Complex interactions of pillar[5]arene with paraquats and bis(pyridinium) derivatives

Abstract

The complexation behavior of a series of paraquats (G1·2PF₆–G5·2PF₆) and bis(pyridinium) derivatives (G6·2PF₆–G14·2PF₆) with pillar[5]arene (P5A) host has been comprehensively investigated by ¹H NMR, ESI mass and UV-vis absorption spectroscopy. It is found that P5A forms 2 : 1 external complexes with N,N′-dialkyl-4,4′-bipyridiniums (G1–G4·2PF₆); while it forms 1 : 1 pseudorotaxane-type inclusion complexes with methylene [–(CH₂)_n–] linked bis(pyridinium) derivatives possessing appropriate chain lengths (n = 3–6, G7–G10·2PF₆). Host–guest association constants in dimethyl sulfoxide (DMSO) were determined, indicating G7–G10·2PF₆ axles form stable [2]pseudorotaxanes with P5A wheel in this very high polarity solvent and 1,4-bis(pyridinium)butane (G8·2PF₆) was the most suitable axle unit. Meanwhile, the nature of the substituents attached to 1,4-bis(pyridinium)butane dramatically affects the molecular recognition behavior. The introduction of pyridyls (G13·2PF₆) increases not only the K_a value (4.5 × 10²→7.4 × 10² M⁻¹), but also the charge transfer (CT) absorption (colorless→yellow). Furthermore, the solvent effects have also been investigated, showing they significantly influence the association strength during the course of host–guest complexation. Particularly, the K_a value of P5A–G13·2PF₆ in 1 : 1 (v:v) acetone-d₆/DMSO-d₆ is enhanced by a factor of 7.3 compared with pure DMSO-d₆ (7.4 × 10²→5.4 × 10³ M⁻¹).