Vanadium Clusters Research Articles

Theoretical calculations on a series of dinuclear vanadium and niobium clusters

Electronic structures of chalcogenide-bridged binuclear clusters of vanadium and niobium with the {M2(μ-Q2)2}4+, {M2(μ-Q)2}4+ and {M2(μ-Q)(μ-Q)2}4+ cores (Q=S, Se, Te) have been studied by density functional theory methods. In the vanadium clusters, the V–V distances are calculated to be in the range of 2.766–3.193Ǻ, whereas in the niobium clusters the calculated Nb–Nb bond lengths fall in the range of 2.881–3.380Ǻ, in accordance with the experimentally determined values. The calculated M–M bond distances generally decrease in the order {M2(μ-Q2)2}4+>{M2(μ-Q)2}4+>{M2(μ-Q)(μ-Q)2}4+ (M=V, Nb, Q=S, Se). The calculated enthalpies of formation for the V clusters are higher than for the corresponding Nb clusters. On the other hand, the M2Q2 clusters have always higher enthalpies of formation than the M2Q3 species, and also (with the exception of M=V, Q=S) higher values of enthalpy of formation than for the M2Q4 species. The hardness η of the niobium clusters are higher than that of the vanadium analogs, except for the [V2S2(SH2)8]4+ case. The enthalpies ΔH298 and the free energies ΔG298 for the reactions of hydrogen addition to the [V2(μ-S2)2(H2O)8]4+ and the [Nb2(μ-S2)2(H2O)8]4+ clusters at constant pressure are −121.75 and −59.73kJ/mol for the vanadium cluster, and 13.97 and 75.15kJ/mol for the niobium cluster.

Electronic structure and transport properties of early transition metal tripledeckers

The electronic structure and transport properties of the Cp(2)BzM(2) (M = Sc, Ti, and V) tripledeckers are studied by spin polarized density functional theory and nonequilibrium Green's function method considering high-spin and low-spin states. Total energy calculations show that the sandwich structured Cp(2)BzSc(2) exists in a singlet state with no local magnetic moment on the Sc atoms. Cp(2)BzTi(2) in triplet state exists as a distorted tripledecker and is more stable than singlet and quintet states. Cp(2)BzV(2) stabilizes in the quintet state with a spin density of 2.4 on each vanadium atom. Hund's coupling plays a vital role in stabilizing the higher multiplets in case of titanium and vanadium clusters. In bigger clusters like Cp(3)Bz(2)M(4), Sc multidecker has one unpaired spin, Ti multidecker has five unpaired spins, and V multidecker has seven unpaired spins in total. Spin polarized electronic transport is found for all states of vanadium tripledecker and one state of the titanium tripledecker when connected to a gold two probe junction. Moderate to high-spin filter efficiencies are calculated for these states. Cp(2)BzSc(2) shows spin-independent electronic transport for all electronic states when introduced in the gold two probe junction. Current versus voltage curves are reported for selected clusters in the two probe setup.

Tripodal Thiols as Ligands for Molecular Magnets: Very Strong Antiferromagnetic Exchange Interactions in Vanadium(III) Clusters

The synthesis, structural and magnetic characterisation of [V(III)(3)O(tmme)(2)(diimine)(2)Cl] [diimine=2,2'-bipyridine (1) or 1,10-phenanthroline (2)] and (HNEt(3))(2)[V(III)(4)O(tmme)(4)] (3) is reported, in which H(3)tmme is tris(mercaptomethyl)ethane, MeC(CH(2)SH)(3), the thiol analogue of the famous tripodal alcohol ligands typified by H(3)thme [tris(hydroxymethyl)ethane, MeC(CH(2)OH)(3)]. Complexes 1 and 3 have "T-shaped" and square topologies, respectively, and the latter is centred on a rare example of a square-planar oxide. The tri-thiolate ligands bind the periphery of the clusters and provide such strong antiferromagnetic exchange pathways that in both cases only a single total spin state is occupied up to room temperature, in the absence of metal-metal bonding. Magnetic data, electronic structure calculations and electrochemical data are reported.

