Polymorphous transformation of a clathrate in the pyridinecadmium nitrate system

Abstract

The polymorphous transformation of the [CdPy ₄(NO₃)2]·2Py clathrate (Py is pyridine) at 221 K is investigated by analyzing the temperature dependences of the Raman spectra and the lattice parameters in the range 100–293 K The crystal structures of the high- and low-temperature modifications of the clathrate were determined. Above the transition point the clathrate crystallizes in space group Ccca (at 293 K: a = 12.360(4), b = 15.456(3), c = 16.931(3) å, V = 3234(1) å³, Z = 4, R = 0.050 for 879 reflections and 126 refined parameters). Below 221K the unit cell of the clathrate is transformed into the primitive cell, space group Pcca (at 215 K: a = 11.953(3), b = 15.129(3), c = 17.234(3) A, V = 3234(1) å³, Z =4, R = 0.057 for 1596 reflections and 210 refined parameters). In both modifications, the host complex involves four pyridine ligands and two monodentate trans-nitrato groups forming a distorted octahedron around the central atom. The guest pyridine molecules lie in the cavities of the molecular framework of the complex. As the clathrate is cooled, the a and b parameters decrease and the c parameter increases; at transition point, each of these parameters abruptly changes by about 0.2 A. The nitrate ligand undergoes ordering and inclination to the equatorial plane of the complex by 20‡.