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

[Display omitted] • • Two novel water-soluble, proton-ionizable, isomeric tetraamino-dihydroxy-tetraoxacalix[4]arene were synthesized. • • Six protonation constants were determined, and the distribution diagrams for the seven protonated species were calculated. • • The phloroglucinol-derived oxacalix[4]arene 1 ·H 2 was successfully employed to complex paraquat both at low and high pH. • • Combination of protonation and apparent binding constants gave association constants for the differently-charged complexes. Tetraamino-dihydroxy-oxacalix[4]arenes 1 ·H 6 4+·4Cl– and 2 ·H 6 4+·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 ·H 6 4+·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 ·H n (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 (K app) for the formation of the host–guest complexes. K app values, combined with the protonation constants, in turn provided the stability constant for the individual complexes between paraquat and 1 ·H n (n–2)+ in the different states of ionization, and allowed to identify 1 ·H– as the most efficient paraquat receptor. [ABSTRACT FROM AUTHOR]