Charles M. Gordon

New developments in catalysis using ionic liquids

Ionic liquids are low melting point salts that represent an exciting new class of reaction solvents for catalysis. Being composed entirely of ions, they possess negligible vapour pressures, and the wide range of possible cations and anions means that other solvent properties may be easily controlled. There is currently great interest in the use of these materials as solvents for a wide range of applications, including catalysis. Many reactions show advantages when carried out in ionic liquids, either with regard to enhanced reaction rates, improved selectivity, or easier reuse of catalysts. This review is intended to bring the reader up to date on the developments involving ionic liquids in catalytic applications over the previous 18 months. Recent spectroscopic investigations into the solvent properties of ionic liquids with relevance to catalysis are discussed first, followed by a critical review of the major developments in transition metal, Lewis acid, and enzyme catalysed processes in these solvents. Particular emphasis is given to the combination of ionic liquids with supercritical fluids, Pd-based catalysts, and enzymes. Wherever possible, the results gained in ionic liquids are critically compared with those obtained using other catalytic systems.

Ionic liquid crystals: hexafluorophosphate salts

A series of novel hexafluorophosphate salts, based on N,N′-dialkylimidazolium and substituted N-alkylpyridinium cations, display liquid crystalline behaviour at temperatures above their melting point. The temperature range over which liquid crystalline behaviour is observed increases markedly with increasing alkyl chain length. Alkyl substitution at the 3- and 4-positions on the pyridinium ring results in a decrease in the melting point compared with the equivalent unsubstituted salt, but also leads to a large decrease in the tendency towards liquid crystalline behaviour (or mesogenicity). The salts prepared are fully characterised using a wide variety of techniques, including NMR and IR spectroscopy, DSC, and single crystal X-ray diffraction in the case of 1-dodecyl-3-methylimidazolium hexafluorophosphate. The effect of preparing mixtures containing different proportions of two cations is also reported.

Investigations of solvent–solute interactions in room temperature ionic liquids using solvatochromic dyes

Different measures of polarity and nucleophilicity of a range of ionic liquids have been investigated using two solvatochromic dyes; the polarity appears to be largely cation controlled, while the donor strength is entirely anion dependent.

The effects of anion and cation substitution on the ultrafast solvent dynamics of ionic liquids: A time-resolved optical Kerr-effect spectroscopic study

Ultrafast solvent dynamics of room-temperature ionic liquids have been investigated by optical heterodyne-detected Raman-induced Kerr-effect spectroscopy (OHD-RIKES) by studying the effects of cation and anion substitution on the low frequency librational modes. The spectra of two series of imidazolium salts are presented. The first series is based on the 1-butyl-3-methylimidazolium salts [bmim]+ containing the anions trifluoromethanesulfate [TfO]−, bis(trifluoromethanesulfonyl)imide [Tf2N]−, and hexafluorophosphate [PF6]−. The second series is based on [Tf2N]− salts containing the three cations 1-butyl-2,3-dimethylimidazolium [bmmim]+, 1-methyl-3-octylimidazolium [omim]+, and [bmim]+. It is found in all five samples that the signal is due to libration of the imidazolium ring at three frequencies around 30, 65, and 100 cm−1 corresponding to three local configurations of the anion with respect to the cation.

Quasielastic neutron scattering characterization of the relaxation processes in a room temperature ionic liquid

We report the first quasielastic neutron scattering measurements on a room temperature ionic liquid: 1-n-butyl-3-methylimidazolium hexafluorophosphate, [bmim][PF6]. Data were collected using a medium resolution spectrometer as a function of temperature in the range 250–320 K. The data unequivocally indicate the existence of two different relaxation processes: a fast, localized motion occurring in the subpicosecond range and a slower process spanning the subnanosecond regime. These results provide experimental support to recently published molecular dynamics simulations. Evidence for slower, unresolved dynamics (under the present experimental conditions) is also obtained. Both temperature and momentum transfer dependence of the intermediate incoherent dynamic structure factor were investigated, after Fourier transformation into the temporal domain. The fast process shows no appreciable Q- and T-dependence. On the other hand the slow process shows evidence of a complex, non-Debye and non-Arrhenius behavior.

Bimolecular rate constants for diffusion in ionic liquidsElectronic supplementary information (ESI) available: Fig. S1: isokinetic plot obtained for the energy transfer reaction of 3BP* and N in five ionic liquids, toluene and acetonitrile. See http://www.rsc.org/suppdata/cc/b2/b202944h/

The temperature dependence of the bimolecular rate constants for a diffusion controlled reaction involving neutral reactants have been directly determined in five commonly used ionic liquids over the temperature range 5-70 degrees C.

Estimates of internal energies of vaporisation of some room temperature ionic liquids.

Systematic variation in the bimolecular rate constant, k(q), for the reaction of singlet oxygen and 1,4-dimethylnaphthalene has led to estimates of the Hildebrand solubility parameter, cohesive pressures and internal energies of vaporisation of some room temperature ionic liquids.

High-pressure CO2-induced reduction of the melting temperature of ionic liquidsElectronic supplementary information (ESI) available: Fig. S1: ATR-IR spectrum of [C16mim][PF6] after it has been subjected to solution of ferrocene in CO2 at 50 °C and pressure of ca. 110 bar. See http://www.rsc.org/suppdata/cc/b2/b202759c/

An in situ ATR-IR spectroscopic study has shown that high-pressure CO2 reduces melting temperature of ionic liquids such as [C16mim][PF6].

Indium and tin-mediated allylation in ionic liquids

The first report of the use of indium metal and allyl bromide for the allylation of aldehydes and ketones in ionic liquids is given in this paper. The homoallylic alcohol products are obtained in high yields on reaction at room temperature using stoichiometric quantities of indium. Initial results gained using a catalytic system of indium, manganese and TMSCl are also reported. The effect of ionic liquid solvents on the stereochemical selectivity of allylation of 2-methoxycyclohexanone and benzoin methyl ether has been investigated, showing a higher selectivity towards the chelation-controlled mechanism in ionic liquids than in conventional solvents such as water and THF. Using tetraallyltin as the allylating agent, the same reaction occurs with modest improvements in stereoselectivity compared with conventional solvents. The addition of 5 mol% Sc(OTf)3, however, results in a large increase in both reaction rate and selectivity, and permits the isolation of high yields of product via a simple workup procedure. The ionic liquid/Sc(OTf)3 mixture may also easily be recycled with good maintenance of yield and stereoselectivity.

Ionic liquids: polar, but weakly coordinating solvents for the first biphasic oligomerisation of ethene to higher α-olefins with cationic Ni complexes

Ethylene oligomerisation in ionic liquids gives predominately alk-1-ene products with better reactivity and selectivity than in conventional solvents; turnover frequencies are correlated with polarity data obtained using solvatochromic dyes.