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 described and, in this context, the family of the meta-bridged tetranitro-oxacalix[4]arenes comes up as a very 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 broader 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.
One oxacalix[4]arene, seven protonated species: a macrocyclic receptor for paraquat
Daniele CrisafulliPrimo
;Francesca Mancuso;Marco Milone;Anna Irto;Rosalia Maria Cigala;Gabriele Lando;Ilenia Pisagatti;Anna Notti;Giuseppe Gattuso
2022-01-01
Abstract
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 described and, in this context, the family of the meta-bridged tetranitro-oxacalix[4]arenes comes up as a very 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 broader 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.Pubblicazioni consigliate
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