Tetrabromidocuprates(II)—Synthesis, Structure and EPR

Metal-containing ionic liquids (ILs) are of interest for a variety of technical applications, e.g., particle synthesis and materials with magnetic or thermochromic properties. In this paper we report the synthesis of, and two structures for, some new tetrabromidocuprates(II) with several “onium” cations in comparison to the results of electron paramagnetic resonance (EPR) spectroscopic analyses. The sterically demanding cations were used to separate the paramagnetic Cu(II) ions for EPR measurements. The EPR hyperfine structure in the spectra of these new compounds is not resolved, due to the line broadening resulting from magnetic exchange between the still-incomplete separated paramagnetic Cu(II) centres. For the majority of compounds, the principal g values (g‖ and g⊥) of the tensors could be determined and information on the structural changes in the [CuBr4]2− anions can be obtained. The complexes have high potential, e.g., as ionic liquids, as precursors for the synthesis of copper bromide particles, as catalytically active or paramagnetic ionic liquids.


Bond Lengths/Å Bond Angles/°
Cu1-Br1 2.4500(9)   Symmetry codes: (i) 0,5 + x, 0.5 − y, 0.5 + z; (ii) 1 + x, y, z; (iii) 1 − x, y, z.    Figure 3 shows a spectrum of (EtPh 3 P) 2 [CuBr 4 ] (7) at 150 K, characteristic for most of the recorded spectra. In general the spectra are of poor resolution, due to the magnetic interactions between the paramagnetic centers and no hyperfine structure can be observed. Some of the spectra are of axial symmetry and g || as well as g K can be determined; for a few samples only an isotropic signal (g iso ) could be extracted. The EPR data are shown in Table 6. The g av -values correspond to the isotropic g iso -value of liquid systems as long as strong solvents are excluded (e.g., solutions or ionic liquids).  Figure 3 shows a spectrum of (EtPh3P)2[CuBr4] (7) at 150 K, characteristic for most of the recorded spectra. In general the spectra are of poor resolution, due to the magnetic interactions between the paramagnetic centers and no hyperfine structure can be observed. Some of the spectra are of axial symmetry and g‖ as well as g⊥ can be determined; for a few samples only an isotropic signal (giso) could be extracted. The EPR data are shown in Table 6. The gav-values correspond to the isotropic giso-value of liquid systems as long as strong solvents are excluded (e.g., solutions or ionic liquids).  The averaged g-values gav for axial symmetry are calculated by the following expression:

Electron Paramagnetic Resonance (EPR) Spectroscopy
For rhombic symmetry the averaged g-values gav are calculated as follows:  Table 6. Experimental g-values (g || , g K and g iso ), calculated values g av and known cis-angles (φ av ) for this series of tetrabromidocuprates(II). The averaged g-values g av for axial symmetry are calculated by the following expression:

Cation/Compound
For rhombic symmetry the averaged g-values g av are calculated as follows: The variation of the structural parameter (φ av ) reflects the degree of structural flexibility of the tetrahalidocuprate(II) moiety. With a series of known X-ray structures combined with EPR parameters of tetrabromidocuprates(II) it could be possible to classify the degree of distortion between square planar and tetrahedral geometries, as well as those of complexes with unknown structures as was recently shown for tetrabromidocuprates(II) [34]. With larger the cis-angles the g av or the isotropic g iso -values have a tendency to increase (see Figure 4). This is also supported by calculated values from DFT calculations. planar and tetrahedral geometries, as well as those of complexes with unknown structures as was recently shown for tetrabromidocuprates(II) [34]. With larger the cis-angles the gav or the isotropic giso-values have a tendency to increase (see Figure 4). This is also supported by calculated values from DFT calculations.  Table 6 (squares); the circles represent the calculated data (DFT). Figure 5 shows the temperature-dependent EPR spectra of (EtPh3P)2[CuBr4] (7) recorded in a temperature range from 300 to 430 K. The signal intensity decreases with rising temperature until it is fully distinct, due to spin saturation effects. This thermal cycle is completely reversible without any signs of decomposition. Interestingly, after cooling to room temperature, only an isotropic EPR signal remains ( Figure 6), which returned to the starting axial symmetric spectrum after a couple of days ( Figure 7). Obviously the re-crystallization process is kinetically enhanced. Compounds of this type might be useful as ionic liquids for higher temperatures.  . Correlation of EPR parameter (g av /g iso ) with the coordination geometry (averaged cis-angle) including the four new data sets from Table 6 (squares); the circles represent the calculated data (DFT). Figure 5 shows the temperature-dependent EPR spectra of (EtPh 3 P) 2 [CuBr 4 ] (7) recorded in a temperature range from 300 to 430 K. The signal intensity decreases with rising temperature until it is fully distinct, due to spin saturation effects. This thermal cycle is completely reversible without any signs of decomposition. Interestingly, after cooling to room temperature, only an isotropic EPR signal remains ( Figure 6), which returned to the starting axial symmetric spectrum after a couple of days (Figure 7). Obviously the re-crystallization process is kinetically enhanced. Compounds of this type might be useful as ionic liquids for higher temperatures. planar and tetrahedral geometries, as well as those of complexes with unknown structures as was recently shown for tetrabromidocuprates(II) [34]. With larger the cis-angles the gav or the isotropic giso-values have a tendency to increase (see Figure 4). This is also supported by calculated values from DFT calculations.  Table 6 (squares); the circles represent the calculated data (DFT). Figure 5 shows the temperature-dependent EPR spectra of (EtPh3P)2[CuBr4] (7) recorded in a temperature range from 300 to 430 K. The signal intensity decreases with rising temperature until it is fully distinct, due to spin saturation effects. This thermal cycle is completely reversible without any signs of decomposition. Interestingly, after cooling to room temperature, only an isotropic EPR signal remains ( Figure 6), which returned to the starting axial symmetric spectrum after a couple of days (Figure 7). Obviously the re-crystallization process is kinetically enhanced. Compounds of this type might be useful as ionic liquids for higher temperatures.    The EPR spectrum of (C12H25Me3N)2[CuBr4] (9) at 110 K ( Figure 7) reflects a rhombic symmetry of the g-tensor with three different g-values and indicates a change in coordination geometry or a possible lamellar structure.

Differential Scanning Calorimetry (DSC)
(EtPh3P)2[CuBr4] (7) shows a glass transitions at 306 K, a melting point at 432 K and a cold crystallization by 369 K. Figure 8 shows the second heat run of the compound. The third measurement comes to the same result as the second. It can be concluded that the compound has a temperature reversibility at least up to 443 K. The results of the differential scanning calorimetry of (7) are in good agreement with the data from EPR-spectroscopy.  The EPR spectrum of (C12H25Me3N)2[CuBr4] (9) at 110 K ( Figure 7) reflects a rhombic symmetry of the g-tensor with three different g-values and indicates a change in coordination geometry or a possible lamellar structure.

Differential Scanning Calorimetry (DSC)
(EtPh3P)2[CuBr4] (7) shows a glass transitions at 306 K, a melting point at 432 K and a cold crystallization by 369 K. Figure 8 shows the second heat run of the compound. The third measurement comes to the same result as the second. It can be concluded that the compound has a temperature reversibility at least up to 443 K. The results of the differential scanning calorimetry of (7) are in good agreement with the data from EPR-spectroscopy. The EPR spectrum of (C 12 H 25 Me 3 N) 2 [CuBr 4 ] (9) at 110 K ( Figure 7) reflects a rhombic symmetry of the g-tensor with three different g-values and indicates a change in coordination geometry or a possible lamellar structure.

Differential Scanning Calorimetry (DSC)
(EtPh 3 P) 2 [CuBr 4 ] (7) shows a glass transitions at 306 K, a melting point at 432 K and a cold crystallization by 369 K. Figure 8 shows the second heat run of the compound. The third measurement comes to the same result as the second. It can be concluded that the compound has a temperature reversibility at least up to 443 K. The results of the differential scanning calorimetry of (7) are in good agreement with the data from EPR-spectroscopy.

