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Various coordination modes in mercury(II) complexes with quinoline-2-carboxylic acid. Preparation and structural characterization

Several mercury(II) complexes of 2-quinaldic acid (quinH ; IUPAC name: quinoline-2-carboxylic acid) were synthesized and characterized. [Hg(quin)_2(OH2)] (1) was obtained as the reaction product of mercury(II) chloride and 2-quinaldic acid in an aqueous media, while a mixture of 1, [(quinH_2)(HgCl_3)]• 2quinH• H_2O (2), and [Hg(quin)_2(EtOH)]• H_2O (3) was obtained from a 96% ethanol solution. Pure [HgCl(quin)(quinH)] (4) was obtained as the first reaction product when the reaction was performed in absolute alcohol. After isolation of 4 from the filtrate the mixture of 1, 2, and 3 was again obtained. The X-ray single-crystal structure analysis reveals that the ligand, 2-quinaldic acid is coordinated to the Hg2+ ion as a N, O-chelating quinaldate ion in structures 1, 3 and 4. In the structure of 2 polymeric (HgCl_3)_n^n- anions are formed with a very deformed trigonal-bipyramidal 3+2 mercury coordination sphere while the ligand, 2-quinaldic acid exists in the zwitterionic form as quinH, and as a cation, quinH^2+. The coordination polyhedron in 1 and 4 can be described as 2+3 and 4+1 deformed trigonal-bipyramidal and square-pyramidal geometry with the water molecule and chlorine atom at the fifth site in 1 and 4, respectively. The coordination sphere in 3 is square pyramidal with the ethanol molecule at the vertex, however through additional Hg • O contacts centrosymmetric dimers are formed resulting in a 5+1 effective octahedral mercury coordination. The crystal structures of 1-4 are characterized by O-H• O and N-H• O intermolecular hydrogen bonds forming various packing motifs, which are discussed in details.

Mercury(II) complexes ; quinoline-2-carboxylic acid ; X-ray single crystal structure analysis ; NMR spectroscopy

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