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

Abstract

Lanthanum(III), gadolinium(III) and dysprosium- (III) complexes with pyridine-2,6-dicarboxylic acid (H 2pdc) of composition Na 3[Ln(pdc) 3]· nH 2O and Ln(pdc)(Hpdc)· mH 2O (n = 5−10; m = 5−8) have been prepared by reaction of aqueous solutions of Ln(NO 3) 3· nH 2O and H 2pdc in the presence of NaOH and their physico-chemical properties studied. The thermal behaviour of Ln(pdc)(Hpdc)· mH 2O has been investigated and the structure of La(pdc)(Hpdc)· 6H 2O determined by X-ray crystallography. The compound is monoclinic, space group P2 1/ c, with a = 14.120(5), b = 11.280(5), c = 13.038(5) Å, β = 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 (H 2tdc) with Ln(NO 3) 3· nH 2O (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 H 2tdc towards lanthanide with respect to H 2pdc.