STRUCTURAL LINKS BETWEEN ZEOLITE-TYPE AND CLATHRATE HYDRATE-TYPE MATERIALS - SYNTHESIS AND CRYSTAL-STRUCTURE OF [N(CH3)4]6[ALXSI8-XO18-X(OH)2+X] . 44H2O (X = 3-4)

A novel tetramethylammonium aluminosilicate hydrate with the approximate composition [NMe4]6[AlxSi8-xO18-x(OH)2+x] . 44H2O (x = 3-4) has been identified by powder X-ray diffraction as a component in a polyphasic solid mixture which crystallized at room temperature from an aqueous NMe4OH-Al2O3 -SiO2 solution. Large crystals of the novel hydrate phase could be mechanically selected from that mixture. The crystal structure has been determined from 1196 unique MoKalphaBAR diffraction data measured at 180 K: Tetragonal crystal system, cell constants a = 16.181(4) and c = 17.450(4) angstrom, space group P4/mnc with Z = 2 formula units per unit cell, R = 0.072. The host-guest compound is of polyhedral clathrate type with a mixed three-dimensional, (mainly) four-connected network composed of oligomeric aluminosilicate anions [AlxSi8-xO18-x(OH)2+x]6- and H2O molecules linked via hydrogen bonds O-H ... O. The aluminosilicate anions possess a cube-shaped (double four-ring) structure. Orientationally disordered cationic guest species NMe4+ are enclosed in the large [4(6)6(8)] and [4(1)5(10)06(7)] polyhedral voids of the host framework; the small [46] cages (i.e. the double four-ring anions) and [4(3)5(6)] cages are empty. The hydrate is a further member in a recently discovered series of clathrates with mixed tetrahedral networks, which provides a structure-chemical link between zeolite- and clathrate hydrate-type host-guest compounds.