Chromium(III) and Calcium(II) Complexes Obtained from Dipicolinic Acid: Synthesis,Characterization, X-Ray Crystal Structure and Solution Studies

The Cr(III) and Ca(II) complexes (dmpH)[Cr(pydc)2]·H2O (1) and [Ca2(pydc)2(H2O)6].2pydcH2 (2) were synthesized by reaction of 2,9-dimethyl-1,10-phenanthroline (dmp) and pyridine-2,6-dicarboxylic acid (pydcH2) with Cr(NO3)3 and Ca(NO3)2,respectively, and characterized using IR spectroscopy, single crystal X-ray diffraction method and solution studies. The space group and crystal system of these two compounds are P21/c and monoclinic. The crystal dimensions are a = 9.785(3) Å, b = 25.671(4) Å, c = 9.3402(16) Å, β = 90.790(17)° for (1) and a = 9.1319(4) Å, b = 14.8430(8) Å, c = 12.2449(7) Å, β = 98.227(5)° for (2). In complex (1), a water molecule presents in the crystal packing, linking the anionic and cationic fragments together by hydrogen bonding and thus increases the stabilization of crystal lattices. In complex (2), the coordinated water molecules relate each dimer to adjacent dimers forming infinite molecular ribbons by strong hydrogen bondings. Hydrogen bonding and ion pairing play an important role in stabilizing these crystals. The complexation reactions of pydc, dmp and pydc+dmp with Cr3+ and Ca2+ ions in aqueous solution were investigated by potentiometric pH titrations and the equilibrium constants for all major complexes formed were evaluated