Synthesis, Crystal Structure, Thermal Analysis, and DFT Calculations of Molecular Copper(II) Chloride Complexes with Bitopic Ligand 1,1,2,2-tetrakis(pyrazol-1-yl)ethane

Abstract

Two binuclear coordination compounds of Cu(II) chloride with the bitopic ligand 1,1,2,2-tetrakis(pyrazol-1-yl)ethane (Pz⁴) of the composition [Cu₂(µ₂-Pz⁴)(DMSO)₂Cl₄]·4H₂O and [Cu₂(µ₂-Pz⁴)(DMSO)₂Cl₄]∙2DMSO were prepared and characterized by elemental analysis, IR spectroscopy, thermogravimetric analysis, single-crystal X-ray diffraction, and powder diffraction analysis. It was shown that in contrast to silver(I) and copper(II) nitrates, copper(II) chloride forms discrete complexes instead of coordination polymers. The supramolecular structure of the complex [Cu₂(µ₂-Pz⁴)(DMSO)₂Cl₄]·4H₂O with lattice water molecules is formed by OH···Cl and OH···O hydrogen bonds. Density functional theory (DFT) calculations of vibrational frequencies of the ligand and its copper(II) complex allowed for assigning IR bands to specific vibrations.

1. Introduction

Multitopic ligands are a class of ligands that nowadays have attracted an increasing amount of interest in many applications of coordination chemistry. These ligands contain two or more separated metal-binding sites that allow them to form a wide variety of structures [1,2,3]. The design of a linker structure between the binding sites permits tuning the properties of the materials based on coordination compounds with multitopic ligands [4,5,6,7]. Applications of functional materials containing such ligands include gas storage [8,9,10], membrane separations [11], electrochemical devices [12], sensing materials [13,14], drug delivery systems [15,16], enantiomer separations [17], and catalysis in fine organic synthesis [18,19].

Bitopic bis- and tris(pyrazol-1yl)methanes have served as ligands for the preparation of various silver coordination polymers of different topologies [20,21,22,23,24,25,26,27,28]. In our recent work, we synthesized silver coordination polymers containing the bitopic ligand 1,1,2,2-tetra(pyrazol-1-yl)ethane (Pz⁴) and showed that silver-based compounds are more likely to form polymeric rather than discrete structures [29]. For the case of Cu²⁺ ions, we demonstrated that both molecular copper(II) complexes and coordination polymers can be prepared [30,31]. Catalytic activity of the binuclear palladium(II) complex of Pz⁴ [32] as well as antibacterial and antifungal activity of the ligand itself [33] were reported recently. In this work, we focused on the synthesis and crystal structure peculiarities of Cu(II) coordination compounds containing the Pz⁴ ligand. Herein, we report the first X-ray crystal structures of the pure ligand and two new copper(II) coordination compounds and discuss their peculiarities.

2. Materials and Methods

2.1. Instrumental Characterization Methods

Elemental analysis (C, H, N, and S) was carried out on Euro EA 3000 analyzer (Eurovector SPA, Redavalle, Italy) following the standard procedures.

IR spectra of the complexes as mineral and fluorinated oil mulls or polyethylene pellets were recorded on Scimitar FTS 2000 (Digilab LLC, Canton, MA, USA) and Vertex 80 (Bruker Corporation, Billerica, MA, USA) spectrometers in the range of 4000–100 cm⁻¹.

X-ray powder diffraction (XRD) patterns were recorded at room temperature on a DRON-RM4 diffractometer (Burevestnik, Saint Petersburg, Russia) using CuK_α irradiation and graphite monochromator d₀₀₁ = 3345 Å. The scanning range was 5°–60° (2θ) for complex salts and 5°–90° (2θ) for products of thermolysis. The experimental diffraction data were processed using the PowderCell v.2.4 program [34], which allowed a calculation of the quantitative phase composition and the lattice parameters. Data from the PDF database [35] were used as reference.

