Voltammetric studies of polyoxometalate microparticles in contact with the reactive distillable ionic liquid DIMCARB

The voltammetry of the polyoxometalate salts [Bu₄N]₄[SiW₁₂O₄₀] (α, β, γ* isomers), [Bu₄N]₄[S₂W₁₈O₆₂] (α, γ* isomers) and [Bu₄N]₄[S₂Mo₁₈O₆₂] (α isomer) has been studied in the “distillable ionic liquid” DIMCARB (a mixture of adducts, both ionic and neutral, of dimethylamine and carbon dioxide). Dissolution of these salts tended to be slow and the anions, except [α-SiW₁₂O₄₀]⁴⁻, reacted with DIMCARB. However, high quality voltammograms were obtained from microparticles of the salts adhered to a glassy carbon electrode in contact with DIMCARB. Rapid dissolution of the reduced forms of each of the polyoxometalate anions provided reversible potential data. The higher basicity of the more highly reduced anions promoted interactions with the acidic solvent component of this medium. Comparison of reversible potentials indicated that the polarity of DIMCARB is similar to CH₃CN, but smaller than that of conventional non-volatile ionic liquids and water.     

Electrodeposition of lead on glassy carbon and mercury film electrodes from a distillable room temperature ionic liquid,DIMCARB

The electrochemical reduction of Pb²⁺ has been studied in the ‘distillable’ ionic liquid DIMCARB (a mixture of adducts of dimethylamine and carbon dioxide, comprising both neutral and ionic moieties). Voltammetric results show that Pb²⁺ is reduced in a single step to form Pb metal via a nucleation and growth mechanism on a glassy carbon electrode. Ex situ powder X-ray diffraction studies on deposited lead show the presence of both α- and β-PbO₂, as well as elemental lead, suggesting the finely deposited lead particles are in an active rather than passive state. Chronamperometric and scanning electron microscope measurements show that the nucleation and growth follows a progressive nucleation mechanism on glassy carbon. Large peak–peak separations for the Pb reduction and oxidation are consistent with this mechanism and do not suggest electrochemical reversibility. However, experiments with co-deposition of Hg show that this irreversibility is a result of deposition onto a solid glassy carbon surface rather than a solvent effect. The diffusion coefficient of Pb²⁺ in DIMCARB has been calculated to be 1.8±0.4×10⁻⁷ cm² s⁻¹. Electronic supplementary material Supplementary material is available in the online version of this article at and is accessible for authorized users. This work is dedicated to Piero Zanello on the occasion of his 65th birthday in recognition of his numerous contributions to inorganic electrochemistry.     

Tetraalkylphosphonium polyoxometalates: electroactive, "task-specific" ionic liquids

The pairing of selected polyoxometalate (POM) anions with appropriate tetraalkylphosphonium cations is shown to yield an original family of ionic liquids, among them an ambient-temperature "liquid POM".     

High performance "ionic liquid" chromatography

We firstly developed high performance liquid chromatography methods with "ionic liquids" as eluents (HPILC) and successfully demonstrated the analysis of various biopolymers including scarcely soluble highly polymerized cellulose by means of HPILC with highly polar ionic liquid.     

Direct catalytic asymmetric cross-aldol reactions in ionic liquid media

Enantioselective proline-catalyzed direct asymmetric cross-aldol reactions with aldehydes were performed in ionic liquid 1-n-butyl-3-methylimidazolium hexafluorophosphate media, which simplified product isolation and catalyst recycling, affording 3-hydroxy aldehydes in high yield with excellent stereoselectivity. In addition, the enhanced reactivity of the asymmetric cross-aldol reactions in ionic liquid allowed the catalyst loading to be lowered significantly.     

Atom transfer carbonylation using ionic liquids as reaction media

Photo-induced ATC reactions of RI, CO, and amines to produce amides, were examined using ionic liquids, such as [bmim]PF₆ and [bmim]NTf₂, as reaction media in the presence of a catalytic amount of a Pd–carbene complex. When the primary alkyl iodide was used, the yield of the amide was lowered due to competing S_N2 reactions between RI and amines, whereas the reaction of the tertiary alkyl iodides was dependent on the structure of the substrates. ATC reactions of a wide variety of secondary RI proceeded smoothly when ionic liquids were used as reaction media. The Pd-catalyst and ionic liquid could also be recycled.     

