring {110} facet could not take place at low temperatures. In addition, the calculated energy barrier for an adatom diffusing along the step edge is lower than that for an adatom on the flat (110) surface. The results show that the adatom could diffuse easily along the step edge, and could be trapped by the facet corner. Taking all of this evidence together, we infer that the {110} facet starts to grow from the facet corner, and then along the step edge, and finally toward the {110} facet center. So the tungsten rhombohedron can grow epitaxially along the {110} facet one facet at a time and the rhombohedron should be the stable structure for both large and small tungsten clusters.

Phosphorus‐Centered and Phosphinidene‐Capped Tungsten Chloride Clusters

AbstractBlack crystalline powders of W6PCl17 and W4(PCl)Cl10 were obtained after reducing WCl6 with red phosphorus at 370 and 500 °C. The crystal structures were determined by single‐crystal and powder X‐ray diffraction analyses. The structure of W6PCl17 is represented by a phosphorus‐centered hexanuclear tungsten cluster, whose (W6PCl11)Cl4aCl4/2a–a chains form a hexagonal stick packing structure. The structure of W4(PCl)Cl10 is represented by a Jahn–Teller distorted tetranuclear tungsten cluster that is interconnected into a layered [W4(μ4‐PCl)Cl6i]Cl8/2a–a structure containing a chloro‐phosphinidene ligand. Solid‐state 31P magic‐angle spinning (MAS) NMR spectra, electronic structures, and magnetic properties are reported.

Nanosized Vanadium, Tungsten and Molybdenum Oxide Clusters Grown in Porous Chitosan Microspheres as Promising Hybrid Materials for Selective Alcohol Oxidation

The ability of chitosan biopolymer to coordinate vanadium, tungsten and molybdenum metallic species and to control their mineralisation growth provides a new family of surface-reactive organic-inorganic hybrid microspheres. Drying the resulting materials under supercritical conditions allowed the gel network dispersion to be retained, thereby leading to a macroporous catalyst with surface areas ranging from 253 to 278 m(2) g(-1). On account of the open framework structure of these microspheres, the redox species entangled within the fibrillar network of the polysaccharide aerogels were found to be active, selective and reusable catalysts for cinamylalcohol oxidations.

Tungsten cluster migration on nanoparticles: minimum energy pathway and migration mechanism

Saddle point searches have been employed to investigate the migration mechanisms of W clusters on W nanoparticles, and to determine the corresponding migration energies for the possible migration paths of these clusters. The tungsten clusters containing up to four adatoms are found to prefer 2D-compact structures with relatively low binding energies. The effect of interface and vertex regions on the migration behavior of the clusters is significantly strong, as compared to that of nanoparticle size. The migration mechanisms are quite different when the clusters are located at the center of the nanoparticle and near the interface or vertex areas. Near the interfaces and vertex areas, the substrate atoms tend to participate in the migration processes of the clusters, and can join the adatoms to form a larger cluster or lead to the dissociation of a cluster via the exchange mechanism, which results in the adatom crossing the facets. The lowest energy paths are used to be determined the energy barriers for W cluster migrations (from 1- to 4-atoms) on the facets, edges and vertex regions. The calculated energy barriers for the trimers suggest that the concerted migration is more probable than the successive jumping of a single adatom in the clusters. In addition, it of interest to note that the dimer shearing is a dominant migration mechanism for the tetramer, but needs to overcome a relatively higher migration energy than other clusters.

Mechanism of the Catalytic Hydrodefluorination of Pentafluoropyridine by Group Six Triangular Cluster Hydrides Containing Phosphines: A Combined Experimental and Theoretical Study

The catalytic hydrodefluorination (HDF) of pentafluoropyridine in the presence of arylsilanes is catalyzed by the tungsten and molybdenum(IV) cluster hydrides of formula [M3S4H3(dmpe)3]+, W-1+ for M = W and Mo-1+ for M = Mo (dmpe = 1,2-(bis)dimethylphosphinoethane). The reaction proceeds regioselectively at the 4-position under microwave radiation to yield the 2,3,5,6-tetrafluoropyridine. Catalytic activity is higher for the tungsten complexes with turnover numbers close to 100, while reactions catalyzed by molybdenum compounds are faster. A mechanism for the HDF reaction has been proposed that explains these differences based on DFT calculations. The mechanism involves partial decoordination of the diphosphine ligand that generates an empty position in the metal coordination sphere. This position together with its neighbor M−H site are used to activate the C−F bond of the pentafluoropyridine through a M−H/C−F σ-bond metathesis mechanism involving a four-center transition state to give 2,3,5,6-tetrafluoro...

