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

Host⋅guest complexes between cucurbit[7] (CB[7]) or CB[8] and diamantane diammonium ion guests 3 or 6 were studied by 1 H NMR spectroscopy and X‐ray crystallography. 1 H NMR competition experiments revealed that CB[7]⋅ 6 is among the tightest monovalent non‐covalent complexes ever reported in water with K a =7.2×10 17 M −1 in pure D 2 O and 1.9×10 15 M −1 in D 2 O buffered with NaO 2 CCD 3 (50 m M ). 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 NMe 3 + 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 C 7 axis of CB[7] with the N + ⋅⋅⋅N + line in 6 . This work further blurs the lines of distinction between natural and synthetic receptors.