Showing papers by "Vadim G. Kessler published in 2007"

Synthesis of Nanocrystalline Zirconium Titanate and its Dielectric Properties

Abstract: The thermal decomposition of a ZrTi2[(OC2H4)2NH]3(OC3H7)6 precursor by the RAPET (reaction under autogenic pressure at elevated temperature) method provided the formation of crystalline zirconium titanate nanoparticles. These as-prepared nanoparticles are embedded in a carbon shell, which can be removed completely by calcination at 500 °C under air for 3 h, resulting in pure white crystalline nanoparticles. At a reaction temperature of 700 °C, the nanoparticles are mainly ZrTi2O6 (srilankite), whereas at 800 °C, the product is predominately Zr5Ti7O24. The structural, morphological, compositional, magnetic, and AC electrical properties are measured for the as-prepared ZrTi2O6 embedded in carbon (ZTEC), as well as the crystalline ZrTi2O6 nanoparticles (ZTN) obtained after sintering. The reaction mechanism is based on the decomposition products containing pyrrol and pyrazine. The presence of these compounds provides an understanding of the decomposition of the diethanolamine ligands and the formation of the nanoparticles in general.

Oxoethoxide Chlorides – Representatives of Oligonuclear Alkoxide Complexes of Gallium: Penta- and Dodecanuclear Molecules

Abstract: Metathesis of solid GaCl 3 with NaOEt/EtOH (1:3) provided, after separation of the NaCl precipitate, a solution containing oxoalkoxide chloride complexes of gallium. Residues from evacuation of the latter were redissolved in either toluene/ MeCN (solution 1) or toluene/pyridine (solution 2). Storage of the first solution provided crystals of Ga 5 (μ 5 -O)(μ-OEt) 8 Cl 5 (1), and of the second solution - [Ga 12 (μ 4 -O)2(μ 3 -O) 5 (μ-OEt) 10 -Cl 12 Py 4 ]Py (2), both at room temperature and in relatively high yields. Compound 1 crystallizes as a vertically contracted tetragonal pyramid with the μ 5 -O ligand situated in the center of the equatorial plane defined by four trigonal bipyramidal coordinated gallium atoms. The axial Ga atom has octahedral coordination. All eight edges of the pyramid are capped by μ-OEt groups, and all the chloride ligands are situated in terminal positions. Crystals of [Ga 12 (μ 4 -O) 2 (μ 3 -O) 5 (μ-OEt) 10 Cl 12 Py 4 ]Py (2) contain two [Ga 5 (μ 4 -O) (μ 3 -O) 2 (μ-OEt) 5 -Cl 5 Py] fragments connected through a cyclic [Ga 4 (μ 3 -O) 5 -Cl 2 Py 2 ) unit with a μ 3 -O ligand in its center. Of the metal atoms, three possess tetrahedral, seven trigonal bipyramidal, and two octahedral coordination. The mass spectrum of 1 contains penta- and tetranuclear fragments. The spectrum of 2 contains heptameric fragments in addition to the pentanuclear fragments that are analogous to those in the spectrum of 1. The factors leading to formation of polymeric M(OR) n and related oligonuclear oxocomplexes are discussed.

Isolation and single crystal study of [Nb2(μ-OMe)2(OiPr)8]. Can alcohol interchange provide the homoleptic niobium isopropoxide?

Abstract: Niobium isopropoxide, Nb(OiPr)5, is an attractive precursor of simple and complex niobium oxides in sol-gel technology. This compound cannot, unfortunately, be obtained by alcohol interchange starting from linear chain homologues such as Nb(OMe)5 or Nb(OEt)5. The equilibrium in the latter reaction favours formation of mixed-ligand complexes, [Nb2(OR)2(OiPr)8], R = Me, Et. In particular, [Nb2(OMe)2(OPri)8] (1) has been isolated in high yield from repeated treatment of Nb2(OMe)10 with excess of isopropanol. The X-ray single crystal study reveals a dinuclear structure containing a pair of edge-sharing octahedra with methoxide ligands in the bridging position. Infrared (IR) and mass spectroscopy (MS) studies confirmed the incomplete ligand substitution. The 1H-NMR spectra suggest equilibrium between different molecular forms in solution. Solvothermal interaction of 1 with La chips in toluene/isopropanol media results in formation of a mixture of LaNb2(OiPr)13 and La2Nb4(μ4−O)4(OH)2(μ−OiPr)8(OiPr)8 (2).

Electrosynthesis of tin(II) alkoxides

Abstract: The anodic dissolution of tin metal in absolute alcohols yields oxoalkoxides with the general formula Sn6O4(OR)4 (R = Me (1), Et (2), i-Pr (3)) and orthoalkoxides with the general formula Sn(OR)2 (R = n-Bu (4), C2H4OMe (5)). According to X-ray crystallographic data, the molecule of Sn6O4(OEt)4 (2) is an octahedral [Sn6] cluster with alternating oxo and OEt groups on its faces. The mass spectra of 1 and 2 indicate the presence of fragments of the hexanuclear oxoalkoxides.

