N. Ya. Turova

Physico-chemical and structural investigation of aluminium isopropoxide

Abstract The crystal structure of tetrameric Al(OPr-iso) 3 has been determined by single crystal X-ray diffraction. The results obtained confirm the structure suggested by Bradley. A strucrural model of the polymerization equilibrium is suggested. A strict correlation between melting point values and degrees of association (or sample “age”) is established and an attempt is made to treat the physico-chemical properties of Al(OPr-iso) 3 in terms of a pseudo-binary trimer/tetramer system. The interaction between Al(OPr-iso) 3 and iso-PrOH has been studied with consideration of the interconversion equilibrium.

Molybdenum and tungsten-VI) bimetallic alkoxides. Decomposition accompanied by dialkylether elimination

Abstract Conductometric titration curves of MO(OR)4, and MO2(OR)2 (M = Mo, W; R = Et, iPr, C2H4OMe) by sodium (or lithium) alkoxides have ultimately proved the existence of the (1:1) bimetallic complexes in alcohol solutions. Upon evaporation of the solvents from the solutions, decomposition of the complexes occurs, accompanied by the elimination of ether, resulting in the precipitation of Na2MoO4; the molybdenum derivatives decompose more readily than the corresponding tungsten species. Under special conditions the crystals of [NaWO(OEt)5(EtOH)2]2 (I) and [NaMo2O4(OR)5(ROH)]2 [R =iPr (VI), Et (VIII)] complexes have been isolated. [NaMoO2(OC2H4OMe)3]2 (XI) proved to be the only stable complex. Solubility isotherms in the systems WO(OEt)4-M(OEt)n-EtOH and MoO2(OC2H4OMe)2- M(OC2H4OMe)2-M(OC2H4OMe)n-MeOC2H4OH (M = Na, Bi) were measured which allowed us to establish the regions of stability of the complexes. The nature of the MO bonding in the synthesized bimetallic complexes is discussed on the basis of their crystal structures and IR spectra.

Bimetallic alkoxides of niobium

Abstract The formation of the I–III group metal alkoxoniobates has been studied using the physicochemical analysis method, namely the investigation of solubility isotherms for the M(OR) n -Nb(OR) 5 -ROH systems (M = Li, Ba, Sc, La; R = 2Et, Pr i ). The crystallization of the following complexes has been observed: LiNb(OEt) 6 ( I ), [LiNbO(OEt) 4 (EtOH)] 2 ( II ), [BaNbO(OEt) 5 (EtOH) 1,5 ] 4 ( III ), BaNb 2 (OPr i ) 12 (Pr i OH) 2 ( IV ) and LaNb 2 (OPr i ) 13 ( V ). It has been found that the bimetallic isopropoxide of Sc and Nb does not exist in the solid state. The oxo complexes II and III are formed on storage (or more rapidly on refluxing) of the solutions of non-oxo compounds in alcohols in the presence of dry air and are, in fact, the products of these compounds oxidation by oxygen. The structure of II is built of dimeric molecules with the metal-oxygen coer analogous to that of Ti 4 (OMe) 16 . The molecule of III is constructed around the [Ba 4 O 4 ] cubane-like unit, whose oxygen vertices are occupied by the oxo atoms of NbO(OEt) 5 octahedra. The structures of the molecules of IV, V and recently studied Nb(OPr i ) 5 are based on the common motif featuring the M 2 O 2 four-membered cycles in edge sharing octahedra. All the observed [NbMNb] units are non-linear, the molecule of V being asymmetric, for the central [LaO 6 ] octahedron is sharing simultaneously an edge with one [NbO 6 ] octahedron and a face with the other. The conditions leading to the formation of oxo groups and the influence of the latter on the complexation process have been discussed. The literature data on the existence of 3 d transition metals alkoxoniobates have not been confirmed. It has been found that interaction of Nb and Co or Ni isopropoxides takes place only due to preliminary microhydrolysis or pyrolysis and leads to formation of oxoalkoxoniobates.

Hydrolysis of molybdenum and tungsten alkoxides: sols, powders and films

Abstract Hydrolysis of molybdenum and tungsten alkoxides of the two series MO(OEt)4 and MO2(OEt)2 (M = Mo, W) has been studied. Hydrolysis products and the partially hydrolyzed solutions have been characterized by means of X-ray diffraction, IR, electron spectroscopy and small-angle X-ray scattering. Preparation of stable sols is reported and their structures are discussed in terms of a fractal model. While ‘tungsten solutions’ may be described as typical polymeric sols, ‘molybdenum solutions’ should be regarded as particulate sols. The reason for this difference has its origin in the structures of alkoxides and mechanisms of their hydrolysis. Sols were used for the preparation of electrochromic and photochromic films by spin-coating techniques. A coloration mechanism is discussed in terms of the percolation model. Some properties of the composite structures WO3TiO2, n-SiWO3 are also reported.

The structure of the crystal solvate of magnesium methoxide with methanol, Mg(OMe)2·3.5MeOH

Abstract The X-ray diffraction study of the crystals deposited from the solution obtained by the reaction of magnesium with methanol allowed the formulation of the product as Mg(OMe)2·3.5MeOH (I). Its structure is built of the residues of four types, namely cubane [Mg4(μ3-OR)4(OR)4(ROH)8] neutral molecules, [Mg4(μ3-OR)4 (OR)2(ROH)10]2+ cations, [(RO)2H]− anions (R = Me) and eight crystallographically independent non-coordinated solvating methanol molecules. All residues are linked into a three-dimensional framework by means of a complicated hydrogen-bonding system. The crystal structure provides the basis for discussion of the possible mechanism of desolvation of I.

