Studies on Clathrasils. VI*

Abstract

Synthetic dodecasil 3C, 136 SiO 2 · 16 M 12 · 8 M 16 , is cubic with space group Fd 3 and a 0 = 19.402(1) Å. Structure refinement of a crystal containing M 12 = N 2 and small amounts of CH 4 and Ar, M 16 = N(CH 3 ) 3 and CO 2 led to a weighted R w = 0.064 for 883 independent reflexions. The clathrate compound is isotypic with the cubic clathrate hydrate type II. The clathrate compound has a silica host framework with guest molecules M in two different types of cage, and it is the cubic three-layer member of the polytypic series of dodecasils. Cornersharing [SiO 4 ] tetrahedra form a 3-dimensional 4-connected net which is built up from pseudohexagonal layers of face-sharing pentagondodecahedra ([5 12 ] cages), the fundamental cage of the dodecasil series. On connecting such layers in the sequence [unk]ABC two types of cagelike void arise, the [5 12 ] cage again and the larger [5 12 6 4 ] cage. The guest molecules M 12 and M 16 which must be present during synthesis show up in difference Fourier synthesis. The entrapped molecules are oriented within the cages such that van der Waals contacts between guest molecules and host framework are optimized. The mean value of the Si – O – Si angles (174.5°) is considerably higher and the mean value of the Si – O distance (1.566 Å) considerably lower than corresponding angles and distances in silica polymorphs. If aspheric guest molecules are used during synthesis, the room temperature phase is of lower symmetry than cubic. Calorimetric and hot-stage microscope experiments with dodecasil 3C containing M 16 = tetrahydrofurane show that a displacive phase transformation to the cubic high symmetry form takes place at about 110°C.