Niobium Clusters Research Articles

Reactions of niobium cluster ions Nb x± (x = 2–16) with NO and NO 2

Gas phase niobium cluster ions, Nb x ± (x = 2–16), are subjected to reactions with NO and NO 2 in a fast-flow reactor. For cationic niobium clusters, the difference mass spectra for the reactions demonstrate that dissociative chemisorption and molecular adsorption take place in different cluster size regimes. Detailed analysis of the reaction products suggests that the cluster cations are fragmented into two parts in the small cluster range: the nitrogen retained clusters and the niobium oxide species. The relative reactivity of the cluster cations shows a decreasing trend with increasing cluster size. The reactions of the anionic niobium clusters with NO and NO 2 are also studied. The results are discussed in terms of electron transfer reactions. Cluster fragmentation is also extensively observed.

Radiative emission of hot metallic clusters

Analysis of experimental data on the relaxation of freely moving hot niobium and tungsten clusters shows that they are cooled as a result of radiative emission. The absorption cross sections per atom of niobium and tungsten clusters in the temperature range 3100–3700 K are (4 −7)×10−18 cm2, and the absorption process loses its resonant character at these temperatures.

Electrochemistry of Centered Hexanuclear Zirconium Halide Clusters in Ambient-Temperature Chloroaluminate Molten Salts.

Ambient temperature AlCl(3)-1-ethyl-3-methylimidazolium chloride (ImCl) molten salts, both basic (40/60 mol % AlCl(3)/ImCl) and acidic (60/40 mol % AlCl(3)/ImCl), were used in an electrochemical investigation of centered hexanuclear zirconium halide clusters. In the basic molten salt, these [(Zr(6)ZCl(12))Cl(6)](n-) (Z = Be, B, C, Mn, Fe) centered clusters exhibit the following electrochemical reactions on a glassy carbon electrode (potentials vs Al/Al(3+)): [(Zr(6)BeCl(12))Cl(6)](n)()(-) + e(-) right harpoon over left harpoon [(Zr(6)BeCl(12))Cl(6)](()(n)()(+1))(-), E(1/2) = -0.613 V (n = 4), E(1/2) = -1.085 V (n = 5); [(Zr(6)BCl(12))Cl(6)](n)()(-) + e(-) right harpoon over left harpoon [(Zr(6)BCl(12))Cl(6)](()(n)()(+1))(-), E(1/2) = -0.365 V (n = 4), E(1/2) = 0.072 V (n = 3); [(Zr(6)CCl(12))Cl(6)](n)()(-) + e(-) right harpoon over left harpoon [(Zr(6)CCl(12))Cl(6)](()(n)()(+1))(-), E(1/2) = 0.230 V (n = 3); [(Zr(6)MnCl(12))Cl(6)](4)(-) + e(-) right harpoon over left harpoon [(Zr(6)MnCl(12))Cl(6)](5)(-), E(1/2) = -0.432 V. In the acidic melt, only electrochemical reactions [(Zr(6)BeCl(12))(AlCl(4))(6)](n)()(-) + e(-) right harpoon over left harpoon [(Zr(6)BeCl(12))(AlCl(4))(6)](()(n)()(+1))(-), E(1/2) = -0.069 V (n = 5), E(1/2) = 0.504 V (n = 4), and [(Zr(6)BCl(12))(AlCl(4))(6)](4)(-) + e(-) right harpoon over left harpoon [(Zr(6)BCl(12))(AlCl(4))(6)](5)(-), E(1/2) = 0.700 V are clearly observed. These data is consistent with less systematic observations of oxidation of these clusters in solution. No unambiguous one-electron electrochemical reduction nor oxidation is observable for the Fe-centered cluster in the ionic liquids. The half-wave potentials of the above reactions of the Be-, B-, C-, and Mn-centered clusters are controlled largely by clusters' charges. This correlation of redox potentials allows a useful direct comparison with data for related hexanuclear niobium clusters in the literature.

ChemInform Abstract: Detailed Structural and Theoretical Studies of the Bonding in Edge‐Bridged Halide and Oxyhalide Octahedral Niobium and Tantalum Clusters.

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The stability of multiply charged vanadium, niobium and tantalum clusters

We have studied the stability of multiply charged vanadium, niobium and tantalum clusters with charges up to z=4. Although the multiply charged clusters are less stable compared with the predictions of the symmetric liquid droplet models, the experimental results can be explained well by an asymmetric fission model assuming an emission of an atomic ion.

