Tetranitro-oxacalix[4]crown-Based Host–Guest Recognition Motif and a Related [2]Rotaxane-Based Molecular Switch

Different from so far reported oxacalix[4]­crown-based host–guest motifs in which oxacalix[4]­crowns act only as hydrogen bond acceptors, a [2]­pseudorotaxane-type tetranitro-oxacalix[4]­crown/urea host–guest recognition motif was developed in which tetranitro-oxacalix[4]­crown played a role as both a hydrogen bond donor and an acceptor to stabilize the resulting supramolecular complex. Furthermore, on the basis of a [2]­pseudorotaxane complex formed from a tetranitro-oxacalix[4]­crown and an axle containing a secondary ammonium ion and a urea group, a [2]­rotaxane-based molecular switch was created, in which the oxacalix[4]­crown wheel was able to reversibly translocate between the secondary ammonium binding site and the urea binding site of the axle under acid–base stimulation.