The Cucurbitnuril Family:  Prime Components for Self-Sorting Systems

We determined the values of Ka for a wide range of host-guest complexes of cucurbitnuril (CBn), 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--CB7 and CB8--individually maintain the size, shape, and functional group selectivity that typifies the recognition behavior of CB6. The cavity of CB7 is found to effectively host trimethylsilyl groups. Remarkably, the values of Ka for the interaction of CB7 with adamantane derivatives 22-24 exceeds 10(12) M(-1)! The high levels of selectivity observed for each CBn individually is also observed for the CBn family collectively. That is, the selectivities of CB6, CB7, and CB8 toward a common guest can be remarkably large. For example, guests 1, 3, and 11 prefer CB8 relative to CB7 by factors greater than 10(7), 10(6), and 3000, respectively. Conversely, guests 23 and 24 prefer CB7 relative to CB8 by factors greater than 5100 and 990, respectively. The high levels of selectivity observed individually and collectively for the CBn family renders them prime components for the preparation of functional biomimetic self-sorting systems.