SYNTHESIS, X-RAY CRYSTAL STRUCTURE, SPECTROSCOPIC CHARACTERIZATION AND HIRSHFELD SURFACE ANALYSIS OF DICHLORO-BIS(3,5-DIMETHYL-4-AMINO-1H-PYRAZOLE) COBALT(II)

The synthesis and characterization of mononuclear Co(II) complex based on 3,5-dime-thyl-4-amino-1H-pyrazole are reported. IR and UV/Vis spectroscopy characterization of the complex are described. The synthesis, results of IR, NMR spectroscopy and elemental analysis of 3,5-dimethyl-4-amino-1H-pyrazole are also reported. X-ray analysis of [Co(C 5 H 9 N 3 ) 2 Cl 2 ] complex reveals that the cobalt atom has a tetrahedral coordination environment formed by two nitrogen atoms belonging to the two 3,5-dimethyl-4-amino-1H-pyrazole ligands [Co1– N1 = 2.005(3) and Co1–N5 = 2.006(3)Å] and two chlorine atoms [Co1–Cl2 = 2.2400(11) and Co1–Cl1 2.2863(12) Å]. In the crystal structure the molecules are linked through intermolecular (N–H···N, N–H···Cl) and intramolecular non-classical (С–H···Cl) hydrogen bonds. Hirshfeld surface analysis of the intermolecular contacts reveals that the most important contributions for the crystal packing are from H···H (47.1%) and H···Cl/Cl···H (28.5%) contacts.

INTRODUCTION. Pyrazole ligands are widely used in different areas of coordination, bioinorganic, supramolecular chemistry and in molecular electronics because of their marked tendency to form high nuclearity species exhibiting specific magnetic properties which also can be used for the development of structural and functional models of active sites of some metalloenzymes [1][2].
Due to the presence of N-N bridging function in the pyrazole ring, these ligands can form polynuclear complexes with specific molecular topology. In this case, triangular azametallacrowns have been a much more common structure type [6][7][8][9][10]. The neutral pyrazole ligands usually bind to metal ions via the pyridine-type nitrogen atom and provide to form of the mononuclear complexes [11][12][13][14]. Despite the considerable amount of avail able data on the 3,5-substituted pyrazole compounds, 3,4,5-substituted pyrazole-based complexes have been studied very unevenly. We have been interested in how the steric factors, namely the presence of NH 2 substitute in the fourth position of the 3,5-dimethyl-1Н-pyrazole, can influence the final crystal structure. In this regard, in this work, we performed the synthesis of 3,5-dimethyl-4-amino-1Hpy razole and Co(II) complex with it was also studied.
Although the X-ray structure of the title compound was previously reported [15], we used another solvent for the preparation of [Co(C 5 H 9 N 3 ) 2 Cl 2 ]. Additionally, IR and UV/Vis spectroscopic characterization of the complex are described. The synthesis, results of IR, NMR spectroscopy and elemental analysis of 3,5-dimethyl-4-amino-1H-pyrazole are also reported. Hirshfeld surface analysis of the intermolecular contacts of the title compound was also performed.
