Complexation between Methyl Viologen (Paraquat) Bis(Hexafluorophosphate) and Dibenzo[24]Crown‐8 Revisited

• Published in: Chemistry : a European journal 2009-01, Vol.15 (1), p.106-116
• Main Authors: Gasa, Travis B., Spruell, Jason M., Dichtel, William R., Sørensen, Thomas J., Philp, Douglas, Stoddart, J. Fraser, Kuzmič, Petr
• Format: Article
• Language: English
• Subjects: crown compounds; Crown Compounds - chemistry; Crown Ethers; donor–acceptor systems; ion pairs; Kinetics; Magnetic Resonance Spectroscopy; Models, Chemical; Molecular Conformation; paraquat; Paraquat - analogs & derivatives; Paraquat - chemistry; Spectrophotometry, Ultraviolet; supramolecular chemistry
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.200801827

Paraquat bis(hexafluorophosphate) undergoes stepwise dissociation in acetone. All three species—the neutral molecule, and the mono‐ and dications—are represented significantly under the experimental conditions typically used in host–guest binding studies. Paraquat forms at least four host–guest complexes with dibenzo[24]crown‐8. They are characterized by both 1:1 and 1:2 stoichiometries, and an overall charge of either zero (neutral molecule) or one (monocation). The monocationic 1:1 host–guest complex is the most abundant species under typical (0.5–20 mM) experimental conditions. The presence of the dicationic 1:1 host–guest complex cannot be excluded on the basis of our experimental data, but neither is it unambiguously confirmed to be present. The two confirmed forms of paraquat that do undergo complexation—the neutral molecule and the monocation—exhibit approximately identical binding affinities toward dibenzo[24]crown‐8. Thus, the relative abundance of neutral, singly, and doubly charged pseudorotaxanes is identical to the relative abundance of neutral, singly, and doubly charged paraquat unbound with respect to the crown ether in acetone. In the specific case of paraquat/dibenzo[24]crown‐8, ion‐pairing does not contribute to host–guest complex formation, as has been suggested previously in the literature. Complex ion‐pairing: Paraquat bis(hexafluorophosphate) in acetone exists as a neutral molecule and as mono‐ and dications, and forms at least four different host–guest complexes with dibenzo[24]crown‐8 (see scheme). In this specific binding system, it is found that ion‐pairing does not affect host–guest complexation.