Anja-Verena Mudring

Temperature-driven mixing-demixing behavior of binary mixtures of the ionic liquid choline bis(trifluoromethylsulfonyl)imide and water.

The ionic liquid (2-hydroxyethylammonium)trimethylammonium) bis(trifluoromethylsulfonyl)imide (choline bistriflimide) was obtained as a supercooled liquid at room temperature (melting point=30 degrees C). Crystals of choline bistriflimide suitable for structure determination were grown from the melt in situ on the X-ray diffractometer. The choline cation adopts a folded conformation, whereas the bistriflimide anion exhibits a transoid conformation. The choline cation and the bistriflimide anion are held together by hydrogen bonds between the hydroxyl proton and a sulfonyl oxygen atom. This hydrogen bonding is of importance for the temperature-dependent solubility properties of the ionic liquid. Choline bistriflimide is not miscible with water at room temperature, but forms one phase with water at temperatures above 72 degrees C (equals upper critical solution temperature). 1H NMR studies show that the hydrogen bonds between the choline cation and the bistriflimide anion are substantially weakened above this temperature. The thermophysical properties of water-choline bistriflimide binary mixtures were furthermore studied by a photopyroelectric technique and by adiabatic scanning calorimetry (ASC). By photothermal analysis, besides highly accurate values for the thermal conductivity and effusivity of choline bistriflimide at 30 degrees C, the detailed temperature dependence of both the thermal conductivity and effusivity of the upper and lower part of a critical water-choline bistriflimide mixture in the neighborhood of the mixing-demixing phase transition could be determined with high resolution and accuracy. Together with high resolution ASC data for the heat capacity, experimental values were obtained for the critical exponents alpha and beta, and for the critical amplitude ratio G+/G-. These three values were found to be consistent with theoretical expectations for a three dimensional Ising-type of critical behavior of binary liquid mixtures.

Synthesis, Structure, and Physico‐optical Properties of Manganate(II)‐Based Ionic Liquids

Several ionic liquids (ILs) based on complex manganate(II) anions with chloro, bromo, and bis(trifluoromethanesulfonyl)amido (Tf(2)N) ligands have been synthesized. As counterions, n-alkyl-methylimidazolium (C(n)mim) cations of different chain length (alkyl=ethyl (C(2)), propyl (C(3)), butyl (C(4)), hexyl (C(6))) were chosen. Except for the 1-hexyl-3-methylimidazolium ILs, all of the prepared compounds could be obtained in a crystalline state at room temperature. However, each of the compounds displayed a strong tendency to form a supercooled liquid. Generally, solidification via a glass transition took place below -40 degrees C. Consequently, all of these compounds can be regarded as ionic liquids. Depending on the local coordination environment of Mn(2+), green (tetrahedrally coordinated Mn(2+)) or red (octahedrally coordinated Mn(2+)) luminescence emission from the (4)T(G) level is observed. The local coordination of the luminescent Mn(2+) centre has been unequivocally established by UV/Vis as well as Raman and IR vibrational spectroscopies. Emission decay times measured at room temperature in the solid state (crystalline or powder) were generally a few ms, although, depending on the ligand, values of up to 25 ms were obtained. For the bromo compounds, the luminescence decay times proved to be almost independent of the physical state and the temperature. However, for the chloro- and bis(trifluoromethanesulfonyl)amido ILs, the emission decay times were found to be dependent on the temperature even in the solid state, indicating that the measured values are strongly influenced by nuclear motion and the vibration of the atoms. In the liquid state, the luminescence of tetrahedrally coordinated Mn(2+) could only be observed when the tetrachloromanganate ILs were diluted with the respective halide ILs. However, for [C(3)mim][Mn(Tf(2)N)(3)], in which Mn(2+) is in an octahedral coordination environment, a weak red emission from the pure compound was found even in the liquid state at elevated temperatures.

