engleski

Pseudopolymorphism in nickel(II) complexes with 6-methylpicolinate. Synthesis, structural, spectroscopic, thermal and density functional theory studies

The reaction between 6-methylpicolinic acid (6-MepicH) and nickel(II) nitrate hexahydrate afforded a new octahedral complex containing two bidentate ligands and two water molecules in cis positions, which crystallized in two pseudopolymorphs, Ni(6-Mepic)2(H2O)2.2H2O (1) with co-crystallized water molecules from water solution, and Ni(6-Mepic)2(H2O)2 (2) in the absence of water. In 1, two molecules of nickel(II) complex are assembled in centrosymmetric dimers by three hydrogen bonds, which form a two-dimensional network by means of water molecules. These layers interact through π -π stacking arrangements between pyridine rings in the third dimension. In 2, centrosymmetric dimers held together by hydrogen bonds form two parallel chains and assemble in a 3-D network by π -π stacking. DFT calculations (ADF) performed on model dimers showed the major role of the O-H...O hydrogen bonds in establishing the whole network, without requiring large distortions from the individual molecules. Raman and IR spectroscopy studies allowed a detailed comparison of the two species being addressed and DFT calculations contributed to the assignment of several bands. Thermogravimetric experiments showed the successive loss of water molecules from 1, starting from the co-crystallized and continuing with the coordinated ones ; in 2, the process started at the second stage.

nickel(II) complexes; 6-methylpicolinate; crystal structures; DFT study

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