New Low-Melting Triply Charged Homoleptic Cr(III)-Based Ionic Liquids in Comparison to Their Singly Charged Heteroleptic Analogues

A series of new low-melting triply charged homoleptic Cr(III)-based ionic liquids of the general formula (RMIm)3[Cr(NCS)6] (R = methyl, ethyl, n-butyl, benzyl) is reported. Their syntheses and properties are described in comparison to their singly charged heteroleptic analogues of the general formula (RMIm)[Cr(NCS)4L2] (R = methyl, ethyl, n-butyl, benzyl; L = pyridine, γ-picoline). In total, sixteen new Reineckate related salts with large imidazolium cations are described. Out of these, five compounds were crystallized, and their structures determined by single-crystal X-ray structure analyses. They all consisted of discrete anions and cations with octahedrally coordinated Cr(III) ions. In the structures, various hydrogen contacts interconnect the entities to build up hydrogen bonded networks. Thermal investigations showed relatively low melting points for the homoleptic complexes. The compounds with the [Cr(NCS)6]3− anion melt without decomposition and are stable up to 200 K above their melting points. The complex salts with the [Cr(NCS)4L2]− anion, in contrast, start to decompose and lose L molecules (Pyr or Pic) already at the melting point.


Introduction
For more than 30 years, ionic liquids (ILs), designated as organic salts with melting points below 100 • C, have been a broad field of investigation due to their in part unique properties. They exhibit, for instance, large electrochemical windows, liquid ranges, hardly measurable vapor pressures, special solubility characteristics, and applications in catalysis [1][2][3][4][5]. ILs have been often called "designer solvents" and have been used in research fields fitting into the "Green Chemistry" approach. Due to their low vapor pressures at room temperature, they are used as substitutes for common organic solvents. In addition, they exhibit partly exceptional solubility properties for organic, inorganic as well as metalorganic compounds. ILs play an important role as starting materials for N-heterocyclic carbenes (NHCs), which can be applied in a variety of different catalytic reactions [6][7][8].
With the introduction of transition metals into ILs, so-called magnetic ionic liquids (MILs) can be prepared. Currently, these ILs containing metal-based paramagnetic complex ions are a highly investigated subclass because they show magnetic response in addition to the phenomena above-mentioned [9][10][11][12][13]. Furthermore, they have been thoroughly investigated as systems assumed to be magnetic and magnetorheological fluids [14,15].

Instrumentation
Elemental analysis for C, H, N, and S was performed on a TruSpec ®® Micro device (Leco, St. Joseph, MI, USA). The MIR (MIR = mid-infrared) spectra were recorded by the attenuated total reflectance (ATR) technique on a Bruker Alpha FTIR spectrometer (Bruker Corporation, Billerica, MA, USA) in the region 4000-600 cm −1 . UV-Vis spectra were recorded on a Lambda 365 device (PerkinElmer, Waltham, MA, USA) in the diffuse reflectance mode for powders or in absorbance mode for solutions. Selective melting points were determined by differential scanning calorimetry (DSC) measurements with a DSC 1 instrument (Mettler-Toledo, Columbus, OH, USA) in the range −60 to 100 • C at a heating rate of 10 K min −1 (Ar atmosphere, Al crucible) or STA 449 F3 Jupiter device (Netzsch, Selb, Germany) in the range 25 to 600 • C at a heating rate of 10 K min −1 (N 2 atmosphere, Al crucible). Single-crystal X-ray diffraction measurements were made with an 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 and polarization effects with the Bruker-Nonius software [44]. Absorption corrections were applied by using the multi-scan method as implemented in SADABS [45].
The structure solutions and refinements were performed with the SHELX program package (vers. 2014) [46]. All 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 Data Center: CCDC 1988743 for (DMIm) 3

