Substituent effects in the binding of bis(4-fluorobenzyl)ammonium ions by dianilino[24]crown-8

A series of para-substituted dianilino[24]crown-8 (DA24C8) macrocycles were synthesized and their ability to form host–guest complexes with bis(4-fluorobenzyl)ammonium ions (DFA+) were investigated. Although these crown ethers contain weakly hydrogen bonding aniline motifs, they do bind DFA+ in CDCl3/CD3NO2 solution, presumably in a pseudorotaxane-like manner. A plot of the values of the relative binding strengths (log[Ka(R)/Ka(H)]) versus the Hammett substituent constants σ+ of the groups at the para-position of the aniline units suggests that a linear free energy correlation exists for this self-assembly process. The strength of the binding between the crown ether and the thread-like ion can be fine-tuned over a narrow range by judicious choice of the substituting groups.