Mixed-valence Vanadium Research Articles

Synthesis, structure and properties of a new unsymmetric tetranuclear mixed-valence vanadium(IV/V) complex containing distinct V 2O 33+ cores

The synthesis, X-ray crystal structure, spectroscopic, and electrochemical properties of a new tetranuclear vanadium(IV,V,IV,V) complex [ ( VO) 4( btppnol) 2( μ-O) 2]( ClO 4) 2·2 DMF·2 H 2 O ( 1 ) utilizing the previously described unsymmetrical dinucleating ligand N-(2-hydroxybenzyl)- N, N ′, N ′-tris(2-pyridylmethyl)-1,3-diaminopropan-2-ol (H 2btppnol) are reported. The complex comprises two different [V 2O 3] 3+ core structures: one [V 2O 3] 3+ core in a syn-dioxo-μ-O(alkoxo) configurarion and another [V 2O 3] 3+ core in an anti-dioxo-μ-oxo configuration, as well as being composed of non-equivalent vanadium centers as a consequence of the use of an unsymmetrical ligand. This complex represents the first example in the literature of a vanadium complex with this particular structure and its interesting electrochemical and spectroscopic properties were investigated.

A novel procedure for the synthesis of NH3 incorporated VPO phases

It has been observed that as a function of a time delay introduced between the onset of the reduction of V2O5 with hydroxylamine hydrochloride and the addition of phosphoric acid, in the aqueous route preparation of the catalytically important VOHPO4·0.5H2O phase, a series of NH3 incorporated phases are obtained. Up to delay times of 3 min the crystalline VOHPO4·0.5H2O phase is obtained whereas a weakly crystalline hemihydrate is formed at a delay time of 4 min. Longer delay times result in X-ray amorphous phases until at delay time of 10 min and above a new crystalline phase containing mixed-valent vanadium emerges. All the phases obtained with delay times of 4 min and above have NH3 incorporated into them. The weakly crystalline VOHPO4·0.5H2O phase as well as the X-ray amorphous phases undergo transformation to a crystalline vanadyl pyrophosphate phase on calcination at 723 K in spite of having NH3 incorporated into them. The in-situ generation of NH3 during the preparation procedure is a hitherto unreported aspect of VPO chemistry and provides a convenient method for the preparation of novel NH3 incorporated VPO phases which could have strong potential as catalysts for alkane oxidation as well as for ammoxidation reactions.

Characterization and catalytic activity of novel palladium-incorporated vanadium phosphates

Palladium has been introduced into the layered vanadyl hydrogen phosphate hemihydrate VOHPO 4·0.5H 2O in different ways depending upon the medium of preparation. In aqueous medium, the introduction of palladium leads to the formation of a new phase containing mixed valent vanadium whereas in the preparation in organic medium, the palladium appears to substitute into the lattice of the parent compound. The palladium-incorporated compounds have been found to be catalytically active, both for oxidative coupling as well as for hydrogenation reactions, with the activity and selectivity varying quite markedly with the mode of incorporation.

Hydrothermal synthesis, structure and characterization of a novel layered vanadium oxide with metal coordination complexes: [Ni(en)2][V6O14

The hydrothermal reaction of V2O5, Ni(NO3)2·6H2O, ethylenediamine and H2O in the molar ratio of 1∶1∶3.5∶519 at 180 °C yields black plates of [Ni(en)2][V6O14]. This compound has been characterized structurally, magnetically, electrically, and spectroscopically. The crystal structure consists of novel mixed-valence vanadium oxide layers and interlayer metal coordination complexes. Each of the three VO5 square pyramids share two edges to form a V3O11 group. The V3O11 groups are connected through sharing edges to produce a linear ribbon. Adjacent strips share corners to form a layer structure. The two oxygen atoms from the neighbouring vanadium oxide layers are coordinated to each nickel atom in a trans-fashion, and form a three-dimensional framework structure. An EHT (Extended Huckel Theory) calculation was performed to analyze the electronic structure, and the electrical resistivity measurements confirmed the expected semiconducting behavior of [Ni(en)2][V6O14].

