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

Abstract

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.

Allosteric and Chelate Cooperativity in Divalent Crown Ether/Ammonium Complexes with Strong Binding Enhancement

“…Althought he structurald ifferences of the two guests G O 2 and G C 2 are small( two ether oxygen atoms in G O 2 insteado ft wo isoelectronic methylene groups in G C 2), they differ significantly with respect to chelate cooperativity.S tudies by Gibsone tal., [39][40][41] Stoddarte tal. [42] and Chiu et al [43] as well as our recent work [44] demonstrated that solvationa nd the nature of the counterion to also have substantial effects on the association behavior of crown/ammonium complexesa nd the corresponding pseudorotaxanes. Hence, as olvent-dependent analysiso fi on-pairing effects on the association processes is included hereasw ell.…”

“…Complex formation was achieved by mixing equimolar amounts of G O 2 and H2 or G C 2 and H2 in a 10:3 (v/v) mixture of chloroform and methanol. The methanol was added to improve the solubility of the charged species and to reduce ion‐pair effects, which have been reported to interfere with complex formation in solvents with low dielectric constants . Indeed, ITC studies performed in pure chloroform with the mono‐ and divalent complexes indicate that the primary ammonium ions and tosylate ions form ion pairs, which strongly interfere with the desired host–guest complex formation.…”

“…Two counterbalancing effects—solvent‐dependent ion pairing and ammonium–crown ether hydrogen bonding—rationalize this behavior. Ion pairing is important in the free guests, but is negligible for their crown ether complexes . The higher the fraction of protic solvent, the lower the competition of the counterion for the ammonium binding site.…”

“…The DFT calculations reported here follow our previously described treatment of monovalent complexes to calculate the association Gibbs energies for charged monovalent crown ether/ammonium complexes in organic solvents (by using COSMO) including counterion effects (for a detailed description of the computational methods, see the Supporting Information) . Since all ITC measurements were performed in various mixtures of chloroform and methanol, that is, solvents with low dielectric constants depending on the amount of methanol, the guest ion pairs are expected to be considerably strong . Therefore, the tosylate counterions must be taken into account for the calculations as well.…”

Thermodynamics of Self‐Assembly of Dicarboxylate Ions with Binuclear Lanthanide Complexes

“…The individual bands in the emission spectra were integrated and plotted against titrant concentration to generate binding isotherms. These data were fitted using DynaFit 4 as previously reported, to determine the affinity constant ( K in m −1 ), defined by Equation (1), where X represents a guest: benzoate, isophthalate, nicotinate, or dinicotinate. …”