Structural links between zeolite-type and clathrate hydrate-type materials: synthesis and crystal structure of [NMe4]16[Si8O20][OH]8·116H2O

Abstract

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)Å, the space group R 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[graphic omitted] 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 corners to form a cube (double four-ring structure). Cationic guest species NMe₄⁺ are enclosed in large polyhedral cavities [4⁶6⁸], [4²5⁸6⁵], [5¹²6⁴] and [4²5⁸6⁷] of the host structure; small [4⁶] cages (i.e. the double-ring anions) and [4³5⁶] 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.