Container Molecules and their Guests
- R. Warmuth
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 (ExCage(6+)) 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 (K(a)) of eight complexes have been measured by isothermal titration calorimetry (ITC) in acetonitrile and were found to span the range from 2.82 × 10(3) for naphthalene up to 5.5 × 10(6) M(-1) for perylene. The barriers to decomplexation, which were measured in DMF-d7 for phenanthrene, pyrene, triphenylene, and coronene by dynamic (1)H 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 (1)H 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 ExCage(6+), 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.