Structural and magnetic phase transitions of AV6O11 (A = Na, Sr, and Pb)

X-ray powder diffraction experiments have been made for mixed-valence vanadium compounds of AV6O11 (A = Na, Sr, and Pb) between 12 and 700 K and their magnetic susceptibility has been measured between 5 and 700 K. It was revealed that each compound of AV6O11 indicates a structural phase transition at Tt = 245 K for A = Na, 320 K for A = Sr and 560 K for A = Pb. This transition is accompanied by atomic displacement, leading to the formation of triangular trimers of the V(1) atoms, which construct a two-dimensional kagome lattice in AV6O11. The Curie constant C, the Weiss temperature Θp and the temperature-independent susceptibility χ0 have been obtained in two temperature regions, T < Tt and T > Tt. The abrupt reduction of C below Tt may indicate that the d-electrons of V(1) show a transition to a spin-singlet ground state at Tt.

Characterization of Vanadium Deposit Formation at a Hydrogen Evolving Electrode in Alkaline Media

Although nickel exhibits a high electrocatalytic activity toward the hydrogen evolution reaction, it undergoes extensive deactivation as a cathode during alkaline water electrolysis. The addition of dissolved to the electrolyte results in partial reactivation of nickel cathodes in 8 mol/L KOH at 70°C by formation of a vanadium-rich deposit. Various analytical techniques were employed to characterize the deposit in terms of surface morphology, phase analysis, and chemical composition. The deposit on nickel had a smooth and compact surface with a thickness of 1-2 μm. X-ray diffraction results indicated an amorphous structure with a Scherrer’s length less than 2 nm. Chemical and thermal analyses led to an empirical compound formula of That formula implied a mixed-valence vanadium compound, later confirmed by X-ray photoelectron spectroscopy. The mechanism of deposit formation was then investigated by cyclic voltammetry on a mercury electrode. A cathodic peak representing the reduction of vanadium species was apparent near the onset of hydrogen evolution on mercury. The behavior of the cathodic peak with concentration and sweep rate revealed key steps in the mechanisms of deposit formation. Deposit formation was modeled as an irreversible precipitation/polymerization reaction following a charge transfer step, to produce a large, mixed-valence vanadium compound. © 2001 The Electrochemical Society. All rights reserved.

A Novel Salt Formed by Mixed-Valence Vanadium(IV, V) [(VO)2O(bpy)2(C2O4)2] Anions and Ferromagnetic [Cu2(bpy)4(C2O4)] Cations: Structure, Spectroscopic Characterization and Magnetic Properties

The new heterometallic, mixed-valence compound [Cu2(bpy)4(C2O4)][(VO)2O(bpy)2(C2O4)2]2·10H2O has been synthesized, its crystal structure determined and its spectroscopic characterization accomplished by means of solid-state vibrational (far and mid FTIR) spectroscopy. The title compound crystallises in the triclinic system, space group P1¯ (No. 2) [a = 13.488(5) A, b = 14.160(6) A, c = 15.829(9) A, α = 87.22(2)°, β = 66.33(2)°, γ = 64.49(2)°, V = 2471(2) A3, Z = 1]. The compound consists of the cationic binuclear copper(II) complex [Cu2(bpy)4(C2O4)]2+, two anionic binuclear mixed valence vanadium(IV)−vanadium(V) complexes [(VO)2O(bpy)2(C2O4)2]−, and ten uncoordinated water molecules. The copper atom exhibits a Jahn−Teller-distorted octahedral coordination. The vanadium atoms adopt a strongly distorted octahedral coordination and form a characteristic O=V−O−V=O moiety with a significantly bent V−O−V link. The temperature dependence of the magnetic susceptibilities has been investigated in the temperature range 2−300 K and explained in terms of the ferromagnetic interaction between CuII ions giving J = 22.7 yJ (1.14 cm−1) and g = 2.014.

23Na-NMR Study in NaV6O11

A mixed valent vanadium compound NaV 6 O 11 shows a magnetic phase transition at T t =245 K. For the microscopic investigation of the transition at T t , 23 Na-NMR measurements from 70 K to 300 K were carried out on NaV 6 O 11 . The Knight shift, K , and the nuclear spin-lattice relaxation rate, 1/ T 1 , were measured. While 1/ T 1 above T t has an almost constant value, 1/ T 1 below T t shows a gap-type temperature dependence. This is the first direct evidence that the d -electrons at V(1) ions, which form a kagomé lattice in NaV 6 O 11 , exhibit a spin-singlet ground state below T t . The formation of the spin-singlet ground state of V(1) seems to be closely related to the frustration effect.

