Coordination modes of 3-hydroxypicolinic acid (OH-picH): Synthesis and characterization of cadmium(II) complexes: Crystal and molecular structures of [CdX(OH-pic)(OH-picH)(H2O)]2 X = Cl−, Br−

Two complexes of 3-hydroxypicolinic acid and cadmium(II) chloride and bromide, namely di-μ-halo-[aqua-(3-hydroxypyridine-2-carboxylato-N,O)(3-hydroxypyridinium-2-carboxylato-O)cadmium(II)], halo = Cl− (1), Br− (2) have been prepared and characterized by spectroscopic, thermal and X-ray crystallographic methods. The IR and thermal data correlate with the structures of the complexes in the solid state. The vibration bands of diagnostic value are compared to the values of the free acid. The complexes 1 and 2 are isostructural. The molecular structures of complexes 1 and 2 comprise centrosymmetrical dimers formed via di-μ-halide bridges. The distorted octahedral cadmium coordination environment contains two halide atoms, one water molecule and two 3-OH-pic ligands, one bonded to the cadmium ion in a monodentate mode through the carboxylate O atom and the other in the bidentate N,O-chelated mode forming a five-membered chelate ring. An extensive network of the O–H⋯O and N–H⋯O hydrogen bonds are found. From 1H and 13C NMR measurements it was determined that in dimethylformamide solution complexes 1 and 2 do not exist. Cd(OH-pic)2 is the only compound that was identified instead.

The cadmium(II) chloride and bromide react almost instantaneously with 3-hydroxypicolinic acid in water solution giving isostructural dinuclear complexes of general formula [CdX(OH-pic)(OH-picH)(H2O)]2; X = Cl−, Br− which is established by the X-ray structural analysis and also supported by the thermal analysis. These compounds are not stable in solutions and NMR spectral analysis revealed the presence of cadmium(II) 3-hydroxypicolinate as the only cadmium(II) compound in solution. Although, the synthesis conditions permit also a formation Cd(OHpic)2, apparently the packing forces in the solid state yield to the title complexes.Figure optionsDownload as PowerPoint slide