Phosphonium ionic liquids as reaction media for strong bases

Phosphonium ionic liquids are compatible with strong bases; for example, solutions composed of commercially available phenylmagnesium bromide in THF are persistent in tetradecyl(trihexyl)phosphonium chloride for several hours-days: their stability appears to be couched in kinetic terms.     

Characterizing ionic liquids as reaction media through a chemical probe

The triplet N,N-dimethylaminophenyl cation, a highly reactive but chemospecific electrophile, has been used as a probe for characterizing the properties of reaction media for a series of imidazolium ILs. With the N-hexyl-N-methyl imidazolium derivatives (not with the N-butyl analogues), hydrogen transfer leading to the aniline was the main process. Trapping by iodide occurred with an inverse dependence on viscosity. Trapping by pi nucleophiles exhibited a more complex behavior. This was explained by the effect of both the bulk viscosity and the structure of the IL cation on both steps of the reaction, namely, initial electrophilic attack and ensuing cation elimination or nucleophile addition. However, with an excellent nucleophile, such as thiophene, or when the latter step was intramolecular, as with 4-pentenol, the difference was obliterated and trapping became uniform. Incorporation of the probe into the IL cation (through insertion into the C--H bond alpha to the imidazolium ring) was demonstrated, while no addition to the anion tested (including bis(trifluoromethanesulfonimide)) took place.     

Rates of ionic reactions with charged nanoparticles in aqueous media

A theory is developed to evaluate the electrostatic correction for the rate of reaction between a small ion and a charged ligand nanoparticle. The particle is assumed to generally consist of an impermeable core and a shell permeable to water and ions. A derivation is proposed for the ion diffusion flux that includes the impact of the equilibrium electrostatic field distribution within and around the shell of the particle. The contribution of the extra- and intraparticulate field is rationalized in terms of a conductive diffusion factor, f(el), that includes the details of the particle geometry (core size and shell thickness), the volume charge density in the shell, and the parameters defining the electrostatic state of the particle core surface. The numerical evaluation of f(el), based on the nonlinear Poisson-Boltzmann equation, is successfully complemented with semianalytical expressions valid under the Debye-Hückel condition in the limits of strong and weak electrostatic screening. The latter limit correctly includes the original result obtained by Debye in his 1942 seminal paper about the effect of electrostatics on the rate of collision between two ions. The significant acceleration and/or retardation possibly experienced by a metal ion diffusing across a soft reactive particle/solution interphase is highlighted by exploring the dependence of f(el) on electrolyte concentration, particle size, particle charge, and particle type (i.e., hard, core/shell, and entirely porous particles).     

Gold-catalyzed hydrative cyclization of 1,6-diynes in ionic liquid media

Gold-catalyzed hydrative cyclization of terminal 1,6-diynes proceeds in ionic liquid with methanol as co-solvent. The solvent-catalyst could be recycled, after separation of the product by extraction with ether.     

Alkylation of the 2-hydroxypyridine anion in ionic liquid media

The alkylation reaction of the ambident 2-hydroxypyridine anion was examined in ionic liquid media. Ionic liquids increase the alkylation reaction rate in comparison with molecular liquids, as well as the level of impact on the reaction rates of the counter ion and/or additives, and the distribution of isomers of the reaction products in trans-formations of the ambident 2-hydroxypyridine anion. __________ Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 5, pp. 699–710, May, 2008.     

Neutral species from "non-protic" N-heterocyclic ionic liquids

Possible isomerisation of 1,2,3-trialkylimidazolium and 1-alkylpyridinium ion pairs by proton transfer and by the nucleophilic addition of the anion to the cation have been investigated at the B3LYP/6-31+G* and B3LYP/6-311+G** levels of density functional theory. The deprotonation energies of 1,2,3-trialkylimidazolium and 1-alkylpyridinium cations to diaza-pentafulvene and pyridinium-ylide, respectively, were only slightly larger than that of 1,3-dialkylimidazolium salts yielding N-heterocyclic carbenes. Accordingly, in the case of 1,2,3-dialkylimidazolium salt ion pairs the stability of the H-bonded complex between the fulvene and the corresponding acid can be comparable to that of the ion pair in the presence of sufficiently basic anions, such as acetate. In the case of the pyridinium salts the nucleophilicity of the cation dominates over the acidity, and the formation of 1,2- or 1,4-dihydropyridine derivatives is preferred over proton transfer.     

