A Novel <i>C</i>₃<i>_v</i>-Symmetrical Calix[6](aza)cryptand with a Remarkably High and Selective Affinity for Small Ammoniums

The three-step synthesis of a calix[6]arene capped with a TAC unit is presented. The novel C3v-symmetrical calix[6](aza)cryptand displayed an exceptionally high affinity for small ammoniums. NMR and X-ray diffraction analyses demonstrated the formation of endo-complexes. These complexes are stabilized thanks to (i) hydrogen bonding to both the aza cap and one phenolic unit of the calixarene and to (ii) cationic and CH−π interactions between the ammonium and the aromatic walls of the host. Combined extraction and competitive binding experiments yielded the free energies of bindings ΔG° in chloroform. The values are the highest ever obtained with a calixarene-type host. Calix[6]TAC displayed the best affinity for EtNH3+. Comparison with other small ammoniums emphasizes the high selectivity of the recognition process.本研究报道了一种经TAC单元（TAC unit）封端的杯[6]芳烃（calix[6]arene）的三步合成路线。这种新型的具有C3v点群对称的杯[6]（氮杂）穴状配体（calix[6](aza)cryptand）对小型铵离子展现出极高的结合亲和力。核磁共振波谱法（NMR）与X射线衍射分析结果证实了内包配合物（endo-complexes）的生成。该类配合物的稳定机制包括两点：一是铵离子与配体的氮杂盖帽以及杯芳烃的一个酚单元形成氢键；二是铵离子与主体的芳香壁之间存在阳离子-CH-π相互作用。通过联合萃取与竞争性结合实验，我们测得该体系在氯仿溶剂中的结合自由能ΔG°。该数值是迄今杯芳烃类主体配体所取得的最高结合自由能值。杯[6]TAC（Calix[6]TAC）对乙基铵阳离子（EtNH3+）展现出最优的结合亲和力。与其他小型铵离子的对比实验进一步凸显了该识别过程的高选择性。