Studies 00 clathrasils . IV * Crystal structure of dodecasillH , a synthetic cIathrate

Synthetic dodecasill H, 34 Si02 . 3 M12 . 2 MI2 . 1 M20, with M12 and M12' = N2 and M20 =piperidine, is hexagonal with space group P6/mmm and a = 13.783(4) and e = 11.190(3) A. Structure refinement with 658 independent reflexions led to a weighted Rw = 0.056. The clathrate compound has a silica host framework and organic guest molecules M within three different types of cages. It is the simplest member (1-layer polytype) of the polytypic series of dodecasils. Comer sharing [Si04] tetrahedra form a 3-dimensionaI4connected net which is built up fram hexagonal layers of face sharing pentagondodecahedra ([512] cages), the fundamental cage of the dodecasil series. Connecting such layers in sequence AA two new types of cagelike voids arise, the [435663]cage and the [51268]cage. The guest molecules M12, M12' and M20, which must be present during synthesis, show up in difference Fourier syntheses. 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 ofthe Si O Si angles (170.6°) is considerably higher and the mean value of Si -O distances (1.565 A) is considerably lower than corresponding angles and distances of silica polymorphs.