Thermogravimetric analysis of coordination compounds was carried out in helium atmosphere on a NETZSCH thermobalance TG 209 F1 Iris (Erich NETZSCH GmbH & Co. Holding KG, Selb, Germany) in open Al₂O₃ crucibles (loads of 5–10 mg, heating rate of 10 K·min⁻¹).

2.2. X-ray Structure Determination

The single crystals of compounds Pz⁴, 1, and 2 were selected directly from the mother liquors and mounted on glass fibers using epoxy resin. Single-crystal X-ray diffraction data were collected on a Bruker-Nonius X8 APEX CCD diffractometer (graphite monochromatized Mo Kα radiation, λ = 0.71073 Å, φ and ω scans of narrow frames (Bruker Corporation, Billerica, MA, USA), equipped with a 4K CCD area detector (

Table 1. Crystallographic data of the compounds 1,1,2,2-tetra(pyrazol-1-yl)ethane (Pz⁴), 1, and 2.

Identification Code	Pz4	1	2
Empirical formula	C14H14N8	C18H34Cl4Cu2N8O6S2	C22H38Cl4Cu2N8O4S4
Formula weight	294.33	791.53	875.72
Temperature/K	150(2)	296(2)	150(2)
Crystalsystem	monoclinic	triclinic	monoclinic
Spacegroup	C2/c	P-1	P21/c
a/Å	15.0087(9)	7.959(3)	8.7218(4)
b/Å	5.4385(3)	8.885(3)	18.2205(7)
c/Å	17.3736(11)	12.269(5)	11.9231(5)
α/°	90	73.239(16)	90
β/°	92.435(2)	72.615(16)	107.0408(16)
γ/°	90	83.178(18)	90
Volume/Å3	1416.84(15)	792.3(5)	1811.58(13)
Z	4	1	2
ρcalcg/cm3	1.380	1.659	1.605
μ/mm−1	0.092	1.857	1.740
F(000)	616.0	404.0	896.0
Crystalsize/mm3	0.35 × 0.1 × 0.09	0.25 × 0.15 × 0.1	0.2 × 0.12 × 0.1
2Θ range for data collection/°	4.694–55.148	4.792–55.588	4.214–52.818
Index ranges	−19 ≤ h ≤ 14, −7 ≤ k ≤ 4, −20 ≤ l ≤ 22	−9 ≤ h ≤ 10, −11 ≤ k ≤ 11, −14 ≤ l ≤ 15	−10 ≤ h ≤ 10, −22 ≤ k ≤ 22, −14 ≤ l ≤ 14
Reflections collected	3290	6095	24618
Independent reflections	1628 [Rint = 0.0167, Rsigma = 0.0257]	3598 [Rint = 0.0391, Rsigma = 0.0579]	3715 [Rint = 0.0328, Rsigma = 0.0217]
Restraints/parameters	0/100	0/189	0/203
Goodness-of-fit on F2	1.053	1.044	1.035
Final R indexes [I ≥ 2σ (I)]	R1 = 0.0386, wR2 = 0.0964	R1 = 0.0424, wR2 = 0.1092	R1 = 0.0252, wR2 = 0.0556
Final R indexes [all data]	R1 = 0.0494, wR2 = 0.1029	R1 = 0.0584, wR2 = 0.1153	R1 = 0.0313, wR2 = 0.0580
Largest diff. peak/hole / e Å−3	0.31/−0.23	0.70/−0.47	0.74/−0.30

). Absorption corrections were applied using the SADABS program [36]. The crystal structures were solved by direct methods and refined by full-matrix least-squares techniques with the use of the SHELXTL package [37] and Olex2 GUI [38]. Atomic thermal displacement parameters for nonhydrogen atoms were refined anisotropically. The positions of H atoms were calculated corresponding to their geometrical conditions and refined using the riding model.

