“The red cage”: implementation of pH-responsiveness within a macrobicyclic pyridinium-based molecular host

Abstract

The “red cage”, a new pyridinium-based macrobicyclic host, has been found to complex model aromatic substrates in aqueous media in a pH-responsive fashion.

“Vermellogens” and the Development of CB[8]-Based Supramolecular Switches Using pH-Responsive and Non-Toxic Viologen Analogues

“…The obtained compounds were fully characterized by 1D/2D NMR techniques, both in CD 3 CN ( R a–e H·2PF 6 ) and D 2 O ( R a–e H·2Cl), showing data in good agreement with that expected. 28 A quite unusual, but well-known, 22 − 24 , 29 − 32 common feature was observed in all cases, with the restricted rotation around the NH-C sp 2 (py) bond within the hydrazinylpyridinium moiety resulting in the non-equivalence of the protons on the upper and lower sides of the heterocycle, which appear in a near-coalescence situation, exchanging moderately slow on the NMR-timescale (e.g., R a H·2PF 6 , Figure 1 d). This end was demonstrated both by the exchange peaks found on the corresponding NOESY/EXSY experiments and by VT 1 H-NMR (inset Figure 1 d).…”

“…As for viologens, ,, the most straightforward methodology for the synthesis of the “vermellogens” would consist of the per- or sequential alkylation of an appropriate bis-pyridine precursor. , Nevertheless, this approach was found not very satisfactory, especially when applied to asymmetrically substituted analogues ( R c,d H·2X, Table ), as the attempted monoalkylation of the precursor yielded complex mixtures of products. Alternatively, considering the high hydrolytic stability of these hydrazone-linked bis-pyridinium salts, − we tackled their synthesis by hydrazone exchange in acidic water of the already alkylated intermediates 3 a–b ·I and 4 a–c ·I. Typically, a mixture of the matching aldehyde/ketone and hydrazone was reacted for 24 h at 60 °C, and the corresponding mixture was cooled down and saturated with solid KPF 6 , producing the precipitation of R a–e H·2PF 6 .…”

“…Following our continuous interest in the supramolecular chemistry of pyridinium salts, we have recently reported a series of hydrazone-based analogues of the V 2+ -containing host “blue box”, − developed by Stoddart and co-workers, in which the two viologen moieties within the model macrocycle are replaced by hydrazones linking the pyridinium rings. In these reports, we have not only found the pseudoviologen moieties acting as archetypic electron acceptors but, additionally, those behaving as acid–base responsive motives, because of an anomalous p K a for the imine protons.…”