Etude de nouveaux complexes hydrosolubles du niobium et de leur mise en oeuvre comme précurseurs d'oxydes multimétalliques

Le niobium est un élément relativement rare, dont la chimie a la réputation d'être peu attractive. Ces dernières années, un grand intérêt a cependant été porté aux oxydes multimétalliques à base de niobium et ce, dans de nombreux domaines de pointe. De plus, du fait des limitations des méthodes de préparation conventionnelles de tels matériaux, une alternative, consistant à combiner en solution des précurseurs moléculaires des métaux, a été considérée. Les avantages de cette méthode sont nombreux : bonne homogénéité du matériau, température de traitement plus faible, surface spécifique plus importante, Toutefois, dans le cas d'oxydes à base de Nb, peu de progrès ont été réalisés dans cette voie du fait de la faible disponibilité de sources appropriées. Dans ce contexte, une étude fondamentale sur les composés de coordination du Nb(V) a d'abord été réalisée. Des complexes hydrosolubles du niobium de type peroxo avec des ligands carboxylates, polyaminocarboxylates et hétérocycliques azotés ont été préparés et caractérisés. La mise en oeuvre de la méthode choisie pour la préparation des oxydes mixtes a également nécessité l'étude de complexes similaires des autres métaux à incorporer, à savoir le tantale(V) et le molybdène(VI). Des composés hétérodinucléaires Nb-Ta ont également été étudiés. La seconde partie a été consacrée à l'utilisation des complexes dans la "méthode des précurseurs moléculaires" pour préparer des oxydes Nb-Ta, Nb-Ta-V et Nb-Ta-Bi sous forme massique et des oxydes Nb-Mo sous forme massique et supportée sur silice. Nous avons ainsi obtenu plusieurs phases mixtes pures qui présentent des surfaces et des morphologies très différentes d'un système à l'autre. De plus, notre méthode permet, dans le cas des oxydes Nb-Mo, d'obtenir la phase Nb2Mo3O14 bien dispersée sur le support. Finalement, une étude exploratrice concernant la capacité potentielle des complexes peroxo du Nb(V) à oxyder un substrat organique en phase liquide a été menée. Plusieurs paramètres expérimentaux ont été modifiés et les résultats obtenus dans le cas de l'oxydation du phényl-1-éthanol et de la dibenzylamine sont encourageants et parfois proches de valeurs issues de la littérature dans le cas de catalyseurs couramment utilisés dans les mêmes réactions.(CHIM 3)--UCL, 200

Molecular precursor route to bulk and silica-supported Nb2Mo3O14 using water-soluble oxo-oxalato complexes.

The catalytically relevant Nb2Mo3O14 phase has been prepared in bulk and silica-supported forms via the so-called "multiple molecular precursors method" from water-soluble oxo-oxalato complexes of Nb and Mo, (NH4)3[NbO(ox)3].H2O, and (NH4)2[MoO3(ox)].H2O. Thermal treatment of the mixed Nb-Mo precursor has been optimized for the formation of the pure Nb2Mo3O14 phase, either as bulk oxide or a silica-supported phase with high specific surface area. A characterization of the bulk phase obtained via the conventional ceramic route has also been carried out and a comparison has been made with the precursors route. According to this route, the Nb2Mo3O14 phase is shown to be formed in a pure form at 700 degrees C (i.e., 100 degrees C below the lowest temperature reported so far for the formation of the phase by the ceramic method). The supported samples have appreciable specific surface areas of 60-70 m(2) g(-1), much larger than those reached in the previous attempts under vacuum in sealed vials. The SEM and EDX analyses reveal a high dispersion of the desired phase on the silica support

Nb-Ta, Nb-Ta-V, and Nb-Ta-Bi oxides prepared from molecular precursors based on EDTA

Multimetallic oxides of general formula (TaxNb1-x)(2)O-5, TaxNb1-xVO5, and BiTaxNb1-xO4 (0 < x < 1) are synthesized by a molecular precursors method from pre-isolated watersoluble EDTA complexes. These materials are obtained by thermal treatment in air of a homogeneous mixture of precursors of the metals to be incorporated in the final oxide. The obtention of a solid solution over the entire range of composition is evidenced, for the three systems studied, by Raman spectroscopy but also by DTA/DSC analyses and powder X-ray diffraction. The incorporation of small amounts of tantalum in the Nb-V-O and Nb-Bi-O systems is shown to stabilize phases difficult to obtain in a pure form, like the NbVO5 and beta-BiNbO4 phases. Moreover, the pore size distribution measurements and SEM pictures of the ternary and quaternary oxides reveal the (TaxNb1-x)(2)O-5 and BiTaNb1-xO4 phases display a disordered porous character. The TaxNb1-xVO5 oxides present a high crystallinity

Peroxo complexes of niobium(V) and tantalum(V)

