STUCTURE OF TRIMETHYLSILYL DERIVATES OF SOME SILICATES WITH CYCLIC ANIONS
Yu.I.Smolin1,Yu.F. Shepelev1, D.Hoebbel2
1Institute for Silicate Chemistry of the
Russian Acad. Sci.,Odoevskaya
str.24-2,St.-Petersburg,199155,Russia,
rsi@isc.nw.ru
2Institute for New Materials University
of Saarbrucken Parallelstrasse30, Saarbrucken66125, Germany.
Keywords: structure, trimethylsilylation, ring silicates, ester
The results of X-ray structure determination of a new class of compounds trimethylsilyl derivates of some silicates with cyclic anyon are discussed. Polysilicic acid esters with cage-like structures have been determined with single crystals obtained by trimethylsilylation of tetraethyl- , tetramethyl- and tetrabuthylammonium silicates.These watersoluble crystals containing great amount of water molecules per unit cell may be considered as the regular models of silicate solutions.The silicon oxygen radicals which exists in cystals are recognized in solution as well. The crystal structures of compounds [(CH3 )3 Si]6 .Si6 O15 , [(CH3)3Si]8.Si8O20 and [(CH3)3Si]10.Si10O25 are discussed.These molecules are double trigonal, tetragonal and pentagonal rings composed of silicon-oxygen tetrahedra with trimethylsilyl groups joined to the terminal oxygen atoms.It was found that the central parts of these types of the molecules are similar in structure to anionic groups of corresponding silicates whose structures contain double trigonal and tetragonal rings.Structures of original and final products of reaction of thrimethylsilylation are compared. Although the structure of tetrabuthylammonium silicate have not been determined , pure silicate anions in this compound have probably the form of double pentagonal ring [Si10 O25] as the central parts of the corresponding ester.
The crystal structures of monocyclic compounds [(CH3)3Si]6.Si3O9, [(CH3)3Si]12Si6O18, [(CH3)3Si]14.Si7O15 obtained by trimethylsilylation of silicates Ba3Si3O9 ,Ba5Si2O6Br and Ba17Si7O21Cl20 are also described. The central parts of the ester molecules are three, six and seven membered rings formed by SiO4-tetrahedra. Trimethylsilyl groups are bonded to terminal oxygen atoms of each tetrahedron. It is known and also show here that thrimethylsilylation does not change the topology of the silicate anions.That is why the pure silicate anions [Si7O21]14- in the barium cloride has probably to be the same form as central part of the ester molecule.