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 H-1 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 (K-a's) from >10(3) to similar to 0 M-1 in acetonitrile. A pronounced decrease in K-a 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. (C) 1999 Elsevier Science Ltd. All rights reserved.