Synthesis of (RMIm) 3 [Cr(NCS) 6 ]
Samples of the general formula (RMIm) 3 [Cr(NCS) 6 ] (R = methyl, ethyl, n-butyl, benzyl; MIm = methyl-imidazolium) were prepared via direct salt metathesis reaction of 3.0 eq. (RMIm)X (X = Cl, Br, I; 3.0 mmol) and 1.0 eq. K 3 [Cr(NCS) 6 ] (0.5 g; 1.0 mmol) in 50 mL acetone with stirring at ambient conditions for 24 h. The precipitate was filtered off and the filtrate solution was evaporated to dryness in vacuum. The residue was dissolved in either dichloromethane or acetone/ethyl acetate (v:v = 2:1), filtered, and evaporated to dryness in vacuum again. The final products were recrystallized by dissolution in small amounts of acetone, followed by subsequent precipitation by the slow addition of water. The solids were filtered off, washed thoroughly with water and n-hexane, and finally dried in vacuum. The final products were obtained as red-violet substances in high yields (> 90%).

Synthesis of (RMIm) 3 [Cr(NCS) 4 L 2 ]
Samples of the general formula (RMIm) 3 [Cr(NCS) 4 L 2 ] (R = methyl, ethyl, n-butyl, benzyl; MIm = methyl-imidazolium; L = Pyr = pyridine, Pic = γ-picoline = 4-methylpyridine) were prepared via treatment of K 3 [Cr(NCS) 6 ] in excessive ligand Pyr or Pic [55,56] to generate substances of the general formula LH[Cr(NCS) 4 L 2 ], followed by the transformation into the corresponding silver salts Ag[Cr(NCS) 4 L 2 ]. 1.0 eq. (RMIm)X (X = Cl, Br, I; 2.6 mmol) and 1.0 eq. Ag[Cr(NCS) 4 L 2 ] were stirred at ambient conditions for 24 h in 100 mL of acetone. The resulting suspension was filtered from AgX (X = Cl, Br, I), precipitated, and the solvent was removed in vacuum. The residue was dissolved in acetone/ethyl acetate (v:v = 2:1), filtered, and evaporated to dryness in vacuum again. The final products were recrystallized by dissolution in small amounts of acetone followed by subsequent precipitation by the slow addition of water. The solid were filtered off, washed thoroughly with water and n-hexane, and finally dried in vacuum. The final products were obtained as red, hygroscopic substances in moderate yields.
PyrH[Cr(NCS) 4 (Pyr) 2 ]: Dried K 3 [Cr(NCS) 6 ] (12.0 g, 23.2 mmol) was suspended in pyridine (11.0 g, 139.0 mmol) and stirred for 4 h at 115 • C in a sealed flask. After cooling to room temperature, the precipitate was washed with portions of acetic acid (10% aqueous solution, 100 mL) and water. Afterward, the precipitate was dissolved in acetone/ethyl acetate (v:v = 1:1, 200 mL) and the solution was combined with an aqueous NH 4 Cl solution (20 g in 100 mL H 2 O) and brought to reflux. The resulting solution was evaporated to dryness in vacuum and the precipitate was extracted with acetone. The combined acetonic solutions were evaporated to dryness in vacuum and the resulting red solid was finally dried for several hours in vacuum at 120 • C (11.4 g, 94%). IR (ν max , cm

Syntheses
The two different routes to generate homoleptic imidazolium-based Cr III complexes of the general formula (RMIm) 3 [Cr(NCS) 6 ] (R = methyl, ethyl, n-butyl, benzyl) are depicted in Scheme 1. The synthetic approach can be divided into two different routes: (I) the rapid, non-organic solvent, aqueous solution-based salt metathesis of ionic liquid precursors (RMIm)X (R = methyl, ethyl, n-butyl, benzyl; X = Cl, Br, I) and K 3 [Cr(NCS) 6 ], or (II) the one-pot, direct reaction of ionic liquid precursors (RMIm)X with CrCl 3 and KSCN in acetonic solution. Both routes lead to the same low-melting Cr(III)-based ionic liquid (RMIm) 3 [Cr(NCS) 6 ] materials with high yields. The overall yield of route I is slightly below the overall yield of route II, mainly because of the work-up strategy. Starting in route II with CrCl 3 and using an organic solvent makes the product separation more effective than in the aqueous strategy used in route I. II with CrCl3 and using an organic solvent makes the product separation more effective than in the aqueous strategy used in route I.