This review covers the literature about niobium(V) and tantalum(V) complexes containing at least one peroxo ligand in their coordination sphere. In addition to the synthesis aspects, the available structural data as well as several results issued from techniques like IR and Raman spectroscopies, NMR, thermal analyses or mass spectrometry are collected and discussed. This paper also includes a brief description of the applications of these compounds in catalysis and as precursors in materials science. (c) 2006 Elsevier B.V. All rights reserved

Crystal structure of tetraguanidinium dioxo-tetraperoxo-mu-tartrato-dimolybdate(VI) dihydrate, (CN3H6)(4)[Mo2O2(O-2)(4)(C4H2O6)]center dot 2H(2)O

C16H60Mo4N24O36, orthorhombic, Pna2(1) (No. 33), a = 29.387(9) Angstrom, b = 9.979(3) Angstrom, c = 19.062(5) Angstrom, V = 5590.0 Angstrom(3), Z = 4, R-gt(F) = 0.094, wR(ref)(F-2) = 0.264, T = 293 K

Crystal structure of trimethylbenzylammonium hexachloroniobate(V), [(CH3)(3)C7H7N][NbCl6]

C10H16Cl6Nb, orthorhombic, Pbcm (no. 57), a = 8.246(3) A, b = 13.104(5) A, c = 16.142(7) A, V = 1744.2 A(3), Z = 4, R-gt(F) = 0.073, wR(ref)(F-2) = 0.307, T = 293 K

Spectroscopic and structural characterizations of novel water-soluble tetraperoxo and diperoxo[polyaminocarboxylato bis(N-oxido)]tantalate(V) complexes.

New water-soluble homoleptic peroxo complexes and heteroleptic peroxo-polyaminocarboxylato (PAC) complexes of tantalum(V) have been prepared. In the case of the peroxo-PAC complexes, the synthesis in the presence of excess H2O2 leads to the oxidation of the nitrogen atoms of the ligand into N-oxides. The compounds correspond to the general formula (gu)3[Ta(O2)2(LO2)] x xH2O (gu = guanidinium, L = edta or pdta) in which H4LO2 refers to the bis(N-oxide) derivative of the PAC ligand. The TaV complexes have been characterized on the basis of elemental and thermal analysis and by IR and 13C and 15N NMR spectroscopy. These last two spectroscopic methods have been used to suggest the coordination mode of the PAC ligand in the complexes. ESI mass spectrometry measurements have also been carried out for the peroxo-PAC compounds. The crystal structures of the homoleptic tetraperoxotantalate, (gu)3[Ta(O2)4] (1), and the heteroleptic complex, (gu)3[Ta(O2)2(edtaO2)] x 2.32H2O x 0.68H2O2 (2b), have been determined, showing, for both cases, an 8-fold-coordinated Ta atom surrounded either by four bidentate peroxides or by two peroxides and one tetradentate edtaO2 ligand

Synthesis and characterization of homo- and heterobimetallic niobium(v) and tantalum(v) peroxo-polyaminocarboxylato complexes and their use as single or multiple molecular precursors for Nb-Ta mixed oxides

New water-soluble bimetallic peroxo complexes of niobium(V) and/or tantalum(V) with high-denticity polyaminocarboxylate ligands have been prepared, characterized from the spectroscopic point of view, and used Lis molecular precursors for Nb-Ta mixed oxides. Four new homobimetallic complexes, (gu)(3)[Nb-2(O-2)(4)(dtpaO(3))]center dot 3H(2)O 1, (gu)(3)[Ta-2(O-2)(4)(dtpaO(3))]center dot 5H(2)O 2, (gu)(3)[Nb-2(O-2)(4) (HtthaO(4))]center dot 2H(2)O 4 and (gu)(3)[Ta-2(O-2)(4)(HtthaO(4))]center dot 3H(2)O 5 and the corresponding heterometallic complexes, (gu)(3)[NbTa(O-2)(4) (dtpaO(3))]center dot 2.5H(2)O 3 and (gu)(3)[NbTa(O-2)(4)(HtthaO4())]center dot 2H(2)O 6 have been obtained. In these compounds, the in situ oxidation of the nitrogen atoms of the PAC ligands into N-oxide groups has been evidenced by IR spectroscopy and mass spectrometry. The thermal treatment of the homonuclear complexes in air at 700 or 800 degrees C, depending on the Ta content, provided Nb2O5 or Ta2O5 while the heteronuclear Compounds led to the solid solution TaNbO5. BET and SEM measurements have been carried out and comparison of the morphology of the samples prepared from homo- and heterometallic precursors is discussed. (c) 2005 Elsevier Inc. All rights reserved

Crystal structure of ammonium oxo(peroxo)(pyridine-2,6-dicarboxylato-N,O,O)aquavanadate(V), (NH4)[VO(O-2)(H2O)(C7H3NO4)]

C7H9N2O8V, orthorhombic, Pcca (No. 54), a = 13.575(4) Angstrom, b = 20.603(6) Angstrom, c = 8.026(3) Angstrom, V = 2244.8 Angstrom(3), Z = 8, R-gt(F) = 0.046, wR(ref)(F-2) = 0.155, T = 293 K