Crystal structure and thermodynamic stability of the (Hg(Pyridine)4(NO3)2] 2(Pyridine) inclusion compound

The studied compound belongs to the family of [MPy4X2] (.) 2Py isomorphous clathrates. Its crystal structure exhibits a van der Waals architecture formed by neutral [HgPy4(NO3)(2)] host molecules, with the guest pyridine molecules included in the cavities of the host lattice. The host complex is formed by coordination of four pyridines, located near the equatorial plane, and two nitrates, located axially, to the Hg(II) cation. One of nitrates ligates as a monodentate ligand and another as a bidentate. The coordination polyhedron is 'HgN4O3', with average Hg-N-Py and Hg-O-nitrate distances of 2.38(5) and 2.68(1) Angstrom, respectively. The crystal structure is complicated with a superlattice and the crystal symmetry reduced to monoclinic, as compared to the structure usually occurring in the [MPy4X2] (.) 2Py clathrates. The pyridine vapor pressure over the clathrate was measured in the 293-369 K temperature range by the static tensimetric method. Thermodynamic parameters of the clathrate dissociation were calculated from these data. For the reaction 1/3[HgPy4(NO3)(2)] (.) 2Py(solid) = 1/3[HgPy3(NO3)(2)](solid) + Py-gas the parameters are as follows: DeltaH(av)(0) = 49.4(2) kJ/mol, DeltaS(av)(0) = 127(2) J/(mol K) and DeltaG(298)(0) = 11.4(3) kJ/mol. The results are compared with previously reported data on compounds of the [MPy4(NO3)(2)] (.) 2Py series.