EFFECT OF VANADIUM CONTENT ON PHOTOLUMINESCENCE AND MAGNETIC PROPERTIES OF DOPED ZnO THIN FILMS

ZnO : V thin films with different doping concentration (0%, 1.8%, 3.9%, 6.8%, 10%, and 13%) were fabricated by direct current magnetron sputtering. The X-ray diffraction patterns show that the wurzite structure changed with doping concentration. Furthermore, we could not find any vanadium cluster or phase separation in the X-ray diffraction patterns. The photoluminescence of ZnO : V with different vanadium concentration was investigated. The room temperature photoluminescence spectrum indicates that the films have purple band with 370 nm and the bands with 475 and 490 nm. The peak intensity of room temperature photoluminescence spectrum was affected by vanadium contents and its position remained stable. The intensity of band with 370 nm increases with raising the vanadium concentration and then decreases. The hysteresis behavior indicates that films were ferromagnetic at 50 K. Room temperature ferromagnetism was observed for the film with the doping concentration at 6.8%. However, in this case almost no hysteresis is noticeable. The results implied that the doping concentration and crystalline microstructure influence strongly the film's magnetic characteristics. Increasing the vanadium content in the film caused the degradation of the magnetic ordering.

A transition-metal-templated 3-D supramolecular framework based on molybdenum–vanadium polyoxoanions

A polyoxometalate [Co(phen)3][Co(en)3][Co(en)2(H2O)2][Co(en)2]0.5[O40(VIVO)2] · 6H2O (1) (en = ethylenediamine, phen = 1,10-phenanthroline) has been hydrothermally synthesized and structurally characterized by single-crystal X-ray diffraction, IR, electron paramagnetic resonance, X-ray photoelectron spectroscopy, elemental analyses, and thermogravimetry. The structure is an interesting 3-D supramolecular network containing bi-capped Keggin-type molybdenum–vanadium clusters with transition metal complexes generated in situ under mild hydrothermal conditions. Compound 1 contains four different counteractions being coordinated complexes of cobalt. The magnetic properties of 1 have also been studied in the temperature range of 4–300 K, and its magnetic susceptibility obeys the Curie–Weiss law, showing ferromagnetic coupling.

Room-temperature ferromagnetism in Ti 1− xV xO 2 nanocrystals synthesized from an organic-free and water-soluble precursor

Ti 1− x V x O 2 nanocrystals were prepared by employing a novel and water-soluble precursor via hydrothermal method, and the microstructure and magnetic properties have been investigated. All the samples belong to a pure anatase structure and exhibit room-temperature ferromagnetism (RTFM) without any trace of vanadium oxides or clusters. After V doping, the anatase structure is retained, while crystal growth is restrained. The homogenous distribution of V, in V 4+ chemical state, in TiO 2 lattice is confirmed by X-ray powder diffraction (XRD), energy-dispersive X-ray spectra (EDS), Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) analyses. Ferromagnetism in Ti 1− x V x O 2 is revealed to be highly dependent on the V content and defect concentrations. Furthermore, the annealing study in various atmospheres indicates that the oxygen vacancies and interstitials play a crucial role in inducing ferromagnetism in Ti 1− x V x O 2 system, and the origin of RTFM can be explained by bound magnetic polaron model.

