Polyhedral clathrate hydrates of a strong base: Phase relations and crystal structures in the system tetramethylammonium hydroxide-water

Abstract

The tetramethylammonium hydroxide-water system has been studied by low-temperature differential thermal analysis and X-ray powder diffraction. The melting diagram was constructed for concentrations between 66.7 and 100 mol% H2O. It shows the existence and stability ranges of as many as eight crystalline hydrate phases:α- andβ-Me4NOH·2H2O (phase transition at −85°C, decomposition atca. 105°C), Me4NOH·4 H2O (melting point 44°C, incongruent),α andβ-Me4NOH·5 H2O (phase transition at 42°C, melting point 68°C, congruent),α- andβ-Me4NOH·7.5 H2O (phase transition at 6°C, melting point 16°C, incongruent), and Me4NOH·10 H2O (melting point −20°C, incongruent). The structures of all these phases, except the already known one ofα-Me4NOH·5 H2O, were determined from single-crystal MoKα diffractometer data. The decahydrate and the high-temperatureβ forms of the 7.5-hydrate and the pentahydrate are genuinepolyhedral clathrate hydrates, the first ones reported of a strong base. Their mostly novel three-dimensional anionic host structures, formed by the hydrogen-bonded OH− ions and H2O molecules, arefour-connected throughout, in spite of their proton deficiency which is apparently leveled by disorder. Disorder also affects the enclosed cationic Me4N+ guest species. Like the low-temperatureα form of the pentahydrate, that of the 7.5-hydrate has a clathrate-related, but not fully polyhedral structure, some of the oxygen atoms being three-connected only. The tetrahydrate presents the rare case of both a hydrogen bond of the type OH−...OH2 and a (deprotonated) water-channel structure. This is fully ordered and apart from that can be derived from the polyhedral one of theβ-pentahydrate just by removing the appropriate number of water molecules from certain positions. The structures ofα- andβ-Me4NOH·2 H2O contain identical one-dimensionalspiro chains [HO−(HOH)/42] with the hydroxide protonnot participating in the hydrogen bonding. The Me4N+ ion is ordered in theα and disordered in theβ phase.