Encapsulation of Linear and Cyclic Alkanes in Water Within a Dimeric Deep-Cavity Sulfonatocalix[4]arene Capsule

The tetrasulfonated deep-cavity calix[4]arene 1, bearing a second row of benzene rings around its upper rim, self-assembles into a dimeric capsule in water upon encapsulation of small hydrophobic guests. ¹H NMR spectroscopy shows that the two calixarene subunits interdigitate, creating an elongated hydrophobic cavity that accommodates linear and cyclic alkanes, as well as benzene and hexafluorobenzene. NMR competition experiments establish a guest size-dependent stability sequence: among the linear alkanes, complex stability peaks at n-hexane and decreases for both shorter and longer homologs, whereas the cycloalkane complexes become steadily more stable from cyclopentane to cyclooctane, with the cyclooctane complex outcompeting the n-octane complex. Benzene forms a comparatively stable complex that is displaced by cyclohexane and cycloheptane, while hexafluorobenzene is in turn displaced by benzene. DFT calculations (BP86-D4(COSMO = water)/def2-SVP) of substitution energies and packing coefficients reproduce these trends qualitatively, particularly for the larger guests, and rationalize the guest size dependence in terms of a shift from single- to bis-subunit CH···π contacts, and, for hexafluorobenzene, a central binding position that minimizes F···π interactions. Together, these results establish deep-cavity calix[4]arene 1 as a versatile, hydrophobic effect-driven capsule for the recognition of small apolar molecules in water.