Departamento de Química Fundamental, Universidade da Coruña, Campus da Zapateira, Alejandro de la Sota 1, 15008 A Coruña, Spain.
Inorg Chem. 2009 Dec 7;48(23):10976-87. doi: 10.1021/ic900838j.
Herein we report the coordination properties toward Cd(II), Pb(II), Ca(II), and Zn(II) of a new octadentate ligand (py-H(2)bcpe) based on a ethane-1,2-diamine unit containing two picolinate and two pyridyl pendants, which is structurally derived from the previous reported ligand bcpe. Potentiometric studies have been carried out to determine the protonation constants of the ligand and the stability constants of the complexes with these cations. The introduction of the pyridyl pendants in bcpe provokes a very important increase of the logK(ML) values obtained for the Pb(II) and Cd(II) complexes, while this effect is less important in the case of the Zn(II) analogue. As a result, py-bcpe shows a certain selectivity for Cd(II) and Pb(II) over Zn(II) while keeping good Pb(II)/Ca(II) and Cd(II)/Ca(II) selectivities, and the new receptor py-bcpe can be considered as a new structural framework for the design of novel Cd(II) and Pb(II) extracting agents. Likewise, the stabilities of the Cd(II) and Pb(II) complexes are higher than those of the corresponding EDTA analogues. The X-ray crystal structure of [Zn(py-bcpe)] shows hexadentate binding of the ligand to the metal ion, the coordination polyhedron being best described as a severely distorted octahedron. However, the X-ray crystal structure of the Pb(II) analogue shows octadentate binding of the ligand to the metal ion. A detailed investigation of the structure in aqueous solution of the complexes by using nuclear magnetic resonance (NMR) techniques and density functional theory (DFT) calculations (B3LYP) shows that while in the Zn(II) complex the metal ion is six-coordinated, in the Pb(II) and Ca(II) analogues the metal ions are eight-coordinated. For the Cd(II) complex, our results suggest that in solution the complex exists as a mixture of seven- and eight-coordinated species. DFT calculations performed both in the gas phase and in aqueous solution have been also used to investigate the role of the Pb(II) lone pair in the structure of the [Pb(py-bcpe)] complex.
在此，我们报告了一种新的八齿配位体（py-H(2)bcpe）与 Cd(II)、Pb(II)、Ca(II)和 Zn(II)的配位性质。该配位体基于一个包含两个吡啶盐和两个吡啶基侧基的乙烷-1,2-二胺单元，结构上来源于之前报道的 bcpe 配体。通过电位法研究了配体的质子化常数和与这些阳离子形成的配合物的稳定性常数。在 bcpe 中引入吡啶基侧基会导致 Pb(II)和 Cd(II)配合物的 logK(ML)值显著增加，而在 Zn(II)类似物的情况下，这种影响则不那么重要。因此，py-bcpe 对 Cd(II)和 Pb(II)具有一定的选择性，而对 Zn(II)的选择性则较低，同时保持了良好的 Pb(II)/Ca(II)和 Cd(II)/Ca(II)选择性，新型受体 py-bcpe 可以被认为是设计新型 Cd(II)和 Pb(II)提取剂的新结构框架。同样，Cd(II)和 Pb(II)配合物的稳定性高于相应的 EDTA 类似物。[Zn(py-bcpe)]的 X 射线晶体结构表明，配体与金属离子呈六配位，配位多面体最好描述为严重扭曲的八面体。然而，Pb(II)类似物的 X 射线晶体结构表明配体与金属离子呈八配位。通过使用核磁共振（NMR）技术和密度泛函理论（DFT）计算（B3LYP）对配合物在水溶液中的结构进行详细研究表明，虽然在 Zn(II)配合物中，金属离子是六配位的，但在 Pb(II)和 Ca(II)类似物中，金属离子是八配位的。对于 Cd(II)配合物，我们的结果表明，在溶液中，该配合物存在于七配位和八配位物种的混合物中。在气相和水溶液中进行的 DFT 计算也被用于研究 Pb(II)孤对电子在[Pb(py-bcpe)]配合物结构中的作用。
