Ion speciation driving chirality transfer in imidazolium-based camphorsulfonate ionic liquid solutions

Abstract

The underlying principle of the chirality transfer in imidazolium-based camphorsulfonate ionic liquids is rationalized by linking catalytic results from the hydrogenation of [N-(3'-oxobutyl)-N-methylimidazolium] [(+)-camphorsulfonate] to [N-(3'-hydroxybutyl)-N-methylimidazolium] [(+)-camphorsulfonate] in tetrahydrofuran with electrolyte theory by the help of dielectric relaxation spectroscopy. Using this approach we are able to explain why the maximum of the enantiomeric excess of the hydrogenation reaction in tetrahydrofuran is found at a medium concentration of 0.15 mol L(-1), whereas it declines at both, lower and higher concentrations. Dielectric spectra in the concentration range between 0.05 and 1.0 mol L(-1) reveal a solute mode due to dipolar ion pairs and larger dipolar ion clusters. They verify that at very low concentrations the ionic liquid ions are fully solvated with an increasing tendency to form neutral ion pairs with increasing concentration. Already at 0.025 mol L(-1) the degree of dissociation reaches a minimum reflecting a maximum of neutral ion pair formation. With increasing ionic liquid concentration ordered ion clusters are formed by two and more ion pairs. At high concentrations these clusters collapse by dilution in the excess ionic liquid and the defined ion contact necessary for the chirality transfer is lost to a great extent.

Complexity in Acid–Base Titrations: Multimer Formation Between Phosphoric Acids and Imines

“…with the real part, ϵ′ , being the frequency dependent permittivity and the imaginary part, ϵ′′ , representing the dielectric loss (i.e. absorption) . DRS spectra were recorded at frequencies ranging from 10 MHz to 110 GHz, using an Anritsu MS4647A Vector Network Analyzer.…”

The ion speciation of ionic liquids in molecular solvents of low and medium polarity

“…The origin of the conductance minimum may be further elucidated by DRS, which can be used to characterize the ion speciation in an IL solution. 6,10,11 Due to rather high IL concentrations required for meaningful experiments, the concentration range covered by DRS experiments (C $ 0.05 mol L À1 ) is typically above the concentration of the conductance minimum. Thus, we observe the fate of the ion pairs along the increasing branch of the conductance curves, but by some extrapolation, and in combination with conductance data for low IL concentrations, a detailed molecular picture can be developed.…”

“…In parts, our interest in solutions in THF resulted from a preceding study, in which we have employed DRS to probe the ion speciation driving chirality transfer in the ruthenium-catalyzed hydrogenation of a keto-functionalized prochiral imidazolium cation. 10 In this hydrogenation reaction the chiral camphorsulfonate anion was found to induce chirality at the cation via ion pairing. In THF a maximum enantiomeric excess was found at an IL concentration of C ¼ 0.15 mol L À1 , which could be linked to the characteristics of the ion speciation in this regime.…”

“…DRS has been employed to neat ILs, [6][7][8][9] and is just beginning to be applied to ILs in molecular solvents. 10,11 Theories which quantitatively describe electrolytes at the conditions of interest are not available now. A beneficial tool for elucidating properties of electrolytes over wide ranges of concentration, temperature and dielectric constant of the solvent is found in the corresponding-states principle, which can be used to relate the properties of real systems to those of a reference system.…”

Ion Speciation of Protic Ionic Liquids in Water: Transition from Contact to Solvent‐Separated Ion Pairs

“…Ion pairing in solution is the result of a subtle balance between ion-solvent interaction and cation-anion interaction. [5][6][7][8][9] This is in particular true for ionic liquids where a mix of Coulomb forces, hydrogen bonds, and dispersion forces between the cation and anion leads to unique macroscopic properties of these Coulomb fluids. [10][11][12] If the anion-cation interaction is sufficiently strong, contact ion pairs (CIPs) are the dominant structures in the mixtures.…”