Physicochemical Properties: Electronic and Infrared Data
Octahedral Cr III complexes exhibit two distinct spin allowed transitions in the visible region, namely 4 T 2g ← 4 A 2g (low energy) and 4 T 1g (F) ← 4 A 2g (higher energy). An additional third short-wave band, 4 T 1g (P) ← 4 A 2g (highest energy), is usually superimposed by charge-transfer bands [57]. Electronic spectra for complexes bearing the [Cr(NCS) 4  N-donor ligands, the spectrochemical series is reported to be NCS − < Pyr < NH 3 [58]. For Reineckates (L = NH 3 ), transitions are reported at 392 and 503 nm, and for L = Pyr at 545 nm, respectively, in the solid state under diffuse reflectance conditions [58].
UV/Vis spectra in diffuse reflectance as well as in an acetonic solution of selected (EMIm) x [Cr(NCS) 4
UV/Vis spectra in diffuse reflectance as well as in an acetonic solution of selected (EMIm)x[Cr(NCS)4L2] (x = 3: L = NCS; x = 1: L = Pyr, Pic) compounds are depicted in Figure  1. Full information on all transitions are given in the Experimental section and maxima for compounds exhibiting the [Cr(NCS)6] -anion are found at 413 (416) and 552 (556) nm, for L = Pyr at 535 (543) nm, and for L = Pic at 537 (541) nm, respectively. These values closely resemble the ones found in the literature and the ligand field strength is also in accordance [31][32][33]56,58].

Thermal Properties
All substances were subjected to thermogravimetric analyses (TGA) and differential scanning calorimetry (DSC) measurements to investigate their thermal as well as melting behavior. The melting points were detected as endothermic peaks in the DSC measurements and are listed in the Experimental section and in Table 1.

Thermal Properties
All substances were subjected to thermogravimetric analyses (TGA) and differential scanning calorimetry (DSC) measurements to investigate their thermal as well as melting behavior. The melting points were detected as endothermic peaks in the DSC measurements and are listed in the Experimental section and in Table 1.  Compounds of the formula (RMIm) 3 [Cr(NCS) 6 ] (R = methyl, ethyl, n-butyl, benzyl) can be melted and recrystallized without decomposition, whereas the comparable compounds with pyridine or picoline ligands melt with decomposition. The melting points of the hexaisothiocyanato complexes can be lowered by extending the alkyl chain in the imidazolium-based cation from 138 • C in (DMIm) 3 [Cr(NCS) 6 ] to 45 • C in (BMIm) 3 [Cr(NCS) 6 ]. This behavior can be found in a variety of other ionic liquids and ionic liquid complexes, but these melting points are exceptionally low for triply charged Cr(III)-based compounds. A recent study has shown that low charge density can be attributed to the low melting points of systems with high charges [59]. Hence, this behavior could be present, particularly in systems also exhibiting the symmetric [Cr(NCS) 6 ] 3− complex anion, leading to the observed low melting points. In contrast, the melting points were not as low as for the comparable doubly charged complexes (e.g., the low-viscosity isothiocyanato-based compound [EMIm] 2 [Co(NCS) 4 ]) [26].
Compounds of the formula (RMIm)3[Cr(NCS)6] (R = methyl, ethyl, n-butyl, benzyl) can be melted and recrystallized without decomposition, whereas the comparable compounds with pyridine or picoline ligands melt with decomposition. The melting points of the hexaisothiocyanato complexes can be lowered by extending the alkyl chain in the imidazolium-based cation from 138 °C in (DMIm)3[Cr(NCS)6] to 45 °C in (BMIm)3[Cr(NCS)6]. This behavior can be found in a variety of other ionic liquids and ionic liquid complexes, but these melting points are exceptionally low for triply charged Cr(III)-based compounds. A recent study has shown that low charge density can be attributed to the low melting points of systems with high charges [59]. Hence, this behavior could be present, particularly in systems also exhibiting the symmetric [Cr(NCS)6] 3− complex anion, leading to the observed low melting points. In contrast, the melting points were not as low as for the comparable doubly charged complexes (e.g., the low-viscosity isothiocyanato-based compound [EMIm]2[Co(NCS)4]) [26].
Decomposition curves of the three selected substances (EMIm)x[Cr(NCS)4L2] (x = 3: L = NCS; x = 1: L = Pyr, Pic) in a nitrogen atmosphere in the temperature range 25 to 400 °C are shown in Figure 2. From Figure 2, it can be seen that the decomposition of (EMIm)3[Cr(NCS)6] starts at ca. 300 °C and the compounds stay undecomposed in the liquid state of a range of more than 200 K. Compounds (EMIm)[Cr(NCS)4L2] (L = Pyr, Pic) melt just below 200 °C by loss of ca. 5% mass. This can be attributed to the beginning release and evaporation of pyridine or picoline ligands into the gas phase. The gradual decomposition is further accelerated above 300 °C, which is comparable to the behavior of (EMIm)3[Cr(NCS)6] and comparable amine-based isothiocyanate chromate(III) complexes [56].

