Low-Melting Manganese(II)-Based Ionic Liquids: Syntheses, Structures, Properties and Influence of Trace Impurities

The synthesis of more than 10 new magnetic ionic liquids with [MnX4]2− anions, X = Cl, NCS, NCO, is presented. Detailed structural information through single-crystal X-ray diffraction is given for (DMDIm)[Mn(NCS)4], (BnEt3N)2[Mn(NCS)4], and {(Ph3P)2N}2[Mn(NCO4)]·0.6H2O, respectively. All compounds consist of discrete anions and cations with tetrahedrally coordinated Mn(II) atoms. They show paramagnetic behavior as expected for spin-only systems. Melting points are found for several systems below 100 °C classifying them as ionic liquids. Thermal properties are investigated using differential scanning calorimetry (DSC) measurements. The physicochemical properties of density, dynamic viscosity, electrolytic conductivity, and surface tension were measured temperature-dependent of selected samples. These properties are discussed in comparison to similar Co containing systems. An increasing amount of bromide impurity is found to affect the surface tension only up to 3.3%.


Introduction
For the last 20 years, the ongoing tremendous interest in ionic liquids (ILs) as salts with low melting points is based on their partly unique properties which lead to a variety of possible applications. Therefore, ILs have been thoroughly investigated as systems with large electrochemical windows and liquid ranges, hardly measurable vapor pressures, unusual solubility characteristics, and applications in catalysis [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15]. ILs containing metal-based complex ions, so-called magnetic ionic liquids (MILs), are a highly investigated subclass of ILs, because they show magnetic response in addition to the characteristics mentioned above [16][17][18][19][20]. Such ILs with paramagnetic transition-metal complex anions have also been assumed to be magnetic and magnetorheological fluids [21][22][23]. 2 of 15 In the last 10 years, we have reported on different classes of transition metal-containing low-melting ILs with complex anions of the 3d metals Cr, Co, and Ni, respectively, of which a series of physicochemical data is available [24][25][26][27][28][29][30][31][32][33][34]. The astonishing results from imidazolium-based ILs containing the [Co(NCS) 4 ] 2− anion [25] led us to extending our research for new low-melting, transparent ILs which contain tetrahedrally coordinated manganese(II) complex anions which could also be suitable as catalysts in chemical reactions. Since the structural motifs especially for manganese complexes is inexhaustible due to the wide range of possible oxidation states (+I to +VII), our research has been focused on homoleptic Mn(II) complex anions only for better comparison with already known MIL subclasses.
The subclass of compounds with (pseudo)tetrahedrally coordinated [Mn II X 4 ] 2− (X = halide, SCN, OCN) complex anions has been known for more than 100 years [35][36][37][38][39][40][41][42][43], but crystal and molecular structure investigations are still scarce. In order to get a better understanding of solid-state properties of these salts, it is necessary to investigate their crystal and molecular structures. There has been a study on manganate(II)-based ionic liquids with main focus on physico-optical properties of [Mn II X 4 ] 2− -based systems (X = Cl, Br, NTf 2 [NTf 2 : bis (trifluoromethanesulfonyl) amido]) which are solids at room temperature and no single crystal data has been presented [44].
In this contribution, we report on the synthesis, properties and physicochemical investigation of a series of A x [MnX 4 ] (A = ammonium-, iminium-and imidazolium-based cation; X = Cl, NCS, NCO) complexes with comparably low melting points. The series was divided into two subclasses: a) non-hygroscopic A x [Mn(NCS) 4 ] compounds, and b) slightly hygroscopic A x [MnX 4 ] (X = Cl, NCO) compounds, respectively. All compounds were investigated by means of elemental analysis and mid-infrared (MIR) spectroscopy. The first subclass was further investigated by nuclear magnetic resonance (NMR) spectroscopy to obtain magnetic data. Additional measurements of density, dynamic viscosity, conductivity, and surface tension were done exemplarily for the low-melting non-hygroscopic substance (EMIm) 2 [Mn(NCS) 4 ] (EMIm = 1-ethyl-3-methylimidazolium). The influence