Tetrabromidocuprates(II)—Synthesis, Structure and EPR

Abstract

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.

1. Introduction

Metal-containing ionic liquids (ILs) or ionic liquid crystals (ILCs) are of interest for a variety of technical applications, e.g., as precursors for the synthesis of (nano)particles, magnetic or thermochromic materials [1,2,3] or as extraction agents [4] and catalysts [5,6,7,8]. Tetrahalidometalate complexes exhibit many of these interesting properties. Also liquid-crystalline ILs based on tetrahalidometalates have been reported [9,10,11,12,13]. Some ILs and ILCs combining tetrahalidocuprate dianions with alkylpyridinium cations can serve as precursors for inorganic materials [2,14,15,16,17]. A variety of these metal-containing ionic liquids show interesting electrochemical properties as electrochromy and/or magnetic behaviour [18,19]. The properties of ionic liquid materials such as tuneable acidity, polarity, amphiphilic character, coordinating ability, and miscibility with many compounds might be combined with the unique properties introduced by the metal ions. ILs of this type are also of interest as catalysts, as alternative solvents and even morphology templates for inorganic materials simultaneously. Reports on tetrahalidometalate-based ILs/ILCs and their applications as well as their structural characterization are still relatively rare [2,10,11,12,13,20,21,22,23,24,25,26,27,28,29,30,31,32]. We present the syntheses, electron paramagnetic resonance (EPR) spectra and two X-ray structures of some new tetrabromidocuprate(II) compounds. The following cations with different steric demand were used to separate the paramagnetic Cu(II) ions for the EPR measurements (see Scheme 1): the homogenously substituted tetraalkyl onium cations with varying chain lengths tetraethylammonium, (Et₄N⁺), tetrabutylphosphonium, (Bu₄P⁺), tetrahexylammonium, (Hex₄N⁺), the benzyl/alkyl substituted ammonium cations benzyltrimethylammonium, (BzlMe₃N⁺), benzyltriethylammonium, (BzlEt₃N⁺), benzyltributylammonium, (BzlBu₃N⁺), the alkyltriphenylphosphonium cations ethyltriphenylphosphonium, (EtPh₃P⁺), hexyltriphenylphosphonium, (HexPh₃P⁺) and the long chain cations dodecyltrimethylammonium, (C₁₂H₂₅Me₃N⁺), hexadecylpyridinium, (C₁₆H₃₃-py⁺), hexadecyldimethylammonium, (C₁₆H₃₃)₂Me₂N⁺), giving raise to lamellar structures [33]. It is known that tetrahalidocuprate(II)-ions present a high structural flexibility. Thus, the complex anion adopts coordination geometries between square-planar and tetrahedral. Recently we have shown that there is a correlation of EPR parameters and the isotropic or averaged g-values with the degree of geometrical distortion in the coordination sphere of tetrahalidocuprates expressed by the cis-angle, the average of the four smallest X-Cu-X angles of the tetrahalidocuprate dianion [33,34,35].

Scheme 1.

Cations of the [CuBr₄]²⁻ salts.

2. Results and Discussion

2.1. X-ray Crystallography

Table 1 summarizes the crystallographic data and refinement parameters for the structures of (BzlEt₃N)₂[CuBr₄] (5) and (HexPh₃P)₂[CuBr₄] (8).

Table 1.

Crystallographic data and refinement parameters for (5) and (8).

Compound	(BzlEt3N)2[CuBr4] (5)	(HexPh3P)2[CuBr4] (8)
Empirical formula	C26H44N2CuBr4	C48H56P2CuBr4
MW/g·mol−1	767.81	1078.04
Crystal description	purple plate	purple needle
Crystal size/mm	0.7 × 0.5 × 0.1	0.8 × 0.2 × 0.1
Crystal system	monoclinic	monoclinic
Space group	P21/c	P21/n
a/Å	15.1019(5)	10.7410(3)
b/Å	11.6664(5)	22.4775(9)
c/Å	17.5517(6)	19.6455(6)
α/°	90	90
β/°	99.166(3)	99.807(2)
γ/°	90	90
V/Å3	3052.9(2)	4673.7(3)
Z	4	4
F(000)	1532	2172
Density/mg·m−3	1.67	1.532
μ/mm−1	5.97	3.987
Θ range/°	2.10–25.00	2.15–24.998
Rint	0.1338	0.0740
Reflections measured	38,767	73,469
Reflections independent	5372	8223
Parameters	299	496
R1/wR2 (I > 2σ(I))	0.0370/0.0893	0.0480/0.1183
R1/wR2 (all data)	0.0529/0.0975	0.0711/0.1293
Goodness of fit	1.015	1.031
Max. difference peak/hole/e Å−3	0.71/−0.83	2.231/−1.220

