Vanadyl Groups Research Articles

Analysis of spin-polarized solutions in the basis set of paired orbitals

This work is a brief review of the authors’ applications of Lowdin–Amos–Hall paired orbitals for the analysis of spin-polarized DFT solutions. The possibilities of this approach are demonstrated on the example (1) of models of Fe(III) hydroxocomplexes with two forms of the terminal oxo center, which are involved in the detachment of methane hydrogen, and (2) models of vanadium oxide experiencing the dissociation of the vanadyl group.

The Significance of Lewis Acid Sites for the Selective Catalytic Reduction of Nitric Oxide on Vanadium‐Based Catalysts

AbstractThe long debated reaction mechanisms of the selective catalytic reduction (SCR) of nitric oxide with ammonia (NH3) on vanadium‐based catalysts rely on the involvement of Brønsted or Lewis acid sites. This issue has been clearly elucidated using a combination of transient perturbations of the catalyst environment with operando time‐resolved spectroscopy to obtain unique molecular level insights. Nitric oxide reacts predominantly with NH3 coordinated to Lewis sites on vanadia on tungsta–titania (V2O5‐WO3‐TiO2), while Brønsted sites are not involved in the catalytic cycle. The Lewis site is a mono‐oxo vanadyl group that reduces only in the presence of both nitric oxide and NH3. We were also able to verify the formation of the nitrosamide (NH2NO) intermediate, which forms in tandem with vanadium reduction, and thus the entire mechanism of SCR. Our experimental approach, demonstrated in the specific case of SCR, promises to progress the understanding of chemical reactions of technological relevance.

The Significance of Lewis Acid Sites for the Selective Catalytic Reduction of Nitric Oxide on Vanadium-Based Catalysts.

The long debated reaction mechanisms of the selective catalytic reduction (SCR) of nitric oxide with ammonia (NH3 ) on vanadium-based catalysts rely on the involvement of Brønsted or Lewis acid sites. This issue has been clearly elucidated using a combination of transient perturbations of the catalyst environment with operando time-resolved spectroscopy to obtain unique molecular level insights. Nitric oxide reacts predominantly with NH3 coordinated to Lewis sites on vanadia on tungsta-titania (V2 O5 -WO3 -TiO2 ), while Brønsted sites are not involved in the catalytic cycle. The Lewis site is a mono-oxo vanadyl group that reduces only in the presence of both nitric oxide and NH3 . We were also able to verify the formation of the nitrosamide (NH2 NO) intermediate, which forms in tandem with vanadium reduction, and thus the entire mechanism of SCR. Our experimental approach, demonstrated in the specific case of SCR, promises to progress the understanding of chemical reactions of technological relevance.

Controllable Assembly of Vanadium-Containing Polyoxoniobate-Based Three-Dimensional Organic-Inorganic Hybrid Compounds and Their Photocatalytic Properties.

The controllable synthesis of two vanadium-containing polyoxoniobate-based three-dimensional organic-inorganic hybrid compounds, [Co(pn)2]4[HPNb10V(IV)2O40(V(IV)O)4]·17H2O (1) and [Co(pn)2]5[PNb12O40(V(IV)O)6](OH)7·15H2O (2), where pn = 1,2-diaminopropane, is realized by changing the hydrothermal temperature or adding N-(aminoethyl)piperazine as an additive. Both compounds 1 and 2 are structurally characterized by single-crystal/powder X-ray diffraction and IR and X-ray photoelectron spectroscopy. Compound 1 features a new divanadium-substituted Keggin polyoxoniobate capped by four vanadyl groups, and the polyanion in 2 exhibits the highest coordination number (10-connected) in polyoxoniobate chemistry. Moreover, the photocatalytic activities of 1 and 2 for hydrogen evolution are preliminarily assessed.

Structure of V2O5·nH2O Xerogels

Vanadium oxide is a layered compound that forms V2O5·nH2O xerogel when intercalated by water. The xerogel consists of V2O5 bilayers with water between them. The structure of each V2O5 layer in the bilayer is close to the structure of a single layer in bulk V2O5. However, the distance between the two layers in the bilayer is much smaller than the distance between single layers in the bulk. The xerogel is a Bronsted acid that has been used as an acid catalyst and whose protons are mobile and can be exchanged with other cations. Here, we use density functional theory to examine five possible models for the structure of the xerogel. In the model that has the properties established by experiments, the vanadyl groups in the two layers point toward the outside of the bilayer, while in the bulk V2O5 they point toward the space between layers. This change in the vanadyl positions allows the two layers to get unusually close to each other. This structure is unstable in the absence of water. Water stabilizes it by r...