Mixed-valence vanadium oxides studied by XPS

We have investigated several mixed-valence vanadium oxides, single-crystals by X-ray photoelectron spectroscopy (XPS). Many studies have been performed on V 2O 5, VO 2 and V 2O 3. Less investigated are the mixed-valence compounds, as the V 3O 5, V 4O 7 and V 6O 13 systems, which are discussed here. They exhibit as a function of temperature metal–insulator transitions (MIT). A detailed analysis for these systems, above and below the MIT temperature is given. The core-level spectra for the mixed-valence compounds are compared with those of the related systems, V 2O 5, VO 2 and V 2O 3 having V 5+, V 4+ and V 3+ ions, respectively. The electronic structures for all investigated oxides are characterized by strong V 3d–O 2p hybridization and they are discussed in relation with similar studies.

Organically Templated Vanadium Oxide Phases: Synthesis and Structures of [H3NCH2CH2NH3][V2O6] and [HN(CH2CH2)3NH][VV2VIV4O14]·H2O

The synthesis and crystal structures of [H3NCH2CH2NH3][V2O6] (1) and [HN(CH2CH2)3NH][VV2VIV4O14]·H2O (2) are described. The structure of the oxidized compound 1 consists of parallel stacks of vanadium oxide chains of corner sharing {VO4} tetrahedra. The chains are stabilized by extensive hydrogen bonding involving oxide ligands of the chains and ethylenediammonium ions which fill the space between the stacks of chains. The structure of compound 2 consists of vanadium oxide layers separated by doubly protonated 1,4-diazabicyclo[2.2.2]octane and lattice water. The vanadium oxide layers, containing mixed-valence vanadium (VV and VIV) centers, are composed of zigzag ribbons of edge-sharing {VO5} square pyramids interconnected by {VO4} tetrahedra. Crystal data. C2H10N2O6V2 , 1: monoclinic, space group P21/c (No. 14), a=5.5359(5), b=12.9430(12), c=5.6856(5) A, α=90, β=97.460(2), γ=90°, V=403.93(6) A3, Z=2. A total of 2506 reflections (θmax=27.89°) was collected, of which 954 were used to resolve the structure. The structure was solved by direct methods and least-squares refinement converged at R=0.0592. C6H16N2O15V6, 2: monoclinic, space group C2 (No. 5), a=19.303(4), b=6.667(2), c=7.579(2) A, α=90, β=111.31(2), γ=90°, V=908.4(4) A3, Z=2. A total of 1779 reflections was collected, of which 1591 unique reflections were used for structural elucidation. The structure was solved by direct methods and least-squares refinement converged at R=0.0314.

Crystal Structure of the Low-Temperature Form and Evolution during the Phase Transition of the Electronic Properties of a Mixed-Valence Vanadium Oxide Inserting Quinuclidinium Cations

The layered solid VIVVV3O10, NC7H14 is obtained either by thermal decomposition of the decavanadate H2V10O28, (NC7H14)4 or by hydrothermal synthesis in the quaternary system V2O5:HF:quinuclidine:H2O. It undergoes a reversible phase transition between 200 and 230 K evidenced by single-crystal X-ray diffraction, DSC, conductivity, and ESR measurements. The low-temperature two-dimensional form is orthorhombic ( space group P21cn (nο. 33)) with lattice parameters a = 11.770(2) A, b = 18.289(4) A, c = 19.740(4) A, V = 4249(3) A3. The structure was solved from 4533 reflections (I ≥ 2σ(I)) with reliability factors R1(Fo) = 0.0573 and wR2(Fo2) = 0.1572. The ordering of the amines in the interlayer space is responsible for the transition and implies some changes in the geometry of the layers. The onset of a partial VIV/VV segregation is evidenced, which could explain the different regimes observed in the ESR and conductivity characteristics above and below the transition.

Mixed-Valence Ammonium Trivanadate with a Tunnel Structure Prepared by Pyrolysis of Polyoxovanadate

Abstract The pyrolysis of a polyoxovanadate solid, (NH4)12[V18O42(H2O)]·nH2O (n ≈ 11), at 300 °C in an Ar+NH3 atmosphere gave a mixed-valence vanadium oxide, (NH4)xV3O7 (x ≈ 0.6), isostructural with Cs0.37V3O7. The crystal structure of (NH4)xV3O7 was refined by the Rietveld method (hexagonal, P63/m, a = 9.8436(6), c = 3.6165(1) Å, V = 303.47(3) Å3, Z = 2). (NH4)xV3O7 comprises edge- and corner-sharing VIV/VO5 square-pyramids with an approximate ratio of VIV : VV ≈ 0.49 : 0.51, to form a columnar cavity along the c-axis, in which ammonium N atoms are hydrogen-bonded to apical O atoms of the VIV/VO5 square-pyramids. A reflux of the (NH4)xV3O7 powder in a 0.6 M LiOH/2-methoxyethanol solution brought about Li-insertion into the columnar cavity without any structural change in the {V3O7} framework.

