Moreau Juliette, Pierrard Jean-Claude, Rimbault Jean, Guillon Emmanuel, Port Marc, Aplincourt Michel
GRECI, Université de Reims Champagne-Ardenne, B. P. 1039, 51687 Reims Cedex 2, France.
Dalton Trans. 2007 Apr 28(16):1611-20. doi: 10.1039/b700673j. Epub 2007 Mar 14.
A new polyazamacrocyclic ligand (called pctga) containing pyridine and N-glutaryl arms has been synthesized as a potential agent for MRI (magnetic resonance imaging). Three series of successive complexes formed with Eu(3+) were characterized by at least two of the following methods: potentiometry, EXAFS or luminescence spectrometry. In the immediate complexes EuH(h)(pctga)(H2O)6+**, the metal ion is bound to the oxygen atoms of the three internal carboxylate groups and to six water molecules. As the lanthanide moves into the macrocyclic cavity, these species rapidly evolve into the intermediate metastable complexes EuH(h)(pctga)(H2O)4+*. The formation of two new bonds with the nitrogen atoms of the tetraazamacrocycle decreased the number of coordinated water molecules to four. In the final thermodynamically stable complexes EuH(h)(pctga)(H2O)(2)+, the pctga is bound to the europium(III) in a heptadentate manner, via the four nitrogen atoms of the tetraazamacrocycle and the three oxygen atoms of the internal carboxylate groups. The coordination number of the metal ion is completed to nine with two inner-sphere water molecules. The mean hydration numbers were calculated from the values of the bimolecular quenching constant k(q) of the luminescence species. The thermodynamic parameters corresponding to the protonation constants of the ligand and to the formation constants of the various intermediate and final complexes were determined from potentiometric measurements. They show that the complex species have some specific thermodynamic and structural properties inherent to the N-glutaryl groups and to the pyridine cycle. The insertion of this aromatic substructure rigidifies the ligand and sensibly diminishes the value of the overall formation constant (log beta(110) = 18.66(5)). This whole study allows us to propose a complexation mechanism for the system Eu(3+)/pctga in solution which is a mixture of the ones determined for the ligands dota and tced.
一种含有吡啶和N - 戊二酰基臂的新型聚氮杂大环配体（称为pctga）已被合成出来，作为一种潜在的磁共振成像（MRI）剂。与铕（III）形成的三个系列的连续配合物通过以下至少两种方法进行了表征：电位滴定法、扩展X射线吸收精细结构谱（EXAFS）或发光光谱法。在直接配合物[EuH(h)(pctga)(H2O)6](h - 3)+*中，金属离子与三个内部羧酸根基团的氧原子以及六个水分子相连。随着镧系元素进入大环腔，这些物种迅速演变成中间亚稳配合物[EuH(h)(pctga)(H2O)4](h - 3)+。与四氮杂大环的氮原子形成的两个新键使配位水分子的数量减少到四个。在最终的热力学稳定配合物[EuH(h)(pctga)(H2O)(2)](h - 3)+中，pctga通过四氮杂大环的四个氮原子和内部羧酸根基团的三个氧原子以七齿方式与铕（III）结合。金属离子的配位数通过两个内球水分子达到九个。平均水合数是根据发光物种的双分子猝灭常数k(q)的值计算得出。与配体的质子化常数以及各种中间和最终配合物的形成常数相对应的热力学参数是通过电位滴定测量确定的。结果表明，配合物物种具有一些N - 戊二酰基基团和吡啶环所固有的特定热力学和结构性质。这种芳香亚结构的插入使配体刚性增强，并显著降低了总形成常数的值（log beta(110) = 18.66(5)）。整个研究使我们能够提出溶液中Eu(3+)/pctga体系的络合机制，该机制是为配体dota和tced所确定的机制的混合。