2.3. Computational Chemistry

Experimental X-Ray structures of the ligand Pz⁴ and complex 1 (without uncoordinated dimethyl sulfoxide (DMSO) molecules) were used as starting points for density functional theory (DFT) geometry optimizations. Singlet- (for Pz⁴) or triplet-state (for complex 1) gas-phase geometry optimizations were carried out at the DFT level of theory employing the three-parameter hybrid B3LYP functional [39,40,41,42] and 6-31+G(d) basis set [43] in the Gaussian 09 package [44]. Frequency calculations were performed for both molecules in order to ensure the lack of imaginary vibration frequencies, which indicates that the optimized structures correspond to minima on the potential energy surfaces. Cartesian atomic coordinates for all of the optimized structures are provided in Supplementary Tables S1 and S2.

Hirshfeld promolecular surfaces mapped over d_norm plots of the complexes were built using the Crystal Explorer (version 17.5) program [45].

2.4. Synthesis of Compounds

Analysis-grade copper(II) chloride dihydrate was used for the synthesis of the complexes. Pz⁴ was prepared as described previously [46]. Solvents were of reagent-grade purity and were used as received.

2.4.1. Synthesis of [Cu₂(µ₂–Pz⁴)(DMSO)₂Cl₄]·4H₂O (1)

A hot suspension containing Pz⁴ (0.3 mmol, 0.09 g) in 5 ml of DMSO was added with stirring to an ethanol solution (7 ml) of CuCl₂·2H₂O (1.5 mmol, 0.25 g). Green crystals of 1 suitable for X-ray diffraction analysis were obtained by slow crystallization over the course of 24 h. The light-green precipitate formed from the solution was filtered, washed with ethanol, and then dried in the air. The yield of the product was 0.13 g (55%). Elemental analysis: found, %: C 27.6; H 4.2; N 14.4; S 8.0; for C₁₈H₃₄Cl₄Cu₂N₈O₆S₂ calculated, %: C 27.3; H 4.3; N 14.2; S 8.1. IR bands, cm⁻¹: 3513, 3424 (νOH (H₂O)), 3125, 3101, 2987 (νCH (Pz)), 1630 (δOH (H₂O)), 1513 (νCH (Pz)), 467 (ν(Cu–O)), 295 (ν(Cu–N)), 254 (ν(Cu–Cl)).

2.4.2. Synthesis of [Cu₂(µ₂–Pz⁴)(DMSO)₂Cl₄]∙2DMSO (2).

A suspension of 26.0 mg Pz⁴ (0.09 mmol) in 1 ml of DMSO was added to 34.0 mg of CuCl₂∙2H₂O (0.2 mmol) in a glass vial. The mixture was stirred for 10 min at room temperature. During the stirring process, a light-green precipitate formed. The vial with the formed powder was placed into an oven at 95 °C. After 18 h of heating, the vial was allowed to cool to room temperature. After one day, the crystals initially formed on the bottom of the vial were filtered and washed once with 1 ml of DMSO and then air-dried for a few days. The yield of the product was 39.9 mg (52%). Elemental analysis: found, %: C 29.6; H 4.0; N 12.9; S 14.3; for C₂₂H₃₈Cl₄Cu₂N₈O₄S₄ calculated, %: C 30.2; H 4.4; N 12.8; S 14.6. IR bands, cm⁻¹: 3141, 3094, 3000 (νCH (Pz)), 1513 (νCH (Pz)), 481 (ν(Cu–O)), 294 (ν(Cu–N)), 260 (ν(Cu–Cl)).

3. Results and Discussion

3.1. Synthesis of the Complexes

The two copper(II) chloride complexes with ligand Pz⁴ were synthesized by the reaction of the ethanol solution of the CuCl₂·2H₂O (for 1) or solid copper salt (for 2) with the DMSO suspension of 1,1,2,2-tetrakis(pyrazol-1-yl)ethane according to Scheme 1.

