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。在无卤化物阴离子存在时，游离的共价笼呈现扁平构象：其空腔沿穿过叔胺（tertiary amines）的C3轴被压缩，且两个三苄胺平台呈重叠式构象。卤化物包埋可诱导该共价笼发生显著的构象重排，表现为空腔沿C3轴伸展、沿赤道平面收缩。值得注意的是，该构象重排伴随叔胺的锥形反转，以及三苄胺基平台的旋转，此时平台转为交错式构象。含咪唑鎓的臂状结构环绕球形阴离子，最终得到M型与P型螺旋配合物（helical complexes）的外消旋混合物。正如该共价笼的柔性结构所预期，两种极限构象间的切换可在交替化学刺激（硝酸银AgNO3/四正丁基氯化铵TBACl）下循环多次。该结果与计算研究所得到的低活化能垒相符；计算研究还量化了收缩与伸展两种笼状构象间的能量差，并推测了构象重排所经历的能量路径。