Methods
The melting points were determined using a Mikroheiztisch Boetius (VEB Wägetechnik, Radebeul, DDR). Elemental analyses were carried out on an Elementar Vario EL III analyzer (elementar Analysensysteme GmbH, Hanau, Germany). Infrared spectra were recorded on a Perkin-Elmer type 16PC FT-IR spectrophotometer (Perkin-Elmer GmbH, Überlingen, GErmany) between 4000 and 400 cm −1 as KBr-pellets (reference KBr). The measurements of the magnetic susceptibility were performed with a Magnetic Susceptibility Balance-Auto from Johnson Matthey GmbH (Matthey GmbH, Cambridge, UK) at room temperature, for diamagnetic correction the increment system of Pascal and Pacault [38] was applied. EPR spectra were recorded at 9.4 GHz (X-band) using a Bruker CW Elexsys E 500 spectrometer (Bruker BioSpin GmbH, Rheinstetten, Germany) For X-ray structure determinations, the crystals were embedded in perfluoropolyalkylether oil and mounted on a glass fibre (5) or within a MicroGripper (8). For structure analysis of (5), the intensity data were collected at 210 K using an Imaging Plate Diffraction System IPDS-2 (Stoe, Darmstadt, Germany) with graphite monochromatized Mo-Kα radiation (λ = 0.71073 Å) at 50 kV and 40 mA. The data collection for (8) was performed on a StadiVari diffractometer (Stoe, Darmstadt, Germany) equipped with a four-circle goniometer (open Eulerian cradle), a Genix Microfocus X-ray source (Mo) with a graded multilayer mirror and a Pilatus 200 K detector (Dectris, Baden-Daettwil, Switzerland). The data were corrected for absorption as well as for Lorentz polarization and extinction effects using the program X-Area (Stoe, 2004) [39]. The structures were solved by direct methods using SHELXS-2013/1 [40] and refined by full-matrix least squares on F 2 using the program SHELXL-2014/7 [41]. All non-hydrogen atoms were refined anisotropically. The hydrogen atoms were calculated in their expected positions and refined with a riding model. CCDC 1459578 (5) and CCDC 1459612 (8) contain the supplementary crystallographic data for this paper. These data are provided free of charge by The Cambridge Crystallographic Centre (Cambridge, UK).
Differential scanning calorimetry (DSC) measurements were performed with a DSC 214 Polyma (Netzsch GmbH & Co. KG, Selb, Germany) by NETZSCH operating with a scan rate of 5-10 °C·min −1 under a nitrogen flow.

Methods
The melting points were determined using a Mikroheiztisch Boetius (VEB Wägetechnik, Radebeul, DDR). Elemental analyses were carried out on an Elementar Vario EL III analyzer (elementar Analysensysteme GmbH, Hanau, Germany). Infrared spectra were recorded on a Perkin-Elmer type 16PC FT-IR spectrophotometer (Perkin-Elmer GmbH, Überlingen, GErmany) between 4000 and 400 cm´1 as KBr-pellets (reference KBr). The measurements of the magnetic susceptibility were performed with a Magnetic Susceptibility Balance-Auto from Johnson Matthey GmbH (Matthey GmbH, Cambridge, UK) at room temperature, for diamagnetic correction the increment system of Pascal and Pacault [38] was applied. EPR spectra were recorded at 9.4 GHz (X-band) using a Bruker CW Elexsys E 500 spectrometer (Bruker BioSpin GmbH, Rheinstetten, Germany) For X-ray structure determinations, the crystals were embedded in perfluoropolyalkylether oil and mounted on a glass fibre (5) or within a MicroGripper (8). For structure analysis of (5), the intensity data were collected at 210 K using an Imaging Plate Diffraction System IPDS-2 (Stoe, Darmstadt, Germany) with graphite monochromatized Mo-Kα radiation (λ = 0.71073 Å) at 50 kV and 40 mA. The data collection for (8) was performed on a StadiVari diffractometer (Stoe, Darmstadt, Germany) equipped with a four-circle goniometer (open Eulerian cradle), a Genix Microfocus X-ray source (Mo) with a graded multilayer mirror and a Pilatus 200 K detector (Dectris, Baden-Daettwil, Switzerland). The data were corrected for absorption as well as for Lorentz polarization and extinction effects using the program X-Area (Stoe, 2004) [39]. The structures were solved by direct methods using SHELXS-2013/1 [40] and refined by full-matrix least squares on F 2 using the program SHELXL-2014/7 [41]. All non-hydrogen atoms were refined anisotropically. The hydrogen atoms were calculated in their expected positions and refined with a riding model. CCDC 1459578 (5) and CCDC 1459612 (8) contain the supplementary crystallographic data for this paper. These data are provided free of charge by The Cambridge Crystallographic Centre (Cambridge, UK).
Differential scanning calorimetry (DSC) measurements were performed with a DSC 214 Polyma (Netzsch GmbH & Co. KG, Selb, Germany) by NETZSCH operating with a scan rate of 5-10˝C¨min´1 under a nitrogen flow.

