Contact stabilization of host complex molecules during clathrate formation: The pyridine-zinc nitrate and the pyridine-cadmium nitrate systems

Clathrate formation ranges of the phase diagrams of two binary systems Py-Zn(NO,), and Py-Cd(NO3)(2) (Py = pyridine) were studied. A clathrate of composition [MPy4(NO3)(2)]. 2Py (M = Zn, Cd) was observed in each of the systems. The space group Ccca (orthorhombic system) and the parameters of the unit cells of both clathrates were determined by X-ray analysis of their single crystals. The data obtained show them to be isostructural with the clathrate [NiPy4(NO3)(2)]. 2Py whose structure is known and suggest the actual presence of the host molecules trans-[MPy4(NO3)(2)] (M = Zn,Cd) inside the clathrate phases. Host complexes do not form as separate compounds but can only arise in clathrate phases due to contact stabilization by the guest molecules. Both Zn- and Cd-clathrates are of constant composition and melt incongruently at 62.3(6) and 106.0(5)degrees C, respectively, yielding the complexes [ZnPy3(NO3)(2)] and [CdPy3(NO3)(2)], these melting congruently at 131.4(5) and 169.5(5)degrees C, respectively. During thermal decomposition under quasi-equilibrium conditions with different pressures of the liberating pyridine both clathrates also decompose in one stage, giving [MPy3(NO3)(2)] complexes. The results obtained are discussed in relation to a number of other systems with Schaeffer's and Hofmann-lwamoto's clathrates in which contact stabilization occurs or might be expected to occur.