The Cucurbit[<i>n</i>]uril Family: Prime Components for Self-Sorting Systems

We determined the values of Ka for a wide range of host−guest complexes of cucurbit[n]uril (CB[n]), where n = 6−8, using 1H NMR competition experiments referenced to absolute binding constants measured by UV/vis titration. We find that the larger homologuesCB[7] and CB[8]individually maintain the size, shape, and functional group selectivity that typifies the recognition behavior of CB[6]. The cavity of CB[7] is found to effectively host trimethylsilyl groups. Remarkably, the values of Ka for the interaction of CB[7] with adamantane derivatives 22−24 exceeds 1012 M-1! The high levels of selectivity observed for each CB[n] individually is also observed for the CB[n] family collectively. That is, the selectivities of CB[6], CB[7], and CB[8] toward a common guest can be remarkably large. For example, guests 1, 3, and 11 prefer CB[8] relative to CB[7] by factors greater than 107, 106, and 3000, respectively. Conversely, guests 23 and 24 prefer CB[7] relative to CB[8] by factors greater than 5100 and 990, respectively. The high levels of selectivity observed individually and collectively for the CB[n] family renders them prime components for the preparation of functional biomimetic self-sorting systems.