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

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-ortho-carborane that binds to CB[7] with
a Ka ≈ 1010 M–1. Upon basic treatment, ortho-carborane readily
undergoes deboronation to yield anionic nido-carborane,
a poor guest for 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.