Silicon, chelates with catechol

Compared with 5-coordinated silicon, 6-coordinated silicon can form more chelates. For example, silicon can form octahedral 6-coordinated chelates with catechol and 2,3-naphthalenediol. The stability of these chelates is significantly affected by the pH of the solution. Bauman and coworkers studied the stability of silicon-catechol chelate and obtained a value of 2.1 x 10 11 (pH 8.25) for the stability constant. 

Reaction of functionalized ketones such as a-hydroxy ketones or 1,3-ketones with allyltrifluorosilane in the presence of Et3N has also been investigated [95]. Although catecholate does not play a key role in the pentacoordination of silicon, chelating hexacoordinate intermediates are formed to give allylation products with high yields and selectivity (Sch. 54). 

An apparently unique type of compound that strongly interacts with silicic acid and silica surfaces by hydrogen bonding is l-hydroxypyridine A -oxide. It will be noted that, as in catechol, there are two oxygen atoms closely spaced, so that they can either chelate with a silicon atom or hydrogen-bond to adjacent SiOH groups on polysilicic acid. 

In animals it is possible that silicon metabolism involves another type of chelate (i.e., catechol type anionic complexes). In this case, according to Baumann (390b), the chelate is stable only above pH 7 and liberates silicic acid below this pH. No chelates of silicon have been bolated from animal tissues. However, the presence of a variety of molecules with catechol-like structure such as catecholamines makes it possible that compounds of this type could be involved in animals. Such chelates have been discussed in Chapters 1 and 2. 

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