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Host⋅guest complexes between cucurbit[7] (CB[7]) or CB[8] and diamantane diammonium ion guests 3 or 6 were studied by ¹H NMR spectroscopy and X‐ray crystallography. ¹H NMR competition experiments revealed that CB[7]⋅6 is among the tightest monovalent non‐covalent complexes ever reported in water with K_a=7.2×10¹⁷ M⁻¹ in pure D₂O and 1.9×10¹⁵ M⁻¹ in D₂O buffered with NaO₂CCD₃ (50 mM). The crystal structure of CB[7]⋅6 allowed us to identify some of the structural features responsible for the ultratight binding, including the distance between the NMe₃⁺ groups of 6 (7.78 Å), which allows it to establish 14 optimal ion‐dipole interactions with CB[7], the complementarity of the convex van der Waals surface contours of 6 with the corresponding concave surfaces of CB[7], desolvation of the CO portals within the CB[7]⋅6 complex, and the co‐linearity of the C₇ axis of CB[7] with the N⁺⋅⋅ …