Bis{4-methylbenzyl 2-[4-(propan-2-yl)benzylidene]hydrazinecarbodithioato-κ2 N 2,S}nickel(II): crystal structure and Hirshfeld surface analysis

Two N,S-chelating hydrazinecarbodithioate ligands provide a trans-N2S2 donor set and a distorted square-planar geometry for the NiII atom. In the crystal, a three-dimensional network is sustained by C—H⋯π and π–π interactions.

Schiff bases that react with different metal ions often show different types of coordination modes. Metal complexes are versatile molecules with a wide range of pharmacological properties due to the inherent characteristics of both the ISSN 2056-9890 central metal atoms and ligands (Meggers, 2009). Various transition metal complexes have been reported to induce DNA cleavage by attacking the sugar or base moieties of DNA through the formation of reactive oxygen species (ROS) (Burrows & Muller, 1998). A nickel(II) bis-dithiocarbazate complex has been used in the photo-catalytic production of hydrogen as a catalyst (Wise et al., 2015). Nickel(II) dithiocarbazate has also been reported to have non-linear optical (NLO) properties (Liu et al., 2016) with the potential to be used in signal processing (Bort et al., 2013;Hales et al., 2014), ultrafast optical communication, data storage, optical limiting (Price et al., 2015;Bouit et al., 2007), optical switching (Gieseking et al. 2014;Thorley et al., 2008), logic devices and bio-imaging (Ahn et al., 2012;Zhu et al., 2016). In line with our interest in evaluating the structures of different isomeric dithiocarbazate Schiff bases and their metal complexes, we report herein the synthesis of the title complex, (I), its X-ray crystal structure determination and a detailed study of the supramolecular association by an analysis of its Hirshfeld surface.

Structural commentary
The Ni II atom in (I), Fig. 1, is located on a crystallographic centre of inversion and is coordinated by two S,N-chelating hydrazinecarbodithioate anions. From symmetry, the resulting N 2 S 2 donor set has like atoms trans, and the square-planar coordination geometry is strictly planar. Distortions from the ideal geometry are related to the deviations of angles subtended at nickel by the donor atoms, Table 1. The C1-N1-N2-C2 backbone of the ligand exhibits a twist as seen in the value of the torsion angle, i.e. À165.61 (17) . Despite being involved in a formal bond to the Ni II atom, the C1-S1 bond length of 1.7296 (19) Å is still significantly shorter than those formed by the S2 atom, i.e. C1-S2 = 1.7479 (18) Å and C12-S2 = 1.824 (2) Å .
The planarity of the N 2 S 2 donor set does not extend to the five-membered chelate ring, which has an envelope conformation with the nickel atom lying 0.465 (2) Å above the leastsquares plane through the remaining atoms [r.m.s. deviation = 0.0016 Å ]. The sequence of C1 N1, N1-N2 and N2 C2 bond lengths of 1.294 (2), 1.408 (2) and 1.300 (2) Å , respectively, suggests limited conjugation across this residue. Each of the benzene rings of the S-and N-bound substituents is twisted with respect to the least-squares plane through the chelate ring. Thus, a nearly orthogonal relationship exists between the chelate and p-tolyl rings, with the dihedral angle 398 Yusof et al. [Ni(C 19  The molecular structure of (I), showing the atom-labelling scheme and displacement ellipsoids at the 70% probability level. The Ni II atom is situated on a centre of inversion. Unlabelled atoms are related by the symmetry operation (1 À x, 1 À y, 1 À z).  Hydrogen-bond geometry (Å , ).

Figure 2
The molecular packing in (I): a view of the unit-cell contents shown in projection down the b axis. Theand C-HÁ Á Á interactions are shown as orange and purple dashed lines, respectively. being 89.72 (5) . Less dramatic is the twist of the i Pr-substituted ring with the dihedral angle being 13.83 (9) . The dihedral angle between the aromatic rings is 84.31 (6) .

Supramolecular features
The two sites potentially available for hydrogen bonding in (I), i.e. the S1 and N1 atoms, are involved in intramolecular interactions, Table 2. The only discernible contacts in the crystal involve -systems (Spek, 2009). Thus, each of the independent rings is involved in C-HÁ Á Á contacts, i.e. ptolyl-C-HÁ Á Á( i Pr-benzene) and i Pr-benzene-C-HÁ Á Á(ptolyl) contacts, Table 2. In addition, centrosymmetrically related p-tolyl rings self-associate via face-to-face, -, interactions [inter-centroid distance = 3.8051 (12) Å for symmetry operation Àx, À1 À y, 1 À z], indicating the p-tolyl ring participates in two distinct interactions. The result of the supramolecular association is the formation of a threedimensional architecture, Fig. 2.

Analysis of the Hirshfeld surfaces
The Hirshfeld surface analysis for (I) was performed as described in a recent publication of a heavy-atom structure (Mohamad et al., 2017). The non-appearance of characteristic red spots on the Hirshfeld surface mapped over d norm ( A view of the Hirshfeld surface for (I) mapped over the electrostatic potential over the range AE0.025 au.

Figure 4
The view of the Hirshfeld surface mapped over d e . The bright-orange spots near rings indicate their involvement in C-HÁ Á Á interactions.

