ExCage

Cyclophanes, especially
those where pyridinium units in conjugation
with each other are linked up face-to-face within platforms that are
held approximately 7 Å apart by rigid linkers, are capable of
forming inclusion complexes with polycyclic aromatic hydrocarbons
(PAHs) with high binding affinities as a result of a combination of
noncovalent bonding interactions, including face-to-face [π···π]
stacking and orthogonal [C–H···π] interactions.
Here, we report the template-directed, catalyst-assisted synthesis
of a three-fold symmetric, extended pyridinium-based, cage-like host
(ExCage6+) containing a total of six π-electron-deficient
pyridinium units connected in a pairwise fashion by three bridging p-xylylene linkers, displayed in a trigonal (1,3,5) fashion
around two opposing and parallel 1,3,5-tris­(4‑pyridinium)­benzene
platforms. The association constants (Ka) of eight complexes have been measured by isothermal titration calorimetry
(ITC) in acetonitrile and were found to span the range from 2.82 ×
103 for naphthalene up to 5.5 × 106 M–1 for perylene. The barriers to decomplexation, which
were measured in DMF-d7 for phenanthrene,
pyrene, triphenylene, and coronene by dynamic 1H NMR spectroscopy
undergo significant stepwise increases from 11.8 → 13.6 →
15.5 → >18.7 kcal mol–1, respectively,
while
complexation experiments using rapid injection 1H NMR spectroscopy
in DMF-d7 at −55 °C revealed
the barriers to complexation for pyrene and coronene to be 6.7 and
>8 kcal mol–1, respectively. The kinetic and
thermodynamic
data reveal that, in the case of ExCage6+,
while the smaller PAHs form complexes faster than the larger ones,
the larger PAHs form stronger complexes than the smaller ones. It
is also worthy of note that, as the complexes become stronger in the
case of the larger and larger PAHs, the Rebek 55% solution formula
for molecular recognition in the liquid state becomes less and less
relevant.

环番（Cyclophanes），尤其是共轭吡啶鎓单元通过刚性连接臂固定为约7埃间距的面对面骨架结构的环番，可通过多种非共价相互作用（包括面对面[π···π]堆积与正交[C–H···π]相互作用）与多环芳烃（PAHs）形成具有高结合亲和力的包结配合物。
本文报道了一种经模板导向、催化剂辅助合成的三重对称延伸型吡啶鎓基笼状主体（ExCage⁶⁺）：该主体共包含六个缺π电子的吡啶鎓单元，通过三个桥联对二甲苯连接臂两两相连，并围绕两个相对且平行的1,3,5-三(4-吡啶鎓)苯骨架平台以三角（1,3,5）构型排布。
通过等温滴定量热法（ITC）在乙腈溶剂中测定了八种配合物的结合常数（Ka），其取值范围从萘的2.82×10³ M⁻¹到苝的5.5×10⁶ M⁻¹。采用动态氢核磁共振（¹H NMR）光谱法在氘代N,N-二甲基甲酰胺（DMF-d7）中测定了菲、芘、三亚苯与蔻的解络合能垒，其值分别为11.8、13.6、15.5和>18.7 kcal mol⁻¹，呈显著阶梯式上升；而在-55 ℃的DMF-d7体系中通过快速注射氢核磁共振光谱法开展络合实验的结果显示，芘与蔻的络合能垒分别为6.7和>8 kcal mol⁻¹。
动力学与热力学数据表明，对于ExCage⁶⁺而言，小分子多环芳烃的络合速率快于大分子多环芳烃，但大分子多环芳烃所形成的配合物稳定性高于小分子多环芳烃形成的配合物。此外值得注意的是，随着多环芳烃尺寸逐渐增大、配合物稳定性不断提升，雷贝克（Rebek）提出的液态分子识别55%溶液法则的适用性会逐步降低。