Rotaxane or pseudorotaxane? That is the question!

A series of secondary dialkylammonium ions (RCH2)(2)NH2+ have been prepared, and their binding properties toward the macrocyclic polyether dibenzo[24]crown-8 (DB24C8) evaluated. By using this information, a route to a kinetically stable rotaxane-like entity-stabilized by noncovalent bonding interactions between the DB24C8 macroring and the ammonium center-was established, in which the crown ether slips over a dialkylammonium ion's stopper groups (R). However, we have found that the kinetic stability of this rotaxane-like entity is extremely dependent on the nature of the solvent in which it is dissolved, suggesting that pseudorotaxanes lie in the fuzzy domain between two sets of extremes, wherein a beadlike macrocycle and a dumbbell-like component may either (1) exist as a rotaxane or (2) be completely disassociated from one another.