V. G. Kessler

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 Mue5f8O bonding in the synthesized bimetallic complexes is discussed on the basis of their crystal structures and IR spectra.

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 WO3ue5f8TiO2, n-Siue5f8WO3 are also reported.

Synthesis, crystal and molecular structure of calcium oxo ethoxide, [Ca6(µ4-O)2(µ3-OEt)4(OEt)4]·14EtOH

The hexanuclear complex of calcium oxo ethoxide ethanol solvate, [Ca6O2(OEt)8]·14EtOH 1 has been isolated after the prolonged refluxing of an ethanol solution of calcium ethoxide in the presence of the oxygen in air and studied by X-ray diffraction; the metal-oxygen framework is built of two [Ca4O4] cubes sharing a common [Ca2O2] face, which involves both oxo groups.

The synthesis and X-ray crystal structure of molybdenum oxomethoxide [MoO(OMe)4]2

MoO(OMe) 4 has been prepared by anodic oxidation of molybdenum metal in anhydrous methanol containing LiCl as an electrolyte. The X-ray single crystal study showed the complex to consist of pseudo-centrosymmetric dimeric molecules, [MoO(OMe) 3 (μ-OMe)] 2 . The general effect of the M(μ-OR) 2 M ring upon the arrangement of bonds with increased multiplicity is discussed in this and related molecules.

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 ue5fb 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 (Rue5fbPri, 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 (Mue5fbMg, 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.

Application of metal alkoxides in the synthesis of oxides

Abstract Powders and thin films of the following simple and complex oxides have been obtained from metal alkoxides: MIITiO3 (M= Mg, Ca-Ba), MM1/3B2/3O3 (M= Sr, Ba, M = Mg-Zn; B= Nb, Ta); MNbO3(M= Li, Na); ZrTiO4; PbTiO3; PZT, PLZT-materials; M2BO4 (M= Li, Na; B= Mo, W), Bi2BO6(B= Mo, W); LixWO3, WO3, MoO3, Bi2O3-WO3(solid solutions), YFeO3, Y3Fe5O12; YBa2Cu3O7-; Bi2Sr2CaCu2Ox; Bi2Sr2Ca2Cu3Ox. For synthesis of metal alkoxides and solutions containing two or more elements electrochemical technique (the anodic dissolution of metals in alcohols) has been used. Analysis of different examples emphasises that considerable attention in sol-gel chemistry of oxides should be paid to decomposition of M(OR)n with elimination of ether and formation of oxoalkoxides, containing M-O-M bonds-the basis of future oxide phases.

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.

Synthesis, X-ray structure and thermal decomposition of lanthanum [dioxoisopropoxomolybdate] [La2Mo4O4(μ4-O)4(μ-OPri)8(Opri)6]

Abstract The reaction of lanthanum isopropoxide with the isopropoxide of molybdenum(VI), MoO(OPr i ) 4 , in 1:2 molar ratio in hexane, leads to the formation of the mixed-metal hexanuclear complex La 2 Mo 4 O 4 ( μ 4 -O) 4 ( μ -OPr i ) 8 (OPr i ) 6 ( 1 ), formally derived from the dioxomolybdenum species MoO 2 (OPr i ) 2 . The molecule of 1 contains an unusual tetragonal bipyramidal La 2 Mo 4 metal core supported by four tetradentate oxobridging ligands binding two neighbouring molybdenum and both lanthanum atoms each. Complex 1 readily decomposes either in solution in iso-propanol or on heating in vacuo to 90°C. The mechanism of the latter process is discussed.

Synthesis and crystal structure of the double barium–titanium isopropoxide [Ba4Ti4(µ4-O)4(µ3-OR)2(µ-OR)8(OR)6(ROH)4][Ba4Ti4(µ4-O)4(µ3-OR)2(µ-OR)9(OR)5(ROH)3]

An X-ray structural study of the crystals isolated from the solutions obtained by the reaction of Ba metal with Ti(OPri)4 in isopropyl alcohol has been carried out; the crystals of the compound, which is a precursor for the synthesis of BaTiO3, contain molecules of two different kinds [Ba4Ti4O4(OR)16(ROH)4] and [Ba4Ti4O4(OR)16(ROH)3], where R = Pri.

Isolation, X-ray single crystal and theoretical study of quinquevalent metal oxoisopropoxides, Nb6O8(iPrO)14(iPrOH)2 and Re4O6(OiPr)10

Oxoisopropoxide complexes of niobium, Nb 6 O 8 ( i PrO) 14 ( i PrOH) 2 ( I ), and rhenium, Re 4 O 6 (O i Pr) 10 ( II ), were isolated as byproducts of anodic oxidation of these metals in i PrOH in the presence of LiCl as conductive additive. The common feature of both structures consists in the occurrence in their molecules of an M 4 O 16 planar core formed in the formal absence of M–M bonding for I and in the presence of electron-deficient but surprisingly short (Re–Re 2.52–2.54 A) metal–metal bonds in II . The stability of this core for the oxoalkoxide derivatives of rhenium(V,VI) and niobium(V) and poor stability for those of tantalum(V) are discussed based on the results of quantum-chemical calculations.