Université de Rennes 1-ENSCR, Institut des Sciences Chimiques de Rennes, UMR CNRS 6226, 263 avenue du Général Leclerc, CS 74205, 35042 Rennes Cedex, France.
Chemistry. 2013 Aug 12;19(33):11021-38. doi: 10.1002/chem.201300881. Epub 2013 Jun 28.
Overhanging carboxylic acid porphyrins have revealed promising ditopic ligands offering a new entry in the field of supramolecular coordination chemistry of porphyrinoids. Notably, the adjunction of a so-called hanging-atop (HAT) Pb(II) cation to regular Pb(II) porphyrin complexes allowed a stereoselective incorporation of the N-core bound cation, and an allosterically controlled Newton's cradle-like motion of the two Pb(II) ions also emerged from such bimetallic complexes. In this contribution, we have extended this work to other ligands and metal ions, aiming at understanding the parameters that control the HAT Pb(II) coordination. The nature of the N-core bound metal ion (Zn(II), Cd(II)), the influence of the deprotonation state of the overhanging COOH group and the presence of a neutral ligand on the opposite side (exogenous or intramolecular), have been examined through (1)H NMR spectroscopic experiments with the help of radiocrystallographic structures and DFT calculations. Single and bis-strap ligands have been considered. They all incorporate a COOH group hung over the N-core on one side. For the bis-strap ligands, either an ester or an amide group has been introduced on the other side. In the presence of a base, the mononuclear Zn(II) or Cd(II) complexes incorporate the carbonyl of the overhanging carboxylate as apical ligand, decreasing its availability for the binding of a HAT Pb(II). An allosteric effector (e.g., 4-dimethylaminopyridine (DMAP), in the case of a single-strap ligand) or an intramolecular ligand (e.g., an amide group), strong enough to compete with the carbonyl of the hung COO(-), is required to switch the N-core bound cation to the opposite side with concomitant release of the COO(-), thereby allowing HAT Pb(II) complexation. In the absence of a base, Zn(II) or Cd(II) binds preferentially the carbonyl of the intramolecular ester or amide groups in apical position rather than that of the COOH. This better preorganization, with the overhanging COOH fully available, is responsible for a stronger binding of the HAT Pb(II). Thus, either allosteric or acid-base control is achieved through stereoselective metalation of Zn(II) or Cd(II). In the latter case, according to the deprotonation state of the COOH group, the best electron-donating ligand is located on one or the other side of the porphyrin (COO(-)>CONHR>COOR>COOH): the lower affinity of COOH for Zn(II) and Cd(II), the higher for a HAT Pb(II). These insights provide new opportunities for the elaboration of innovative bimetallic molecular switches.
悬垂羧酸卟啉已被证明是一种很有前途的双齿配体，为卟啉类超分子配位化学领域提供了新的切入点。值得注意的是，在常规的 Pb(II) 卟啉配合物中加入所谓的悬垂 atop (HAT) Pb(II) 阳离子，允许 N-核结合的阳离子选择性掺入，并且来自这种双金属配合物的两个 Pb(II) 离子的变构控制的牛顿摇篮样运动也出现了。在本研究中，我们将这项工作扩展到其他配体和金属离子，旨在了解控制 HAT Pb(II) 配位的参数。通过 (1)H NMR 光谱实验，结合晶体学结构和密度泛函理论计算，研究了 N-核结合金属离子（Zn(II)、Cd(II)）的性质、悬垂 COOH 基团的去质子化状态的影响以及在相反侧存在中性配体（外源性或内源性）对 HAT Pb(II) 配位的影响。研究了单环和双环配体。它们都在一侧包含一个悬垂在 N-核上的 COOH 基团。对于双环配体，在另一侧引入酯或酰胺基团。在存在碱的情况下，单核 Zn(II) 或 Cd(II) 配合物将悬垂羧酸盐的羰基作为顶端配体纳入，从而降低其与 HAT Pb(II) 结合的可用性。需要足够强的变构效应物（例如，单环配体中的 4-二甲氨基吡啶 (DMAP)）或内源性配体（例如酰胺基团）与悬垂 COO(-) 竞争，从而将 N-核结合的阳离子切换到另一侧，同时释放 COO(-)，从而允许 HAT Pb(II) 配位。在没有碱的情况下，Zn(II) 或 Cd(II) 优先结合在顶端位置的内源性酯或酰胺基团的羰基，而不是 COOH 的羰基。这种更好的预组织，悬垂 COOH 完全可用，导致 HAT Pb(II) 的更强结合。因此，通过 Zn(II) 或 Cd(II) 的立体选择性金属化实现变构或酸碱控制。在后一种情况下，根据 COOH 基团的去质子化状态，最佳的供电子配体位于卟啉的一侧或另一侧（COO(-)>CONHR>COOR>COOH）：COOH 对 Zn(II) 和 Cd(II) 的亲和力越低，对 HAT Pb(II) 的亲和力越高。这些见解为创新的双金属分子开关的设计提供了新的机会。