Small, beautiful and magnetically exotic: {V4W2}- and {V4W4}-type polyoxometalates.

Minimal-nuclearity vanadato-tungstate clusters in [{V(IV)(dien)}4WO14]·4H2O () and [{V(IV)(dien)}4WO20]·6H2O () feature cores of edge-sharing WO6 octahedra, surrounded by a ring of four vanadyl groups. Surprisingly, the V(iv) centers in both and are ferromagnetically coupled, in contrast to all other known vanadato-polyoxotungstates featuring the ubiquituos V-O-W-O-V exchange pathways.

Surface Structure of V2O3(0001): A Combined I/V-LEED and STM Study

Using I/V-LEED and scanning tunneling microscopy, we have investigated the surface structure of ∼100 Å thick V2O3(0001) films on Au(111). Both methods clearly show that the surface is terminated by a layer of vanadyl groups. I/V-LEED quantitative structure determination applied to differently prepared films always leads to a Pendry R-factor for the V═O termination close to 0.11 while the R-factor for a reconstructed O3 termination is always larger than 0.2 and increases with increasing data set size. These results are at variance with a recent publication by Window et al. [Phys. Rev. Lett. 2015, 114, 216101] in which the authors propose that the V2O3(0001) surface is terminated by a reconstructed O3 structure. Surface oxidation experiments also contradict the conclusions of Window et al. since oxidation leads to a previously identified structure with a (√3 × √3)R30° LEED pattern which is not expected for oxidation of an O3-terminated surface. In the course of the I/V-LEED calculations the individual Debye temperatures of the surface atoms were determined as part of the structural optimization procedure. We show that this approach is superior to the kinematical analysis of temperature-dependent LEED measurements.

Surface structure of V_{2}O_{3}(0001) revisited.

In a recent paper [A. J. Window etal., Phys. Rev. Lett. 107, 016105 (2011)], it was proposed that V_{2}O_{3}(0001) is terminated by the so-called O_{3} termination, a reconstruction with a terminating distorted hexagonal oxygen layer. We show that the surface is terminated by vanadyl (V═O) groups instead. This conclusion is based on quantitative low-energy electron diffraction combined with scanning tunneling microscopy, fast atom scattering, and density functional theory employing the Heyd-Scuseria-Ernzerhof functional. New insights into the subsurface sensitivity of ion beam triangulation show that results previously interpreted in favor of the O_{3} termination are reconcilable with vanadyl termination as well.

DFT cluster model study of Mo[sbnd]V[sbnd]O-type mixed-metal oxides

Cluster models of the proposed active site of MoVO mixed-metal oxides (MMOs) have been explored computationally using density functional methods. Besides the density functional approximation, the size of the cluster models, the number of reduced metal centers, and the effect of the crystal environment on anionic cluster models have been examined. We studied the electronic structure of the MMO and, as a model reaction, the hydrogenation of a surface vanadyl group as well as the subsequent formation of a water molecule. For an adequate description of the electronic structure and the resulting reaction energies, it is necessary to use a density functional that accounts, at least partially, for the self-interaction error, e.g., the hybrid functional PBE0. To predict adequate reaction energies, one has to employ cluster models featuring at least two layers of the MoVO structure, so that inter-layer interactions can be modeled.

Size-dependent catalytic activity of supported vanadium oxide species: oxidative dehydrogenation of propane.

Possible reaction pathways for the oxidative dehydrogenation of propane by vanadium oxide catalysts supported on silica are examined by density functional theory. Monomeric and dimeric vanadium oxide species are both considered and modeled by vanadyl-substituted silsesquioxanes. The reaction proceeds in two subsequent steps. In a first step, hydrogen abstraction from propane by a vanadyl (O═V) group yields a propyl radical bound to a HOV(IV) surface site. Propene is formed by a second hydrogen abstraction, either at the same vanadia site or at a different one. V(V)/V(IV) redox cycles are preferred over V(V)/V(III) cycles. Under the assumption of fast reoxidation, microkinetic simulations show that the first step is rate-determining and yields Arrhenius barriers that are lower for dimers (114 kJ/mol at 750 K) than for monomers (124 kJ/mol). The rate constants predicted for a mixture of monomers and dimers are 14% larger (750 K) than for monomers only, although the increase remains within experimental uncertainty limits. Direct calculations of energy barriers also yield lower values for dimeric species than for monomeric ones. Reactivity descriptors indicate that this trend will continue also for larger oligomers. The size distribution of oligomeric species is predicted to be rather statistical. This, together with the small increase in the rate constants, explains that turnover frequencies observed for submonolayer coverages of vanadia on silica do not vary with the loading within the experimental uncertainty limits.