Solidification of Ionic Liquids: Theory and Techniques

Ionic liquids (ILs) have become an important class of solvents and soft materials over the past decades. Despite being salts built by discrete cations and anions, many of them are liquid at room temperature and below. They have been used in a wide variety of applications such as electrochemistry, separation science, chemical synthesis and catalysis, for breaking azeotropes, as thermal fluids, lubricants and additives, for gas storage, for cellulose processing, and photovoltaics. It has been realized that the true advantage of ILs is their modular character. Each specific cation–anion combination is characterized by a unique, characteristic set of chemical and physical properties. Although ILs have been known for roughly a century, they are still a novel class of compounds to exploit due to the vast number of possible ion combinations and one fundamental question remains still inadequately answered: why do certain salts like ILs have such a low melting point and do not crystallize readily? This Review aims to give an insight into the liquid–solid phase transition of ILs from the viewpoint of a solid-state chemist and hopes to contribute to a better understanding of this intriguing class of compounds. It will introduce the fundamental theories of liquid–solid-phase transition and crystallization from melt and solution. Aside form the formation of ideal crystals the development of solid phases with disorder and of lower order like plastic crystals and liquid crystals by ionic liquid compounds are addressed. The formation of ionic liquid glasses is discussed and finally practical techniques, strategies and methods for crystallization of ionic liquids are given.

Lanthanide coordination polymers with tetrafluoroterephthalate as a bridging ligand: thermal and optical properties.

By slow diffusion of triethylamine into a solution of 2,3,5,6-tetrafluoroterephthalic acid (H2tfBDC) and the respective lanthanide salt in EtOH/DMF single crystals of seven nonporous coordination polymers, (∞)(2)[Ln(tfBDC)(NO(3))(DMF)(2)]·DMF (Ln(3+) = Ce, Pr, Nd, Sm, Dy, Er, Yb; C2/c, Z = 8) have been obtained. In the crystal structures, two-dimensional square grids are found, which are composed of binuclear lanthanide nodes connected by tfBDC(2-) as a linking ligand. The coordination sphere of each lanthanide cation is completed by a nitrate anion and two DMF molecules (CN = 9). This crystal structure is unprecedented in the crystal chemistry of coordination polymers based on nonfluorinated terephthalate (BDC(2-)) as a bridging ligand; as for tfBDC(2-), a nonplanar conformation of the ligand is energetically more favorable, whereas for BDC(2-), a planar conformation is preferred. Differential thermal analysis/thermogravimetric analysis (DTA/TGA) investigations reveal that the noncoordinating DMF molecule is released first at temperatures of 100-200 °C. Subsequent endothermal weight losses correspond to the release of the coordinating DMF molecules. Between 350 and 400 °C, a strong exothermal weight loss is found, which is probably due to a decomposition of the tfBDC(2-) ligand. The residues could not be identified. The emission spectra of the (∞)(2)[Ln(tfBDC)(NO(3))(DMF)(2)]·DMF compounds reveal intense emission in the visible region of light for Pr, Sm, and Dy with colors from orange, orange-red, to warm white.

Ultrasound-assisted synthesis of mesoporous β-Ni(OH)2 and NiO nano-sheets using ionic liquids

