Electrostatic Repulsion between Cucurbit[7]urils Can Be Overcome in [3]Pseudorotaxane without Adding Salts

The host–guest chemistry between cucurbit[7]­uril (CB7) and a series of bolaform (B<i>n</i>) surfactants with different chain lengths, <i>n</i> = 12–22, was the target of our study. [3]­Pseudorotaxanes are formed when the alkyl chain of the bolaform has more than 14 carbon atoms. In these cases, two CB7 molecules can be accommodated between the two head groups of the bolaform without addition of electrolytes to the medium. In the case of a bolaform with 12 carbon atoms, the electrostatic repulsion between the carbonyl groups of the CB7 molecules avoids the threading of a second CB7 molecule yielding a mixed structure formed by a [2]­pseudorotaxane and an external host–guest complex. The assembly behavior was investigated using NMR spectroscopy, isothermal titration calorimetry (ITC), and kinetic measurements.