Investigation of the New P′3-Na0.60VO2 Phase: Structural and Physical Properties

A new layered phase Na(0.60)VO(2) was synthesized by chemical deintercalation of sodium from the pristine compound O3-NaVO(2). The Na(0.60)VO(2) compound exhibits a distorted P'3-type oxygen stacking (AABBCC) with an average monoclinic unit cell containing a = 4.9862(14) A, b = 2.8708(8) A, c = 5.917(2) A, and beta = 104.36(3) degrees. A modulated structure was observed by transmission electron microscopy and X-ray diffraction (XRD) measurements. Indexation of the XRD pattern was achieved by using the q vector equal to 0.44b*, and the 4D superspace group C2/m (0 beta 0) s0 was then deduced. The specific heat measurement showed a strong correlated system with a gamma value of around 20 mJ x mol(-1) x K(-2). The electrical conductivity shows a semiconductor-like behavior with an activation energy of 0.52 eV. A paramagnetic behavior of the susceptibility is observed below room temperature with a Curie constant equal to C = 0.076 emu x K(-1) x mol(-1) x Oe(-1). To explain this small value, a model of pseudotriangular clusters of vanadium with a random distribution of V(3+) and V(4+) was considered.

Pronounced Cluster-Size Effects: Gas-Phase Reactivity of Bare Vanadium Cluster Cations Vn+(n= 1−7) Toward Methanol

Mass spectrometric experiments are used to examine the size-dependent interactions of bare vanadium cluster cations V(n)(+) (n = 1-7) with methanol. The reactivity patterns exhibit enormous size effects throughout the range of clusters investigated. For example, dehydrogenation of methanol to produce V(n)OC(+) is only brought about by clusters with n > or = 3. Atomic vanadium cation V(+) also is reactive, but instead of dehydrogenation of the alcohol, expulsions of either methane or a methyl radical take place. In marked contrast, the reaction efficiency of the dinuclear cluster V(2)(+) is extremely low. For the cluster cations V(n)(+) (n = 3-7), complete and efficient dehydrogenation of methanol to produce V(n)OC(+) and two hydrogen molecules prevails. DFT calculations shed light on the mechanism of the dehydrogenation of methanol by the smallest reactive cluster cation V(3)(+) and propose the occurrence of chemisorption concomitant with C-O bond cleavage rather than adsorption of an intact carbon monoxide molecule by the cluster.

A Magneto-Structural Study of a Hexanuclear (VIV═O)6-Complex and a Tetranuclear Mixed-Valent [VIII2VIV2] Species

Syntheses, crystal structures, and magnetic properties are reported for a hexametallic and a mixed-valent tetrametallic vanadium cluster, namely [Na(L(1))(6)(V(IV)=O)(6)]ClO(4) (1) and [(L(2))(2)V(III)(2)V(IV)(2)(mu-Cl)(6)Cl(4)] (2), where H(2)L(1) represents N-methyldiethanolamine and H(2)L(2) N'N'-di(3,5-di-tert-butyl-salicylidene)-1,3-diaminobenzene. The structure of the cation in 1 is comprised of a hexagon of six (V(IV)=O) units, in which a sodium ion is encapsulated in the center of the hexagon. Thus, the hexametalate ring contains six d(1) ions, each of which is in an octahedral environment of O(5)N atoms. Magnetic studies reveal the cluster to exhibit ferromagnetic exchange interactions (J = +16.7 +/- 0.3 cm(-1)), which is rationalized by the orthogonal neighboring xy planes of d(1) V(IV)=O ions. The structure of complex 2 can be considered as two mixed-valent face-sharing bioctahedral units of [V(III)(mu-Cl)(3)V(IV)] bridged by two m-phenylene linkers. The V(1)(III)...V(2)(IV) distances of av. 3.10 A preclude metal-metal bonding. Variable-temperature magnetic susceptibility data are fitted to obtain the parameters J(1) = -47.6 cm(-1), J(2) = +1.5 cm(-1), g(V(III)) = 1.95, g(V(IV)) = 1.85 (H = -2J x S(i).S(j)). The ferromagnetic coupling J(2) operates at a distance of approximately 7.09 A between two vanadium centers through the spin polarization mechanism due probably to the topology (1,3-substitution) of the m-phenylene bridges. The X-band electron paramagnetic resonance (EPR) spectrum analyzed with S(t) = 1 shows that the mixed-valent V(III)V(IV) species has indeed been formed and the tetranuclear core remains intact in solution. The hyperfine coupling constants indicate the localized mixed-valence nature of 2.

