The effect of pyridinecarboxylate chelating groups on the stability and electronic relaxation of gadolinium complexes

The ligand N,N'-bis[6-carboxy-2-pyridylmethyl]ethylenediamine-N,N'-diacetic acid (H4bpeda) was synthesized using an improved procedure which requires a reduced no. of steps and leads to a higher yield with respect to the published procedure. It was obtained in three steps from diethylpyridine-2,6-dicarboxylate and com. available ethylenediamine-N,N'-diacetic acid with a total yield of .apprx.20%. The crystal structure of the hexa-protonated form of the ligand which was detd. by x-ray diffraction shows that the four carboxylates and the two amines are protonated. The crystal structure of polynuclear [Gd(bpeda)(H2O)2]3[Gd(H2O)6]2Cl3 (2), isolated by slow evapn. of a 1:1 mixt. of GdCl3 and H4bpeda at pH .apprx. 1, was detd. by x-ray diffraction. In complex 2 three [Gd(bpeda)(H2O)2] units, contg. a Gd(III) ion ten-coordinated by the octadentate bpeda and two water mols., are connected in a pentametallic structure by two hexa-aqua Gd3+ cations through four carboxylato bridges. The protonation consts. (pKa1 = 2.9(1), pKa2 = 3.5(1), pKa3 = 5.2(2), and pKa4 = 8.5(1)) and the stability consts. of the complexes formed between Gd(III) and Ca(II) ions and H4bpeda (log bGdL = 15.1(3); log bCaL = 9.4(1)) were detd. by potentiometric titrn. The unexpected decrease in the stability of the gadolinium complex and of the calcium complex of the octadentate ligand bpeda4- with respect to the hexadentate ligand EDTA4- was interpreted in terms of an overall lower contribution to stability of the metal-nitrogen interactions. The EPR spectra display very broad lines (apparent DHpp .apprx.800-1200 G at X-band and 90-110 G at Q-band depending on the temp.), indicating a rapid transverse electron spin relaxation. At X-band, Gd(bpeda) is among the fastest relaxing Gd3+ complexes to date suggesting that the presence of pyridinecarboxylate chelating groups in itself does not lead to slow electron relaxation.