Assemblies of salen-type oxidovanadium(IV) complexes: substituent effects and in vitro protein tyrosine phosphatase inhibition.

Oxidovanadium(IV) complexes with substituted chiral tetradentate dianionic N,N'-bis-o-hydroxybenzylidene-1,2-propylenediamines were synthesized and their physicochemical properties were characterized using single crystal X-ray diffraction, elemental analysis, ATR FTIR, UV-VIS and EPR spectroscopy, cyclic voltammetry, spectroelectrochemistry and preliminary in vitro protein-tyrosine phosphatase inhibition activity studies. Different 5-substituents in the salicylaldehyde (condensed with 1,2-diaminopropane; 2 : 1) were tested, namely 5-Br (complex 1), 5-Cl (2), 5-NO2 (3) and 5-OCH3 (4). The crystal structures of 1 and 2 show square pyramidal coordination of vanadium and parallel arrangement of monomeric exo isomers in supramolecular dimers. The halogen-halogen interaction of substituents in 5,5'-positions leads to weakening of axial interaction between phenolate O and V in 2, compared to 1. The Br atom takes part in halogen bonding with a vanadyl group in 1. Complex 3 has a linear polymeric structure with a V-O-V asymmetric bridge motif (IR absorption band at 873 cm(-1), separated d-d bands and broad EPR band structure in frozen solution pointing to oligomeric nature) while 4 is monomeric (V=O stretching at 976 cm(-1), broad d-d band structure). Redox potentials of the V(4+)/V(5+) couple lie in the range of -0.14 to 0.21 V (vs. Fc/Fc(+)) and show substantial dependence on the electron withdrawing properties of the substituents. The charge transfer character of the bands present in the range 365-395 nm was confirmed based on UV-VIS spectroelectrochemical experiments. Different assemblies of complex molecules are influenced by the electron withdrawing properties of the 5,5'-substituents, leading to supramolecular dimers (1, 2 and 4) and linear polymeric self-assembly (3). An in vitro study of representative complex 1 showed protein tyrosine phosphatase activity inhibition higher than that of suramin but lower than those of oxidovanadium(IV) sulphate and bis(maltolato)oxidovanadium(IV).

Crystal lattice effect on the quenching of the intracluster magnetic interaction in [V12B18O60H6](10-) polyoxometalate.

In the present work, the synthesis and structural characterization of four new polyoxovanadoborate (BVO) frameworks based on the [V12B18O60H6](10-) polyanion are reported: (NH4)8(1,3-diapH2)[V12B18O60H6]·5H2O (1), K8(NH4)2[V12B18O60H6]·18H2O (2), K10[V12B18O60H6]·10H2O (3) and K8Cs2[V12B18O60H6]·10H2O (4). A global antiferromagnetic behaviour is observed for these 10V(IV)/2V(V) mixed valence clusters. The magnetic data of 1, 2 and 3, which present different countercation environments, show that 1 is more coupled than 2 and 3. DFT calculations show that the positive charges strongly influence the polarization mechanism of the spin density of the vanadyl groups and the extent of the magnetic orbitals, therefore corroborating the experimental observation of the quenching effect of the magnetic coupling between vanadium centres of 2 and 3.

Mitochondria‐Targeting Oxidovanadium(IV) Complex as a Near‐IR Light Photocytotoxic Agent

