engleski

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.

nickel(II) complexes; 6-bromopicolinic acid; π – π interactions; pseudopolymorphism; TGA/DTA methods

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