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

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.


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].

X-ray Structure Determination
The single crystals of compounds Pz 4 , 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).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.

Computational Chemistry
Experimental X-Ray structures of the ligand Pz 4 and complex 1 (without uncoordinated dimethyl sulfoxide (DMSO) molecules) were used as starting points for density functional theory (DFT) geometry optimizations.Singlet-(for Pz 4 ) 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].

Synthesis of Compounds
Analysis-grade copper(II) chloride dihydrate was used for the synthesis of the complexes.Pz 4 was prepared as described previously [46].Solvents were of reagent-grade purity and were used as received.
A hot suspension containing Pz 4 (0.3 mmol, 0.09 g) in 5 ml of DMSO was added with stirring to an ethanol solution (7 ml) of CuCl 2 •2H 2 O (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%  A suspension of 26.0 mg Pz 4 (0.09 mmol) in 1 ml of DMSO was added to 34.0 mg of CuCl 2 •2H 2 O (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.

Synthesis of the Complexes
The two copper(II) chloride complexes with ligand Pz 4 were synthesized by the reaction of the ethanol solution of the CuCl 2 •2H 2 O (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.

Synthesis of the Complexes
The two copper(II) chloride complexes with ligand Pz 4 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 Cu 2+ :Pz 4 = 5:1.Complex 2 was obtained after 18 h of heating at 95 °C of the reaction mixture using solid copper salt at the Cu 2+ :Pz 4 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.

Crystal Structures of the Complexes
The compounds [Cu2(Pz 4 )(DMSO)2Cl4]•4H2O (1) and [Cu2(Pz 4 )(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 Pz 4 .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).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(Pz 4 )(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 Pz 4 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(Pz 4 )(NO3)2]n and [{Cu(Pz 4 )(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 Pz 4 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 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 Cu 2+ :Pz 4 = 5:1.Complex 2 was obtained after 18 h of heating at 95 • C of the reaction mixture using solid copper salt at the Cu 2+ :Pz 4 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.

Crystal Structures of the Complexes
The compounds [Cu 2 (Pz 4 )(DMSO) 2 Cl 4 ]•4H 2 O (1) and [Cu 2 (Pz 4 )(DMSO) 2 Cl 4 ]•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 Pz 4 .Addison's τ 5 criterion [47] was used to determine the geometry of coordination polyhedra of copper(II) ions in complexes 1 and 2 (Table 2).As suggested by the τ 5 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 [Cu 2 (Pz 4 )(H 2 O) 2 (NO 3 ) 4 ] [30,31], the τ 5 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 Pz 4 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(Pz 4 )(NO 3 ) 2 ] n and [{Cu(Pz 4 )(H 2 O)(NO 3 ) 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 Pz 4 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     In complex 1, both crystallographically independent Cl atoms form weak intermolecular hydrogen bonds with two water molecules (Figure 3 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 Pz 4 )•••Cu mean line (Supplementary Materials Figure S4).The same type of chain was observed in the coordination polymers [Cu(Pz 4 )(NO 3 ) 2 ] n [31], [Ag(Pz 4 )(NO 3 )] n , and {[Ag(Pz 4 )(NO 3 )]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.

Crystal Structure of the Ligand Pz 4
Conformation of the Pz 4 molecule in the solid state differs from that in the complexes.In free Pz 4 , 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).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 Cu 2+ .

Crystal Structure of the Ligand Pz 4
Conformation of the Pz 4 molecule in the solid state differs from that in the complexes.In free Pz 4 , 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).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 Cu 2+ .

Crystal Structure of the Ligand Pz 4
Conformation of the Pz 4 molecule in the solid state differs from that in the complexes.In free Pz 4 , 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).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 Cu 2+ .

IR Spectroscopy and DFT Calculations
For more accurate interpretation of IR spectra of the Pz 4 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.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. 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.

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].
Crystals 2019, 9, x FOR PEER REVIEW 10 of 13 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(Pz 4 )(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.
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(Pz 4 )(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.
(a) (b)   Supplementary Materials: The following are available online at www.mdpi.com/xxx/s1,Table S1: Atomic coordinates for optimized geometry of complex 1, Table S2: Atomic coordinates for optimized geometry of ligand Pz 4 , Figure S1: Overlay of the metallocycles of 1 and 2, [Cu2(Pz 4 )(H2O)2(NO3)4] (refcode XUDWUQ), 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 [Cu 2 (Pz 4 )(DMSO) 2 Cl 4 ].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.
(d norm ) mapping on the promolecular Hirshfeld surface of the Pz 4 ligand revealed intramolecular C-H•••Cl contacts between the ethane unit of Pz 4 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(Pz 4 )(NO 3 ) 2 ] n and [{Cu(Pz 4 )(H 2 O)(NO 3 ) 2 } 2 ] n [31].In other words, Cl − and NO 3 − are inclined to C-H of the ethane unit.However, two polymorphs [Cu 2 (Pz 4 )(H 2 O) 2 (NO 3 ) 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.
) (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

Figure 2 .
Figure 2. The dnorm Hirshfeld surface of Pz 4 ligand in the copper(II) complexes showing intramolecular C-H•••D (D = Cl, O) contacts (marked dashed green).Area with intermolecular contacts closer than the sum of the atoms' van der Waals radii are red, longer contacts are blue, and the contacts around the sum of van der Waals radii are white.For polymeric complexes, only the

Figure 2 .
Figure 2. The d norm Hirshfeld surface of Pz 4 ligand in the copper(II) complexes showing intramolecular C-H•••D (D = Cl, O) contacts (marked dashed green).Area with intermolecular contacts closer than the sum of the atoms' van der Waals radii are red, longer contacts are blue, and the contacts around the sum of van der Waals radii are white.For polymeric complexes, only the ligand and coordination environment of the central atom are shown.For [Cu 2 (Pz 4 )(H 2 O) 2 (NO 3 ) 4 ] (Refcode XUDWUQ01), only one of two crystallographically independent molecules is shown.

Figure 5 .
Figure 5. TG curves of 1 (a) and 2 (b) in inert atmosphere at 10 K/min speed rate.

Figure 5 .
Figure 5. TG curves of 1 (a) and 2 (b) in inert atmosphere at 10 K/min speed rate.

Table 2 .
Addison's τ5 criterion for coordination centers in the complexes in copper(II) complexes with Pz 4 ligand.

Table 2 .
Addison's τ 5 criterion for coordination centers in the complexes in copper(II) complexes with Pz 4 ligand.

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