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~A host•guest complex between cucurbit[7]uril (CB[7] and bisquaternary diamantane diaminium salt, diam-4,9-di(NMe3I) 1, was studied by 1H NMR spectroscopy and X-ray crystallography. 1H NMR competition experiments revealed that CB[7]/1 is among the tightest monovalent non-covalent complexes ever reported in water with Ka = 7.2 × 1017 M-1 in pure D2O and 2.1 × 1015 M-1 in 50 mM NaO2CCD3 buffered D2O. The impetus for this synthesis were previously reported X-ray crystallographic investigations of CB[7] with adamantane ammonium ions. 'Crystal structure based molecular design suggested that each of the two cations of a new i-symmetry diamantane diaminium guest would simultaneously interact with a different portal. The crystal structure of CB[7]/1 allowed us to identify some of the structural features responsible for the ultratight binding including the distance between the NMe3+ groups of 1 (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 1 with the corresponding concave surfaces of CB[7], desolvation of the C=O portals within the CB[7]/1 complex, and the co-linearity of the C7-axis of CB[7] with the N+...N+ line in 1.
The induced fit principle is involved in the binding process. As part of the design process (using published and in-house) crystal structures, ΔΔit became apparent that primary ammonium guests would tilt towards the portal walls in order to generate hydrogen-bonds with a very limited number of portal carbonyl oxygens. On the other hand, quaternary ammonium guests undergo +N-CH3….–.δO=C and +N-CquatCH2….–.δO=C electrostatic interactions with all the binding sites of the host.