2.2. Bis(benzyltriethylammonium)tetrabromidocuprate(II) (5)

(BzlEt₃N)₂[CuBr₄] (5) crystallizes in the monoclinic space group P2₁/c with four formula units per unit cell. The corresponding lattice parameters are a = 15.1019(5) Å, b = 11.6664(5) Å, c = 17.5517(6) Å, and β = 99,166(3)°. The complex dianion has a distorted tetrahedral geometry with Br–Cu–Br angles between 98.2° and 131.8° (see Table 2), the resulting cis-angle, the average of the four smallest angles, is 99.8°. The shortest Cu···Cu distances are 9.12 and 9.44 Å. Figure 1 shows the molecular structure of (BzlEt₃N)₂[CuBr₄] (5). The structure is stabilized by a series of hydrogen bonds (see Table 3).

Table 2.

Selected bond lengths and bond angles of (BzlEt₃N)₂[CuBr₄] (5).

Bond Lengths/Å		Bond Angles/°
Cu1–Br1	2.3981(7)	Br1–Cu1–Br2	99.73(2)
		Br1–Cu1–Br3	98.38(2)
		Br1–Cu1–Br4	130.80(3)
Cu1–Br2	2.3747(7)	Br2–Cu1–Br3	131.84(3)
Cu1–Br3	2.3918(7)	Br2–Cu1–Br4	98.19(2)
Cu1–Br4	2.3984(7)	Br3–Cu1–Br4	102.80(2)

Figure 1.

Molecular structure of (BzlEt₃N)₂[CuBr₄] (5), hydrogen contacts are presented as fragmented lines.

Table 3.

Hydrogen contacts for (BzlEt₃N)₂[CuBr₄] (5).

C–H∙∙∙Br	H∙∙∙Br/Å	C∙∙∙Br/Å	Angles C–H∙∙∙Br/°
C1–H1A···Br2 i	2.93	3.568(4)	124.2
C2–H2A···Br2	3.09	3.955(6)	151.1
C6–H6A···Br2 ii	2.82	3.742(6)	161.1
C6–H6A···Br1 ii	3.01	3.621(6)	132.2
C7–H7A···Br2 ii	2.99	3.935(4)	164.6
C7–H7B···Br3	3.04	3.982(5)	165.4
C14–H14A···Br1 iii	3.08	3.564(4)	112.3
C14–H14B···Br3	2.92	3.720(5)	140.3
C16–H16A···Br4	3.02	3.849(5)	144.6
C16–H16B···Br1	2.96	3.868(5)	156.9
C20–H20A···Br1 ii	3.08	3.982(4)	154.6

Symmetry codes: (i) 1 − x, 2 − y, 1 − z; (ii) 1 − x, 0.5 + y, 0.5 − z; (iii) −x, −0.5 + y, 0.5 − z.

2.3. Bis(hexyltriphenylphosphonium)tetrabromidocuprate(II) (8)

(HexPh₃P)₂[CuBr₄] (8) crystallizes in the monoclinic space group P2₁/n with Z = 4. The corresponding lattice parameters are a = 10.7410(3) Å, b = 22.4775(9) Å, c = 19.6455(6) Å, and β = 99,807(2)°. This complex dianion has also a distorted tetrahedral geometry with Br–Cu–Br angles between 100.4° and 124.8° (Table 4), with a resulting cis-angle of 103.3°. The shortest Cu···Cu distances are 10.62 and 10.74 Å. Figure 2 shows the molecular structure of (HexPh₃P)₂[CuBr₄] (8). The structure is stabilized by a variety of hydrogen contacts (Table 5).

