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

Abstract

The ligand N,N′-bis[(6-carboxy-2-pyridylmethyl]ethylenediamine-N,N′-diacetic acid (H₄bpeda) was synthesised using an improved procedure which requires a reduced number 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 commercially available ethylenediamine-N,N′-diacetic acid with a total yield of ∼20%. The crystal structure of the hexa-protonated form of the ligand which was determined by X-ray diffraction shows that the four carboxylates and the two amines are protonated. The crystal structure of the polynuclear complex [Gd(bpeda)(H₂O)₂]₃[Gd(H₂O)₆]₂Cl₃ (2), isolated by slow evaporation of a 1 ∶ 1 mixture of GdCl₃ and H₄bpeda at pH ∼ 1, was determined by X-ray diffraction. In complex 2 three [Gd(bpeda)(H₂O)₂] units, containing a Gd(III) ion ten-coordinated by the octadentate bpeda and two water molecules, are connected in a pentametallic structure by two hexa-aquo Gd³⁺ cations through four carboxylato bridges. The protonation constants (pK_a1 = 2.9(1), pK_a2 = 3.5(1), pK_a3 = 5.2(2), and pK_a4 = 8.5(1)) and the stability constants of the complexes formed between Gd(III) and Ca(II) ions and H₄bpeda (logβ_GdL = 15.1(3); logβ_CaL = 9.4(1)) were determined by potentiometric titration. 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⁻ has been interpreted in terms of an overall lower contribution to stability of the metal-nitrogen interactions. The EPR spectra display very broad lines (apparent ΔH_pp ∼800–1200 G at X-band and 90–110 G at Q-band depending on the temperature), 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.