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

The title compound, [Co(NCS)(2)(C6H7N)(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. B33, 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 center dot center dot center dot N and C-H center dot center dot center dot 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 degrees C and that the coligands are removed in separate steps without any sign of a polymorphic transition before decomposition.