The synthesis of complex 1 was carried out with stirring at room temperature followed by slow crystallization from solution over the course of 24 h. Complex 1 was synthesized at the ratio Cu²⁺:Pz⁴ = 5:1. Complex 2 was obtained after 18 h of heating at 95 °C of the reaction mixture using solid copper salt at the Cu²⁺:Pz⁴ ratio of 2.2:1. Although complexes 1 and 2 differed only in outer-sphere solvate molecules, they demonstrated different crystal packing and different thermal behavior.

3.2. Crystal Structures of the Complexes

The compounds [Cu₂(Pz⁴)(DMSO)₂Cl₄]·4H₂O (1) and [Cu₂(Pz⁴)(DMSO)₂Cl₄]·2DMSO (2) include water and DMSO solvate molecules, respectively, and are not isostructural. Both complexes are centrosymmetric binuclear (Figure 1) and reveal very similar molecular geometries. Cu atoms coordinate two Cl⁻ ligands, one O atom of DMSO ligand, and two N atoms of Pz⁴. Addison’s τ₅ criterion [47] was used to determine the geometry of coordination polyhedra of copper(II) ions in complexes 1 and 2 (

Table 2. Addison’s τ₅ criterion for coordination centers in the complexes in copper(II) complexes with Pz⁴ ligand.

Parameter	α	β	τ5 = (β − α)/60	References
[Cu2(Pz4)(DMSO)2Cl4]·4H2O (1)	129.0	171.7	0.71	This paper
[Cu2(Pz4)(DMSO)2Cl4]·2DMSO (2)	153.1	168.7	0.26	This paper
[Cu2(Pz4)(H2O)2(NO3)4]	176.0, 178.3	168.1, 172.2	0.06, 0.17	[30]
[Cu2(Pz4)(H2O)2(NO3)4]	179.3, 176.1, 177.2, 179.5	170.2, 175.4, 173.1, 168.2	0.15, 0.01, 0.07, 0.19	[31]

). As suggested by the τ₅ criterion, the environment of the central atom in complex 1 is a slightly distorted trigonal bipyramid with O and N atoms located in axial positions, while in complex 2, the arrangement is more distorted toward square pyramidal. In previously reported structures of two binuclear copper(II) nitrate polymorphs [Cu₂(Pz⁴)(H₂O)₂(NO₃)₄] [30,31], the τ₅ criterion had values in the range of 0.01–0.19, indicative of a square planar geometry of coordination sphere comprising two monodentate nitrate ions, two nitrogen atoms of the ligand, and one water molecule. Thus, the Pz⁴ ligand shields the central atom in a chelate mode, leaving enough space for three more ligands (chloride/nitrate and solvent molecules). However, it can provide space even for four ligands, as shown by the example of polymeric nitrate complexes [Cu(Pz⁴)(NO₃)₂]_n and [{Cu(Pz⁴)(H₂O)(NO₃)₂}₂]_n [31], which reveal a highly distorted (4+2) octahedral environment. Copper–donor-atom bond lengths and bond angles are very similar in both complexes 1 and 2, with the exception of the Cu–Cl distance being larger by 0.10 Å for complex 2 in respect to 1. The metallocycles in 1, 2, and known copper(II) complexes with Pz⁴ have similar geometries, implying their inflexibility (Supplementary Materials Figure S1). The relative positions of one pyrazole unit and DMSO ligand are slightly different for 1 and 2 (Supplementary Materials Figure S2), which is likely due to intermolecular interactions with the solvent molecules and/or packing effects. Analysis of normalized contact distance (d_norm) mapping on the promolecular Hirshfeld surface of the Pz⁴ ligand revealed intramolecular C–H···Cl contacts between the ethane unit of Pz⁴ and chloride in both compounds 1 and 2 (Figure 2), being noticeably shorter for the latter. The same type of C–H···O contacts with nitrates in place of chlorides has been observed in known complexes [Cu(Pz⁴)(NO₃)₂]_n and [{Cu(Pz⁴)(H₂O)(NO₃)₂}₂]_n [31]. In other words, Cl⁻ and NO₃⁻ are inclined to C–H of the ethane unit. However, two polymorphs [Cu₂(Pz⁴)(H₂O)₂(NO₃)₄] [30,31] show different arrangement of nitrates, which lie on both sides of the corresponding C–H line. In this way, they are inclined to C–H of the pyrazole unit. Considering the large distance between C–H and the donor atoms as well as the very low polarity of the C–H bonds, these types of contacts are likely to have a steric nature.