EXPERIMENT AND DISCUSSION OF THE RESULTS. All chemicals and solvents were commercial products of reagent grade and used without further purification. Microanalyses were performed with a Perkin -Elmer 2400 CHN. IR spectra (KBr pellets) were recorded with a Perkin -Elmer Spectrum BX FT-IR in the range of 400-4000 cm -1 . Absorbance UV/ Vis spectra were registered with a Varian Cary 50 spectrophotometer in the range of 200-1000 nm at room temperature. 1 H NMR spectra were recorded on a Bruker AC-400 spectrometer (400 MHz) at room temperature. For X-ray structure determination, single crystals of [Co(C 5 H 9 N 3 ) 2 Cl 2 ] were mounted on a Nonius four circle diffractometer equipped with a CCD camera and a graphite monochromated Mo К α radiation source (λ = 0.71073 Å). Data were collected at 293K. Effective absorption correction was performed (SCALEPACK [16]). The structure of the complex was solved with the direct method using SIR-97 [17] software and was refined with full matrix least squares method on F2 using SHELXL-97 program [18]. H atoms were treated by a riding model. Crystallographic data are summarized in Table 1 The synthesis of 3,5-dimethyl-4-amino-1Hpyrazole (L) was carried out according to the following method. 50 g (0.50 mol) of freshly distilled acetylacetone, 45 mL of concentrated HCl and 250 mL of water were placed into a four-neck 1L round bottom flask equipped with a stirrer, reflux condenser, thermometer, and addition funnel. The mixture was cooled with stirring in an ice bath to 5 o C and a solution of NaNO 2 (36 g; 0.52 mol) in 100 mL of water was dropwise added to it over 10-15 minutes. After consecutive stirring for 20 min, 27.5 g (0.55 mol) of 85% hydrazine hydrate was added to the reaction mixture. After that 85 g of NaCl was added to the reaction flask to preci pitate the product and the mixture was stirred for 1.5 hours at r.t. The obtained blue crystals were filtered off and air-dried. Subsequent treatment of the isolated product by refluxing of its mixture with anhydrous benzene (800 mL) during 5-7 min allowed to selectively dissolve 3,5-dime thyl-4-nitroso-1H-pyrazole (1), in Scheme 1, whereas NaCl was insoluble in these conditions and was filtered off from the hot solution. Slow cooling of the filtrate to 10 o C led to the crystallization of 3,5-dimethyl-4-nitroso-1H-pyrazole (m.p. 128-129 o C), which turned green in the air. To increase the yield of the product, the filtered off (on the first purification step) solid was mixed with the filtrate and obtained mixture was refluxed, filtered and the new filtrate was cooled to 10 o C, which led to the second portion of the product. Yield: 62.5 g; 90%.
Hydrazine hydrate (90%, 93 mL; 0.5 mol) was added to 3,5-dimethyl-4-nitroso-1H-pyrazole (62.5 g; 0.5 mol) in ethanol (650 mL) and placed into a three-neck round-bottom flask equipped with a stirrer, reflux condenser, and thermometer. At that temperature of reaction mixture increased to 60-65 o C. The mixture was refluxed under constant stirring for 3 hours, which was accompanied by its color change from green to yellow-brown. The solvent was completely distilled off and the resulting crystalline residue was washed with cold ethanol (20-25 mL) and air-dried. The yield of the crystalline product (2), in Scheme 1, was 52.0 g (93%). 1

INORGANIC CHEMISTRY
The preparation of the title compound was carried out according to the following me thod. Cobalt (II) chloride hexahydrate (0,0071 g, 3·10 -5 mol) and 3,5-dimethyl-4-amino-1H-pyrazole (0,0033 g, 3·10 -5 mol) were dissolved in methanol (1 mL To confirm the fact of coordination of the ligand to the metal ion, an IR study of the obtained complex [Co(C 5 H 9 N 3 ) 2 Cl 2 ] was compared with the spectra of the ligand. (Figure 1). In the comparative analysis of the IR spectra of the synthesized complex and ligand, the main attention was paid to the absorption bands corresponding to the oscillations of the functional groups of the ligand. Accordingly, in the IR spectra of [Co(C 5 H 9 N 3 ) 2 Cl 2 ] and 3,5-dimethyl-4-amino-1H-pyrazole observed an intense sharp band of the valence oscillations of ν(NH) in the region of 3350 cm -1 (for complex), 3347 cm -1 (for L). Both spectra have several bands in the region of 2880-3280 cm -1 corresponding to the asymmetric and symmetric stretch of ν(NH 2 ); scissoring oscillations of (NH 2 ) for complex 1561 cm -1 , for 3,5dimethyl-4-amino-1H-pyrazole -1537 cm -1 . The re are several bands in the region of 2700-2900 cm -1 corresponding to the oscillations of ν(СH 3 ) asymmetric and symmetric stretch, in the region of 1386-1427 cm -1 of ρ(СH 3 ) asymmetric and symmetric stretch. A rather intense and sharp peak in the region of 1232 cm -1 (for complex) and 1299 cm -1 (for L) indicates the presence of ν(С-N) oscillations. A band is pre-sent in the absorption region of ν(С=N) oscillations: 1609 cm -1 (for complex) and 1607 cm -1 (for L) [19]. In view of the above facts, it can be stated indicating the presence of a coordinated but not deprotonated ligand.
