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

High affinity guest have been reported for the macrocyclic host cucurbit[7]uril (CB[7]), enabling widespread applications, but preventing 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 and 9-ammonium modified ortho-carboranes that bind to CB[7] with a K_a=10¹⁰ M^-1. Upon treatment with base, the ortho-carboranes readily undergo deboronation to yield anionic nido-carborane, a poor guest of CB[7], facilitating recovery of guest-free CB[7]. We showcase the utility of the modified ortho-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.