Sequential attachment of hydrogen molecules to niobium cluster cations

The reactivity of niobium cluster cations Nb n + ( n=6–12) towards molecular hydrogen was studied using an FT-ICR mass spectrometer combined with a laser-ablation/supersonic-jet external ion source. The reaction rate patterns for the sequential adsorption of hydrogen on niobium cluster cations as well as the H 2 saturation coverage pattern were measured. It was found that for the Nb 8 + and Nb 11 + clusters, the adsorption of an H 2 molecule can increase the reactivity of the cluster toward another H 2 molecule. This suggests that the adsorption of a hydrogen molecule can substantially change the electronic configuration and/or geometric structure of a niobium cluster and therefore supports the chemisorption mechanism for H 2 adsorption on Nb n + clusters.

Structural, electronic, and vibrational properties of neutral and chargedNbn (n=8,9,10)clusters

We present a theoretical study of the structural, vibrational, and electronic properties of small niobium clusters $({\mathrm{Nb}}_{8},$ ${\mathrm{Nb}}_{9},$ and ${\mathrm{Nb}}_{10})$ using density-functional theory with local as well as gradient-corrected exchange correlation density functionals. For ${\mathrm{Nb}}_{8}$ and ${\mathrm{Nb}}_{9},$ several low-energy structures were obtained, whereas for ${\mathrm{Nb}}_{10}$ there seems to be only one isomer that is energetically preferred. Vibrational spectra are in the range 100--300 ${\mathrm{cm}}^{\ensuremath{-}1}$ and are sensitive to the structure. Electronic properties are investigated by means of the densities of states, which depend strongly on the geometry. The theoretical results for the ionization potentials and vertical detachment energies are in good agreement with available experimental data.

Dehydrogenation and physisorption of saturated hydrocarbons (n-butane and isobutane) on Nbx+

Niobium cluster cations, Nbx+, are produced from a laser vaporization/supersonic expansion cluster ion source, and their reactivities are studied using a fast-flow reactor. The reaction patterns of n-butane and isobutane are found to be quite similar. In general, the relative reactivity of the cluster cations shows a decreasing trend with increasing cluster size (x=1–9). Dehydrogenation products from the butane molecules are observed within the small cluster size range (x=1–6). The degree of dehydrogenation increases with cluster size (x=1–6); but right after x=7, no dehydrogenation is observed and only molecular adsorption takes place. The product distribution for isobutane is found to be more scattered to channels of different extent of dehydrogenation than that for n-butane. The results are explained on the basis of the prompt insertion and the lowering of the activation barrier by the charge-induced polarization.

Detailed Structural and Theoretical Studies of the Bonding in Edge-Bridged Halide and Oxyhalide Octahedral Niobium and Tantalum Clusters

New AxREM6X18 (A = monovalent cation; RE = rare-earth metal; M = Nb, Ta; X = Cl, Br; x = 0, 1, 2), A2REM6X18-yOy (y = 1, 3) and REM6X13O3 containing edge-bridged octahedral (M6Xi12)Xa6 units are ob...

Effect of charge upon metal cluster chemistry: Reactions of Nbn and Rhn anions and cations with benzene

The reactions of anionic niobium and rhodium clusters Mn−, M=Nb, Rh, n=3–28, with C6H6 are investigated under single collision conditions in a Fourier-transform ion-cyclotron-resonance mass spectrometer and compared with the results of previous studies on corresponding cationic species. This reveals strong effects of the cluster charge state on hydrocarbon activation as a function of cluster size. Both differences and parallels are observed for reactions of anions and cations. Niobium clusters with a given number of atoms react quite differently than those with a single atom more or less. The fact that almost identical such effects are in the present work found for anion clusters, as for cations with the same number of atoms but two less electrons, suggests that the observed reactivity patterns are more a function of the cluster shape and geometry, than of the details of their electronic structure. The variety of interesting trends and effects observed is interpreted in terms of simple physical models.

Synthesis and properties of [M 6X 12(C 3H 7NO) 6]X 2 (MNb, Ta; XCl, Br)

Reactions of the complexes [M 6X 12(C 2H 5OH) 6]X 2 (MNb, Ta; XCl, Br) with N, N-dimethylformamide afforded a new series of clusters of the composition [M 6X 12(C 3H 7NO) 6]X 2. These microcrystalline substances are not isotypic with any other series of niobium and tantalum clusters with the [M 6X 12] 2+ unit. The analytical and spectral data point to the conclusion that all octahedral coordination sites in the hexanuclear cluster units are occupied by the oxygen bonded N, N-dimethylformamide molecules.

