Pseudopolymorphism in nickel(II) complexes with 6-bromopicolinic acid. Synthesis, structural and thermal studies

Nickel(II) complexes of 6-bromopicolinic acid, namely [Ni(6-Brpic)2(H2O)2]·3H2O (1) and [Ni(6-Brpic)2(H2O)2]·H2O (2) 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. Compound 1 was prepared by reaction of nickel(II) nitrate and 6-bromopicolinic acid in aqueous solution at pH value of 8, while 2 was obtained by spontaneous recrystallization of 1 in its mother liquor during few months. These complexes are an example of pseudopolymorphs which differ in number of co-crystallized water molecules but both crystallize in the same crystal system (triclinic). 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 nickel(II) ion is completed by two coordinated water molecules. These pseudopolymorphs differ in their crystal structures which are dominated by intermolecular O-H⋯O hydrogen bonds and π-π stacking interactions in both cases. The infrared spectra of 1 and 2 are very similar so these pseudopolymorphs cannot be unambiguously distinguished by IR spectroscopy. Thermal methods confirm number of co-crystallized water molecules in both pseudopolymorphs.