Trimorphism of Zn(NCS)2(4-dimethylaminopyridine)2: Crystal Structures, Thermodynamic Relations, and Comparison with the Co(II) Polymorphs

Abstract

Reaction of Zn(NCS)2 with 4-dimethylaminopyridine (DMAP) leads to the formation of three polymorphic modifications of Zn(NCS)2(DMAP)2 (I–Zn, II–Zn, and III–Zn), of which only the crystal structure of I–Zn is reported in literature. In all three modifications the Zn(II) cations are tetrahedrally coordinated by two terminal N-bonded thiocyanate anions and two DMAP ligands forming discrete complexes, which differ in the dihedral angle between the DMAP ring planes and the arrangement of the complexes in the crystals. All three modifications show melting, and the thermodynamic relations and the transition behavior were investigated by temperature-dependent X-ray powder diffraction, differential scanning calorimetry, thermomicroscopy, and solvent-mediated conversion experiments. These reveal that I–Zn and II–Zn as well as II–Zn and III–Zn are related by enantiotropism, whereas I–Zn and III–Zn are related by monotropism. The thermodynamic transition temperature between I–Zn and II–Zn is much lower than that betw...