Reactivity of niobium clusters with nitrogen and deuterium

Absolute rate coefficients are reported for reactions of Nbn clusters (n=2–20) with D2 and N2 at 280, 300, and 370 K. Most clusters are highly reactive but there are conspicuous exceptions at n=8, 10, and 16 for both D2 and N2. The origin of this trend in reactivity with cluster size and the reason why D2 and N2 show similar trends are discussed. Density functional theory (DFT) electronic structure calculations have been used to investigate the details of the reactions for the smallest clusters Nb2 and Nb8 with H2 and N2. The steric and electronic requirements for dissociation of H2 and N2 are described in terms of frontier orbital interactions. The main conclusion from the DFT calculations is that complete dissociation of H2 or N2 requires charge transfer by transit of an avoided crossing between neutral and ionic potentials. This idea is extended to larger clusters by using a simple charge transfer model that predicts an inverse correlation between reactivity and an appropriately defined effective ionization potential. Such a correlation is observed and indicates that the effective ionization potential is the dominant influence on reactivity.

Ti2 Nb6 Cl14 O4 : A Unique 2D-1D Network Combination in Niobium Cluster Chemistry.

Layers of niobium clusters that are linked to each other through zigzag chains of edge-sharing [TiCl4 O2 ] ocatahedra are the central structural features of the title compound. Bridging of the chains by [(Nb6 Cl8i O4i )Cl6a ]6- clusters results in the formation of empty tunnels (a section of the structure is shown on the right).

Mass spectrometric study of thermal decomposition process of niobium clusters Nb3Cl8 and Nb6Cl14

The thermal decomposition process of solid niobium clusters Nb3Cl8 (NbCl2.67) and Nb6Cl14 (NbCl2.33) was studied by effusion methods with mass spectral registration of the vapour composition and pressure and their dependence on temperature. It is shown that the NbCl2.67 cluster decomposes to the NbCl2.33 cluster with formation of niobium tetrachloride in the gas phase. The process is monovariant. The NbCl2.33 cluster decomposes to niobium dichloride. The process is bivariant because the homogeneity region is formed on the basis of the NbCl2.33 crystal lattice. The composition of the vapour phase changes with an Nb: Cl atomic ratio which decreases during the process. At the beginning of the process the vapour consists of niobium tetrachloride and at the end the vapour is only niobium trichloride. The enthalpy of thermal decomposition of NbCl2.67 (1.67 Nb3Cl8 (s) = 0.67 Nb6Cl14 (s) + NbCl4 (g)) which is ΔrH°(700K) = 287.4 ± 3.8 kJ mol−1 and the formation enthalpy of NbCl2.33, which is ΔfH°(Nb6Cl14, 298K) = – 479 ± 10 kJ mol−1, were calculated from experimental data.

Oxidation induced ionization and reactions of metal carbide clusters (Nb, Zr, V, Ta)

Following our recent report of the oxidation induced formation of Ti 8C 12 + (H.T. Deng, K.P. Kerns, and A.W. Castleman, Jr., J. Chem. Phys. 104 (1996) 4862), the oxidation induced ionization of niobium and zirconium carbide clusters are studied using a triple quadrupole mass spectrometer coupled with a laser induced plasma reaction source. It was found that reactions of both of these neutral carbide clusters with dioxygen leads to formation of carbide ions. The ion product distributions show that zirconium carbide clusters mainly take the form of Met—Car cations, but niobium carbide clusters favor a cubic-like crystalline pattern. Furthermore, reactions of mass-selected Nb xC y + with dioxygen result in a sequential loss of C 2 units from Nb xC y +, and leads to formation of Nb x + and Nb xC + depending on y being an even or odd number. However, Nb xC y + shows comparably low reactivity towards nitrous oxide through a single oxygen abstraction mechanism. In comparison with the reaction products of V xC y + with dioxygen, the complementary information obtained in the present study suggests that the C 2 unit is a basic building block for formation of small early transition metal carbide clusters. The oxidation induced ionization mechanisms are also discussed in relation with the stability, ionization potentials, and structures of the clusters.

Amide C−N Bond Cleavage and Formation of Nitride Promoted by a Niobium(II) Cluster

Reaction of the diamagnetic cluster (TMEDA)2Nb2Cl5Li(TMEDA) with the (3,5-Me2Ph)(Ad)NLi (Ad = adamantyl) amide led to a rare case of C−N bond activation and formation of tetravalent, dinuclear, and diamagnetic {(3,5-Me2Ph)(Ad)N]Nb(3,5-Me2Ph)}2(μ-NAd)2·ether (1) containing a Nb−Nb single bond. Conversely, a similar reaction carried out with [Cy2NLi]4 (Cy = cyclohexyl) yielded dinuclear {[(Cy2N)2Nb]2(μ-N)}(μ3-N)Li(TMEDA) (2).