Table 4.

Selected bond lengths and bond angles for (HexPh₃P)₂[CuBr₄] (8).

Bond Lengths/Å		Bond Angles/°
Cu1–Br1	2.4500(9)	Br1–Cu1–Br2	120.66(4)
		Br1–Cu1–Br3	103.25(3)
		Br1–Cu1–Br4	104.45(3)
Cu1–Br2	2.3867(9)	Br2–Cu1–Br3	104.95(3)
Cu1–Br3	2.4207(9)	Br2–Cu1–Br4	100.39(3)
Cu1–Br4	2.3677(9)	Br3–Cu1–Br4	124.76(4)

Figure 2.

Molecular structure of (HexPh₃P)₂[CuBr₄] (8), hydrogen contacts are presented as fragmented lines.

Table 5.

Hydrogen bond geometry for (HexPh₃P)₂[CuBr₄] (8).

C–H∙∙∙Br	H∙∙∙Br/Å	C∙∙∙Br/Å	Angles C–H∙∙∙Br/°
C12–H12∙∙∙Br2	2.82	3.744(6)	166.4
C19–H19B∙∙∙Br3 iii	3.10	4.064(5)	166.4
C26–H26∙∙∙Br1	3.13	3.840(5)	133.8
C26–H26∙∙∙Br3	3.07	3.718(6)	127.2
C32–H32∙∙∙Br4 ii	2.90	3.673(5)	140.8
C36–H36∙∙∙Br2 i	2.84	3.776(5)	176.7
C43–H43B∙∙∙Br1	2.88	3.834(5)	165.9
C43–H43A∙∙∙Br2 i	3.06	4.000(6)	161.1

Symmetry codes: (i) 0,5 + x, 0.5 − y, 0.5 + z; (ii) 1 + x, y, z; (iii) 1 − x, y, z.

2.4. Electron Paramagnetic Resonance (EPR) Spectroscopy

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 (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.

Electron paramagnetic resonance (EPR) spectrum of (EtPh₃P)₂[CuBr₄] (7) at 150 K.

Table 6.

Experimental g-values (g_‖, g_⊥ and g_iso), calculated values g_av and known cis-angles (ϕ_av) for this series of tetrabromidocuprates(II).

Cation/Compound	g‖	g⊥	giso	gav	ϕav/°	Reference (ϕav)
Et4N+/(1)	-	-	2.18 a	-	108.06 *	[36]
Bu4P+/(2)	-	-	2.089	-	-
Hex4P+/(3)	2.34 a	2.079	2.11 a	2.166	-
BzlMe3N+/(4)	2.221	2.058	2.101	2.109	99.64 *	[37]
BzlEt3N+/(5)	-	-	2.11 a	-	99.77 *
BzlBu3P+/(6)	-	-	2.108	-	-
EtPh3P+/(7)	2.217	2.062	-	2.114	-
BuPh3P+/(8)	2.280	2.069	-	2.139	-
HexPh3P+/(9)	-	-	2.13 a	-	103.26 *
(C12H25)Me3N+/(10)	2.26 a, 2.124, 2.040			2.141	-
(C16H33)py+/(11)	-	-	2.15 a	-	-
(C16H33)2Me2N+/(12)	2.28 a	2.079	-	2.146	-

Experimental errors: g-values: ±0.005; ^a g: ±0.01; *: new added EPR/structure data sets.

The averaged g-values g_av for axial symmetry are calculated by the following expression:

$$g_{av}=\frac{g_{II}+2\times g_{\perp }}{3}$$ (1)

For rhombic symmetry the averaged g-values g_av are calculated as follows:

$$g_{\mathrm{a}\mathrm{v}}=\frac{g_1 + g_2+g_3}{3}$$ (2)

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.

Figure 4.

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 (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.

Figure 5.

Temperature dependent EPR spectra of (EtPh₃P)₂[CuBr₄] (7) recorded in the temperature range from 300 to 430 K (10 K steps).

Figure 6.