In complex 1, both crystallographically independent Cl atoms form weak intermolecular hydrogen bonds with two water molecules (Figure 3) (O···Cl distances are 3.21 and 3.35 Å). Hydrogen bonds between water molecules were also observed (O···O distances are 2.78 and 2.80 Å), revealing a layered supramolecular structure. Analysis of the Hirshfeld surface revealed intermolecular contacts C–H···D (D = Cl, O) between molecules of the complexes and solvates in 1 and 2 (Supplementary Materials Figure S3) as well as in known copper(II) complexes [30,31], but they are likely to have a steric nature due to the low polarity of the C–H bonds. Overall crystal packing of 1 and 2 is somewhat close: one can distinguish chains built from the molecules, which are arranged along Cu··· (center of Pz⁴)···Cu mean line (Supplementary Materials Figure S4). The same type of chain was observed in the coordination polymers [Cu(Pz⁴)(NO₃)₂]_n [31], [Ag(Pz⁴)(NO₃)]_n, and {[Ag(Pz⁴)(NO₃)]DMF}_n [29]. In 1 and 2, the Cu···Cu distances within the molecule were 6.78 and 6.81 Å, while those between neighboring molecules in the chain were 7.14 and 7.94 Å, correspondingly.

3.3. Crystal Structure of the Ligand Pz⁴

Conformation of the Pz⁴ molecule in the solid state differs from that in the complexes. In free Pz⁴, the C1–C1′–N11–N12 torsion angle (of 130.1°) characteristic for the rotation of one of the pyrazole rings around the C1′–N11 bond is obtuse (Figure 4). In contrast, all known complexes revealed acute torsion angles for both pyrazole rings due to chelate coordination of the ligand (

Table 3. Torsion angles of Pz⁴ in solid state and in copper(II) and silver(I) complexes.

Compound	C1–C1′–Nn1–Nn2 (n = 1, 2) Torsion Angle,°	References
Pz4	±130.1, ±50.9	This paper
[Cu2(Pz4)(DMSO)2Cl4]·4H2O (1)	±71,8; ±59.5	This paper
[Cu2(Pz4)(DMSO)2Cl4]·2DMSO (2)	±69,6; ±64.4	This paper
[Cu2(Pz4)(H2O)2(NO3)4]	60.8; −70.4; 69.1; −61.7 70.4; −61.5; 63.2; −67.0	[30]
[Cu2(Pz4)(H2O)2(NO3)4]	59.5; −74.9; 67.7; −61.0.	[31]
[Cu(Pz4)(NO3)2]n	±72.2; ±61.5;	[31]
[{Cu(Pz4)(H2O)(NO3)2}2]n	±70.0; ±72.5	[31]
{[Ag(Pz4)(NO3)]DMF}n	±57.9; ±53.8; ±50.4; ±54.8	[29]
[{Ag(Pz4)(NO3)}n]	±57.6; ±58.1; ±50.5; ±58.2	[29]

). Note that the angle in copper(II) complexes falls within the 59°–72° range, while that in silver(I) compounds lies in the range of 50°–58°, which is in accordance with the increased ionic radius of Ag⁺ as compared with Cu²⁺.

3.4. IR Spectroscopy and DFT Calculations

For more accurate interpretation of IR spectra of the Pz⁴ ligand and its copper(II) chloride complexes, DFT calculations of geometries of isolated molecules and normal mode vibration frequencies were carried out.