Synthesis, Structure, and Properties of LiNb6Cl15

AbstractBlack single crystals of LiNb6Cl15 were obtained from reactions of Nb powder, NbCl5 and LiCl in sealed niobium tubes at 850°C. The heavy atom structure of LiNb6Cl15 (Ia3d (no. 230), Z = 16, a = 2055.5(2) pm at 100 K, R(F) = 0.028, R(F2) = 0.053) is isotypic with the structure of Ta6Cl15. The [Nb6Cl12]2+ clusters in the structure are three‐dimensionally interconnected via three additional Cla−a. These bridge all vertices of niobium clusters, consistent with [Nb6Cl12iCl]− (the NbCla–aNb angles are 140°). At low temperatures the lithium ions are disordered over one‐third of sites (48 f), occupying cavities inside of two distorted corner‐sharing tetrahedra of chloride. These positions for lithium represent the narrowest sections of infinite channels in the structure built up by chloride. An increased lithium ion mobility is obtained above 170 K by 7Li‐NMR measurements.

Reactions of Hexanuclear Niobium and Tantalum Halide Clusters with Mercury(II) Halides. I. Synthesis and structures of the semiconducting compounds [M6Br12(H2O)6][HgBr4] · 12H2O, MNb, Ta

AbstractA method for the preparation of new solvated clusters of the composition [M6Br12(H2O)6][HgBr2X2] · 12H2O (MNb, Ta; XCl, Br, I) is given. The cubic crystals of [Nb6Br12(H2O)6][HgBr4] · 12H2O 1 and [Ta6Br12(H2O)6][HgBr4] · 12H2O 2 were characterized by the X‐ray structure analysis: 1: cubic, space group Fd3m, a = 21.0072(6) Å, Z = 8, R = 0.051 (Rw = 0.066); 2: cubic, space group Fd3m, a = 20.9698(1) Å, Z = 8, R = 0.038 (Rw = 0.050). 1 and 2 contain octahedral cluster cation [M6Br12(H2O)6]2+ and tetrahedrally arranged [HgBr4]2− anion. The MM bond distances are 2.949(1) Å for 1 and 2.9000(8) Å for 2. The HgBr bond distances in [HgBr4]2− anion are 2.614(2) Å in 1 and 2.622(2) Å in 2. The crystal packing patterns of the isostructural clusters 1 and 2 involve the three‐dimensional hydrogen bond network; the crystalline water molecules act as donors of hydrogen to the bromine atoms of the cluster and [HgBr4]2− units, whereas the coordinated water molecules form hydrogen bonds to the crystalline water molecules. [Nb6Br12(H2O)6][HgBr4] · 12H2O is diamagnetic and semiconducting with the activation energy, Ea = 0.20 eV.

Chemical reactivity of size-selected supported clusters: An experimental setup

We describe an experimental setup for the investigation of the chemical reactivity of size-selected, supported clusters. The clusters are generated by a high frequency laser vaporization source. After mass selection and energy control the clusters are deposited under ultrahigh-vacuum conditions and with low kinetic energy onto thin oxide films grown on metal single crystals. Such films are ideal in their function as supports for size-selected deposited metal clusters. Measured currents of mass-selected niobium cluster ions were of the order of 1 nA and are high enough for fast deposition rates. Adsorbed molecules used on the one hand as probes to characterize the supported clusters and on the other hand for studying chemical reactions on these clusters are investigated in situ by Fourier transform infrared spectroscopy, temperature desorption spectroscopy, and Auger electron spectroscopy. The performance of these analytical tools is illustrated with CO adsorbed on Ni atoms supported on thin MgO(100) films grown on a Mo(100) single crystal.

Theoretical study of chemisorption on niobium clusters: carbon monoxide and oxygen

We have studied chemisorption on niobium clusters based on the local spin density approximation in a scheme which uses pseudopotentials and a plane wave expansion of the electronic wave functions. Results are presented for geometries and the electronic structure of Nb4 and Nb8, and compared with experimental data of ionization potentials. Key issues concerning atomic and molecular adsorption on metal clusters are the nature of the binding, the preferred configurations, and the changes induced on the cluster properties. We have examined these questions in the case of carbon monoxide and oxygen. For carbon monoxide, we calculate a significant reduction of the stretch vibration frequency and, in the case of oxygen, shifts of the ionization potentials were obtained. Our results for oxygen are in agreement with experimental data, indicating only a minor shift of the ionization potential due to oxidation, also as a function of coverage.