Inorganic/Organic Hybrid Materials: Layered Vanadium Oxides with Interlayer Metal Coordination Complexes

The hydrothermal reactions of V2O5, H2C2O4, Ni(NO3)2, ethylenediamine(en) or 1,2-diaminopropane(enMe) and water yield layered vanadium oxides with interlayer metal coordination complexes [Ni(en)2]0.5[V3O7] (1) and [Ni(enMe)2]0.5[H2enMe]0.5[V6O14] (2). 1 crystallizes in an orthorhombic system with space group Pbcn and cell parameters a = 16.617(3) A, b = 6.6329(9) A, and c = 17.138(4) A. 2 crystallizes in a triclinic system with space group P1 and cell parameters a = 6.6698(7) A, b = 10.8679(11) A, and c = 13.6097(13) A. Both compounds are inorganic/organic compositions and consist of layered mixed-valence vanadium oxides with interlayers {Ni(en)2}2+ or {Ni(enMe)}2+ and H2enMe2+. In the structure of 1, a novel arrangement of vanadium and oxygen atoms of {V3O7}- was found different from that in its isomer [Zn(en)2][V6O14]. In 2, both {Ni(enMe)}2+ and H2enMe2+ serving as unusual pillars between {V6O14}2- layers were observed. Their infrared characters and thermal stabilities were also given.

Formation of SrVO 3 precursors from Sr(OH) 2 and V mO n ( [formula omitted]) by mechanical stressing under ambient or reducing atmosphere

Mechanochemical and subsequent heating processes to produce SrVO 3 are studied, starting from a mixture of Sr(OH) 2 and V m O n (2/5≦ m/n≦2/3) by milling and heating in an ambient or reducing atmosphere. Single phase perovskite SrVO 3 was obtained by firing the milled mixtures at 1300°C under reducing atmosphere with four dehydration steps via two intermediates, Sr 2V 2O 7 and Sr 8V 8O 25. The fourth dehydration step, corresponding to transformation of Sr 8V 8O 25 to SrVO 3, appears at around 1100°C by heating mixtures containing trivalent vanadium and milled in a reducing atmosphere. Dehydration of the mixtures was promoted by milling under N 2–4% H 2, due to reduction of precursors by hydrogen. The precursor from the mixture with III–V mixed-valent vanadium milled under N 2–4% H 2 is closer to the final product SrVO 3 than any other precursors.

Crystal structure and magnetism of SrV6O11 single crystals

Magnetic property and specific heat of mixed valence vanadium oxide SrV6O11 were investigated. Single crystals and polycrystals were synthesized with a solid state reaction. Four-circle x-ray structural analysis discovered a structural transition at a temperature range between room temperature and 353 K. This transition corresponds to that of NaV6O11 at 245 K. Specific heat measurement of SrV6O11 was performed between 20 and 150 K and two anomalies were observed at 42 and 73 K. These anomalies are consistent with the two peaks in the temperature dependence of magnetization. Below 42 K, SrV6O11 shows metamagnetism and its critical field is about 4 T at 5 K. Its basic magnetic structure at 73 K is considered to be an antiferromagnetic state formed by the localized electron system.

An Air-Synthesized, Mixed-Valent Vanadium Tungsten Monophosphate with Intersecting Tunnels: Cs5VW4O9VO4(PO4)4

A new mixed-valent vanadium tungsten monophosphate Cs5V2W4O13(PO4)4has been synthesized in air in spite of the presence of V(IV). It crystallizes in the space groupCmc21witha=14.948(1),b=20.008(2), andc=9.942(1) Å. Its original structure, built up from W2O11bioctahedra, P2VO10tritetrahedra units, and VO5pyramids, can be described from [W2P2O13]∞layers parallel to (100) interconnected through VO4tetrahedra and VO5pyramids. This open framework forms intersecting tunnels running along a→ and c→, where the Cs+cations are located. Bond strength–bond length calculations and magnetic measurements show that the electrons are localized, V(IV) sitting in the VO5pyramid, whereas the VO4tetrahedra and WO6octahedra are occupied by V(V) and W(VI), respectively.

