Complexation between methyl viologen (paraquat) bis(hexafluorophosphate) and dibenzo[24]crown-8 revisited.

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.