of trace impurities (residual bromide) is also discussed on the basis of the physicochemical properties. Electronic spectra are not presented in detail due to the already existing large amount of such data [33,34,36,55] determined by differential scanning calorimetry (DSC) measurements with a DSC823 e instrument (Mettler-Toledo, Columbus, OH, USA) in the range −100 to 200 • C at a heating rate of 10 K min −1 (Ar atmosphere, Al crucible) or with a STA 449 F3 Jupiter device (Netzsch, Selb, Germany) in the temperature range 25 to 600 • C at a heating rate of 10 K min −1 (N 2 atmosphere, Al crucible), respectively. Single-crystal X-ray diffraction measurements were made with a Apex X8 diffractometer (Bruker-Nonius, Billerica, MA, USA) equipped with a CCD detector. The measurements were performed with monochromatic Mo-Kα radiation (λ = 0.71073 Å). The preliminary unit-cell data were obtained from the reflection positions of 36 frames, measured in different directions of reciprocal space. After the completion of the data measurements, the intensities were corrected for Lorentz, polarization, and absorption effects with the Bruker-Nonius software [56]. The structure solutions and refinements were performed with the SHELX-97 program package [57]. All of the non-hydrogen atoms were refined anisotropically. The hydrogen atoms were added at idealized positions and refined in riding models. These data can be obtained free of charge from the Cambridge Crystallographic  [58,59]. Molar susceptibilities were corrected by applying Pascal constants [60]. Effective magnetic moments µ eff /µ B were determined by applying the Langevin equation [61]. Trace impurities ([pseudo-]halides, sulphate) were determined by an ionic chromatography (IC) system equipped with a Alltech 550 model conductivity detector. A Metrosep Anion Dual 2 75/4.6 mm column was employed and the mobile phase consisted of a mixture of 1.3 mmol/L NaHCO 3 and 2.0 mmol/L Na 2 CO 3 in pure water with a flow rate of 0.8 mL/min. The column temperature was 25 • C. For sample preparation approximately 100 mg of ionic liquid was weighed into a 100 mL volumetric flask and dissolved in pure water (1:1000). The solution was transferred to a sample vial, manually injected and analysed with the IC system. Water contents were determined by Karl Fischer titration on a TitroLine KF Trace device (SI Analytics, Weilheim, Germany). Densities (ρ) and dynamic viscosities (η) were measured with an Stabinger viscometer SVM 3000 (Anton Paar, Graz, Austria). The specification provided by the manufacturer estimates the reproducibility as 0.35% for viscosities and 5 × 10 −4 g·cm −3 for densities with repeatability on the level 0.1% and 2 × 10 −4 g·cm −3 for viscosity and density, respectively. Surface tensions (γ) were measured using the pendant drop method with the DSA 10 Tensiometer with Drop Shape Analysis software (Krüss GmbH, Hamburg, Germany). The instrumental procedures and the experimental details have been published previously [34,62]. The general uncertainty of the method given by the manufacturer is 0.1 mN·m −1 . Conductivities (κ) were measured with a Microprocessor Conductivity Meter LF 537 with a Tetracon 96 electrode with the general uncertainty 0.5% (WTW, Weilheim, Germany).

Materials
All commercially available chemicals were used as received (Sigma-Aldrich, St. Louis, MO, USA, purities >99%). N-methylimidazole was freshly distilled from KOH in vacuo before use. Monocationic ionic liquid precursors were prepared by literature procedures [63,64]. 4 ] was synthesized on a multi-gram level: Manganese(II) chloride (MnCl 2 , 10.0 g, 79.5 mmol), potassium thiocyanate (KSCN, 31.7 g, 325.8 mmol) were placed in a round-bottom flask and heated under reflux overnight in 250 mL acetone. The precipitate (KCl) was filtered off and the solvent of the light green filtrate was completely removed in vacuo. The residue was extracted with 100 mL dichloromethane, filtered and the solvent was removed again in vacuo. The purified solid was dried overnight at 120 • C, yielding an almost white hygroscopic powder in high yield (26.5

Syntheses
The schematic synthetic approach to obtain low-melting salts consisting of [MnX4] 2− complex anions (X = Cl, NCS, NCO) is depicted in Scheme 1.  1II). The respective chlorido-as well as isocyanato complexes show significant tendencies to deliquescence.