Intramolecular Condensation of 1,2-C6H4(CH2RH)2 (R = O, S, and NH) to Yield Heterocyclic Compounds over Halide-cluster Catalysts

Abstract 1,2-Benzenedimethanol was reacted under a helium stream in the presence of [(Nb6Cl12)Cl2(H2O)4]·4H2O supported on silica gel. When the temperature was raised above 200 °C, catalytic activity of the cluster for cyclization appeared, yielding 1,3-dihydroisobenzofuran in 91% selectivity at 350 °C. The corresponding halide clusters of tantalum and tungsten also catalyzed the reaction. cis-1,2-Cyclohexanedimethanol and 1,4-butanediol exclusively produced the corresponding furans. 1,2-Benzenedimethanamine and 1,2-benzenedimethanethiol selectively afforded isoindoline and 1,3-dihydrobenzo[c]thiophene, respectively.

Vapor-phase Beckmann rearrangement of cyclohexanone oxime over halide cluster catalysts

When a silica gel-supported tungsten halide cluster with an octahedral metal framework, (H 3O) 2[(W 6Cl 8)Cl 6]·6H 2O/SiO 2, is treated in a helium stream in the temperature range 250–350 °C, catalytic activity for the Beckmann rearrangement of cyclohexanone oxime develops. Niobium and tantalum clusters with the same metal framework also catalyze the reaction. Cyclopentanone oxime and acetone oxime also undergo Beckmann rearrangements over the tungsten cluster. The weak Brønsted acidity ( H 0 ≈ +1.5) of the hydroxo ligand, which is developed on the activated cluster, is favorable for the rearrangement.

The Trigonal Prismatic Cluster Compound W6CCl15 and a Carambolage of Tungsten Clusters in the Structure of the Heteroleptic Cluster Compound W30C2(Cl,Br)68

Reactions between W(6)Cl(12) and carbon halides can initiate a cascade of reactions and reaction products, yielding W(6)Cl(18), W(6)CCl(18), and W(6)CCl(15) with increasing temperature, before decomposition into tungsten carbide is obtained. The new compound W(6)CCl(15) and the new heteroleptic compound W(30)C(2)(Cl,Br)(68) obtained from these reactions were structurally characterized. The structure of W(30)C(2)X(68) combines a carambolage of two distinct octahedral and one centered trigonal prismatic cluster in one structure as refined by X-ray single-crystal diffraction (P1, Z = 1; a = 12.003(2) A, b = 14.862(3) A, c = 15.792(3) A, alpha = 88.75(2) degrees, beta = 68.85(2) degrees, gamma = 71.19(2) degrees). The unit cell content W(30)C(2)X(68) accommodates five hexanuclear tungsten clusters, similar by a total of three octahedral [W(6)X(8)] type clusters and two carbon-centered trigonal prismatic [W(6)CX(12)] type clusters, sharing terminal halogen atoms to form a network structure. The trigonal prismatic cluster compound W(6)CCl(15) (P2(1)/c, Z = 4; a = 9.8830(4) A, b = 11.8945(4) A, c = 17.8670(7) A, beta = 107.883(2) degrees) is related to the already known compound W(6)CCl(16). According to X-ray powder structure refinement, the structure is showing a special connectivity pattern with short intercluster W-W contacts between trigonal prismatic cluster units.

Closed-Cage Tungsten Oxide Clusters in the Gas Phase

During the course of a study on the clustering of W-Se and W-S mixtures in the gas phase using laser desorption ionization (LDI) mass spectrometry, we observed several anionic W-O clusters. Three distinct species, W(6)O(19)(-), W(13)O(29)(-), and W(14)O(32)(-), stand out as intense peaks in the regular mass spectral pattern of tungsten oxide clusters suggesting unusual stabilities for them. Moreover, these clusters do not fragment in the postsource decay analysis. While trying to understand the precursor material, which produced these clusters, we found the presence of nanoscale forms of tungsten oxide. The structure and thermodynamic parameters of tungsten clusters have been explored using relativistic quantum chemical methods. Our computed results of atomization energy are consistent with the observed LDI mass spectra. The computational results suggest that the clusters observed have closed-cage structure. These distinct W(13) and W(14) clusters were observed for the first time in the gas phase.

