Tetraamino-dihydroxy-tetraoxacalix[4]arene as a versatile paraquat receptor

Oxacalix[n]arenes are macrocyclic compounds composed of aromatic units linked together by oxygen atoms, that were firstly reported in 1966. Since their discovery a very wide variety of derivatives has been reported and, among them, the family of the meta-bridged tetranitro-oxacalix[4]arenes emerged as intriguing molecular receptors. We already showed that reduction of the nitro groups allowed a tetraamino-calix[4]arene to form 1:1 complex with paraquat (N,N’-dimethyl-4,4’-bypiridinium, PQT2+) in aqueous solution in the 1–2 pH range. We now present two new tetraamino-calix[4]arenes, 1·H2 and 2·H2 that, owing to the presence of two oppositely-oriented hydroxyl groups are able to operate in a wider pH interval. Tetraamino-calix[4]arene 1·H2 on the left, tetraamino-calix[4]arene 2·H2 on the right These two novel macrocycles have been synthesized in two steps from 1,3-dinitro-4,6-dichloro-benzene and pyrogallol or phloroglucinol. 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 for the formation of the host-guest complexes that, 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.