Crystal structure and Hirshfeld surface analysis of dichloridotetrakis(4-methyl-1 H -pyrazole- j N 2 )-nickel(II) acetonitrile disolvate

is a mononuclear octahedral Ni II pyrazole-based complex. Two acetonitrile molecules are linked to the Ni II complex by N—H (cid:2) (cid:2) (cid:2) N hydrogen bonds. The Ni II atom is octahedrally coordinated by four N atoms of four 4-methyl-1 H -pyrazole ligands, forming the equatorial plane. The axial positions are occupied by two Cl atoms.

The title compound, [NiCl 2 (C 4 H 6 N 2 ) 4 ]Á2CH 3 CN, is a mononuclear octahedral Ni II pyrazole-based complex. Two acetonitrile molecules are linked to the Ni II complex by N-HÁ Á ÁN hydrogen bonds. The Ni II atom is octahedrally coordinated by four N atoms of four 4-methyl-1H-pyrazole ligands, forming the equatorial plane. The axial positions are occupied by two Cl atoms.

Chemical context
Pyrazoles as ligands are widely used for the synthesis of coordination compounds because of their rich coordinative flexibility (Trofimenko, 1972;Mukherjee, 2000;Monica & Ardizzoia, 2007;Halcrow, 2009;Viciano-Chumillas et al., 2010;Klingele et al., 2009). Numerous studies of the synthesis and structure of transition-metal complexes such as Cu, Fe, Co, Ni, and Zn with pyrazole ligands indicate such compounds exhibit promising properties (Evans et al., 2004;Kirthan et al., 2020;Govor et al., 2012;Kulkarni et al., 2011;Dias et al., 2020;Naik et al., 2016;Malinkin et al., 2012). For example, Cu II pyrazolebased complexes are very promising as antioxidants (Kupcewicz, Sobiesiak et al., 2013;Chkirate et al., 2019) and anticancer agents because of their cytotoxic activity (Kupcewicz, Ciolkowski et al., 2013;Aljuhani et al., 2021;Santini et al., 2014). Iron pyrazole-containing complexes have extraordinary electronic properties (Kulmaczewski et al., 2021;Olguín & Brooker, 2011) and catalytic activity in the hydrosilylation of organocarbonyl substrates (Lin et al., 2018). Cobalt complexes with pyrazole ligands are used as catalyst precursors for the peroxidative oxidation of cyclohexane (Silva et al., 2014) and have useful optical and photoluminescence properties (Direm et al., 2021). Zinc complexes with pyrazoles also exhibit antioxidative activity (Barta Holló et al., 2022) and have useful luminescent properties (Li et al., 2004;Singh et al., 2009). The study of the synthesis, structure and properties of nickel complexes with pyrazoles is also important. Nickel(II) pyrazolate complexes can be synthesized by the reaction between nickel(II) salts and pyrazoles in water or organic solvents (Nicholls & Warburton, 1970;Sun et al., 2002;Małecka et al., 2001;Chen et al., 2009). Nickel complexes incorporating pyrazole-based ligands are used for ethylene dimerization (Wang et al., 2015) or polymerization (Nelana et al., 2004;Moreno-Lara et al., 2015). Mononuclear nickel(II) coordination compounds with pyrazoles show anticancer activity. The cytotoxic and apoptotic effects of such compounds suggested that they could be good candidates for further pharmacological research in the field of the development of effective anticancer agents (Gogoi et al., 2019;Sobiesiak et al., 2011). There is also a report on the activation of some organonitriles by transition-metal centers, such as Ni, toward nucleophilic addition of pyrazole . Ni II complexes can activate the pyrazole-nitrile coupling reaction. As part of our continuing interest in multifunctional transition-metal complexes with pyrazole ligands, we report herein the synthesis and crystal structure of a new mononuclear octahedral nickel(II) coordination compound based on 4-methyl-1Hpyrazole.