Via a facile ultrasound synthesis from nickel acetate and sodium hydroxide with ionic liquids as the solvent and template it is possible to obtain nano-β-Ni(OH)2 of various dimensionalities depending on the reaction conditions with the ionic liquid (IL) being the most important factor. Scanning electron microscopy (SEM) imaging showed β-Ni(OH)2 to form as nanosheets, nanorods and nanospheres depending on the IL. ILs with strong to moderate hydrogen bonding capability like [C3mimOH][Tf2N] (1-(3-hydroxypropyl)-3-methylimidazolium bis(trifluoromethanesulfonylamide)), [C4mim][Tf2N] (1-butyl-3-methylimidazolium bis(trifluoromethanesulfonylamide)) and [Edimim][Tf2N] (1-ethyl-2,3-diemethylimidazolium bis(trifluoromethanesulfonylamide)) lead to the formation of nanosheets whilst [Py4][Tf2N] (butyl-pyridinium bis(trifluoromethanesulfonylamide)) leads to nanoparticles and [N1888][Tf2N] (methyltrioctylammonium bis(trifluoromethanesulfonylamide)) to nanorods. Subsequent calcination of the materials at elevated temperatures (285–425 °C) leads to the conversion of β-Ni(OH)2 to NiO under preservation of the nanostructure. Scanning electron microscopy (SEM), X-ray diffraction (XRD), TG-DTA, X-ray photoelectron spectroscopy (XPS), and energy dispersive X-ray spectroscopy (EDX) were used to observe the morphology, crystallinity, and chemical composition in more detail. Mesoporous NiO nanosheets obtained in [C4mim][Tf2N] possess an exceptionally high surface area of 141.28 m2 g−1 and a pore volume of 0.2 cm3 g−1 at 285 °C. As a result of calcination at 425 °C the surface area decreased to 92.84 m2 g−1, but the pore volume increased to 0.48 cm3 g−1. In addition, the product has an extraordinarily high saturation magnetization of 1.38 emu g−1, a coercivity of 117 Oe and an excellent specific capacitance of 199.4 F g−1 which renders the material highly interesting for application in supercapacitors.

Bis-cationic ionic liquid crystals

A series of bis-cationic imidazolium-based compounds are reported, where two imidazolium head groups are bridged by different types of spacers: a saturated C6 hydrocarbon spacer (type A), an ether bridge (type B), a benzylic spacer (type C) and a 2-butyne spacer (type D). The effect on mesophase formation of (1) the type of spacer, (2) the alkyl side chain length, and (3) the nature of the anion was investigated. The structural flexibility of the spacers seems to be responsible for the temperature range over which mesophases could be observed. Compared to types A, C, and D, the liquid crystalline state in type B is reached at much lower temperature and extends over a broader temperature range. As type B showed the best mesophase properties, we also combined the [B-C12] (bis(n-alkyl)-1,1-(oxydi-2,1-ethane-diyl)bis-imidazolium) cation with other anions such as ClO4−, BF4−, PF6− and the large, weakly coordinating anion bis(trifluoromethanesulfonyl)amide, NTf2−. For compounds [B-C12][BF4]2 and [B-C12][ClO4]2, with an anion of Td symmetry, a rare higher order smectic T phase could be observed, which transited into a smectic A phase during heating. Unexpectedly, mesophase formation was found for compound [B-C12][NTf2]2, which has not previously been observed for compounds with mono-cationic structures paired with this anion. In addition the rheological behavior of the bromide ILCs was investigated. The viscosities of all ILCs depend on the shear rate applied to the sample when the salt is in its mesophase. The viscosity increases drastically following the shear rate decrease, and non-Newtonian viscosity behavior is displayed by the mesophase.

Efficient quantum cutting in hexagonal NaGdF4:Eu3+ nanorods

An ionic liquid (IL) assisted solvothermal method is employed to prepare single phase, oxygen free, hexagonal NaGdF4:Eu3+ (2 mol%) nanorods with a visible quantum efficiency of 187%. In contrast, for mixed materials containing cubic and hexagonal NaGdF4:Eu3+, the quantum efficiency is much less (127%). Thus, synthesis parameters have to be carefully chosen in order to get the high performance hexagonal material. Not only the influence of the IL but also of the Gd:F ratio during synthesis as well as the temperature were studied. It is found that the IL stabilizes the formation of hexagonal NaGdF4:Eu3+, likewise a fluoride excess (Gd:F = 1:8) and elevated reaction temperatures (200 °C).