Crystal Structures
Single crystals suitable for X-ray structure determinations of (RMIm)3[Cr(NCS)6] (R = methyl, ethyl, benzyl), (BMIm)[Cr(NCS)4(Pyr)2], and (EMIm)[Cr(NCS)4(Pic)2] were obtained by slow evaporation acetonic solutions of the three compounds at ambient pressure and temperatures of 25 °C over a period of one week. Crystallographic data and structurerefinement parameters of all five compounds can be found in Table 2. Additional selected

Crystal Structures
Single crystals suitable for X-ray structure determinations of (RMIm) 3 [Cr(NCS) 6 ] (R = methyl, ethyl, benzyl), (BMIm)[Cr(NCS) 4 (Pyr) 2 ], and (EMIm)[Cr(NCS) 4 (Pic) 2 ] were obtained by slow evaporation acetonic solutions of the three compounds at ambient pressure and temperatures of 25 • C over a period of one week. Crystallographic data and structure-refinement parameters of all five compounds can be found in Table 2. Additional selected interatomic distances for the complex anions and hydrogen-bond geometries are given in Table 3. Table 2. Crystallographic data and structure-refinement parameters for (RMIm) 3
All angles and bond lengths within the methylimidazolium-based cations (RMIm) + (R = methyl, ethyl, n-butyl, benzyl) are in accordance with values reported in the literature for the corresponding substances (DMIm)[Co(NCO) 4

Conclusions
The subclass of ionic liquids bearing Reineckate-derived complex anions (Reinecke's salt, (NH 4 )[Cr(NCS) 4 (NH 3 ) 2 ]·H 2 O) could be successfully extended to substances of the general formula (RMIm) x [Cr(NCS) 4 L 2 ] (R = methyl, ethyl, n-butyl, benzyl; x = 3: L = NCS; x = 1: L = Pyr, Pic). The homoleptic triply charged salts of the general formula (RMIm) 3 [Cr(NCS) 6 ] exhibit relatively low melting points and can be transferred into the liquid state without decomposition. These model substances could find a broader application in Cr(III)-based catalytic transformations (e.g., olefin polymerizations or tri-and tetramerization reaction, respectively). Comparable (RMIm)[Cr(NCS) 4 L 2 ] complexes have shown that they undergo decomposition by the release of amine ligands upon melting. The melting points are almost 100 K higher than the melting point of the comparable (RMIm) 3 [Cr(NCS) 6 ] compounds. Thorough single-crystal X-ray investigations revealed that the substances are designated by extended hydrogen contact networks in the solid state, whereas additional anion-anion contacts are present in the respective (RMIm)[Cr(NCS) 4 (Pic) 2 ] compounds. This is the first example of such behavior to be reported for this subclass of Reineckate-related substances.