Departamento de Química Fundamental, Universidade da Coruña, Campus da Zapateira-Rúa da Fraga 10, 15008 A Coruña, Spain.
Inorg Chem. 2011 May 2;50(9):4125-41. doi: 10.1021/ic2001915. Epub 2011 Apr 1.
In this contribution we present two ligands based on a do3a platform containing a picolinate group attached to the fourth nitrogen atom of the cyclen unit, which are designed for stable lanthanide complexation in aqueous solutions. Potentiometric measurements reveal that the thermodynamic stability of the complexes is very high (log K = 21.2-23.5), being comparable to that of the dota analogues. Luminescence lifetime measurements performed on solutions of the Eu(III) and Tb(III) complexes indicate that the complexes are nine coordinate with no inner-sphere water molecules. A combination of density functional theory (DFT) calculations and NMR measurements shows that for the complexes of the heaviest lanthanides there is a major isomer in solution consisting of the enantiomeric pair Λ(δδδδ) and Δ(λλλλ), which provides square antiprismatic coordination (SAP) around the metal ion. Analysis of the Yb(III)-induced paramagnetic shifts unambiguously confirms that these complexes have SAP coordination in aqueous solution. For the light lanthanide ions however both the SAP and twisted-square antiprismatic (TSAP) isomers are present in solution. Inversion of the cyclen ring appears to be the rate-determining step for the Λ(δδδδ) ↔ Δ(λλλλ) enantiomerization process observed in the Lu(III) complexes. The energy barriers obtained from NMR measurements for this dynamic process are in excellent agreement with those predicted by DFT calculations. The energy barriers calculated for the arm-rotation process are considerably lower than those obtained for the ring-inversion path. Kinetic studies show that replacement of an acetate arm of dota by a picolinate pendant results in a 3-fold increase in the formation rate of the corresponding Eu(III) complexes and a significant increase of the rates of acid-catalyzed dissociation of the complexes. However, these rates are 1-2 orders of magnitude lower than those of do3a analogues, which shows that the complexes reported herein are remarkably inert with respect to metal ion dissociation.
在本贡献中，我们展示了两种基于 do3a 平台的配体，它们包含一个连接在环戊二烯单元第四个氮原子上的吡啶甲酸基团，旨在稳定镧系元素在水溶液中的配合物。电位测定表明，配合物的热力学稳定性非常高（log K = 21.2-23.5），与 dota 类似物相当。对 Eu(III)和 Tb(III)配合物溶液进行的荧光寿命测量表明，配合物是九配位的，没有内球水分子。密度泛函理论（DFT）计算和 NMR 测量的组合表明，对于最重的镧系元素的配合物，在溶液中有一个主要的异构体，由对映体对 Λ(δδδδ)和 Δ(λλλλ)组成，这为金属离子提供了四方反棱柱配位（SAP）。Yb(III)诱导的顺磁位移分析明确证实，这些配合物在水溶液中具有 SAP 配位。然而，对于轻镧系元素离子，SAP 和扭曲四方反棱柱（TSAP）异构体都存在于溶液中。环戊二烯环的反转似乎是 Lu(III)配合物中观察到的 Λ(δδδδ)↔Δ(λλλλ)对映体化过程的速率决定步骤。从 NMR 测量得到的这个动态过程的能垒与 DFT 计算预测的非常吻合。臂旋转过程的能垒远低于环反转路径的能垒。动力学研究表明，用吡啶甲酸侧链取代 dota 的一个乙酸盐臂会导致相应的 Eu(III)配合物形成速率增加 3 倍，并显著增加配合物的酸催化解离速率。然而，这些速率比 do3a 类似物低 1-2 个数量级，这表明本文报道的配合物在金属离子解离方面非常惰性。