[Ni(tmen)(acac)][B(Ph)4] a probe for the anion basicity of ionic liquids

An optical basicity scale of various ionic liquids (IL) anions has been determined by UV-Vis absorption spectroscopy. The indicator complex [Ni(tmen)(acac)][B(Ph)(4)] (tmen = tetramethylethylendiamine, acac = acetylacetonate) shows to be a sensitive probe as the position of the (3)A(2g)(F) --> (3)T(1g)(F) absorption maximum of square planar to octahedral Ni(2+) shifts distinctly depending on the electron donor ability of the respective ionic liquid anion. If the Lewis basicity of the anion becomes stronger a change to a tetrahedral coordination around Ni(2+) is observed. Here the (3)T(1)(F) --> (3)T(2)(F) and (3)T(1)(F) --> (3)T(1)(P) transitions are helpful for monitoring the IL anion basicity. Evaluating the optical spectra allows ordering the ionic liquids according to rising Lewis basicity: PF(6)(-) < BF(4)(-) < N(Tf)(2)(-) < OTf(-) << HCOO(-) << DCA(-) << CF(3)COO(-) << PO(2)(OEt)(2)(-) << Cl(-). No substantial influence of the ionic liquid counter cation on the Ni(2+) absorption spectra could be detected. In consequence, [Ni(tmen)(acac)][B(Ph)(4)] allows evaluation of the donor power, hence, the Lewis basicity of the ionic liquid anion independent from the IL cation and an optical basicity scale of ionic liquid anions could be successfully established. In addition, [Ni(tmen)(acac)][B(Ph)(4)] turns out to be a sensitive probe to detect even small chloride impurities in ionic liquids which are a commonly encountered contamination in ILs.

Sonochemical synthesis of 0D, 1D, and 2D zinc oxide nanostructures in ionic liquids and their photocatalytic activity.

Ultrasound synthesis of zinc oxide from zinc acetate and sodium hydroxide in ionic liquids (ILs) is a fast, facile, and effective, yet highly morphology- and size-selective route to zinc oxide nanostructures of various dimensionalities. No additional organic solvents, water, surfactants, or templating agents are required. Depending on the synthetic conditions, the selective manufacturing of 0D, 1D, and 2D ZnO nanostructures is possible: Whereas the formation of rodlike structures is typically favored, ZnO nanoparticles can be obtained either under strongly basic conditions or by use of ILs with a long alkyl chain, such as 1-n-alkyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([C(n)mim][Tf(2)N]; n>8). A short ultrasound irradiation time favors the formation of ZnO nanosheets. Prolonged irradiation leads to the conversion of the ZnO nanosheets into nanorods. In contrast, ionothermal synthesis (conventional heating) does not allow for morphology tuning by variation of the IL or other synthesis conditions, as the longer reaction times required lead always to the formation of well-developed hexagonal nanocrystals with prismatic tips. The ZnO nanostructures synthesized by using ultrasound were efficient photocatalysts in the photodegradation of methyl orange. The photoactivity was observed to be as high as 95 % for ZnO nanoparticles obtained in [C(10)mim][Tf(2)N].

Small nickel nanoparticle arrays from long chain imidazolium ionic liquids

A series of six long chain alkyl mono- and bi-cationic imidazolium based salts with bis(trifluoromethylsulfonyl)imide (NTf2(-)) as the anion were synthesized and characterized. The single crystal structure of 1-methyl-3-octadecylimidazolium bis(trifluoromethylsulfonyl)imide could be obtained by X-ray analysis. All these long chain alkyl imidazolium based ILs were applied in the synthesis of nickel nanoparticles via chemical decomposition of an organometallic precursor of nickel. In these media, spontaneous decomposition of Ni(COD)2 (COD = 1,5-cyclooctadiene) in the absence of H2 occurred giving small NPs (≤4 nm) with narrow size distributions. Interestingly, formation of regularly interspaced NP arrays was also observed in long chain ILs. Such array formation could be interesting for potential applications such as carbon nanotube growth.