EPR spectrum of (EtPh₃P)₂[CuBr₄] (7) at 150 K directly after the thermal treatment; only an isotropic signal remains.

Figure 7.

EPR spectrum of (C₁₂H₂₅Me₃N)₂[CuBr₄] (9) at 110 K.

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.

2.5. 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.

Figure 8.

Second DSC scan from (EtPh₃P)₂[CuBr₄] (7).

3. Materials and Methods

3.1. 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 F² 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.

3.2. Chemicals

The following chemicals were used without further purification. Copper(II) bromide (99%), ethanol anhydrous (99%), n-hexane (96%), potassium bromide (Uvasol, Merk KGaA, Darmstadt, Germany, for IR spectroscopy) tetraethylammonium bromide (98%), tetrabutylphosphonium bromide (99%), tetrahexylammonium bromide (99%), benzyltrimethylammonium bromide (98%), benzyltriethylammonium bromide (98%), benzyltributylammonium bromide (99%), ethyltriphenylphosphonium bromide (98%), hexyltriphenylphosphonium bromide (>99%), dodecyltrimethylammonium bromide (>98%), hexadecylpyridinium bromide (98%), and dihexadecyldimethylammonium bromide (99%).

3.3. Syntheses

3.3.1. General Preparation

In general, tetrabromidocuprate(II) complexes can be achieved by different procedures [42,43,44]. In the current work, the [CuBr₄]²⁻ moiety was synthesized according to a protocol by N. S. Gill and R. S. Nyholm [44]: an ethanolic solution of a stoichiometric amount of CuBr₂ 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.

3.3.2. Bis(tetraethylammonium)tetrabromidocuprate(II), (Et₄N)₂[CuBr₄] (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.

Melting point: 241–242 °C. Yield: 0.22 g (67%). Elemental analysis calculated for C₁₆H₄₀N₂CuBr₄ (643.66): C 29.85, H 6.26, N 4.35 (%); found: C 29.80, H 6.20, N 4.34 (%). IR (KBr, cm⁻¹): 3443 s, 2976 m, 2921 m, 2852 wm, 1628 wm, 1478 s, 1402 m, 1307 w, 1183 m, 1032 m, 1006 m, 793 m.

(IR: s = strong, ms = medium strong, m = medium, wm = weak medium, w = weak).

3.3.3. Bis(tetrabutylphosphonium)tetrabromidocuprate(II), (Bu₄P)₂[CuBr₄] (2)

Solutions of 0.5 mmol (0.11 g) copper(II) bromide and 1.0 mmol (0.32 g) tetrabutylphosphonium bromide, each dissolved in 2 mL ethanol, were combined and stirred at room temperature for 1 h. The resulting violet precipitate is filtered off and dried.

Melting point: 49–52 °C. Yield: 0.38 g (84%). Elemental analysis calculated for C₃₂H₇₂P₂CuBr₄ (902.13): C 42.60, H 8.04 (%), found: C 42.08, H 8.16 (%). IR (KBr, cm⁻¹): 2928 s, 2891 ms, 2870 ms, 1463 ms, 1407 m, 1375 m, 1237 w, 1098 m, 968 w, 920 m, 722 m. μ_eff = 1.5 B.M.

3.3.4. Bis(tetrahexylammonium)tetrabromidocuprate(II), (Hex₄N)₂[CuBr₄] (3)

A solution of 1.0 mmol (0.22 g) CuBr₂ in 5 mL ethanol was added to a solution of 2.0 mmol (0.87 g) tetrahexylammonium bromide in 5 mL ethanol. The mixed solution was stirred at room temperature for 1 h. The resulting violet precipitate was filtered off and dried.

Melting point: 93–95 °C. Yield: 0.58 g (53%). Elemental analysis calculated for C₄₈H₁₀₄N₂CuBr₄ (1092.51): C 52.77, H 9.60, N 2.56 (%), found: C 53.06, H 9.70, N 2.67 (%). IR (KBr, cm⁻¹): 2928 s, 2553 m, 1634 w, 1480 m, 1383 mw, 1050 w, 728 w. μ_eff = 1.5 B.M.

