Synthesis, crystal structure and Hirshfeld and thermal analysis of bis[benzyl 2-(heptan-4-ylidene)hydrazine-1-carboxylate-κ2
 N
 2,O]bis(thiocyanato)nickel(II)

The molecular and crystal structure of bis[benzyl 2-(heptan-4-ylidene)hydrazine-1-carboxylate-κ2 N 2,O]bis(thiocyanato)nickel(II) is reported. Hirshfeld surface and simultaneous TGA–DTA analyses are also described


Chemical context
Investigations of the Schiff base complexes of benzyl carbazate are scarce except for our own reports (Nithya et al., 2016(Nithya et al., , 2017a(Nithya et al., ,b, 2018a. These complexes are formed by Schiff base carbazate ligands in their keto form with N,O chelation to give complexes with octahedral geometry. The coordination chemistry of benzyl carbazate Schiff base complexes has gained importance not only from the inorganic point of view, but also because of their biological and thermal properties. In the course of our recent studies on such complexes, we reported the cobalt(II) complex of a Schiff base derived from benzyl carbazate and heptan-4-one with thiocyanates as the charge-compensating ligands (Nithya et al., 2019). In this work, we report the synthesis, molecular and crystal structures, Hirshfeld surface analysis and thermal properties of the corresponding nickel complex, bis[benzyl-2-(heptan-4-ylidene)hydrazine-1-carboxylate]bis(thiocyanato)nickel(II), 1. ISSN 2056-9890

Structural commentary
The title compound, 1, crystallizes in the space group P2 1 /c with one half of the complex in the asymmetric unit as the Ni II cation lies on an inversion centre, Fig. 1. This contrasts with the previously determined Co II analogue (Nithya et al., 2019) that crystallizes with two unique, centrosymmetric complex molecules in the asymmetric unit. Two inversion-related intramolecular C13-H13AÁ Á ÁO1 hydrogen bonds, Table 1, influence the conformation of the benzyl-2-(heptan-4-ylidene)hydrazine-1-carboxylate ligands and enclose R 2 2 (14) ring motifs. Two hydrazine-carboxylate ligands chelate the Ni atom with N1 and O1 donor atoms; these chelating ligands lie trans to one another in the equatorial plane of the slightly distorted octahedral complex. The axial positions are occupied by two thiocyanato ligands bound to the metal through their N3 atoms. The NCS ligands are kinked away from the alkane chains of the other ligands with C16-N3-Ni1 angles of 163.23 (11) . Bond lengths and angles in the closely related Ni and Co complexes are generally similar, although the Ni1-N1 bond [2.1332 (12) Å ] is significantly shorter here than the corresponding  and 2.248 (6) Å respectively].

Supramolecular features
In the crystal structure, atom S1 acts as a trifurcated acceptor forming N2-H2NÁ Á ÁS1 and weaker C8-H8Á Á ÁS1 and C10-     The molecular structure of 1 showing the atom numbering with ellipsoids drawn at the 50% probability level. Labelled atoms are related to unlabelled atoms by the symmetry operation Àx + 1, Ày, Àz + 2. Intramolecular hydrogen bonds are shown as dashed black lines.

Figure 4
Chains of molecules of 1 along a. C-HÁ Á Á contacts are drawn as dashed magenta lines with the centroids (Cg) of the C3-C8 rings shown as magenta spheres.

Figure 5
Overall packing of 1 viewed along the b-axis direction.
H10AÁ Á ÁS1 hydrogen bonds, Table 1, that form chains of complex molecules along the bc diagonal, Fig. 2. Inversionrelated pairs of C10-H10BÁ Á ÁS1 hydrogen bonds link adjacent molecules into rows along the b-axis direction, Fig. 3, while rows also form along a, through C2-H2AÁ Á ÁCg3, C-HÁ Á Á contacts, Fig. 4; Cg3 is the centroid of the C3-C8 phenyl ring. These contacts combine to stack molecules of the complex in a regular fashion along the b-axis direction, Fig. 5.

Hirshfeld surface analysis
Further details of the intermolecular interactions in 1 were obtained using Hirshfeld surface analysis (Spackman & Jayatilaka, 2009) with Hirshfeld surfaces and two-dimensional fingerprint plots generated with CrystalExplorer17 (Turner et al., 2017). Hirshfeld surfaces for opposite faces of 1 are shown in Fig. 6(a) and (b). Bold red circles on the Hirshfeld surfaces correspond to the N-HÁ Á ÁS hydrogen bonds while the weaker C-HÁ Á ÁS and C-HÁ Á Á contacts appear as faint red circles. Fingerprint plots, Fig. 7, reveal that while HÁ Á ÁH interactions make the greatest contributions to the surface contacts, as would be expected for a molecule with such a predominance of H atoms, HÁ Á ÁC/CÁ Á ÁH and HÁ Á ÁS/SÁ Á ÁH contacts are also substantial, Table 2. HÁ Á ÁN/NÁ Á ÁH and HÁ Á ÁO/OÁ Á ÁH contacts are less significant, with the OÁ Á ÁC/CÁ Á ÁO and OÁ Á ÁS/SÁ Á ÁO contacts being essentially trivial with contributions of 0.7% and 0.6%, respectively. These are not shown in Fig. 7 but are included in Table 3 Table 1 Hydrogen-bond geometry (Å , ).