Surface structure of a "non-amphiphilic" protic ionic liquid

The nanostructure of the ethanolammonium nitrate (EtAN)-air surface has been investigated using X-ray reflectometry (XRR), vibrational sum frequency spectroscopy (VSFS) and neutral impact collision ion scattering spectroscopy (NICISS). The XRR data decays more rapidly than expected for a perfectly sharp interface, indicating a diffuse electron (scattering length) density profile. Modelling of the XRR data using three different fitting routines produced consistent interfacial profiles that suggest the formation of interfacial EtAN clusters. Consistent with this, VSFS reveals that the EtAN surface is predominantly covered by -CH(2)- moieties, with the -NH(3)(+) and -OH groups of the cation buried slightly deeper in the interface. The elemental profiles determined using NICISS also show enrichment of carbon relative to nitrogen and oxygen in the outermost surface layer, which is consistent with the surface cation orientation deduced from VSFS, and with the presence of EtAN aggregates at the liquid surface.     

Enantioselective hydrogenation of imines in ionic liquid/carbon dioxide media

The enantioselective hydrogenation of N-(1-phenylethylidene)aniline using cationic iridium complexes with chiral phosphinooxazoline ligands was studied as a chemical probe to assess the potential of ionic liquid/carbon dioxide (IL/CO2) media for, multiphase catalysis. The biphasic system leads to activation, tuning, and immobilization of the catalyst that would be impossible in classical organic solvent systems or in either of the two unconventional media separately. In particular it is demonstrated that (i) the presence of CO2 can be beneficial or even mandatory for efficient hydrogenation in the IL; (ii) the precursor is activated in the IL by anion exchange allowing one to use in situ catalysts; (iii) the anion of the IL greatly influences the selectivity of the catalyst; (iv) the products are readily isolated from the catalyst solution by CO2 extraction without cross contamination of IL or catalyst; and (v) the IL leads to enhanced stability of the catalyst. These results are corroborated and rationalized on the basis of the physicochemical properties of the biphasic medium and the chemical characteristics of the catalytic systems.     

Formation constants at high ionic strength-I. Potentiometric determination of protonation constants for succinic, propionic and mono-methyl succinic acid in different ionic media

The protonation constants, K(r), for the ligands succinic acid (SA), mono-methyl succinate (MS) and propionate (PA) have been determined, at 25 degrees C, by glass electrode potentiometry in 3 mol/dm(3) (M) NaNO(3), KNO(3), NH(4)NO(3), Ca(NO(3))(2) and Et(4)NBr aqueous media. Results are compared with literature constants determined in 3M NaClO(4). The order of stability was found to be K(1)(SA) > K(1)(PA) > K(1)(MS) > K(2)(SA) and for the ligands in the different media K(r) followed the general trend with respect to the background electrolyte Et(4)NBr > NaClO(4) > KNO(3) > NaNO(3) > NH(4)NO(3) > Ca(NO(3))(2).     

"Nonsolvent" applications of ionic liquids in biotransformations and organocatalysis

The application of room-temperature ionic liquids (RTILs) as (co)solvents and/or reagents is well documented. However, RTILS also have "nonsolvent" applications in biotransformations and organocatalysis. Examples are the anchoring of substrates to RTILs; ionic-liquid-coated enzymes (ILCE) and enzyme-IL colyophilization; the construction of biocatalytic ternary reaction systems; the combination of enzymes, RTILs, membranes, and (bio)electrochemistry; and ionic-liquid-supported organocatalysts. These strategies provide more robust, more efficient, and more enantioselective bio- and organocatalysts with many practical applications. As shown herein, RTILs offer a wide range of promising alternatives to conventional chemistry.     

Charge-tagged polar phosphine ligands in Pd-catalysed reactions in aqueous and ionic media

A new range of polar imidazolium and phosphate-containing ligands was synthesised from readily available starting materials in high yielding multi-step transformations. These ligands were used to generate Pd catalysts for Suzuki and Heck C–C coupling reactions in organic and organic/aqueous media. The catalysts performed well in aqueous media in the Suzuki reaction and less well in the Heck reaction, related to substrate solubility in the aqueous media. When moving to ionic liquids, the Heck reaction dramatically improved, especially in media compatible with the polar catalysts and the non-polar reagents. In all cases, the catalysts were stable to the formation of Pd black, a form of degradation that frequently befalls Pd catalysts. The catalysts could be successfully recycled without loss of activity.