Oxidovanadium(IV) complexes [VO(L(1))(phen)]·Cl (1) and [VO(L(2))(L(3))]·Cl (2), in which HL(1) is 2-{[(benzimidazol-2-yl)methylimino]-methyl}phenol (sal-ambmz), HL(2) is 2-[({1-[(anthracen-9-yl)methyl]-benzimidazol-2-yl}methylimino)-methyl]phenol (sal-an-ambmz), phen is 1,10-phenanthroline and L(3) is dipyrido[3,2-a:2',3'-c]phenazine (dppz) conjugated to a Gly-Gly-OMe dipeptide moiety, were prepared, characterized, and their DNA binding, photoinduced DNA-cleavage, and photocytotoxic properties were studied. Fluorescence microscopy studies were performed by using complex 2 in HeLa and HaCaT cells. Complex 1, structurally characterized by X-ray crystallography, has a vanadyl group in VO2N4 core with the VO(2+) moiety bonded to N,N-donor phen and a N,N,O-donor Schiff base. Complex 2, having an anthracenyl fluorophore, showed fluorescence emission bands at 397, 419, and 443 nm. The complexes are redox-active exhibiting the V(IV)/V(III) redox couple near -0.85 V versus SCE in DMF 0.1 M tetrabutylammonium perchlorate (TBAP). Complex 2, having a dipeptide moiety, showed specific binding towards poly(dAdT)2 sequence. The dppz-Gly-Gly-OMe complex showed significant DNA photocleavage activity in red light of 705 nm through a hydroxyl radical ((.) OH) pathway. Complex 2 showed photocytotoxicity in HaCaT and HeLa cells in visible light (400-700 nm) and red light (620-700 nm), however, the complex was less toxic in the dark. Fluorescence microscopy revealed the localization of complex 2 primarily in mitochondria. Apoptosis was found to occur inside mitochondria (intrinsic pathway) caused by ROS generation.

Atomic Structure and Special Reactivity Toward Methanol Oxidation of Vanadia Nanoclusters on TiO2(110)

We have grown highly controlled VOx nanoclusters on rutile TiO2(110). The combination of photoemission and photoelectron diffraction techniques based on synchrotron radiation with DFT calculations has allowed identifying these nanostructures as exotic V4O6 nanoclusters, which hold vanadyl groups, even if vanadium oxidation state is formally +3. Our theoretical investigation also indicates that on the surface of titania, vanadia mononuclear species, with oxidation states ranging from +2 to +4, can be strongly stabilized by aggregation into tetramers that are characterized by a charge transfer to the titania substrate and a consequent decrease of the electron density in the vanadium 3d levels. We then performed temperature programmed desorption experiments using methanol as probe molecule to understand the impact of these unusual electronic and structural properties on the chemical reactivity, obtaining that the V4O6 nanoclusters can selectively convert methanol to formaldehyde at an unprecedented low temperature (300 K).

Reaction of [VO(OPr)3] with hexamethyldisylthiane in the presence of β-diketones

The reaction of [VO(OPr)3] (Pr is n-propyl) with hexamethyldisylthiane Me3SiSSiMe3 in the presence of β-diketones (acetylacetone (HAcac), hexafluoroacetylacetone (Hfac), and dipivaloylmethane (Dpm)), is studied. In all cases, vanadium(IV) and vanadium(III) β-diketonate complexes of different types are formed. New crystalline modification [V(Acac)3] is obtained in the reaction with HAcac. The mixedligand vanadium(III) complex of the composition [V2(Hfac)2(μ-OPr)]2 is formed with Hfac. In the presence of Dpm, the known vanadium(IV) complex [V2O2(Dpm)2(μ-OPr)2] is obtained in which two vanadyl groups VO2+ are linked by two bridging propoxy groups. The structures of all products are determined by X-ray diffraction analysis.

Catalysts Transform While Molecules React: An Atomic-Scale View.

We explore how the atomic-scale structural and chemical properties of an oxide-supported monolayer (ML) catalyst are related to catalytic behavior. This case study is for vanadium oxide deposited on a rutile α-TiO2(110) single-crystal surface by atomic layer deposition (ALD) undergoing a redox reaction cycle in the oxidative dehydrogenation (ODH) of cyclohexane. For measurements that require a greater effective surface area, we include a comparative set of ALD-processed rutile powder samples. In situ single-crystal X-ray standing wave (XSW) analysis shows a reversible vanadium oxide structural change through the redox cycle. Ex situ X-ray photoelectron spectroscopy (XPS) shows that V cations are 5+ in the oxidized state and primarily 4+ in the reduced state for both the (110) single-crystal surface and the multifaceted surfaces of the powder sample. In situ diffuse reflectance infrared Fourier transform spectroscopy, which could only achieve a measurable signal level from the powder sample, indicates that these structural and chemical state changes are associated with the change of the V═O vanadyl group. Catalytic tests on the powder-supported VOx revealed benzene as the major product. This study not only provides atomic-scale models for cyclohexane molecules interacting with V sites on the rutile surface but also demonstrates a general strategy for linking the processing, structure, properties, and performance of oxide-supported catalysts.

