Department of Chemistry, The University of Texas at Austin , 105 E. 24th Street-Stop A5300, Austin, Texas 78712-1224, United States.
Acc Chem Res. 2014 Aug 19;47(8):2525-36. doi: 10.1021/ar500157a. Epub 2014 Jun 30.
Ion pair receptors, which are able to bind concurrently both a cation and an anion, often display higher selectivity and affinity for specific ion pairs than simple ion receptors capable of recognizing primarily either a cation or an anion. This enhancement in recognition function is attributable to direct or indirect cooperative interactions between cobound ions via electrostatic attractions between oppositely charged ions, as well as to positive allosteric effects. In addition, by virtue of binding the counterions of the targeted ion, ion pair receptors can minimize the solvation of the counterions, which can otherwise have a negative effect on the interactions between the receptors and the targeted ions. As a result of their more favorable interactions, ion pair receptors are attractive for use in applications, such as extraction and sensing, where control of the binding interactions is advantageous. In this Account, we illustrate this potential in the context of ion pair receptors based on the calix[4]pyrrole scaffold. Both simple ditopic ion pair receptors, containing sites for the recognition of a single anion and single cation, and so-called multitopic ion pair receptors will be discussed. The latter systems differ from conventional, so-called ditopic ion pair receptors in that they contain more than one binding site for a given targeted ion (e.g., a cation). This permits a level of selectivity and control over binding function not normally seen for simple ion or ion pair receptors containing one or two binding sites, respectively. Calix[4]pyrroles are macrocyclic compounds consisting of four pyrrole units linked via fully substituted sp(3) hybridized meso carbon atoms. They are effective receptors for Lewis basic anions (e.g., halides) in typical organic media and under certain conditions will recognize ion pairs containing charge diffuse cations, such as a small alkylammonium, imidazolium, or cesium cations. The calix[4]pyrrole framework is further attractive in that it is relatively easy to modify. In particular, functionalization of the β-pyrrolic carbon and meso-carbon atoms with simple crown ethers or calix[4]arene crown ethers can produce heteromultitopic ion pair receptors containing more than two cation binding sites. This allows the interactions between receptors and ions to be manipulated on a higher level than can be achieved using simple ion receptors or heteroditopic ion pair receptors and has made these systems attractive for use in ion transport, recognition, and extraction. Recent progress in developing calix[4]pyrroles as both multitopic and more conventional ion pair receptors is summarized in this Account. The emphasis will be on our own work.
离子对受体能够同时结合阳离子和阴离子，通常比仅能识别主要是阳离子或阴离子的简单离子受体对特定的离子对具有更高的选择性和亲和力。这种识别功能的增强归因于通过相反电荷离子之间的静电吸引力以及正变构效应，在共结合离子之间的直接或间接协同相互作用，以及通过结合目标离子的抗衡离子，离子对受体可以最小化抗衡离子的溶剂化，否则这会对受体和目标离子之间的相互作用产生负面影响。由于它们具有更有利的相互作用，因此离子对受体在提取和传感等应用中很有吸引力，在这些应用中，控制结合相互作用是有利的。在本文中，我们将基于杯[4]吡咯骨架来阐述这种潜力。将讨论基于杯[4]吡咯骨架的简单双齿离子对受体和所谓的多齿离子对受体。后一种系统与传统的、所谓的双齿离子对受体不同，因为它们含有针对给定目标离子（例如阳离子）的不止一个结合位点。这允许对结合功能进行选择性和控制程度，而这在分别包含一个或两个结合位点的简单离子或离子对受体中通常是看不到的。杯[4]吡咯是由通过完全取代的 sp(3)杂化的中位碳原子连接的四个吡咯单元组成的大环化合物。它们是路易斯碱性阴离子（例如卤化物）在典型有机介质中的有效受体，并且在某些条件下，它们将识别包含电荷扩散阳离子的离子对，例如小的烷基铵、咪唑鎓或铯阳离子。杯[4]吡咯骨架进一步具有吸引力，因为它相对容易修饰。特别是，β-吡咯碳原子和中位碳原子的功能化与简单冠醚或杯[4]冠醚一起可以产生含有两个以上阳离子结合位点的杂多齿离子对受体。这使得可以在比使用简单离子受体或杂双齿离子对受体更高的水平上操纵受体和离子之间的相互作用，并且使这些系统在离子传输、识别和提取中具有吸引力。本文总结了作为多齿和更传统的离子对受体的杯[4]吡咯的最新进展。重点将放在我们自己的工作上。
