Secondary dibenzylammonium ion binding by [24]crown-8 and [25]crown-8 macrocycles

A range of crown ethers with either [24]crown-8 or [25]crown-8 constitutions have been demonstrated to form pseudorotaxanes with para-disubstituted dibenzylammonium ions, both in solution and in the “gas phase” by 1H NMR spectroscopy and liquid secondary ion mass spectrometry, respectively. Substitution of the [24]crown-8 framework with increasing numbers of benzo rings is observed to lower the stability constants (Ka's) from >103 to ∼0 M−1 in acetonitrile. A pronounced decrease in Ka values also occurs when the [24]crown-8 constitution is expanded to give a macroring containing 25 atoms — a modification that not only increases the size of the macrocyclic cavity but also disrupts the OCCO repeating unit in the macrocycle's backbone.

Relatively subtle changes in the constitutions of macrocyclic polyethers and dibenzylammonium ions alters dramatically their recognition for one another.