Halide-Controlled Extending–Shrinking Motion of a Covalent Cage

Herein, we present an example of covalent cages, whose flexible framework undergoes extending–shrinking motion under halide control. In the absence of halide anions, the free cage assumes a flattened conformation: the cavity is compressed along the C3 axis passing through the tertiary amines, and the two tribenzylamine platforms are eclipsed. Halide encapsulation promotes a large conformational rearrangement of the cage, involving an extension of the cavity along the C3 axis and shrinkage along the equatorial plane. Interestingly, the rearrangement is accompanied by the pyramidal inversion of the tertiary amines and by the rotation of the tribenzylamine-based platforms, which become staggered. The imidazolium-containing arms wrap around the spherical anion, leading to a racemic mixture of the M and P helical complexes. As expected from the flexible structure of the cage, the switch between the two limit conformations can be repeated for several cycles under alternating chemical stimuli (AgNO3/TBACl). This result is consistent with the low activation barriers determined by computational investigations. These also allowed us to quantify the energy difference between the shrunk and expanded cage conformations and to hypothesize an energetic pathway along which the conformational rearrangement can occur.本文报道一例共价笼（covalent cages）体系，其柔性骨架可在卤化物（halide）调控下发生伸展-收缩运动。在无卤阴离子存在时，游离态共价笼呈现扁平构象：空腔沿穿过叔胺（tertiary amines）的C3轴被压缩，且两个三苄胺（tribenzylamine）平台处于重叠构象。卤化物包封可诱导共价笼发生显著构象重排，表现为空腔沿C3轴伸长、沿赤道平面收缩。值得注意的是，该构象重排同时伴随叔胺的锥型反转，以及三苄胺基平台的旋转，最终使其呈交错构象。含咪唑鎓（imidazolium）的臂会环绕球形阴离子，最终生成M型与P型螺旋配合物（helical complexes）的外消旋混合物（racemic mixture）。正如该共价笼的柔性结构所预期，两种极限构象间的转换可在交替化学刺激（硝酸银/四丁基氯化铵，AgNO3/TBACl）下重复多个循环。该结果与计算研究测得的低活化能垒相符，同时计算研究还使我们得以量化收缩态与伸展态共价笼构象间的能量差，并推测出构象重排可遵循的能量路径。