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首页> 外文期刊>Dalton transactions: An international journal of inorganic chemistry >The effect of pyridinecarboxylate chelating groups on the stability and electronic relaxation of gadolinium complexes
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The effect of pyridinecarboxylate chelating groups on the stability and electronic relaxation of gadolinium complexes

机译：吡啶羧酸盐螯合基团对of配合物的稳定性和电子弛豫的影响

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摘要

The ligand N,N'-bis[(6-carboxy-2-pyridylmethyl]ethylenediamine-N,N'-diacetic acid (H_4bpeda) 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 approx 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_2O)_2]_3[Gd(H_2O)_6]_2Cl_3 (2),isolated by slow evaporation of a 1 : 1 mixture of GdCl_3 and H_4bpeda at pH approx 1,was determined by X-ray diffraction.In complex 2 three [Gd(bpeda)(H_2O)_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_(al) = 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_4bpeda (logbeta_(GdL) = 15.1(3);logbeta_(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) approx 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.
机译：使用改进的方法合成了配体N，N'-双[（6-羧基-2-吡啶基甲基）乙二胺-N，N'-二乙酸（H_4bpeda），该步骤所需的步骤数减少，并且收率较高。它是由三乙基-2,6-二羧酸二乙酯和市售的乙二胺-N，N'-二乙酸组成的三步法获得的，总产率约为20％。六质子化形式的晶体结构通过X射线衍射确定的配体的结构表明，四个羧酸盐和两个胺均已质子化。多核络合物[Gd（bpeda）（H_2O）_2] _3 [Gd（H_2O）_6] _2Cl_3（ 2），通过X射线衍射确定在pH大约为1的条件下缓慢蒸发GdCl_3和H_4bpeda的1：1混合物而分离得到。在络合物2中，三个[Gd（bpeda）（H_2O）_2]单元含有Gd（ III）由八齿的bpeda和两个水分子十配位的离子，通过两个六水合Gd〜（3+ ）阳离子通过四个羧基连接。质子化常数（pK_（al）= 2.9（1），pK_（a2）= 3.5（1），pK_（a3）= 5.2（2），pK_（a4）= 8.5（1 ）），并通过电位滴定法确定Gd（III）和Ca（II）离子与H_4bpeda（logbeta_（GdL）= 15.1（3）; logbeta_（CaL）= 9.4（1））之间形成的配合物的稳定性常数。 dent配体bpeda〜（4-）相对于六配体edta〜（4-）的complex配合物和钙配合物的稳定性出乎意料地降低，这是因为总体上对of稳定性的贡献较低。 EPR光谱显示出很宽的线（视温度在X波段的表观DELTAH_（pp）大约为800-1200 G，在Q波段的表观DELTAH_（pp）大约为90-110 G），表明快速的横向电子自旋弛豫在X波段，Gd（bpeda）是迄今为止最快的弛豫Gd〜（3+）配合物，这表明吡啶羧酸盐螯合基团本身的存在不会导致慢速反应。电子弛豫。

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