The electronic spectra of the title compound (Figure 2, B) and CоCl 2 ·6H 2 O (Figure 2, A) in the same solvent (methanol solution) indicate absorption maxima at 600 and 670 nm for CоCl 2 ·H 2 O, and at 600 and 650 nm for the complex. The first of these bands in each case appear as shoulders and may be attributed to charge transfer from the ligand to cobalt, and the second can be assigned as the d-d transitions of cobalt(II) ions. During complex formation, the most intense band is shifted to the short-wavelength region by 20 nm in compa rison with Cо-Cl 2 ·6H 2 O. Since the expected data for octahedral cobalt complexes are 400-500 nm (chromophore CoO6 -CoN6), and for tetrahedral complexes are 600-700 nm (chromophore CoN4), the results suggest the tetrahedral structure of the obtained complex [20].
Due to such organization of crystal packing the intermolecular hydrogen bonds N-H···N take part in forming the sixteen-and twenty-membered cycles, which are interchangeable in the molecular structure. The four mo-lecules of the ligand with two Co atoms are connected by hydrogen bonds N4-H4···N3 and take part in forming the sixteen-membered cycle. The interchangeable hydrogen bonds N2-H2···N6 and N4-H4···N3 from four molecules of the ligand take part in the formation of the twenty-membered cycle. In the crystal packing the shortest intrachain Co... Co separations are 7.407(27) Å). The Hirshfeld surface analysis and the associated two-dimensional fingerprint plots were performed using Crystal Explorer 17.5 software [23], with a standard resolution of the three-dimensional d norm surfaces plotted over a fixed color scale of -0.6232 (red) to 1.6217 (blue) a.u. There are 12 red spots on the d norm surface ( Figure 5). The dark-red spots arise as a result, of short interatomic contacts and represent negative d norm values on the surface, while the other weaker intermolecular interactions appear as light-red spots. The Hirshfeld surfaces mapped over d norm are shown for the H···H, H···Cl/Cl···H, H···N/N···H and H···C/C···H contacts, the overall two-dimensional fingerprint plot and the decomposed two-dimensional fingerprint plots are given in Figure 6.
All short interatomic contacts are in the range of 1.829-2.782 Å. The shortest contacts are NH···N and the longest contacts are NH···Cl. The most significant contributions to the overall crystal packing are from H···H (47.1%), H···Cl/Cl···H (28.5%), H···N/N···H (12.9%) and   CONCLUSIONS. The present work describes the synthesis and characterization of the mononuclear coordination compound [Co(C 5 H 9 N 3 ) 2 Cl 2 ]. X-ray analysis of the complex reveals that the cobalt atom has a tetrahedral coordination environment formed by two nitrogen atoms belonging to the two 3,5-dimethyl-4-amino-1H-pyrazole and two chlorine atoms. In the crystal structure the molecules are linked through intermolecular hydrogen bonds (N-H···N, N-H···Cl) and non-classical intramolecular hydrogen bonds (С-H···Cl). It was found that the introduction to the 4th position of the 3,5-dimethyl-1H-pyrazole NH 2 group does not contribute to the participation of the latter in coordination. The reason for this fact may be the weak σ-donor capacity of the NH 2 group in the pyrazole cycle. Hirshfeld surface analysis was used to study intermole cular interactions in the crystal. The 2D-fingerprint plot calculations displayed the H⋯H and H···-Cl/Cl···H contacts that were the most significant interaction with the Hirshfeld surface.