Cucurbit[7]uril⋅Guest Pair with an Attomolar Dissociation Constant

Host?guest complexes between cucurbit7] (CB7]) or CB8] and diamantane diammonium ion guests 3 or 6 were studied by ¹H NMR spectroscopy and X‐ray crystallography. ¹H NMR competition experiments revealed that CB7]? 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 CB7]? 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 CB7], the complementarity of the convex van der Waals surface contours of 6 with the corresponding concave surfaces of CB7], desolvation of the C?O portals within the CB7]? 6 complex, and the co‐linearity of the C₇ axis of CB7] with the N⁺???N⁺ line in 6 . This work further blurs the lines of distinction between natural and synthetic receptors.