VO⋯C interactions in crystal structures of oxovanadium-coordination compounds

A survey of the Cambridge Structural Database (CSD) revealed the existence of a new supramolecular homosynthon in oxovanadium coordination compounds (OCCs) that is based on non-covalent VO⋯C interactions. VO⋯C close contacts were found in 38.7% of structures containing vanadyl or pervanadyl groups. The VO⋯C interaction is clearly distinguishable from a putative VO⋯H–C hydrogen bond upon analysis of OCC crystal structures deposited in the CSD. In the majority of structures, the OCCs assemble to form dimers or polymers. The VO⋯C interactions were also found to lead to self-assembly of two new oxovanadium β-diketonate complexes into dimers in the solid state.

V6O13films by control of the oxidation state from aqueous precursor to crystalline phase

An aqueous deposition process for V(6)O(13) films is developed whereby the vanadium oxidation state is continuously controlled throughout the entire process. In the precursor stage, a controlled wet chemical reduction of the vanadium(V) source with oxalic acid is achieved and monitored by (51)Vanadium Nuclear Magnetic Resonance ((51)V-NMR) and Ultraviolet-Visible (UV-Vis) spectroscopy. The resulting vanadium(IV) species in the aqueous solution are identified as mononuclear citrato-oxovanadate(IV) complexes by Electron Paramagnetic Resonance (EPR) and Fourier Transform Infra-Red (FTIR) spectroscopy. This precursor is successfully employed for the deposition of uniform, thin films. The optimal deposition and annealing conditions for the formation of crystalline V(6)O(13), including the control of the vanadium oxidation state, are determined through an elaborate study of processing temperature and O(2) partial pressure. To ensure a sub 100 nm adjustable film thickness, a non-oxidative intermediate thermal treatment is carried out at the end of each deposition cycle, allowing maximal precursor decomposition while still avoiding V(IV) oxidation. The resulting surface hydrophilicity, indispensable for the homogeneous deposition of the next layer, is explained by an increased surface roughness and the increased availability of surface vanadyl groups. Crystalline V(6)O(13) with a preferential (002) orientation is obtained after a post deposition annealing in a 0.1% O(2) ambient for thin films with a thickness of 20 nm.

A new ring-like arrangement of vanadyl(IV) groups bridged by monodentate alkoxides:cyclo-decakis(μ-cyclohexylmethanolato)pentakis[oxidovanadium(IV)

The pentanuclear title compound, [V(5)(C(7)H(13)O)(10)O(5)], has a metal-oxygen core that consists of five vanadyl(IV) centres bridged by the O atoms of cyclohexylmethanolate ligands. This particular ring topology is new to oxovanadium(IV) chemistry and resembles the structure proposed for [V(5)O(15)](5-) on the basis of (51)V NMR studies in aqueous solution. The bulky cyclohexylmethanolate ligands adopt chair-like conformations and project outwards from the central cyclic core. The title compound crystallizes in a centrosymmetric triclinic unit cell, which contains four independent but chemically identical molecules in the asymmetric unit. The crystal structure is devoid of any significant intermolecular interactions.

Sol–gel synthesis and luminescent properties of SiO2/Zn2SiO4 and SiO2/Zn2SiO4:V composite materials

Undoped and vanadium-doped Zn2SiO4 particles embedded in silica host matrix were prepared by a simple solid-phase reaction after the incorporation of ZnO and ZnO:V nanoparticles, respectively, in silica monolith using the sol–gel method with supercritical drying of ethyl alcohol in two steps. After supercritical drying and annealing in the temperature range between 1423 and 1473K in an air atmosphere, the photoluminescence (PL) measurements show a band centered at about 760nm in the case of non-doped Zn2SiO4 which is attributed to energy transfer from Zn2SiO4 particles to NBOHs interface defects. In the case of vanadium doped Zn2SiO4, the PL reveals a band centered at about 540nm attributed to the vanadium in the interfaces between Zn2SiO4 particles and SiO2 host matrix. Photoluminescence excitation (PLE) measurements show different origins of the emission bands. The PLE band (∼240–350nm) may be understood as an energy transfer process from O2− to V5+ which occurs intrinsically in the vanadyl group.