Metal alkoxides monomeric

Double alkoxides of zirconium with alkah metals of the type MZr2(OR) have been obtained by reaction of alkah metal alkoxides with zirconium alkoxides (220). Although these usually are monomeric derivatives, the reaction between zirconium tetra-/-butoxide [1071 -76-7] and sodium /-butoxide was found (221) to form dimeric NaZr(OC(CH2)2) ]2. 

The most common sol-gel process employs metal alkoxides of network forming elements (M(0R) where M is Si, B, Ti, Al, etc. and R is often an alkyl group) as monomeric precursors. In alcohol/water solutions the alkoxide groups are removed stepwise by hydrolysis reactions, generally employing acid or base catalysts, and are... 

Furukawa and his coworkers (20) considered the stereospecifically active catalyst speties of the polymerization catalyzed by the metal alkyl MR to be the metal alkoxide derived easily by the reaction with monomeric acetaldehyde. 

The orthoesters of nonmetals E(OR) , E = B, Si, P, C - in contrast to metal alkoxides - are monomeric, the stabilization of their molecules being achieved via the increase in multiplicity of E-0 bonds due to p,-d, interaction of the filled orbitals of O-atoms and vacant orbitals of the E atoms. 

The properties of Ge(OR)4 allow them to be considered more likely to be the esters of an inorganic acid than metal alkoxides these are colorless volatile liquids, containing monomeric tetrahedral molecules. The solid crystalline form is known only for R = Bu, OC6Hnc, and also 2,6-substituted phenoxides. All the members of the Ge(OR)4 homologous series are characterized by thoroughly determined physical characteristics — density, refraction index, surface tension, viscosity (and calculated parachor values), dipole moments in different solvents [222, 857, 1537] (Table 12.9). The results of the investigation of vapor pressure, density, viscosity polytherms, and so on. permitted rectification for the preparation of samples of high purity for sol-gel and MOCVD applications [682, 884]. 

Hydrolysis and condensation rates depend on the molecular structure of metal alkoxides and alkoxide precursors have to be chosen as a function of the desired material final product. In the case of Ti02, for instance, monomeric precursors such as Ti(OPF)4, in which Ti is fourfold coordinated, react very quickly with water leading to the uncontrolled precipitation of polydispersed Ti02. The reaction is much slower with oligomeric precursors such as [Ti(OEt)4] in which Ti has a higher coordination number. Spherical monodispersed Ti02 powders can be produced via the controlled hydrolysis of diluted solutions of Ti(OEt)4 in EtOH. On the contrary, monomeric precursors are more convenient for the sol-gel synthesis of multicomponent oxides. The perovskite phase BaTiOs is formed upon heating around 800 °C when [Ti(OEt)4] is used as a precursor. This temperature decreases down to 600 °C with the monomeric precursor Ti(OPT)4 which favors the formation of Ti-O-Ba bonds. ... 

Hybrid siloxane-oxide materials were prepared (573) by the SG techniques by using either monomeric species such as Me2Si(OEt)2 or by OH-terminated PDMS cross-linked by metallic alkoxides for example, M(OR)4 with M = Si, Ti, Zr, and so on. 

These compounds (Table IV) can be distilled or sublimed in vacuo and are soluble in common organic solvents, readily hydrolyzed, and converted by alcoholysis into metal alkoxides. Molecular weight determinations in boiling benzene show that the dimethylamino derivatives are significantly polymerized, whereas the diethylamino derivatives are monomeric. 

Sharing alkoxy groups is the easiest way for metal alkoxides to increase the coordination of the metal atom without changing their stoichiometry. In pure alkoxides, coordination expansion currently occurs via the formation of OR bridges. Therefore oligomeric as well as monomeric molecular precursors can be found. Oligomerization depends on physical parameters (concentration and temperature) and chemical factors (solvent and oxidation state of the metal atom or steric hindrance of alkoxide groups) [7]. 

The molecular complexity of metal alkoxides also depends on the steric hindrance of alkoxy groups. Bulky secondary or tertiary alkoxy groups tend to prevent oligomerization. Trimeric species [Ti(OEt)4]3 have been evidenced in pure liquid titanium ethoxide (Fig. lb) whereas titanium iso-propoxide Ti(OPr )4 remains monomeric (Fig.la). This is no more the case for zirconium iso-propoxide which is dimeric because of the larger size of Zr(rV). Moreover solvent molecules can also be used for coordination expansion leading to solvated dimers [Zr(OPri)4(Pr OH)]2 when the alkoxide is dissolved in its parent alcohol (Fig.lc). 

In connection with the mechanistic interpretations of stereocontrol, either (LXII) or (LXIII) is consistent with the effect of a bulky alkoxide group. This is because alkyl-alkoxide exchange reactions could place an alkoxide group on aluminum where growth occurs. Structure (LXIII) seems reasonable in view of the known tendency of aluminum alkoxides to dimerize (McElvain and Davie, 1951 Hoffmann, 1960) however, a monomeric structure (LXII) cannot presently be excluded. The metal alkoxides, like alkyllithiums, probably involve equilibria among monomeric and associated species. Therefore, one or more active species may be involved. Whatever the species, it is not too difficult to imagine a complex composed of a tetravalent aluminum, in which monomer-chain or monomer-chain-alkoxide interactions favor one path for monomer addition. 

The controlled hydrolysis of metal alkoxides has since been used to prepare fine powders of several simple oxides. We mentioned in Chapter 1 the work of Barringer and Bowen (57) for the preparation, packing, and sintering of monodis-perse Ti02 powders. Later work (58) provided some insight into the mechanism of hydrolysis of Ti(OC2Fl5)4. The alkoxide reacts with water to produce a monomeric hydrolysis species according to... 

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