Structures and Properties of Three Polymorphic Modifications based on Tetrahedral Building Blocks of Dichlorobis(pyridazine-N) Zinc(II)

Reaction of zinc(II) chloride with pyridazine under different conditions leads to the formation of three polymorphic modifications of dichlorobis(pyridazine-N) zinc(II). Forms <b>I</b> and <b>II</b> crystallize monoclinic in space group <i>Cc</i> (<b>I</b>) and <i>P</i>2<sub>1</sub>/<i>c</i> (<b>II</b>), respectively, whereas form <b>III</b> crystallizes orthorhombic in space group <i>Pna</i>2<sub>1</sub>. In all three forms, the zinc atoms are surrounded by two chloro atoms and two pyridazine ligands within distorted tetrahedra, and the orientations of the pyridazine rings within these tetrahedra are different. In the crystal structure, the tetrahedral building blocks are packed differently and are connected by intermolecular C–H···Cl and C–H···N interactions. Crystallization experiments clearly show that form <b>I</b> represents the thermodynamically most stable form at room temperature, whereas forms <b>II</b> and <b>III</b> are metastable. Theoretical calculations show that in form <b>II</b> the most stable molecular structure is found, whereas the molecular structure in form <b>III</b> is less stable by only 8 kJ/mol. Differential thermoanalysis and thermogravimetry measurements reveal that all forms decompose into a new ligand-deficient compound [ZnCl<sub>2</sub>(pyridazine)], which can also be prepared in solution. Form <b>I</b> can be transformed into form <b>II</b>, which is more stable at higher temperatures as evidenced by differential scanning calorimetry (DSC) measurements. On some of the DSC measurements, form <b>I</b> decomposes without further transformation into form <b>II</b>. Both forms <b>I</b> and <b>II</b> behave enantiotropic. The DSC thermogram of form <b>III</b> gave no indication of a polymorphic transformation.