Preparation and properties of lanthanide complexes with pyridine-2,6-dicarboxylic and thiophene-2,5-dicarboxylic acids. Crystal structure of diaqua-bis(dipicolinate)lanthanum(III) tetrahydrate

Lanthanum(III), gadolinium(III) and dysprosium- (III) complexes with pyridine-2,6-dicarboxylic acid (H2pdc) of composition Na3[Ln(pdc)3]·nH2O and Ln(pdc)(Hpdc)·mH2O (n = 5-10; m = 5-8) have been prepared by reaction of aqueous solutions of Ln(NO3)3·nH2O and H2pdc in the presence of NaOH and their physico-chemical properties studied. The thermal behaviour of Ln(pdc)(Hpdc)·mH2O has been investigated and the structure of La(pdc)(Hpdc)· 6H2O determined by X-ray crystallography. The compound is monoclinic, space group P21/c, with a = 14.120(5), b = 11.280(5), c = 13.038(5) Å, ? = 101.67(3)°, and Z = 4. Refincment of the structure based on the 3217 observed reflexions led to the final R of 0.026. The asymmetric complex unit comprises the nine-coordinate metal ion, two dipicolinato ligands and two coordinated water molecules. One dipicolinato group provides linkage with a carboxylate oxygen, allowing the formation of a polymeric chain. Selected bond distances are La?N 2.67 ) Å, La?O (water) 2.51 Å (mean), La?O (pdc) 2.55 Å (mean). The interaction of thiophene-2, 5-dicarboxylic acid (H2tdc) with Ln(NO3)3·nH2O (Ln = La, Gd, Dy; n = 6, 5, 4) has also been studied; there is a marked difference in the complexes obtained and consequently in the coordination ability of H2tdc towards lanthanide with respect to H2pdc.