A Metallacage Encapsulating Chloride as a Probe for a Solvation Scale in Ionic Liquids

With the purpose of assessing the reactivity of chloride ions dissolved in ionic liquids (ILs), a relative scale for the solvation of chloride is given for a series of ILs based on the bis(trifluoromethane)sulfonimide ([Tf2N]) anion and different cations, 1-butyl-3-methylimidazolium ([bmim]), 1-butyl-2,3-dimethylimidazolium ([bdmim]), 1-butyl-1-methylpyrrolidinium ([bmpy]), 1-butylpyridinium ([bpy]), 1-pentyl-1,1,1-triethylammonium ([C5e3am]), and 1-(2-hydroxy)ethyl-3-methylimidazolium ([mimeOH]). Insights into the solvation of chloride are achieved by the thermodynamic study of the reaction of dissociation of a chloride-templated nickel(II) metallacage performed at various temperatures by UV-visible spectroscopy in each IL. The order of chloride solvation [C5e3am][Tf2N] < [bmpy][Tf2N] < [bmim][Tf2N] ≤ [bdmim][Tf2N] < [bpy][Tf2N] << [mimeOH][Tf2N] was ascertained from the reaction enthalpies determined in the different ILs. The reaction entropies display a concentration-dependent behavior, which has highlighted a correlated motion of the ions in these ILs.

Catalytic hydrogenation of alkenes under biphasic conditions

Corinne Daguenet

Ruthenium(II)-arene diphosphine complexes are shown to be effective precursors for the catalytic hydrogenation of the C=C bond in an aqueous biphase using molecular hydrogen as the hydrogen source. A mechanistic study performed in water, utilizing high gas pressure NMR spectroscopy and electrospray ionization mass spectrometry, and experiments involving the variation of pH, enables indirect identification of the catalytically active species resulting from the substitution of the chloride with the dihydrogen ligand. Similar catalytic experiments are carried out in ionic liquids (ILs), such as [bmim][BF4] and [bmim][TfO] using an analogous precursor, but no conversion is observed and the ruthenium(II) species does not undergo any chemical change during the reaction. The dissociation of chloride, leading to the catalyst, is inhibited in these ILs due to a lower solvation of chloride. An estimate of the bmim-chloride interaction, which is lower than the water-chloride interaction, is determined by 1H NMR spectroscopy. To ensure the formation of the active species in such ILs, the coordinated chloride is abstracted beforehand and replaced by acetonitrile, which restores the catalytic activity. An alternative approach to overcome the chloride dissociation problem consists of improving the solvation of the chloride in the IL medium. To accomplish this, water is added as a co-solvent in the IL phase and, under these conditions, the activity is restored. In order to assess the reactivity of chloride ions in ILs, a relative scale for the solvation of chloride is given for a series of [cation][Tf2N] ILs. This scale, based on the reaction enthalpy for the dissociation of a chloride-templated metallacage, yields the following order of chloride solvation: [C5e3am][Tf2N] < [bmpy][Tf2N] < [bmim][Tf2N] ≤ [bdmim][Tf2N] < [bpy] [Tf2N] ≪ [mimeOH][Tf2N]. In Tf2N-based ILs, the catalytic activity of hydrogenation reactions using the same Ru(II) precursor in the presence of chloride is totally modified. In a series of [cation][Tf2N] ILs, the rate of formation of the catalyst, which is promoted by chlorides, is shown to depend on the solvation of chloride.

EPFL2006

Switching the Mechanism of Catalyst Activation by Ionic Liquids

Corinne Daguenet, Paul Joseph Dyson

In ionic liqs. (ILs), the activation of a ruthenium(II) catalyst precursor occurs via an unexpected mechanism, which is different from the activation mechanism that takes place in water. The effectiveness of a biphasic catalytic hydrogenation reaction using the ruthenium(II) precatalyst is highly dependent on the nature of both anion and cation of the IL. Evidence is provided that chloride ions are required to generate the active catalyst. Moreover, the influence of the IL cation on the rate of hydrogenation is correlated with the differences in capability of the IL to solvate chloride. A kinetic study provides insights into the mechanism of the formation of the active catalytic species.

2006

Synthesis and characterization of ionic liquids incorporating the nitrile functionality

Paul Joseph Dyson, Zhaofu Fei, Dongbin Zhao

A series of imidazolium salts with the nitrile functional group attached to the alkyl side chain, viz. [C(n)CNmim][X] (where C(n)CNmim is the 1-alkylnitrile-3-methylimidazolium cation and C-n = (CH2)(n), n = 1-4; X = Cl, PF6, and BF4) and [C(3)CNdmim][X] (where C(n)CNdmim is the 1-alkylnitrile-2,3-dimethylimidazolium cation and C-n = (CH2)(n), n = 3; X = Cl, PF6, and BF4), have been prepared and characterized using spectroscopic methods. The majority of the nitrile-functionalized imidazolium salts can be classed as ionic liquids since they melt below 100 degreesC. Four of the imidazolium salts have been characterized in the solid state using single-crystal X-ray diffraction analysis to reveal an extensive series of hydrogen bonds between H atoms on the cation and the anion. The relationship between the solid-state structure and the melting point is discussed. Key physical properties (density, viscosity, and solubility in common solvents) of the low melting ionic liquid have been determined and are compared with those of the related 1-alkyl-3-methylimidazolium and 1-alkyl-2,3-dimethylimidazolium ionic liquids. It was envisaged that these ionic liquids could act as both solvent and ligand for catalyzed reactions, and this application is demonstrated in hydrogenation reactions, which show that retention of the catalyst in the ionic liquid during product extraction is extremely high.

2004

Catalytic hydrogenation of alkenes under biphasic conditions

Corinne Daguenet

EPFL2006