Metal alkoxides oligomeric

A variety of anionic initiators, both ionic and covalent, have been used to polymerize lactones [Duda and Penczek, 2001 Jedlinski, 2002 Jerome and Teyssie, 1989 Penczek and Duda, 1993]. Much of the more recent activity involves the use of anionic covalent (coordination) initiators such as alkylmetal alkoxides and metal alkoxides such as R2A OR and Al(OR)3, metal carboxylates such as tin(II) 2-ethylhexanoate, metalloporpyrins (VI), and aluminox-anes such as oligomeric [A1(CH3)0] [Biela et al., 2002 Duda et al., 1990 Endo et al., 1987a,b Gross et al., 1988 Kricheldorf et al., 1990 Penczek et al., 2000a,b Sugimotoa and Inoue, 1999]. 

Alkoxides and aryloxides demonstrate similar chemistry to that of hydroxides in that it is possible to prepare mixed metal or double metal derivatives similar to hydroxo salts such as Na2[Sn(OH)6]. The formation of mixed metal alkoxides, e.g. Na2[Zr(OEt)6] and Ln[Al(OPri)4]3, is typically a result of the electron deficiency (Lewis acidity) of the metal centers in units of the type M(OR)x or M(OAr)JC (x < 5). This then leads either to oligomerization via alkoxide bridges or, in the presence of other alkoxides, to the formation of mixed metal compounds. 

Figure 1 Prototypal structures for small oligomeric metal alkoxides (a) the cubane-M404 unit in [TKOMe) , and [Na(OBu )]4 (b) die planar [CufOBu1) , structure (c) and (d) fused trigonal and fused trigonal-tetrahedral units as seen in [Be(OR)2) compounds, where n = 3 and 2 (e) fused tetrahedral-octahedral units seen in [Al(OPri)3]4 (f) edge-shared tetrahedra as in [Al(OBu )3]2 (g) fused octahedra as in [MfOEt), , where M = Ti, V, W and (h) edge-shared octahedra as...

Comparison of the properties of metal alkoxides with their structures permits a conclusion that the polymeric nature does not always lead to chemical inertness. The major role appears to be played by the nature of the M-OR bonding. Solubility in alcohols and liquid ammonia of the methoxides of alkaline and alkaline earth metals and that in hydrocarbons ofthe isopropoxides of K, Rb, Cs (isostructural with the corresponding methoxides), and also M(OC2H4OMe)n, M = Pb, Bi indicates the easy oligomerization due to solvation or chelation. At the same time the methoxides and ethoxides of Al, Cr, Fe, and so on, forming the strongest covalent bonds in the [MOs/6] octahedra (and not prone to solvation in alcohols), appear almost inert. They can be dissolved only due to complexation or partial destruction with formation ofoxobridges. 

Hydrolysis of metal alkoxides is the basis for the sol-gel method of preparation of oxide materials therefore, reactions of metal alkoxides with water in various solvents, and primarily in alcohols, may be considered as their most important chemical properties. For many years the sol-gel method was mosdy associated with hydrolysis of Si(OR)4, discussed in numerous original papers and reviews [242, 1793,243]. Hydrolysis of M(OR) , in contrast to hydrolysis of Si(OR)4, is an extremely quick process therefore, the main concepts well developed for Si(OR)4 cannot be applied to hydrolysis of alcoholic derivatives of metals. Moreover, it proved impossible to apply classical kinetic approaches successfully used for the hydrolysis of Si(OR)4 to the study of the hydrolysis of metal alkoxides. A higher coordination number of metals in their alcoholic derivatives in comparison with Si(OR)4 leads to the high tendency to oligomerization of metal alkoxides in their solutions prior to hydrolysis step as well as to the continuation of this process of oligomerization and polymerization after first steps of substitution of alkoxide groups by hydroxides in the course of their reactions with water molecules. This results in extremely complicated oligomeric and polymeric structures of the metal alkoxides hydrolysis products. 

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. ... 

Formation of alkoxide from hydroxide is a reverse reaction of hydrolysis of alkoxide, which proceeds easily at room temperature and is a highly exothermic reaction (therefore Equation 2.2 has a positive reaction enthalpy). However, metal hydroxide is usually solid and has a polymeric M-(OH)-M network, while metal alkoxide usually has oligomeric structure. Therefore the former compound has lesser freedom (lower entropy). Consequently the unfavorable enthalpy term is overcome by the entropy term at high temperatures and equilibrium is attained. 

Hexamers (239, 240) and tetramers (34, 238) are common in alkali metal alkoxide chemistry, although with more sterically demanding alkoxo groups, the oligomeric nature becomes lower in the solid state (cf. Section IV.B). Interestingly, [NaO-/-Bu] , crystallizes as two independent molecules in the asymmetric unit as a hexamer (n = 6) and a nonamer (n = 9). 

Volatility, solubility, and stereochemistry of metal alkoxides, [M(OR)J, tend to be limited both by their oligomeric nature and the steric demand of the alkoxide moiety, as a consequence of the formation of /t2 OR//x3-OR bridges... 

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