Chatterton Nicholas, Gateau Christelle, Mazzanti Marinella, Pécaut Jacques, Borel Alain, Helm Lothar, Merbach André
Laboratoire de Reconnaissance Ionique, Service de Chimie Inorganique et Biologique, Departement de Recherche Fondamentale sur la Matiere Condensee, CEA-Grenoble, 38054 Grenoble Cedex 09, France.
Dalton Trans. 2005 Mar 21(6):1129-35. doi: 10.1039/b416150e. Epub 2005 Feb 10.
The ligand N,N'-bis[(6-carboxy-2-pyridylmethyl]ethylenediamine-N,N'-diacetic acid (H(4)bpeda) 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[prime or minute]-diacetic acid with a total yield of approximately 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)(H(2)O)(2)Gd(H(2)O)(6)Cl(3)(2), isolated by slow evaporation of a 1:1 mixture of GdCl(3) and H(4)bpeda at pH approximately 1, was determined by X-ray diffraction. In complex three [Gd(bpeda)(H(2)O)(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 Gd(3+) 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 H(4)bpeda (log beta(GdL)= 15.1(3); log beta(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 bpeda(4-) with respect to the hexadentate ligand edta(4-) 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 DeltaH(pp) approximately 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 Gd(3+) complexes to date suggesting that the presence of pyridinecarboxylate chelating groups in itself does not lead to slow electron relaxation.
配体N,N'-双[(6-羧基-2-吡啶甲基]乙二胺-N,N'-二乙酸(H(4)bpeda)是采用一种改进的方法合成的，该方法所需步骤较少，相对于已发表的方法产率更高。它由2,6-吡啶二甲酸二乙酯和市售的乙二胺-N,N'-二乙酸分三步制得，总产率约为20%。通过X射线衍射测定的该配体六质子化形式的晶体结构表明，四个羧酸盐和两个胺都被质子化了。通过在pH约为1的条件下缓慢蒸发GdCl(3)和H(4)bpeda的1:1混合物分离得到的多核配合物Gd(bpeda)(H(2)O)(2)Gd(H(2)O)(6)Cl(3)(2)的晶体结构通过X射线衍射确定。在配合物中，三个[Gd(bpeda)(H(2)O)(2)]单元，其中一个Gd(III)离子由八齿bpeda和两个水分子十配位，通过四个羧基桥由两个六水合Gd(3+)阳离子连接成五金属结构。通过电位滴定法测定了质子化常数(pK(a1)= 2.9(1)，pK(a2)= 3.5(1)，pK(a3)= 5.2(2)，pK(a4)= 8.5(1))以及Gd(III)和Ca(II)离子与H(4)bpeda形成的配合物的稳定常数(log beta(GdL)= 15.1(3)；log beta(CaL)= 9.4(1))。相对于六齿配体edta(4-)，八齿配体bpeda(4-)的钆配合物和钙配合物稳定性意外降低，这已根据金属-氮相互作用对稳定性的总体较低贡献进行了解释。电子顺磁共振光谱显示出线宽非常宽(在X波段表观ΔH(pp)约为800 - 1200 G，在Q波段为90 - 110 G，取决于温度)，表明横向电子自旋弛豫很快。在X波段，Gd(bpeda)是迄今为止弛豫最快的Gd(3+)配合物之一，这表明吡啶羧酸螯合基团的存在本身并不会导致电子弛豫缓慢。