Hydrothermal synthesis and crystal structures of two novel vanadium oxides containing interlamellar transition metal complexes

Two new vanadium oxide compounds, which contain interlamellar transition metal complexes bound to vanadium oxide sheets, were synthesized hydrothermally. The crystal structure of each reveals unusual coordination geometries and interlayer bonding. [Ni(2,2′-bipyridine)] 2[V 12O 32] ( 1) was prepared from the reaction of V 2O 5, NiO, H 2O and 2,2′-bipyridyl in the molar ratio 4:8:3312:0.5. The isomorphous compounds [M(2,2′-bipy)] 2[V 12O 32] (where M is Co ( 1a) and Cu ( 1b)) were prepared in a similar fashion. The Ni coordination compound 1 crystallizes as amber needles, and the structure was solved in the space group C2/ m (#12) with unit cell parameters a=24.344(3) A ̊ , b=6.897(3) A ̊ , c=12.028(2) A ̊ , β=93.60(2)° ( R( Rw)=0.039(0.038)). This compound is composed of vanadium(V) oxide layers with nickel atoms coordinated within a single layer via four Ni–O–V linkages. Bidentate bipyridine (bpy) ligands complete the coordination sphere of the nickel atoms and interdigitate via π–π stacking with bpy ligands from the adjacent layer. Cu(en) 2[V 6O 14] ( 2) crystallizes as black plates in the space group P2 1/ n (#14) with unit cell parameters a=8.934(2) A ̊ , b=6.558(1) A ̊ , c=15.694(2) A ̊ , β=99.93(2)° ( R( Rw)=0.039(0.035)), from the reaction of CuCl 2·2H 2O, V 2O 5, H 2O and ethylenediamine in the ratio 1:1:447:4. This structure consists of mixed-valent vanadium(IV,V) oxide sheets forming weak O–Cu bonds to intercalated copper bis(ethylenediamine) complexes.

Synthesis and 183W and 51V Nmr Characterization of α- and β-K6H[Gew9V3O40].Xh2O and Esr Spectra of Mixed-Valence Vanadium Polyanions

Trivanadium-substituted heteropolytungstates, α- and β-K6H-[GeW9V3Q40]* xH2O (α- and β-GeW9V3), have been synthesized from their lacunary precursors, α- and β- [GeW9O34]10, and characterized by elemental analysis, cyclic voltammograms and 51V and 183NMR spectra. 183w NMR spectra of the title anions consist of tulo lines (intensity ratio 2 : 1) as expected for the C3V structure of trisubstituted α- and β-Keggin anions. The 2Jw.o.w couplings of the title anions exhibit sharper lines (19.35 and 16.73 Hz. respectively), confirming that the title complexes contain substituted vanadium in adjacent comer-shared octahedra of α- and β-Keggin structures, respectively. The solution ESR spectra of the one-electron-reduced anions consist of more than 40 equally spaced lines, showing that the three VO6 octahedra in both α- and β-fom are comer-shared and one of the bridging oxygen atams is protonated at pH 4.7.

A Mixed-Valent Vanadium Monophosphate with a Chain-Like Structure: Ba3V2O3(PO4)3

A new mixed-valence vanadium monophosphate with a chain-like structure has been synthesized. It crystallizes in theP212121space group witha=5.137(1),b=12.418(1), andc=20.724(3) Å. The structure consists of [V2P3O15]∞ribbons running along a, whose cohesion is ensured by Ba2+cations. Each ribbon is built from two sorts of chains:[VPO7]∞chains, which consist of corner-sharing VO5trigonal bipyramids and PO4tetrahedra, and [VP2O10]∞chains built of VO6octahedra and PO4tetrahedra. The vanadium-centered polyhedra are linked together by corner-sharing, so that V2O10units are formed. Charge ordering is observed on the vanadium species, with V(IV) in the octahedron and V(V) in the bipyramid.

New mixed valent vanadium monophosphate with an intersecting tunnel structure Ba2V5O8(PO4)4

A new mixed valent vanadium monophosphate Ba2V5O8(PO4)4 has been synthesized. It crystallizes in the space group I1m1 with a=7.6405(1), b=22.777(1), c=11.6157(7) A and β=103.38(1)°. Its framework consists of [V4P4O22 ]∞ layers of corner sharing polyhedra, interconnected by VO4 tetrahedra which delimit tunnels in which the barium atoms are located. The vanadium atoms exhibit three kinds of coordination; pyramidal, octahedral and tetrahedral, in the same framework.