Carborane Guests for Cucurbit[7]uril Facilitate Strong Binding and On-Demand Removal

Abstract

High-affinity guests have been reported for the macrocyclic host cucurbit[7]uril (CB[7]), enabling widespread applications, but hindering CB[7] materials from being returned to their guest-free state for reuse. Here, we present polyhedral boron clusters (carboranes) as strongly binding, yet easily removable, guests for CB[7]. Aided by a Pd-catalyzed coupling of an azide anion, we prepared boron-functionalized 9-amino-<i>ortho</i>-carborane that binds to CB[7] with a <i>K</i>_a ≈ 10¹⁰ M^-1. Upon basic treatment, <i>ortho</i>-carborane readily undergoes deboronation to yield anionic <i>nido</i>-carborane, a poor guest for CB[7], facilitating recovery of guest-free CB[7]. We showcase the utility of the modified <i>ortho</i>-carborane guest by recycling a CB[7]-functionalized resin. With this report, we introduce stimuli-responsive decomplexation as an additional consideration in the design of high-affinity host-guest complexes.