Tri-, tetra- and octa-metallic vanadium(III) clusters from new, simple starting materials: interplay of exchange and anisotropy effects

The crystal structure of the monomeric vanadium(III) species mer-[V(bipy)Cl(3)(MeCN)] (1; bipy = 2,2'-bipyridine) is reported. The solvothermal reaction of [V(bipy)Cl(3)(MeCN)]with Na(O(2)CPh) yields the T-shaped cluster [V(3)(O)Cl(3)(O(2)CPh)(2)(bipy)(2)(OEt)(2)], magnetic studies of which show strong intramolecular antiferromagnetic coupling giving a well isolated S = 1 ground state. Solvothermal treatment of 1 with triols yields a series of polymetallic clusters [V(4)Cl(6)(thme)(2)(bipy)(3)], [V(3)Cl(4)(Hcht)(2)(bipy)(2)]Cl and [V(8)(OH)(2)Cl(4)(cht)(4)(O(2)CPh)(6)(bipy)(2)], structurally related to previously reported {M(4)} centred triangles. Magnetic studies of this series reveal very weak intramolecular antiferromagnetic exchange and very strong local zero-field splitting effects.

Two new transition-metal complexes (TMCs)-templated three-dimensional supramolecular networks based on tungstovanadophosphates

Two new polyoxometalates [ Ni ( phen ) 3 ] [ Ni ( en ) 3 ] [ Ni ( en ) 2 ( H 2 O ) 2 ] [ Ni ( en ) 2 ] 0.5 [ PW VI 7 W V 2 V IV 3 O 40 ( V IV O ) 2 ] · 6 H 2 O ( 1) and [ Ni ( phen ) 3 ] 2 [ Ni ( en ) 2 ] Na [ PW VI 7 W V 2 V IV 3 O 40 ( V IV O ) 2 ] · 8 H 2 O ( 2) (en=ethylenediamine, phen=1,10-phenanthroline) have been hydrothermally synthesized and structurally characterized by single-crystal X-ray diffraction, IR, EPR, XPS, elemental analysis and thermogravimetry. Their structures exhibit interesting 3D supramolecular networks, and contain new bicapped (pseudo-) Keggin-type tungsten–vanadium cluster and the template transition-metal complexes (TMCs) being generated in situ under mild hydrothermal conditions. Interestingly, compound 1 contains four different kinds of nickel complex countercations.

Structural investigations of sandwich-type heteropolyoxometalate with dinuclear vanadium cluster

The sandwich-type K10[(VO)2Sb2W20O70]·20H2O heteropolyoxotungstate was investigated by means of elemental analyses, thermogravimetric and spectroscopic (FT-IR, UV–vis and EPR) methods. The stretching vibration of the terminal W=Ot bonds is shifted with 8cm−1 towards higher wave numbers in the FT-IR spectrum of the complex, which indicates the involvement of the terminal oxygen atoms in the coordination to the vanadium ions. The UV electronic spectra of the ligand and complex spectrum presents two bands assigned to ligand to metal charge transfer pπ→dπ transitions in the W=Ot bonds and the electron transition dπ→pπ→dπ between the energetic levels of the tricentric bonds W–Ob–W. The powder EPR spectrum obtained at room temperature is typical for mononuclear oxovanadium species in an axial environment. The spectrum exhibits eight components, both in perpendicular and parallel bands due to the hyperfine coupling of the spin of one unpaired electron with the nuclear spin of the 51V isotope (g⊥=1.908, gII=1.974, A⊥=69G, AII=201G).