Can alcohol interchange provide the homoleptic niobium isopropoxide

Abstract: Niobium isopropoxide, Nb(OPr)5, is an attractive precursor of simple and complex niobium oxides in sol-gel technology. This compound cannot, unfortunately, be obtained by alcohol interchange starting from linear chain homologues such as Nb(OMe)5 or Nb(OEt)5. The equilibrium in the latter reaction favours formation of mixed-ligand complexes, [Nb2(OR)2(O Pr)8], R = Me, Et. In particular, [Nb2(OMe)2(OPr )8] (1) has been isolated in high yield from repeated treatment of Nb2(OMe)10 with excess of isopropanol. The X-ray single crystal study reveals a dinuclear structure containing a pair of edge-sharing octahedra with methoxide ligands in the bridging position. Infrared (IR) and mass spectroscopy (MS) studies confirmed the incomplete ligand substitution. The HNMR spectra suggest equilibrium between different molecular forms in solution. Solvothermal interaction of 1 with La chips in toluene/isopropanol media results in formation of a mixture of LaNb2(O Pr)13 and La2Nb4(l4– O)4(OH)2(l–O Pr)8(O Pr)8 (2).

Comparative study of bimetal alkoxo complexes of rhenium, niobium, and tantalum by single-crystal x-ray diffraction and IR spectroscopy

Abstract: Bimetal alkoxo complexes of rhenium, niobium, and tantalum with the general formula M4O2(OR)14(ReO4)2, where M = Nb or Ta and R = Me or Et, were prepared in ∼80% yield by reacting niobium or tantalum alkoxide M(OR)5 (R = Me or Et) with rhenium heptoxide Re2O7 in toluene. Comparative analysis of the molecular structures of the complexes was carried out by means of single-crystal X-ray diffraction and IR spectroscopy. The effect of the organic ligand and crystallization temperature on the geometry of the perrhenate group was studied. The solubility of the aforementioned alkoxo complexes in organic solvents increases with increasing hydrocarbon chain length of the ligand.

Molecular structure design of single source precursors and multivariate analysis of their evaporation in dynamic vacuum using EI-Mass spectrometry. An approach to Barium–Strontium Titanate–Niobate as a case study

Abstract: Preparation of high purity dense coatings of perovskite materials, in particular (Ba,Sr)TiO 3 and Ba(Ti,Nb)O 3 , requires application of highly solution and even gas phase stable single-source precursors. We report here a new series of bimetallic alkoxide complexes of 1: 1 composition, M II 2 M V 2 (L) 2 (OEt) 12 (EtOH) n , M II = Sr, Ba, M V = Nb, Ta, L = 2,2,6,6-tetramethylheptanedionate (thd, n = 2) or isopropylacetoacetate ( i PrAcAc, n = 0). These products were characterized by X-ray single crystal study in the solid state for the Sr 2 Ta 2 (thd) 2 (OEt) 12 (EtOH) 2 as an example, and for all complexes by NMR in solution. Multivariate analysis of evaporation of these complexes by EI-mass spectrometry shows that they are transformed into gas phase in the molecular form and display high gas phase stability. TG measurements reveal them to be thermally stable up to at least 280 °C and decompose at higher temperatures in a single step without separation of the components. The precursor transformations in solutions (through microhydrolysis) have also been studied.

Metal oxide hydrogels and hydrosols, their preparation and use

Abstract: A process for preparing a hydrosol of one or more metal oxides, e.g. titanium dioxide, comprising preparing a metal alkoxide solution in a water-miscible organic solvent, e.g. an alcohol; providing an aqueous solvent; mixing the metal alkoxide solution with the aqueous solvent in a volume or weight proportion to form a single-phase aqueous sol colloid (hydrosol) of hydrated metal oxide in absence of a non-ionic block polymer surfactant. Also disclosed is a corresponding hydrogel; water-insoluble particles encapsulated in hydrated metal oxide and a process for their encapsulation; uses of the encapsulation products.

Hydrogels et hydrosols d'oxydes métalliques, leur élaboration et leurs applications

Abstract: La presente invention concerne un procede d'elaboration d'un hydrosol d'oxydes metalliques comprenant l'elaboration d'une solution d'alkoxyde metallique dans un solvant organique miscible a l'eau ; le fait de se munir d'un solvant aqueux ; le melange de la solution d'alkoxyde metallique au solvant aqueux dans une proportion en volume ou en poids suffisante pour former un colloide aqueux sol monophasique (hydrosol) d'un oxyde metallique hydrate en l'absence d'un tensioactif polymere bloc nonionique. La presente invention concerne egalement un hydrogel correspondant ; des particules insolubles dans l'eau encapsulees dans un oxyde metallique hydrate et leur procede d'encapsulation ; et les applications des produits encapsules.