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.