Size Dependence ofL2,3Branching Ratio and2pCore-Hole Screening in X-Ray Absorption of Metal Clusters

Resonant 2p x-ray absorption spectra of size-selected transition metal ions and clusters consisting of 1<or=n<or=200 atoms are reported. Remnants of atomic multiplet splitting in L(2,3)-edge x-ray absorption can be resolved up to the trimer, above which the overall line shape is already bulklike. A strong cluster size dependence of the L(2,3) branching ratio was found for titanium, vanadium, and cobalt clusters. While 3d electron delocalization increases with cluster size, the apparent 2p spin-orbit splitting remains constant within the error bars. The size dependence of the L(2,3) branching ratio can be attributed to cluster size specific screening of the 2p-3d Coulomb interaction by 3d/4s valence electrons.

Mixed-Valent Dodecanuclear Vanadium Cluster Encapsulating Chloride Anions and Its Reaction To Form a “Bowl”-Shaped Cluster

Two oxovanadium phosphonate cage compounds have been synthesized in an organic solvent, and their characterization has been done by single-crystal X-ray analysis, IR spectroscopy, and bond valence sum calculations. The simple reaction of a mixed-valent closed V12 cage system produced another quasi-closed system composed of two V6 bowl-type cages.

VFe3S4 Single and Double Cubane Clusters: Synthesis, Structures, and Dependence of Redox Potentials and Electron Distribution on Ligation and Heterometal

Both vanadium and molybdenum cofactor clusters are found in nitrogenase. In biomimetic research, many fewer heterometal MFe3S4 cubane-type clusters have been synthesized with M = V than with M = Mo because of the well-established structural relationship of the latter to the molybdenum coordination unit in the enzyme. In this work, a series of single cubane and edge-bridged double cubane clusters containing the cores [VFe3(mu3-S)4]2+ and [V2Fe6(mu3-S)6(mu4-S)2]2+ have been prepared by ligand substitution of the phosphine clusters [(Tp)VFe3S4(PEt3)3]1+ and [(Tp)2V2Fe6S8(PEt3)4]. The single cubanes [(Tp)VFe3S4L3]2- and double cubanes [(Tp)2V2Fe6S8L4]4- (L= F-, N3-, CN-, PhS-) are shown by X-ray structures to have trigonal symmetry and centrosymmetry, respectively. Single cubanes form the three-member electron transfer series [(Tp)VFe3S4L3]3-,2-,1-. The ligand dependence of redox potentials and electron distribution in cluster cores as sensed by 57Fe isomer shifts (delta) have been determined. Comparison of these results with those previously determined for the analogous molybdenum clusters (Pesavento, Berlinguette, and Holm Inorg. Chem. 2007, 46, 510) allows detection of the influence of heterometal M on the properties. At constant M and variable L, redox potentials are lowest for pi-donor ligands and largest for cyanide and relate approximately with decreasing ferrous character in clusters with constant charge z = 2-. At constant L and z and variable M, EV > E(Mo) and delta(av)V < delta(av)Mo, demonstrating that M = Mo clusters are more readily oxidized and suggesting a qualitative relation between lower potentials (greater ease of oxidation) and ferrous character.

Two new polyoxometalate tri-supported transition metal complexes constructed from bi-capped Keggin molybdenum–vanadium clusters and copper complex fragments

Two new molybdenum–vanadium polyoxometalate tri-supported transition metal complexes [Cu(2,2 ′-bipy)] [Cu(2,2 ′-bipy) 2] 2[PMo 8V 6O 42]· nH 2O ( n = 1.5 ( 1) and n = 2 ( 2); 2,2 ′-bipy = 2,2 ′-bipyridine) have been hydrothermally synthesized and characterized by elemental analyses, IR, XPS and single-crystal X-ray diffraction analyses. Both compound 1 and compound 2 contain molybdenum–vanadium polyoxometalates, each of which support one [Cu(2,2 ′-bipy)] 2+ cation and two [Cu(2,2 ′-bipy) 2] 2+ cations, respectively. The difference between compound 1 and compound 2 is that the polyoxoanion of 1 is a bi-capped α-Keggin cluster and that of 2 is a bi-capped pseudo-Keggin one. Studies of magnetic properties indicated the presence of ferromagnetic behaviors for compounds 1 and 2.