Figure 5
Two views (a) and (b) of the Hirshfeld surface mapped with shape-index property about a reference molecule. The C-HÁ Á Á and Á Á ÁH-C interactions in both views are indicated with red and white dotted lines, respectively. The blue dotted lines in (a) indicatestacking between ptolyl rings.
appearing near i Pr-benzene and p-tolyl rings on the Hirshfeld surface mapped over d e , Fig. 4. The immediate environment about a reference molecule within the Hirshfeld surface mapped with shape-index property is illustrated in Fig. 5. The C-HÁ Á Á and their reciprocal contacts, i.e. Á Á ÁH-C contacts, between i Pr-H11B and the p-tolyl ring are represented by red and white dotted lines, respectively in Fig. 5a; the blue dotted lines in Fig. 5a representstacking between p-tolyl rings at Àx, À1 À y, 1 À z. The other C-HÁ Á Á contacts involving p-tolyl-H17 and i Pr-benzene rings are illustrated in Fig. 5b. The overall two-dimensional fingerprint plot and those delineated into HÁ Á ÁH, CÁ Á ÁH/HÁ Á ÁC, SÁ Á ÁH/HÁ Á ÁS and NÁ Á ÁH/HÁ Á ÁN and CÁ Á ÁC contacts (McKinnon et al., 2007) illustrated in Fig. 6a-f. From the quantitative summary of the relative contributions of the various interatomic contacts given in Table 3, it is important to note the dominant contribution of hydrogen atoms to the Hirshfeld surface, i.e. 95.3%. In the fingerprint plot delineated into HÁ Á ÁH contacts. Fig. 6b, the points are distributed in the major part of the plot, but they do not make significant contributions to the molecular packing as their interatomic separations are greater than sum of their van der Waals radii, i.e. d e + d i > 2.4 Å . The presence of short interatomic CÁ Á ÁH/HÁ Á ÁC contacts, see Table 4, and C-HÁ Á Á interactions contribute to the second largest contribution to the Hirshfeld surface, i.e. 22.2%. This is consistent with the fingerprint plot, Fig. 6c, where the short interatomic CÁ Á ÁH/HÁ Á ÁC contacts appear as a pair of small peaks at d e + d i $ 2.8 Å and also as the blue regions around the participating hydrogen atoms, namely H5 and H10B, on the Hirshfeld surface mapped over electrostatic potential, Fig. 3. The involvement of the chelating S1 and N1 atoms in intramolecular interactions, Table 2, prevents them from forming intermolecular SÁ Á ÁH/HÁ Á ÁS and NÁ Á ÁH/HÁ Á ÁN contacts. However, the symmetrical distribution of points with the usual characteristics in their respective plots, Fig. 6d and e, indicate meaningful contributions to the Hirshfeld surface, Table 3. A small, i.e. 2.1%, but recognizable contribution from CÁ Á ÁC contacts to the Hirshfeld surface is ascribed tostacking interactions between symmetry-related p-tolyl rings, and appear as an arrow-like distribution of points around Table 4 Short interatomic contacts in (I).

Figure 6
The two-dimensional fingerprint plots for (  Simplified molecular structure diagrams of (I)-(IV). All C atoms, except those of the C-N-N-C backbone, are represented as small black spheres and H atoms have been omitted. d e = d i 1.9 Å in Fig. 6f. The other contacts have low percentage contributions to the surface and are likely to have negligible effects on the molecular packing, Table 3.

Database survey
There are three closely related nickel(II) dithiocarbazate complexes in the crystallographic literature (Groom et al., 2016); these are illustrated in simplified form in Fig. 7. Complex (II) differs from (I) only in the nature of the terminal substituents (Tan et al., 2012). Despite there being only small differences in chemical composition, a distinct coordination geometry is observed, with the Ni II atom located on a twofold rotation axis and the N 2 S 2 donor set having cis-dispositions of like atoms. In (III), with a formal link between the two imine functionalities, the cis-N 2 S 2 arrangement is imposed by the geometric requirements of the bis(dithiocarbazate) di-anion (Zhou et al., 2002). The molecular structure of (IV), again with a cis-N 2 S 2 donor set, appears to indicate that steric effects do not preclude a cis-N 2 S 2 coordination geometry (Liu et al., 2000). With the foregoing in mind, it appears that the molecular structure of (I) is unprecedented, suggesting further systematic investigations in this area are warranted.

Synthesis and crystallization
The S-4-methylbenzyldithiocarbazate (S4MDTC) precursor was synthesized by following a procedure adapted from the literature (Omar et al., 2014). The Schiff base was also synthesized using a procedure adapted from the literature (Yusof et al., 2015b) by the reaction of S4MDTC (2.12 g, 0.01 mol), dissolved in hot acetonitrile (100 ml), with an equimolar amount of 4-isopropylbenzaldehyde (1.48 g, 0.01 mol) in absolute ethanol (20 ml). The mixture was then heated at 353 K until half of the mixture solution reduced and allowed to cool to room temperature until a precipitate formed. The compound was recrystallized from ethanol solution and dried over silica gel. The synthesized Schiff base (0.33 g, 1 mmol) was dissolved in hot acetonitrile (50 ml) and added to nickel(II) acetate tetrahydrate (0.13 g, 0.5 mmol) in an ethanolic solution (30 ml). The mixture was heated and stirred to reduce the volume of the solution. Precipitation occurred once the mixture cooled to room temperature. The precipitate then was filtered and dried over silica gel. The complex was recrystallized from its methanol solution. Brown prismatic crystals were formed from the filtrate after being left to stand for a month. The crystals were filtered and washed with absolute ethanol at room temperature. Yield: 70%. M.p.: 479-480 K.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 5. The carbon-bound H-atoms were placed in calculated positions (C-H = 0.95-0.99 Å ) and were included in the refinement in the riding-model approximation, with U iso (H) set to 1.2-1.5U eq (C). Yusof et al. [Ni(C 19 H 21 N 2 S 2 ) 2 ] 401 Table 5 Experimental details.

Bis{4-methylbenzyl 2-[4-(propan-2-yl)benzylidene]hydrazinecarbodithioato-κ 2 N 2 ,S}nickel(II):
Crystal data [Ni(C 19  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.