Silicon compounds, cationic chelates

Tropolonato chelates, 201 (Figure 32) were first reported by Muetterties and Wright (332). All available spectroscopic data (infrared, 1H, and 29Si NMR) suggest a cationic octahedral structure around silicon. Analogous tropolonato chelates seem to have been isolated from plants and compared with the synthetic compounds (323). 

The first few pentacoordinate cationic silicon complexes (153-155) were discussed in an earlier review2. The first two were described as trivalent silicenium ions stabilized by mtermolecularcoordination183 184, while 155 is an intramolecular silicenium ion21a. Since then, a few other compounds were reported. Oxidation of the dihydrido-hexacoordinate chelate 156 by the addition of excess iodine produced 157185. The X-ray structure of 157 showed a slightly distorted TBP for the cation, with a well separated (Si—I distance 5.036 A) I8 2 anion for every two cations. The nitrogens are in the apical positions, and both Si-N distances (2.08 and 2.06 A) are longer than covalent bonds but well within the coordination distance. 

Sihca impregnated with saturated and unsaturated hydrocarbons (squalene, paraffin oil) silicone and plant oils complexing agents (silver ions, boric acid, and borates carbohydrates unsaturated and aromatic compounds) chelating compounds [ethylene diamine tetra-acetic add (EDTA), digitonin] transition metal salt synthetic peptides 18-crown-6 and ammonium sulfate silanized sdica gel impregnated with anionic and cationic surfactants Cross-hnked, polymeric dextran gels (Sephadex)... 

However, for silicon to be specifically transported, concentrated, and deposited, it seems logical to suppose that chelate-type compounds are involved. Weiss and Herzog (127) find that in plants the cationic silicon complexes of tropolone derivatives are formed in the pH region 4.0-6.8. Such a complex might be translocated and concentrated and then the silicic acid liberated and deposited if the pH were slightly raised to 7. The energy involved in transport, concentration, and release may be quite small. It would mainly involve the reactions required to change the pH... 

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