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

The crystal structure of title compound consists of discrete complexes in which the Co^II cations are octa­hedrally coordinated to two terminally N-bonded thio­cyanate anions and four 4-methyl­pyridine coligands and represents a new polymorphic modification of Co(NCS)₂(4-methyl­pyridine)₄, which is already reported in the literature.

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⋯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.