Time-resolved photoelectron spectroscopy on small tungsten cluster anions

We have studied the dynamics of photoexcited tungsten cluster anions \(\mathrm{W}_{n}^{-}\) (n=3,4,…,14) by means of time-resolved two-photon photodetachment spectroscopy. At an excitation energy of hνpump=1.56 eV the photoinduced dynamics is mainly dominated by fast electronic relaxation processes. For the smallest clusters, i.e., \(\mathrm{W}_{3}^{-}\), \(\mathrm{W}_{4}^{-}\), and \(\mathrm{W}_{5}^{-}\), individual relaxation channels have been identified and resolved on a timescale well below 100 fs. The time constants for the decay of nascent and secondary electrons have been deduced from a Bloch model. Complete thermalization takes place for all clusters on a timescale of ∼1 ps.

Kinetic energy spectra in thermionic emission from small tungsten cluster anions: Evidence for nonclassical electron capture

The delayed electron emission from small mass-selected anionic tungsten clusters W(n)(-) has been studied for sizes in the range 9 < or = n < or = 21. Kinetic energy spectra have been measured for delays of about 100 ns after laser excitation by a velocity-map imaging spectrometer. They are analyzed in the framework of microreversible statistical theories. The low-energy behavior shows some significant deviations with respect to the classical Langevin capture model, which we interpret as possibly due to the influence of quantum dynamical effects such as tunneling through the centrifugal barrier, rather than shape effects. The cluster temperature has been extracted from both the experimental kinetic energy spectrum and the absolute decay rate. Discrepancies between the two approaches suggest that the sticking probability can be as low as a few percent for the smallest clusters.

Synthesis and characterization of tetrairon–tungsten cluster complex containing a μ4, η2-carbonyl ligand

Reaction of [(PPh 2C 5H 4)Cp 3Fe 4(CO) 4] ( 1) with (CO) 4W(CH 3CN) 2 at ambient temperature affords [(CO) 4W(PPh 2C 5H 4)Cp 3Fe 4(CO) 4] ( 2) as the major product, together with a small amount of [(CO) 5W(PPh 2C 5H 4)Cp 3Fe 4(CO) 4] ( 3). Compound 3 can be obtained in good yield by treating (CO) 5W(CH 3CN) with equal molar of 1, and reaction of 3 with Me 3NO in acetonitrile solvent produces 2 exclusively. The crystal structures of 2 and 3 have been determined by an X-ray diffraction study. Compound 2 contains an interesting μ 4, η 2-CO ligand, where two electrons donated by the carbon atom are involved to bridge a Fe 3 face and two electrons from oxygen are donated to the tungsten(0) atom.

Cover Picture: ZAAC ‐ Journal of Inorganic and General Chemistry 1/2010

AbstractThe cover picture shows two distinct clusters [(W5I8i)Ia(I3)2a(I3)2/2a–a] bridged by the two terminal I9 atoms. The structure of W15I47 was refined by single crystal X‐ray diffraction with the non‐centrosymmetric space group C2 [Z = 2, a = 4088.3(4) pm, b = 952.3(1) pm, c = 1198.5(2) pm, and β = 103.97(1) °]. The characteristic motif in the structure is the square pyramidal tungsten cluster that may be constructed from an octahedral [(W6I8i)I6a]2– cluster by removing one tungsten atom together with its outer iodide ligand to yield [(W5I8i)I5a]n–. The formula W15I47 represents three square pyramidal tungsten clusters with two of them being distinguishable in [(W5I8i)Ia(I3)2a(I3)2/2a–a][(W5I8i)I2a(I3)3/2a–a]2, by different numbers of iodide and triiodide ligands at the cluster vertices. The background is made up of layer sequences and connectivity pattern of pentagonal tungsten clusters in the W15I47 structure (inner iodides are omitted from the drawing). More details are discussed in the article by H.‐J. Meyer et al. on p. 62ff.

The New Binary Tungsten Iodide W15I47

AbstractThe new tungsten iodide W15I47 was synthesized by reacting tungsten hexacarbonyl with iodine under elevated pressure and temperature conditions. The structure of W15I47 was refined by single‐crystal X‐ray diffraction with the non‐centrosymmetric space group C2 [Z = 2, a = 4088.3(4) pm, b = 952.3(1) pm, c = 1198.5(2) pm, and β = 103.97(1)°]. The characteristic motif in the structure is the square pyramidal tungsten cluster that may be constructed from an octahedral [(W6I8i)I6a]2– cluster by removing one tungsten atom together with its outer iodide ligand to yield [(W5I8i)I5a]n–. The formula W15I47 represents three square pyramidal tungsten clusters with two of them being distinguishable in [(W5I8i)Ia(I3)2a(I3)2/2a–a][(W5I8i)I2a(I3)3/2a–a]2, by different numbers of iodide and tri‐iodide ligands at the cluster vertices.