Formation of LiNbO3 powders and thin-films by hydrolysis of metal alkoxides

Crystalline LiNbO3 was prepared by simultaneous hydrolysis of lithium and niobium alkoxides (in the mole ratio Li : Nb = 1 :1) followed by heat treatment. Crystallization of LiNbO3 occurs in a wide temperature region, 350 to 700° C, directly from an amorphous hydrolysis product containing residual alkoxy and hydroxy groups. Heat treatment at 700° C results in the formation of fully crystalline LiNbO3 powder with completely formed ferroelectric properties. Hydrolysis of metal alkoxide solutions is successfully used to obtain high-quality LiNbO3 crystalline thin films.

PHYSICOCHEMICAL APPROACH TO THE STUDIES OF METAL ALKOXIDES

Abstract The review provides a description of main events in the development of chemistry of metal alkoxides in Russian and summarizes the results obtained by the authors. The studies of solubility and vapor pressure in the M(OR)n-ROH (R  Li, Mg, Ca, Sr, Ba, Tl) systems permitted the determination of the composition of the solvates formed and the optimization of the isolation conditions for distinct derivatives. The electrochemical synthetic approach to the alkoxide and 2-methoxyethoxide derivatives of III–VIII Group elements has been elaborated. Improvement of the MCln with NaOR metathesis conditions led to practically quantitative yields of alkoxides. The examples of the structures of polynuclear oxoalkoxides, earlier erroneously considered to be orthoderivatives, i.e. ‘M(OR)n’, are given. It has been stated that among the alkoxoderivatives of Zr and Hf these are only M(OC2H4OMe)4 and M(OR)4·ROH (RPri, Bui) that exist as individual compounds, while the samples of those with different R contain oxocomplexes of M3O(OR)10 and M4O(OR)14 composition. The questions connected with the origin of oxogroups in the molecules of alkoxides and their influence on the properties of the samples are discussed. The irreproducibility of the physicochemical constants for the samples (physical state, melting points, solubility in alcohols, intensity of coloration) is caused by different ‘chemical’ (synthetic procedures, isolation conditions) and the thermal prehistory and storage times; it originates from the difference in their molecular composition (different fractions of different types of oligomeric and polymeric [M(OR)n]m and MxOy(OR)z aggregates). Formation of bimetallic alkoxides is studied using the plotting to 20°C solubility isotherms in M(OR)n− M′(OR)m-Solvent systems. Different kinds of systems are considered and the composition and structure of bimetallic complexes and oxocomplexes, originating from their decomposition, are described. Decomposition of alkoxomolybdates and -tungstates in solution may result in crystallization of complex oxides as the final products. The example of MTiO3 (MMg, Ba) oxide preparation is discussed as most illustratively demonstrating the need in studies of metal alkoxides interaction in solution for the optimization of synthesis of oxides by the sol-gel technique.

The alkoxides of zirconium and hafnium: direct electrochemical synthesis and mass-spectral study. Do “M(OR)4”, where M=Zr, Hf, Sn, really exist?

The direct electrochemical synthesis of zirconium (1a) and hafnium (1b) alkoxides, M(OPri)4·PriOH, Zr(OBui)4·BuiOH (4a) and M(OR)4, where R=Et (2a,b), Bun (3a), Bus (5a), C2H4OMe (6a,b) has been carried out by anodic oxidation of metals in anhydrous alcohols in the presence of LiCl as a conductive additive to give quantitative yields. The solubility polytherms and dissociation pressure of1a,b have been investigated. It has been proved by means of chemical analysis, X-ray powder, and IR spectral studies that the desolvation of 1a,b and Sn(OPri)4·PriOH (1c) is accompanied by the formation of amorphous oxocompounds M3O(OPri)10. On the basis of1H NMR data it has been proved that the structure of the latter is analogous to that of known triangular cluster molecules M3(μ3-O)(μ3-OR)(μ-OR)3(OR)6, where M=Mo, W, U. Mass-spectral data and the determined physicochemical characteristics of1–5 permit to conclude that the samples of composition M(OR)4, where M=Zr, Hf, and2,3,5 contain tri- and tetranuclear oxocomplexes M3O(OR)10 and M4O(OR)14 respectively, along with Zr(OR)4 oligomers of different molecular complexity.

BaTiO3 films on silicon wafers from metal alkoxides

Barium titanate films on silicon wafers have been obtained by spinning of the alkoxide solution in butyl alcohol. Application and crystallization conditions have been studied. The best characteristics were achieved for the samples with a Si3N4 protective sublayer annealed at 700°C; formation of the ferroelectric tetrahedral phase occurs only after annealing of the films above 900°C.

The structure of the bismuth ethoxide ethanol solvate. A new structural type for octameric alkoxides

Abstract The octanuclear complex of bismuth ethoxide ethanol solvate, [Bi(OEt)3]8·(7+x)EtOH (1) has been isolated on crystallization from the ethanol solution of the products from the BiCl3 metathesis with NaOEt. The molecule of 1 is a [Bi8] cycle with a crown shape (analogous to that of S8 in the elementary sulfur), and is built up of seven [Bi(μ-OEt)4(OEt)] tetragonal pyramids and one [Bi(μ-OEt)4(OEt)(EtOH)] octahedron. The ring is surrounded by EtOH molecules forming hydrogen bonds with the terminal OEt-groups.