3.3.5. Bis(benzyltrimethylammonium)tetrabromidocuprate(II), (BzlMe₃N)₂[CuBr₄] (4)

Compound (4) was also synthesized according to an already published procedure [37].

To a solution of 0.5 mmol (0.11 g) of CuBr₂ 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.

Melting point: 173–175 °C. Yield: 0.35 g (72%). Elemental analysis calculated for C₂₀H₃₂N₂CuBr₄ (969.51): C 35.16, H 4.72, N 4.10 (%); found: C 35.09, H 4.71, N 4.17 (%). IR (KBr, cm⁻¹): 3017 m, 1585 w, 1485 s, 1476 s, 1411 m, 1218 m, 989 m, 974 m, 887 s, 780 s, 703 s. μ_eff = 1.4 B.M.

3.3.6. Bis(benzyltriethylammonium)tetrabromidocuprate(II), (BzlEt₃N)₂[CuBr₄] (5)

0.5 mmol (0.11 g) of CuBr₂ dissolved in 3 mL HBr was heated under reflux for 0.5 h. To this copper solution a solution of 1.0 mmol (0.27 g) BzlEt₃NBr and 2 mL methanol was slowly added. The mixture was stirred for one hour at room temperature. The solvent was reduced and purple crystals were obtained by covering the remaining solution with N-hexane.

Melting point: 110–112 °C. Yield: 0.21 g (54%). Elemental analysis calculated for C₂₆H₄₄N₂CuBr₄ (767.7): C 40.67, H 5.78, N 3.65 (%); found: C 40.56, H 5.76, N 3.69 (%). IR (KBr, cm⁻¹): 2983 m, 1583 w, 1450 s, 1402 ms, 1372 w, 1171 w, 1154 m, 1027 m, 1005 m, 787 m,756 s, 705 s. μ_eff = 1.6 B.M.

3.3.7. Bis(benzyltributylammonium)tetrabromidocuprate(II), (BzlBu₃N)₂[CuBr₄] (6)

A solution of 1.0 mmol (0.22 g) CuBr₂ in 5 mL ethanol was added to a solution of 2.0 mmol (0.71 g) benzyltributylammonium bromide in 5 mL ethanol. The mixed solution was stirred at room temperature for 1 h. The resulting violet precipitate was filtered off and dried.

Melting point: 58–60 °C. Yield: 0.59 g (63%). Elemental analysis calculated for C₃₈H₆₈N₂CuBr₄ (936.12): C 48.75, H 7.32, N 2.99 (%), found: C 48.90, H 7.48, N 3.12 (%). IR (KBr, cm⁻¹): 2959 s, 2871 m, 1654 w, 1561 w, 1474 m, 1458 m, 1380 mw, 1212 w, 869 mw, 725 m, 704 m. μ_eff = 1.5 B.M.

3.3.8. Bis(ethyltriphenylphosphonium)tetrabromidocuprate(II), (EtPh₃P)₂[CuBr₄] (7)

The synthesis of (EtPh₃P)₂[CuBr₄] as follows: 0.5 mmol (0.11 g) copper(II) bromide and 1.0 mmol (0.37 g) EtPh₃PBr solved even in 2 mL ethanol. The combined solutions are stirred for one hour at room temperature. A violet powder was obtained.

Melting point: 135–137 °C. Yield: 0.23 g (48%). Elemental analysis calculated for C₄₀H₄₀P₂CuBr₄ (964.90): C 49.74, H 4.17 (%), found: C 49.74, H 4.19 (%). IR (KBr, cm⁻¹): 2922 m, 1586 m, 1483 m, 1438 s, 1113 s, 996 m, 780 w, 739 s, 691 s, 531 s, 510 s, 482 m. μ_eff = 1.6 B.M.

3.3.9. Bis(hexyltriphenylphosphonium)tetrabromidocuprate(II), (HexPh₃P)₂[CuBr₄] (8)

A solution of 1.0 mmol (0.22 g) copper(II) bromide in 5 mL ethanol is added to 2.0 mmol (0.85 g) hexyltriphenylphosphonium bromide dissolved in 5 mL ethanol. The solution was stirred for 1 h at room temperature. The formed violet precipitate was filtered off and dried. Purple crystals were obtained by covering the remaining solution with n-hexane for slowly interdiffusion.