General Preparation
In general, tetrabromidocuprate(II) complexes can be achieved by different procedures [42][43][44]. In the current work, the [CuBr 4 ] 2´m oiety was synthesized according to a protocol by N. S. Gill and R. S. Nyholm [44]: an ethanolic solution of a stoichiometric amount of CuBr 2 was added to the respective bromide salt of the cation dissolved in a minimum volume of ethanol. The reaction mixture was stirred for one hour at room temperature. The product was precipitated by evaporating the solvent.

Bis(tetraethylammonium)tetrabromidocuprate(II), (Et 4 N) 2 [CuBr 4 ] (1)
Compound (1) was synthesized according to an already published protocol [36]. A solution of 1.5 mmol (0.32 g) of tetraethylammonium bromide in 3 mL of ethanol was mixed with a solution of 0.5 mmol (0.11 g) of copper(II) bromide in 10 mL ethanol. The solution was stirred one hour at room temperature. The solvent was removed and a violet powder was received, filtered off and dried.

Bis(benzyltrimethylammonium)tetrabromidocuprate(II), (BzlMe 3 N) 2 [CuBr 4 ] (4)
Compound (4) was also synthesized according to an already published procedure [37]. To a solution of 0.5 mmol (0.11 g) of CuBr 2 in 3.5 mL methanol a solution of 1.0 mmol (0.23 g) benzyltrimethylammonium bromide, dissolved in 1.5 mL methanol, was added. The mixture was stirred for one hour at room temperature. After a short while the complex precipitated as purple crystals. The synthesis of (EtPh 3 P) 2 [CuBr 4 ] as follows: 0.5 mmol (0.11 g) copper(II) bromide and 1.0 mmol (0.37 g) EtPh 3 PBr solved even in 2 mL ethanol. The combined solutions are stirred for one hour at room temperature. A violet powder was obtained. A solution of 1.0 mmol (0.22 g) of CuBr 2 in 5 mL ethanol and a solution of 2.0 mmol (0.62 g) C 12 H 25 Me 3 NBr in 5 mL ethanol were combined and stirred at room temperature for 1 h. The resulting violet precipitate was filtered off and dried.

Conclusions
Some of the compounds in this series are real ionic liquids-(2), (3), (6), (10) and (11)-with melting points below 100˝C, or very close to it-(5) and (8). All the reported compounds are thermally stable up to at least 430 K. This thermal cycle is completely reversible without any signs of decomposition. Interestingly, after cooling to room temperature, for (7) only an isotropic EPR signal remains, which returned to the axial symmetric spectrum after a couple of days. That means the re-crystallizing process is kinetically enhanced. Compounds of this type might be useful as ionic liquids for higher temperatures.
Two compounds, (5) and (8), could be structurally characterized by X-ray structure analysis. The structures are stabilized by a variety of hydrogen contacts between the [CuBr 4 ] 2´a nions and corresponding onium cations, responsible for the coordination geometry of the tetrahalidocuprates. The EPR parameters also reflect the degree of structural flexibility of the tetrahalidocuprate(II) moiety. With a series available data sets of known X-ray structures combined with EPR parameters of tetrabromidocuprates(II) it is possible to classify the degree of the distortion coordination sphere between square planar and tetrahedral geometries, as was recently shown for tetrabromidocuprates(II) [34].