STRUCTURAL LINKS BETWEEN ZEOLITE-TYPE AND CLATHRATE HYDRATE-TYPE MATERIALS - SYNTHESIS AND CRYSTAL-STRUCTURE OF [NME4]16[SI8O20][OH]8.116H2O

A novel crystalline tetramethylammonium silicate hydroxide polyhydrate has been obtained by fractional crystallization at room temperature from a highly alkaline aqueous NMe4OH-SiO2 solution. X-Ray single-crystal studies at 200 K revealed the trigonal crystal system, the unit-cell constants (hexagonal setting) a = 18.126(2) and c = 57.529(9) angstrom, the space group R3BAR and Z = 3 formula units of [NMe4]16[Si8O20][OH]8.116H2O per unit cell. The crystal structure has been determined from 1416 unique Mo- K-alpha-BAR data and refined to R = 0.104. The host-guest compound is of polyhedral clathrate type with a mixed three-dimensional, (mainly) four-connected tetrahedral network composed of oligomeric silicate anions [Si8O20)]8-, OH- ions and H2O molecules linked via hydrogen bonds O-H ... O. The silicate anions are each built up of eight SiO4 tetrahedra sharing comers to form a cube (double four-ring structure). Cationic guest species NMe4+ are enclosed in large polyhedral cavities [4(6)6(8)], [4(2)5(8)6(5)], [5(12)(6(4)] and [4(2)5(8)6(7)] of the host structure; small [4(6)] cages (ie. the double-ring anions) and [4(3)5(6)] cages are unoccupied. Each silicate anion is the centre of a specific and probably very stable finite cluster [(NMe4)6(Si8O20).24H2O]2-. It is suggested that the polyhydrate may be taken as a crystalline model system for studies on the structures of aqueous tetramethylammonium silicate solutions.