Cucurbit7uril⋅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 (1) H NMR spectroscopy and X-ray crystallography. (1) H NMR competition experiments revealed that CB7⋅6 is among the tightest monovalent non-covalent complexes ever reported in water with Ka =7.2×10(17) M(-1) in pure D2 O and 1.9×10(15) M(-1) in D2 O buffered with NaO2 CCD3 (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.