Calculated and experimental (from X-Ray single-crystal structural analysis) geometrical parameters are listed in

Table 4. Calculated and experimental geometrical parameters of Pz⁴ and complexes.

Parameter	Experimental	Calculated	Parameter	Experimental	Calculated
Pz4			Complex 1
d(C7-N3), Å	1.450(1)	1.452	d(Cu1-Cl1), Å	2.436(1)	2.426
d(N3-N4), Å	1.355(1)	1.359	d(Cu1-Cl2), Å	2.314(1)	2.323
d(N4-C4), Å	1.329(2)	1.330	d(Cu1-O1), Å	1.940(3)	2.012
d(C4-C5), Å	1.392(2)	1.416	d(Cu1-N1), Å	2.074(3)	2.212
d(C5-C6), Å	1.368(2)	1.380	d(Cu1-N3), Å	2.009(3)	2.067
d(C6-N3), Å	1.357(1)	1.366	d(S1=O1), Å	1.533(2)	1.556
d(C7-C7′), Å	1.541(1)	1.558	d(N3-N4), Å	1.363(3)	1.365
			d(C7-C7′), Å	1.551(3)	1.564
			φ(O1-Cu1-N3), °	171.7(1)	168.7
			φ(O1-Cu1-N1), °	83.7(1)	83.4
			φ(O1-Cu1-Cl2), °	94.09(9)	90.3
			φ(Cl2-Cu1-Cl1), °	116.67(3)	126.0
			φ(S1=O1-Cu1), °	121.0(2)	132.5

. In most cases, deviations of calculated parameters from the experimental are rather small and probably due to crystal packing effects.

Experimental and calculated IR bands are listed in

Table 5. Calculated (scaled) and experimental vibration frequencies for Pz⁴ and complex.

Vibration	Pz4		[Cu2(Pz4)(DMSO)2Cl4]
	$\tilde{\mathit{\nu }}$calc., cm−1	$\tilde{\mathit{\nu }}$exp., cm−1	$\tilde{\mathit{\nu }}$calc., cm−1 1	$\tilde{\mathit{\nu }}$exp., cm−1 1	$\tilde{\mathit{\nu }}$exp., cm−1 2
νCH (Pz)	3173	3137	3139	3125	3128
νCH (Pz)	3156	3130	-	-	-
νCH (Pz)	3142	3115	-	-	-
νCH (DMSO)	-	-	3065	2987	3000
νCH	3054	3018	2952	2946	2943
νCH	3042	2994	-	-	-
νPzasym	1509	1520	1509	1513	1513
δCCH	-	-	1458	1471	1469
νPzasym	1420	1437	1400	1404	1405
νPzsym	1292	1311	1291	1304	1302
βCH (Pz)	1380	1391	1240	1251	1254
βCH (Pz)	1203	1216	1192	1199	1200
βCH (Pz)	1154	1172	1086	1095	1095
βCH (Pz)	1073	1092	1078	1067	1068
βCH (Pz)	1029	1053	1048	1035	1032
βCH (Pz)	947	968	-	-	-
δCCH	1273	1293	-	-	-
βCH (Pz)	899	918	-	-	-
γCH	859	890	-	-	-
γCH	812	857	-	-	-
γCH	740	771	747	769	768
γCH	725	754	-	-	-
δNCH	759	783	768	780	789
νSO (DMSO)	-	-	1003	988	991
νSO (DMSO)	-	-	899	944	944
γCH	606	616	603	610	612
δCCH	569	585	548	550	549
δNCC	348	357	-	-	-
δNCC	302	319	-	-	-
δNCC	230	245	-	-	-
δNCC	106	134	-	-	-
νCu-Cl	-	-	244	254	260

