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

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.