Structural commentary
The title compound has a molecular crystal structure, which is built-up from neutral monomeric [NiCl 2 (4-MeHpz) 4 ] units ( Fig. 1) and acetonitrile as interstitial molecules in a 1:2 ratio. All the components of the structure are associated via intermolecular N-HÁ Á ÁN and C-HÁ Á ÁN hydrogen bonds. Intramolecular N-HÁ Á ÁN hydrogen bonding is also observed. The Ni II ion displays a distorted octahedral coordination environment formed by four pyridine-like nitrogen atoms of 4-MeHpz ligands in the equatorial positions with Ni1-N1 = 2.112 (2) Å and Ni1-N3 = 2.092 (2) Å bond distances and two Cl À anions in axial positions with an Ni1-Cl1 distance of 2.4581 (6) Å . Selected bond lengths and bond angles are given in Table 1. The orientation of the pyrazole ligands around the metal ion is different, as indicated by the plane-to-plane angles of pyrazole rings. Two pyrazole ring planes are almost perpendicular to the NiN 4 equatorial plane [86.6 (1) ] whereas two other pyrazole rings are less tilted [43.9 (1) ]. The complex has an NiCl 2 L 4 structure with a trans arrangement of the ligands and crystallographically imposed centrosymmetry.

Supramolecular features
The crystal structure is built up from the parallel packing of discrete supramolecular chains running along the a-axis direction with an NiÁ Á ÁNi separation of 6.9625 (4) Å . A perspective view of a chain is depicted in Fig. 2. Within the chain, the complex molecules interact through N-HÁ Á ÁCl research communications Table 1 Selected geometric parameters (Å , ).

Figure 1
The molecular structure of the title compound with displacement ellipsoids drawn at the 50% probability level. Hydrogen-bond parameters are given in Table 2. Symmetry codes:

Figure 2
View of the one-dimensional supramolecular architecture in the crystal structure of the title compound.
hydrogen bonds, while the association with the interstitial acetonitrile molecules occurs via N-HÁ Á ÁN hydrogen bonds. The geometric parameters of the hydrogen bonds are given in Table 2.

Hirshfeld surface analysis
The Hirshfeld surface analysis was performed and the associated two-dimensional fingerprint plots were generated using Crystal Explorer 17.5 software (Spackman et al., 2021), with a standard resolution of the three-dimensional d norm surfaces plotted over a fixed color scale of À0.3714 (red) to 2.0459 (blue) a.u. There are six red spots on the d norm surface (Fig. 3).
The dark-red spots arise from interatomic contacts less than the sum of the corresponding van der Waals radii 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 HÁ Á ÁH, HÁ Á ÁN/NÁ Á ÁH, HÁ Á ÁC/CÁ Á ÁH, and HÁ Á ÁCl/ClÁ Á ÁH contacts. The Hirshfeld surface representations with the function d norm , which were plotted onto the surface for interactions mentioned above, the overall two-dimensional fingerprint plot, and the decomposed two-dimensional fingerprint plots for the several interactions are given in Fig. 4. The most significant contributions to the overall crystal packing are from HÁ Á ÁH (62.1%), HÁ Á ÁN/NÁ Á ÁH (13.7%), HÁ Á ÁC/CÁ Á ÁH (13.4%), and HÁ Á ÁCl/ClÁ Á ÁH (10.1%). There is also a small contribution from weak ClÁ Á ÁC/CÁ Á ÁCl (0.2%) and CÁ Á ÁC (0.4%) intermolecular contacts. These contacts are not visible as red spots on the Hirshfeld surface. The HÁ Á ÁH contacts are located in the middle region of the two-dimensional fingerprint plot, while HÁ Á ÁCl/ClÁ Á ÁH contacts form sharp wings on the sides of the corresponding two-dimensional plot.
accordingly, have different crystal structures. In addition, all of the above pyrazole-based complexes with terminal chlorine ligands have similar geometric parameters. Finally, the central nickel atom has an octahedral geometric environment in all cases.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. H atoms were placed in calculated positions [C-N = 0.86 Å , C-H = 0.93 Å (0.96 Å for Cmethyl)] and refined as riding with U iso (H) = 1.2U eq (C,N) or 1.5U eq (C-methyl). Reflections with (ÁF 2 /esd) > 10 were omitted from the refinement.

Funding information
Funding for this research was provided by: Ministry of Education and Science of Ukraine (grant No. 22BF037-09).

Dichloridotetrakis(4-methyl-1H-pyrazole-κN 2 )nickel(II) acetonitrile disolvate
Crystal data where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 0.57 e Å −3 Δρ min = −0.45 e Å −3 Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.