Melting point: 102–103 °C. Yield: 0.65 g (60%). Elemental analysis calculated for C₄₈H₅₆P₂CuBr₄ (1078.04): C 53.47, H 5.24 (%), found: C 53.05, H 4.97 (%). IR (KBr, cm⁻¹): 2956 m, 2923 m, 2758 m, 1585 w, 1485 w, 1436 s, 1113 s, 724 s, 689 s, 533s, 498 ms. μ_eff = 1.6 B.M.

3.3.10. Bis(dodecyltrimethylammonium)tetrabromidocuprate(II), (C₁₂H₂₅Me₃N)₂[CuBr₄] (9)

A solution of 1.0 mmol (0.22 g) of CuBr₂ in 5 mL ethanol and a solution of 2.0 mmol (0.62 g) C₁₂H₂₅Me₃NBr in 5 mL ethanol were combined and stirred at room temperature for 1 h. The resulting violet precipitate was filtered off and dried.

Melting point: 94–95 °C. Yield: 0.56 g (85%). Elemental analysis calculated for C₃₀H₄₈N₂CuBr₄ (662.23): C 42.89, H 8.16, N 3.34 (%), found: C 43.19, H 8.23, N 3.37 (%). IR (KBr, cm⁻¹): 2922 ms, 2851 m, 1632 w, 1469 s, 966 ms, 909 m, 722 m. μ_eff = 1.5 B.M.

3.3.11. Bis(hexadecylpyridinium)tetrabromidocuprate(II), (C₁₆-py)₂[CuBr₄] (10)

A solution of 0.5 mmol (0.11 g) copper(II) bromide in 10 mL ethanol is added to 1.0 mmol (0.40 g) hexadecylpyridinium bromide dissolved in 3 mL ethanol. The solution was stirred for 1 h at room temperature. The obtained violet precipitate was filtered off and dried.

Melting point: 73–74 °C. Yield: 0.15 g (30%). Elemental analysis calculated for C₄₂H₇₆N₂CuBr₄ (992.23): C 50.84, H 7.72, N 2.82 (%), found: C 47.67, H 7.20, N 2.95 (%). IR (KBr, cm⁻¹): 3446 wm, 3053 w, 2918 s, 2850 m, 1633 wm, 1485 m, 1468 wm, 1175 w, 768 w, 721 w, 679 w. μ_eff = 1.6 B.M.

3.3.12. Bis(dihexadecyldimethylammonium)tetrabromidocuprate(II), (C₁₆H₃₃)₂Me₂N)₂[CuBr₄] (11)

A solution of 1.0 mmol (0.58 g) dihexadecyldimethylammonium bromide in 3 mL ethanol was slowly added to a solution of 0.5 mmol (0.11 g) copper(II) bromide in 3 mL ethanol. The mixture was stirred at room temperature for 1 h. The formed violet precipitate was filtered off and dried.

Melting point: 36–38 °C. Yield: 0.67 g (98%). Elemental analysis calculated for C₆₈H₁₄₄N₂CuBr₄ (1373.05): C 59.48, H 10.57, N 2.04 (%), found: C 59.11, H 10.89, N 2.15 (%). IR (KBr, cm⁻¹): 2919 m, 2851 m, 1628 w, 1469 s, 1376 w, 1054 w, 990 w, 968 w, 878 w, 718 m.

4. 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 [CuBr₄]²⁻ anions 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].

Abbreviations

DFT	Discrete Fourier Transform
EPR	Electron Paramagnetic Resonance
DSC	Differential Scanning Calorimetry

Author Contributions

André Zabel, Peter Strauch: writing of manuscript, analytical interpretation, literature search; André Zabel, Alette Winter: syntheses; Peter Strauch, Alette Winter: EPR spectroscopy; Alexandra Kelling, Uwe Schilde: X-ray crystallography.

Conflicts of Interest

The authors declare no conflict of interest.