Crystal Structures
Single crystals suitable for X-ray structure determinations of (DMDIm)[Mn(NCS) 4  , dynamic viscosities (η), surface tensions (γ) and conductivities (κ), respectively, and it was synthesized for this purpose at the multi-gram scale (~100-250 mL IL). The reactions were performed in acetonic solutions to ensure complete precipitation of the side product KX (X = Cl, Br).
[MnX4] 2− -based compounds (X = Cl, NCO) were synthesized in a two-step reaction pathway by direct reaction of ammonium-, iminium-or imidazolium-based chloride salts with MnCl2 in ethanol solution and further ligand exchange reaction with freshly prepared AgOCN in nitromethane to generate [Mn(NCO)4] 2− -based complexes (see Scheme 1II). The respective chlorido-as well as isocyanato complexes show significant tendencies to deliquescence.

Crystal Structures
Single crystals suitable for X-ray structure determinations of (DMDIm

Thermal and Magnetic Properties
Thermal properties of all compounds were measured by using DSC techniques in the temperature range −100 to 600 • C in order to investigate melting points as well as glass transitions. DSC measurements were performed in a three cycle repetition mode. All solid materials were heated above their melting points, then cooled down to −100 • C and again heated up showing in some cases additional glass transitions prior re-crystallization and melting behavior.
The ratio of melting point (T m ) and glass transition temperature (T g ) was calculated for those compounds which showed melting and glass transitions in the DSC experiments, for example  4 ], respectively, by applying NMR techniques [58,59]. Both complexes show magnetic moments in the range of µ eff = 5.9-6.0 µ B which resembles closely to the spin only value (µ eff = 5.92 µ B ) and experimental data of comparable tetrahedral high-spin Mn II complexes, for example ( n BuPh 3 P) 2

Physicochemical Properties and Influence of Trace Impurities
The impact of water content on physicochemical properties of ILs is known and has been investigated in detail for a long time [72][73][74], but the analyses led to in part opposite results. For example, the basic estimations of contents of water and other impurities in ILs have deficiencies in different applied methods and apparatuses with different and relatively large uncertainties (~10 %). In general, it is known that the influence of water depends strongly of the nature of the ILs and their structures. The situation for viscosities and conductivities is even more complex, but a general trend is known: The higher the water content in aprotic ILs, the lower the viscosities and higher the conductivities. If the water content is below 0.1% an impact of 5%-15% could be detected [72,73]. More recently, the influence of water was also investigated on the speed of sound and antielectrostatic activity in ILs [75,76]. The findings showed that the water content differed significantly from IL to IL at a really low level. The surface tension and densities were not affected significantly and were in agreement with earlier observations [73]. Further insights for such phenomena with regard of the influence of water on properties of ILs were done by theoretical investigations [77].
The agreement between electrolytic conductivity and viscosity for all three investigated samples is very surprising (the maximum deviation is 7.2% and −8.8%, for conductivity and for viscosity, respectively). Furthermore, at 298.15 K the conductivity and dynamic viscosity of (EMIm) 2 [25].
In summary, the variation of bromide content on the level investigated in this work does not affect seriously measured parameters. The most interesting fact is that all investigated properties for (EMIm) 2 [Mn(NCS) 4 ] mirror the proper values found earlier for (EMIm) 2 [Co(NCS) 4 ] [25].
The temperature dependence of the density of the sample I is as follows: where as the temperature dependence of surface tension is: On the basis of the first dependence the isobaric coefficient of thermal expansion, α p = 1/V·(∂V/∂T) p = −1/ρ·(∂ρ/∂T) p was calculated and compared with α p for other (EMIm) + salts with [Co(NCX) 4 ] 2− (X = O, S) complex anions (see Figure 6).    α p of (EMIm) 2 [Mn(NCS) 4 ] is significantly higher than the other regarded (EMIm) + -based ionic liquids.