Two Polymorphic Modifications of Dibromo-bis(acetonitrile-N)-zinc(II)

Abstract

Two polymorphic modifications of dibromo-bis(acetonitrile-N)-zinc(II) have been characterized by single crystal structure analysis and X-ray powder measurements. Form I cyrstallizes in the orthorhombic space group Pnma with a = 13.101(2), b = 10.210(1), c = 6.8078(7) Å and Z = 4. The structure is composed of discrete molecular complexes in which each zinc cation is coordinated by two bromide anions and two acetonitrile ligands within distorted tetrahedra. Form II crystallizes in the orthorhombic space group Cmcm with a=8.0734(8), b=11.012(1), c=10.204(1) Å and Z =4. In this compound also discrete complexes are found and the coordination of the zinc atoms is identical to that of form I. Two bromide atoms and two acetonitrile ligands coordinate to the zinc cations within distorted tetrahedra. The packing of the discrete molecular complexes in form II is completely different form that in form I. Form I was originally prepared in the presence of 2-chloropyrazine but can also be synthesized phase pure by crystallization from acetonitrile alone. In contrast, form II was only obtained in the presence of 2-chloropyrazine if the crystallization is performed under kinetic control but it immediately transforms into form I. Crystallization experiments reveal that form I is the thermodynamically most stable form between −40 ÅC and the boiling point of acetonitrile of 80 °C, whereas form II is metastable. On storing, the thermodynamically stable form I at room temperature decomposes into zinc(II) bromide within a few hours, but in a saturated acetonitrile atmosphere it is stable over a long period. On heating form I all ligands are liberated in one step leading to zinc bromide