Tetracationic-to-dianionic tetraamino-dihydroxy-tetraoxacalix[4]arene: A paraquat receptor for all seasons

Abstract

Tetraamino-dihydroxy-oxacalix[4]arenes 1·H64+·4Cl– and 2·H64+·4Cl– were synthesized in two steps from off-the-shelf components, and subjected to in-depth investigations on their molecular and supramolecular properties. UV–Vis titrations with aqueous NaOH provided six discrete protonation constants along with the distribution diagram for each of the oxacalixarenes, while a 1H NMR titration of 1·H64+·4Cl– with NaOD(aq), combined with DFT (B3LYP/6–311++G(d,p)) calculations allowed to shed light on the conformational preferences of macrocycle 1·Hn(n–2)+ in the different protonation states. 1H NMR titrations with N,N′-dimethyl-4,4′-bipyridinium (paraquat) dichloride gave, at different pH values, apparent association constants (Kapp) for the formation of the host–guest complexes. Kapp values, combined with the protonation constants, in turn provided the stability constant for the individual complexes between paraquat and 1·Hn(n–2)+ in the different states of ionization, and allowed to identify 1·H– as the most efficient paraquat receptor.