Cueurbit[7]uril·Guest Pair with an Attomolar Dissociation Constant

摘要

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 × 10~(17) M~(-1) in pure D2O and 1.9 × 10~(15) M~(-1) in D2O buffered with NaO2CCD3 (50 km). The crystal structure of CB[7]·6 allowed us to identify some of the structural features responsible for the ultralight binding, including the distance between the NMe3~+ groups of 6 (7.78 A), 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=0 portals within the CB[7]·6 complex, and the co-linearity of the C7 axis of CB[7] with the N~+—N~+ line in 6. This work further blurs the lines of distinction between natural and synthetic receptors.