Alkoxides group, structure

Compound (6) has the structure shown in Fig. 4(d). It formally has 14-electron configurations about each metal atom and all fourteen electrons are involved in the bonds represented by lines in the structural drawing. Either the U2-CO group makes no appreciable demand for tt electron density or there may be an indirect feeding of electrons from the lone-pair orbitals of the alkoxide groups to y2-C0. Alkoxide groups are known to be 7r-electron donors. 

Dimerisation and ester coordination restricts the number of sites available for alkoxides to two only, while maintaining a comparatively Lewis-acidic titanium centre, as needed for the reaction. In the dimer, the methine protons, alkoxide groups, and ester groups are inequivalent, but they show a rapid exchange on the H NMR timescale at room temperature, as the AG for the process is only 64 kl.mol. This process is much faster than the catalytic reaction, but due to the C2-symmetry of the tartaric esters the resulting structures of the dimers are the same. 

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. 

Rate ofhydrolysis depends primarily on the size and structure of the alkoxide group, — for example, it decreases in the series titanium butoxide tertiary >... 

The IR (281-300) and NMR (102, 212, 235, 282, 288, 290, 292, 295, 296, 299) spectral data are usually inconclusive for actual structural elucidation, except in providing preliminary information regarding the presence of alkoxide groups and other inorganic and organic moieties and their different environments. However, in some cases, NMR spectra are capable of providing useful information about the behavior of a metal alkoxide in solution. For example, H NMR spectra (at ambient temperature) of [Th(OCH-i-Pr2)4]2 (299) are consistent with the presence of two species (dimer and monomer) in solution. 

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