Structural links between zeolite-type and clathrate hydrate-type materials: Redetermination of the crystal structure of [N(CH 3) 4] 8[Si 8O 20]·65H 2O by single-crystal X-ray diffraction and variable-temperature MAS NMR spectroscopy

Abstract

The tetramethylammonium silicate hydrate [NMe 4] 8[Si 8O 20]·65H 2O (1 hereafter) crystallizes triclinically with space group P 1 , Z = 1 and the cell constants a = 15.644(3), b = 15.756(4), c = 15.967(5) Å, α = 119.31(2), β = 108.83(2), γ = 91.99(2)° (T = 215 K). It is shown that the polyhydrate is a host—guest compound with a mixed three-dimensional host structure composed of oligomeric [Si 8O 20] 8− anions and H 2O molecules linked via hydrogen bonds OH⋯O (heteronetwork clathrate). The silicate anions possess a cube-shaped double four-ring structure. Four distinct large, irregular, cage-like voids house the guest species NMe 4 +. MAS NMR spectra measured at various temperatures between 170 and 290 K reveal that the 13C resonance of the NMe 4 + cation is shifted continuously to higher field with decreasing temperature and depends primarily on the chemical composition of the cage wall rather than on the cage size. At room temperature the 1H MAS NMR spectrum is well resolved, suggesting that both the H 2O molecules and NMe 4 + cations are highly mobile on the NMR time scale. The water dynamics slow down at temperatures below ca. 250 K which in turn affects the motions of the guest species. The polyhydrate melts incongruently at 345 K with the formation of a liquid phase and a crystalline heteronetwork clathrate, the host structure of which possesses the topology of the zeolite structure type AST.