Figure 6
Hirshfeld surfaces for opposite faces (a) and (b) of 1 mapped over d norm in the range À0.3928 to 2.1718 a.u. Cg3 is the centroid of the C3-C8 phenyl ring.

Figure 7
A full two-dimensional fingerprint plot for 1, (a), together with separate principal contact types for the molecule (b)-(f). These were found to be HÁ Á ÁH, HÁ Á ÁC/CÁ Á ÁH, HÁ Á ÁS/SÁ Á ÁH, HÁ Á ÁN/NÁ Á ÁH and HÁ Á ÁO/OÁ Á ÁH contacts. Simultaneous TGA-DTA analyses were recorded in air on a Perkin-Elmer SII Thermal Analyser over the temperature range 50-800 C. With the equipment used here, the TGA curve shows the temperature range but not the individual peak temperatures. However, peak temperatures can be seen in the DTA curve. In the first step of decomposition, the weight loss of 74% occurs over the temperature range 115-260 C (TGA). This corresponds to the loss of the Schiff base ligands to form Ni II thiocyanate as an intermediate. This was marked by both endothermic (170 C) and exothermic peaks (190 and 210 C) in the DTA curve. As the thermal analysis was carried out under a dynamic flowing air atmosphere, the S and N atoms are oxidized to SO 2 and NO 2 , while nickel ultimately forms nickel oxide. Similar decomposition processes have been observed in our recent wok on numerous similar complexes, see for example (Nithya et al., 2017a(Nithya et al., ,b, 2018a(Nithya et al., ,b, 2019a).

Database survey
As mentioned previously, the most closely related structure to the one reported here is that of the Co II analogue (Nithya et al. 2019) while we have also reported the structures of 18 other Schiff base complexes of various transition metals with ligands based on benzyl carbazate (Nithya et al. 2016(Nithya et al. , 2017a(Nithya et al. ,b, 2018a. A search in the Cambridge Structural Database (version 5.41, November 2019; Groom et al., 2016) for other related transition-metal complexes produced no additional hits. The novelty of the ligands found in these complexes is reinforced by the fact that a search for organic compounds incorporating the PhCH 2 OC(O)NHN C(CH 2 ) 2 unit produced only two hits. One was our own report of the ligand benzyl 2-cyclopentylidenehydrazinecarboxylate (JENFAM; Nithya et al., 2017a). The other was (2E)-1-ethyl 8-methyl 7-(2-(benzyloxycarbonyl)hydrazono)oct-2-enedioate, (VEWMOA; Gergely et al., 2006). In both cases, the bond distances and angles in the structures compare very favourably with those reported here.

Synthesis and crystallization
Equimolar amounts of ammonium thiocyanate (0.076 g, 1 mmol) and benzyl carbazate (0.166 g, 1 mmol) were dissolved in methanol (10 mL). Nickel nitrate, Ni(NO 3 ) 2 Á6H 2 O, (0.146 g, 0.5 mmol) dissolved in 10 mL of doubly distilled water was added to this solution. The resulting blue solution was layered with heptan-4-one (dipropyl ketone) and the solution changed to a green colour. The final solution was left to evaporate at room temperature. After slow evaporation, bluish-green rhombus-shaped crystals suitable for X-ray diffraction analysis were collected, washed with doubly distilled water and air-dried.
The FT-IR spectrum was recorded on a JASCO-4100 FT-IR spectrophotometer from 4000 to 400 cm À1 using KBr pellets: N-H stretch 3152 cm À1 C O stretch 1675 cm À1 C N stretch 1524 cm À1 , N-N stretch 1058 cm À1 . 2108 cm À1 C N stretch of the N-bound thiocyanate ligands.
Simultaneous TGA-DTA analyses were recorded in air on a PerkinElmer SII Thermal Analyser over the temperature range 50-800 C.

Bis[benzyl 2-(heptan-4-ylidene)hydrazine-1-carboxylate-κ 2 N 2 ,O]bis(thiocyanato)nickel(II)
Crystal data 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. Refinement. One reflection with Fo >>> Fc was omitted from the final refinement cycles.