Metal alkoxides properties

Besides pH, other preparative variables that can affect the microstructure of a gel, and consequendy, the properties of the dried and heat-treated product iaclude water content, solvent, precursor type and concentration, and temperature (9). Of these, water content has been studied most extensively because of its large effect on gelation and its relative ease of use as a preparative variable. In general, too Httie water (less than one mole per mole of metal alkoxide) prevents gelation and too much (more than the stoichiometric amount) leads to precipitation (3,9). Other than the amount of water used, the rate at which it is added offers another level of control over gel characteristics. 

The most outstanding property of the metal alkoxides is ease of hydrolysis. 

Table 4 Hsts the manufacturers of some metal alkoxides, and the individual materials are described in the following. Some other properties of metal ethoxides are summarized in Table 1.

Arkles BA (1995) Silicon, germanium, tin and lead compounds metal alkoxides, diketo-nates and carboxylates a survey of properties and chemistry. Gelest, Tullytown PA... 

Miyafuji, H., Saka, S. and Yamamoto, A. (1998). Si02-P205-B203 wood-inorganic composites prepared by metal alkoxides ohgomers and their fire-resisting properties. Holzforschung, 52(4), 410-416. 

Heterocycles treated in this section belong to several structure types. The metal atoms in compounds under discussion can exist not only in the tetravalent but also in the carbenoid divalent state and the heterocycles differ not only in the nature and number of metal atoms but also in the nature of ring bonds of the latter. Thus, besides conventional organometallic compounds having only M—C bonds there are heterocycles with M—M bonds and those with C—M—X or X—M—Y fragments (X and Y—common heteroatoms like O, S, N). The last type is the most numerous and important group of five-membered Ge-, Sn- and Pb-heterocycles in which the metals are involved in M—X and M— Y bonds similar to those in respective metal alkoxides, thiolates and amides. This feature not only affects the structural parameters of these compounds but determines their chemical properties, synthetic routes and applications. 

Perhaps the best studied group of titanium(IV) complexes is the alkoxides. The metal alkoxides generally have received a great deal of attention because of their ease of hydrolysis and reactivity with hydroxylic molecules, and their tendency to increase the coordination number of the metal which is opposed by the steric effect of the alkyl group. These properties result in materials, the characteristics of which range from polymeric solids to volatile liquids. The definitive review of this area is that by Bradley. ... 

PHYSICAL AND SPECTROSCOPIC PROPERTIES OF METAL ALKOXIDES AND ARYLOXIDES, M(OR),... 

Of the vast array of physical techniques available to the modern chemist, many have been applied to the study of metal alkoxides and aryloxides. We have decided to summarize the physical and spectroscopic properties of alkoxides and aryloxides according to stoichiometry and include techniques only where they highlight a particularly interesting characteristic of these compounds. The mass of more routine data that has been accumulated cannot be covered in detail here. 

In order to highlight the major aspects of the physical and spectroscopic properties of metal alkoxides and phenoxides, representative examples of homoleptic compounds of stoichiometries M(OR)v or M(OAr)x will be considered. 

Another method described for the synthesis of TS-1 involves the use of colloidal Si02 and tetrapropylammonium peroxo titanate (Taramasso et al., 1983). Indeed, TS-1 can be produced by this method, but subsequent experience has shown that it has the serious drawback of impurities contained in colloidal Si02, particularly Al3 +. These impurities are incorporated into the crystalline product and modify the catalytic properties, as discussed in Section V. It is certainly possible to obtain pure colloidal Si02 by hydrolysis of TEOS, but in this case the method does not offer advantages over the use of metal alkoxides. 

Metal alkoxides themselves — the derivatives of metal and aliphatic alcohols. The character of M-OR bonding in them is intermediate between phenoxides and esters. Their properties are varied, but almost all of them are sensitive to the action of water and, normally, aggregated in solution, gaseous, and solid phases. 

An outstanding input into the chemistry of metal alkoxides was made by the dissertation of V. E. Tishchenko, which was devoted to aluminium alkoxides. It was published in 1899 (unfortunately only in Russian) [1585] and—in contrast to his works on organic chemistry — is practically unknown by foreign chemists. A number of general synthetic approaches to metal alkoxides have been developed. The main regularities for the variation of the properties in the homologous sales and ramification of the radical were found to determine the main directions of the thermal decomposition. He made also the proposal to use the Al(OR)3 in ester condensation of aldehydes (Tishchenko reaction) (see also Chapter 12, Section 12.5). 

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. 

As has been mentioned above, of the many works devoted to the studies of the chemical properties of metal alkoxides, we here can consider only the reactions of partial substitution for the alkoxide groups (Chapter 7), decomposition with formation of oxocomplexes (Chapter 5), and hydrolysis with the formation of homo- and heterometallic hydroxides (Chapters 9 and 10). We discuss here the complex formation products of metal alkoxides — the adducts with neutral ligands, M(OR) mL, and the bimetallic alkoxides, precursors of complex oxides in sol-gel technology. 

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