Ferreirós-Martínez Raquel, Esteban-Gómez David, Platas-Iglesias Carlos, de Blas Andrés, Rodríguez-Blas Teresa
Departamento de Química Fundamental, Universidade da Coruña, Campus da Zapateira s/n, 15071 A, Coruña, Spain.
Dalton Trans. 2008 Nov 14(42):5754-65. doi: 10.1039/b808631a. Epub 2008 Sep 9.
Herein, we report the coordination properties towards Zn(ii), Cd(ii) and Pb(ii) of two hexadentate ligands containing pyridinecarboxylate groups with ethane-1,2-diamine (bcpe) or cyclohexane-1,2-diamine (bcpc) backbones. The X-ray crystal structures of [Zn(bcpe)], [Cd(bcpe)] and [Cd(bcpc)] show hexadentate binding of the ligand to the metal ions, with the coordination polyhedron being best described as a severely distorted octahedron. The X-ray crystal structure of the Pb(ii) analogue shows the presence of tetrameric structural units [Pb(4)(bcpe)(4)] in which the four Pb(ii) ions are bridged by carboxylate oxygen atoms. While in the Zn(ii) and Cd(ii) complexes the bcpe ligand adopts a twist-wrap (tw) conformation in which the ligand wraps around the metal ion by twisting the pyridyl units relative to each other, for the Pb(ii) complex a twist-fold (tf) conformation, where a slight twisting of the pyridyl units is accompanied by an overall folding of the two pyridine units relative to each other is observed. Theoretical calculations performed at the DFT (B3LYP) level on the [Pb(bcpe)] and [Pb(bcpc)] systems indicate that the tf conformation is more stable than the tw form both in the solid state and in aqueous solution. The analysis of the natural bond orbitals (NBOs) indicate that the Pb(ii) lone-pair is polarized by a substantial 6p contribution, which results in a hemi-directed coordination geometry around the metal ion. Potentiometric studies have been carried out to determine the protonation constants of the ligands and the stability constants of the complexes with Zn(ii), Cd(ii), Pb(ii) and Ca(ii). The replacement of the ethylene backbone of bcpe by a cyclohexylene ring causes a very important increase in the stability constant of the Pb(ii) complex (ca. 2.3 logK units), while this effect is less important for Cd(ii) (ca. 1.4 logK units). However, the introduction of the cyclohexylene ring does not substantially affect the stability of the Zn(ii) and Ca(ii) complexes. The ligand bcpc shows Pb/Ca and Cd/Ca selectivities [10(8.9) and 10(9.8), respectively] superior to those of extracting agents, such as EDTA, already used in Pb(ii) and Cd(ii) removal from contaminated water and soils.
在此，我们报道了两种含有吡啶羧酸酯基团且具有乙二胺（bcpe）或环己二胺（bcpc）骨架的六齿配体与锌（II）、镉（II）和铅（II）的配位性质。[Zn(bcpe)]、[Cd(bcpe)]和[Cd(bcpc)]的X射线晶体结构表明配体以六齿方式与金属离子结合，配位多面体最好描述为严重扭曲的八面体。铅（II）类似物的X射线晶体结构显示存在四聚体结构单元[Pb(4)(bcpe)(4)]，其中四个铅（II）离子由羧酸根氧原子桥连。在锌（II）和镉（II）配合物中，bcpe配体采取扭曲缠绕（tw）构象，其中配体通过使吡啶基单元相互扭曲而围绕金属离子缠绕，而对于铅（II）配合物，观察到一种扭曲折叠（tf）构象，其中吡啶基单元的轻微扭曲伴随着两个吡啶单元相对于彼此的整体折叠。在DFT（B3LYP）水平上对[Pb(bcpe)]和[Pb(bcpc)]体系进行的理论计算表明，无论是在固态还是在水溶液中，tf构象都比tw构象更稳定。自然键轨道（NBO）分析表明，铅（II）的孤对电子有相当大的6p贡献使其极化，这导致金属离子周围形成半定向配位几何结构。已经进行了电位滴定研究以确定配体的质子化常数以及与锌（II）、镉（II）、铅（II）和钙（II）形成的配合物的稳定常数。用环己烯环取代bcpe的乙烯骨架会使铅（II）配合物的稳定常数大幅增加（约2.3个logK单位），而这种影响对镉（II）来说不太重要（约1.4个logK单位）。然而，引入环己烯环对锌（II）和钙（II）配合物的稳定性影响不大。配体bcpc显示出优于已用于从受污染水和土壤中去除铅（II）和镉（II）的萃取剂（如EDTA）的铅/钙和镉/钙选择性[分别为10(8.9)和10(9.8)]。
