Synthesis, crystal structure and thermal properties of a new polymorphic modification of diisothiocyanatotetrakis(4-methylpyridine)cobalt(II)

The title compound, [Co(NCS) 2 (C 6 H 7 N) 4 ] or Co(NCS) 2 (4-methylpyridine) 4 , was prepared by the reaction of Co(NCS) 2 with 4-methylpyridine in water and is isotypic to one of the polymorphs of Ni(NCS) 2 (4-methylpyridine) 4 [Kerr & Williams (1977). Acta Cryst. B 33 , 3589–3592 and Soldatov et al. (2004). Cryst. Growth Des. 4 , 1185–1194]. Comparison of the experimental X-ray powder pattern with that calculated from the single-crystal data proves that a pure phase has been obtained. The asymmetric unit consists of one Co II cation, two crystallographically independent thiocyanate anions and four independent 4-methylpyridine ligands, all located in general positions. The Co II cations are sixfold coordinated to two terminally N-bonded thiocyanate anions and four 4-methylpyridine coligands within slightly distorted octahedra. Between the complexes, a number of weak C—H...N and C—H...S contacts are found. This structure represent a polymorphic modification of Co(NCS) 2 (4-methylpyridine) 4 already reported in the CCD [Harris et al. (2003). NASA Technical Reports , 211890]. In contrast to this form, the crystal structure of the new polymorph shows a denser packing, indicating that it is thermodynamically stable at least at low temperatures. Thermogravimetric and differential thermoanalysis reveal that the title compound starts to decomposes at about 100°C and that the coligands are removed in separate steps without any sign of a polymorphic transition before decomposition.