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

Hostguest complexes between cucurbit[7] (CB[7]) or CB[8] and diamantane diammonium ion guests 3 or 6 were studied by (HNMR)-H-1 spectroscopy and X-ray crystallography. (HNMR)-H-1 competition experiments revealed that CB[7]6 is among the tightest monovalent non-covalent complexes ever reported in water with K-a=7.2x10(17)M(-1) in pure D2O and 1.9x10(15)M(-1) in D2O buffered with NaO2CCD3 (50mM). 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 angstrom), 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 CO 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.