H2 Adsorption on 3d Transition Metal Clusters: A Combined Infrared Spectroscopy and Density Functional Study

The adsorption of H2 on a series of gas-phase transition metal (scandium, vanadium, iron, cobalt, and nickel) clusters containing up to 20 metal atoms is studied using IR-multiple photon dissociation spectroscopy complemented with density functional theory based calculations. Comparison of the experimental and calculated spectra gives information on hydrogen-bonding geometries. The adsorption of H2 is found to be exclusively dissociative on Sc(n)O+, V(n)+, Fe(n)+, and Co(n)+, and both atomic and molecularly chemisorbed hydrogen is present in Ni(n)H(m)+ complexes. It is shown that hydrogen adsorption geometries depend on the elemental composition as well as on the cluster size and that the adsorption sites are different for clusters and extended surfaces. In contrast to what is observed for extended metal surfaces, where hydrogen has a preference for high coordination sites, hydrogen can be both 2- or 3-fold coordinated to cationic metal clusters.

Two new hydrogen bond-supported supramolecular compounds assembly from arsenic vanadates and [ M(H 2O) 6] 2+ cations ( M[dbnd]Co, Ni)

Two new supramolecular compounds based on arsenic vanadates formulated as [H 2As 6V 15O 42(H 2O)][Co(H 2O) 6] 2·2H 2O ( 1) and [H 2As 6V 15O 42(H 2O)][Ni(H 2O) 6] 2·2H 2O ( 2) have been prepared by reacting V 2O 5, H 2C 2O 4·2H 2O, As 2O 3,·H 2SO 4, CoCl 2·6H 2O (NiSO 4·6H 2O) and enMe (enMe=1,2-diaminopropane) under mild hydrothermal conditions and characterized by elemental analyses, IR, ESR, XPS and single crystal X-ray diffraction analyses. Crystal structure analyses reveal that compounds 1 and 2 are isostructural and exhibit novel 2-D supramolecular layer structures constructed from arsenic–vanadium clusters and two different types of secondary building units (SBUs), respectively, the different SBUs are formed by joint of two adjacent [Co(H 2O) 6] 2+ cations in compounds 1 and [Ni(H 2O) 6] 2+ cations in 2, respectively.

Chalcogenide Clusters of Group 5—7 Metals

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Highly Reduced, Polyoxo(alkoxo)vanadium(III/IV) Clusters

A family of high nuclearity oxo(alkoxo)vanadium clusters in unprecedentedly low oxidation states is reported, synthesised from simple vanadium diketonate precursors in alcohols under solvothermal conditions. Crystal structures of [V18(O)12(OH)2(H2O)4(EtO)22(O2CPh)6(acac)2] (1), [V16Na2(O)18(EtO)16(EtOH)2(O2CPh)6(HO2CPh)2]infinity (2), [V13(O)13(EtO)15(EtOH)(RCO2)3] in which R=adamantyl (3) or Ph3C (4), and [V11(O)12(EtO)13(EtOH)(Ph3CCO2)2(MePO3)] (5) are reported, revealing these to be {VIII 16VIV 2} (1), {VIII 9VIV 3VV} (3 and 4) and {VIII 3VIV 8} (5) clusters, while 2 consists of isolated {VIII 8VIV 8} clusters bridged into polymeric chains by {Na2(OEt)2} fragments. Solvothermal conditions are essential to the formation of these species, and the level of oxidation of the isolated clusters is in part controlled by the crystallisation time, with the lowest mean-oxidation-state species being isolated by direct crystallisation on controlled cooling of the reaction solutions.