Polymorphism of cobalt(II) complex with 6-bromopicolinic acid: The influence of the solution pH value on the formation of polymorphs

Triclinic (1) and monoclinic (2) polymorphs of cobalt(II) complex with 6-bromopicolinic acid, namely [Co(6-Brpic)2(H2O)2]·3H2O were prepared and characterized by spectroscopic methods (IR), their molecular and crystal structures were determined by X-ray crystal structure analysis and their thermal stability by TGA/DTA methods. Polymorph 1 was prepared by reaction of cobalt(II) acetate tetrahydrate and 6-bromopicolinic acid in aqueous solution at pH value of 6, while 2 was obtained by the same reaction in aqueous solution at pH value of 8. The pH value was adjusted with the addition of ammonia solution. This is an example of the influence of the solution pH value on the formation of polymorphs. X-ray structural analysis revealed octahedron as a coordination polyhedron in both 1 and 2 and the same N,O-chelated coordination mode of 6-bromopicolinic acid. The coordination sphere of cobalt(II) ion is completed by two coordinated water molecules. The polymorphs differ in their crystal structures, which are dominated by intermolecular O–H ⋯ O hydrogen bonds and π–π stacking interactions, and in the crystal system (triclinic vs. monoclinic). The infrared spectra of 1 and 2 are very similar so the polymorphs cannot be unambiguously distinguished by IR spectroscopy. Thermal methods confirm number of co-crystallized water molecules in both polymorphs.