Abbreviations: νCH (Pz)—stretching vibrations of C–H bonds in pyrazole rings; νCH (DMSO)—stretching vibrations of C–H bonds in DMSO molecules; νCH—stretching vibrations of aliphatic C–H bonds; νPz_asym, νPz_sym—asymmetrical and symmetrical pyrazole ring stretching vibrations; δCCH—bending vibrations of aliphatic C–C–H bonds; βCH (Pz)—in-plane bending vibrations of C–H bonds in pyrazole rings; γCH—out-of-plane bending vibrations of C–H bonds in pyrazole rings; δNCH—bending vibrations of aliphatic N–C–H bonds; νSO (DMSO)—stretching vibrations of S=O bonds in DMSO molecules; δNCC—bending vibrations of aliphatic N–C–C bonds; νCu-Cl—stretching vibrations of Cu–Cl bonds.

. Experimental and simulated IR spectra are shown in Figures S5 and S6. As one can see form Table 5 and Figures S5 and S6, calculated vibration frequencies are in good agreement with experimental values and can thus be used for band assignments. In the spectrum of the ligand, bands at 1520 and 1437 cm⁻¹ were associated with asymmetrical pyrazole ring stretching vibrations, while the band at 1311 cm⁻¹ corresponded to symmetrical pyrazole stretching vibrations. Upon coordination to copper(II), one of the pyrazole stretching bands shifted to a lower frequency region (1404 and 1405 cm⁻¹), while two others remained at essentially the same positions. Bands of in-plane C–H vibrations (βCH) also underwent a coordination-induced low-frequency shift. It is interesting to note that the band assigned to aliphatic CCH bending vibrations demonstrated a considerable high-frequency shift from 1293 to 1471 cm⁻¹, which can be associated with the change of molecular symmetry that takes place upon coordination. Low-frequency bands at 254 and 260 cm⁻¹ in the spectra of complexes 1 and 2 were assigned to Cu–Cl stretching vibrations.

3.5. Thermal and XRD Analyses

The curves of thermal analysis for compounds 1 and 2 are shown in Figure 5. Investigation of thermal properties in helium atmosphere revealed that the first step of thermolysis for both compounds is associated with the removal of solvated molecules in the range of 40–150 °C for 1 and 100–200 °C for 2. The XRD pattern for thermolysis products of 1 at 150 °C differs from the pattern of the initial compound (Figure 6), suggesting a structural rearrangement. Further decomposition leads to formation of Cu and CuCl and amorphous products of ligand degradation at 350 °C, and the final step corresponds to partial sublimation of CuCl and formation of amorphous carbon. Formation of cubic CuCl and copper phases at thermolysis temperature 350 °C was observed previously [48].

It should be noted that dehydration process of 1 proceeds in two separate steps, and it is thus possible to obtain an anhydrous compound with composition [Cu₂(Pz⁴)(DMSO)₂Cl₄]. Thermal decomposition of 2 runs in a more complex manner with a simultaneous loss of outer- and inner-sphere dimethyl sulfoxide molecules in one step (Figure 5). Concurrent loss of both outer- and inner-sphere DMSO molecules can be explained by the absence of intermolecular interactions between solvent molecules in complex 2 in contrast to the hydrogen-bonded network in complex 1.

4. Conclusions

Two binuclear coordination compounds of Cu(II) chloride with 1,1,2,2-tetrakis(pyrazol-1-yl)ethane (Pz⁴) of the composition [Cu₂(µ₂-Pz⁴)(DMSO)₂Cl₄]·4H₂O and [Cu₂(µ₂-Pz⁴)(DMSO)₂Cl₄]∙2DMSO have been synthesized and fully characterized by elemental analysis, IR spectroscopy, thermogravimetric analysis, single-crystal X-ray diffraction, and powder diffraction analysis. Crystal structure analysis of the compound [Cu₂(µ₂-Pz⁴)(DMSO)₂Cl₄]·4H₂O revealed a layered supramolecular structure formed by OH···Cl and OH···O (dashed red lines) hydrogen bonds.