Bis(N′-{(E)-[(2E)-1,3-diphenylprop-2-en-1-ylidene]amino}-N-ethylcarbamimidothioato-κ2 N′,S)zinc(II): crystal structure and Hirshfeld surface analysis

The title thiosemicarbazonate complex has the ligands coordinating the ZnII centre via the thiolate S and imine N atoms in each of the two independent molecules comprising the asymmetric unit, leading to N2S2 donor sets and distorted tetrahedal geometries. The crystal features zigzag chains of molecules sustained by N—H⋯N and amine-N—H⋯S hydrogen bonds.

The title Zn II complex, [Zn(C 18 H 18 N 3 S) 2 ], (I), features two independent but chemically equivalent molecules in the asymmetric unit. In each, the thiosemicarbazonate monoanion coordinates the Zn II atom via the thiolate-S and imine-N atoms, with the resulting N 2 S 2 donor set defining a distorted tetrahedral geometry. The five-membered ZnSCN 2 chelate rings adopt distinct conformations in each independent molecule, i.e. one ring is almost planar while the other is twisted about the Zn-S bond. In the crystal, the two molecules comprising the asymmetric unit are linked by amine-N-HÁ Á ÁN(imine) and amine-N-HÁ Á ÁS(thiolate) hydrogen bonds via an eight-membered heterosynthon, {Á Á ÁHNCNÁ Á ÁHNCS}. The dimeric aggregates are further consolidated by benzene-C-HÁ Á ÁS(thiolate) interactions and are linked into a zigzag supramolecular chain along the c axis via amine-N-HÁ Á ÁS(thiolate) hydrogen bonds. The chains are connected into a three-dimensional architecture via phenyl-C-HÁ Á Á(phenyl) andinteractions, the latter occurring between chelate and phenyl rings [inter-centroid separation = 3.6873 (11) Å ]. The analysis of the Hirshfeld surfaces calculated for (I) emphasizes the different interactions formed by the independent molecules in the crystal and the impact of theinteractions between chelate and phenyl rings.

Chemical context
Thiosemicarbazone molecules, derived from thiosemicarbazide, H 2 N-NH-C( S)-NH 2 , constitute an important class of mixed hard-soft, nitrogen-sulfur donor ligands which have been extensively investigated in their coordination chemistry towards both transition metals (Lobana et al., 2009) and main group elements (Casas et al., 2000). Complexes of thiosemicarbazones, including Zn II complexes (Da Silva et al., 2013), have been evaluated variously as potential anti-cancer (Afrasiabi et al., 2003), anti-viral (Garoufis et al., 2009) and anti-bacterial (Quiroga & Ranninger, 2004) therapeutics for over 50 years (Dilworth & Hueting, 2012). The interesting properties of their metal complexes, such as structural diversity, accessible redox activities, the ability to fine-tune ligand substitution, access to radical species, catalytic properties, distinct spectroscopic properties, etc. afford them many potential advantages over organic-based drugs (van Rijt & Sadler, 2009;Meggers, 2009). Recent studies have focused upon their suitability as single-source precursors for ZnS nanomaterials (Pawar et al., 2017). Thiosemicarbazones can exist as thione-thiol tautomers and can bind to a metal centre in neutral or anionic forms as monodentate, bidentate or bridging ligands (Viñ uelas-Zahínos et al., 2011). The presence of additional, suitably positioned donor atoms can increase their coordination ability/denticity, giving rise to different coordination geometries, such as tetrahedral, octahedral and pentagonal-bipyramidal. (Umamatheswari et al., 2011). As part of a programme investigating thiosemicarbazones and their metal complexes (Tan et al., 2015), the crystal and molecular structures of the title compound (I) are described, complemented by an analysis of the Hirshfeld surface.

Structural commentary
Two independent molecules comprise the asymmetric unit of (I), and these are illustrated in Fig. 1. The mono-anion derived from the thiosemicarbazone ligand is chelating, coordinating the Zn II atom via the thiolate-S and imine-N atoms. Referring to Table 1, the Zn-S bond lengths in the molecules span a narrow range of just over 0.01 Å , i.e. 2.2688 (5) Å for Zn1-S2, to 2.2827 (6) Å for Zn1-S1, whereas the Zn-N bonds show more variability, spanning a range of over 0.02 Å , i.e. 2.0496 (15) Å for Zn2-N12, to 2.0727 (16) Å for Zn2-N9. The similarity in bond lengths extends to the angles subtended at the Zn II atoms which, for the Zn1-containing molecule range from 87.00 (5) for S2-Zn1-N6, to 134.00 (5) for S2-Zn1-N3, i.e. a range of 47 ; the acute angle is associated with the chelate angle. A slightly narrower range is noted for the Zn2-containing molecule, i.e. 85.99 (5) for S3-Zn2-N9, to 131.29 (5) for S3-Zn2-N12, i.e. about 45 . The assignment of four-coordinate geometries can be quantified by the values of 4 , which range from 1.00 for an ideal tetrahedron to 0.00 for perfect square-planar geometry (Yang et al., 2007). The values of 4 in (I) compute to 0.70 and 0.74 for the Zn1and Zn2-containing molecules, respectively, indicating significant distortions from the ideal tetrahedral angles. The conformation about each of the imine C N bonds is E, as are the conformations about the ethylene bonds, The molecular structures of the two molecules comprising the asymmetric unit of (I) showing the atom-labelling scheme and displacement ellipsoids at the 70% probability level.  Fig. 2. Some physical properties for the two independent molecules in (I), calculated in Crystal Explorer (Wolff et al., 2012) and PLATON (Spek, 2009), are included in Table 3. These data indicate small but significant differences between the independent molecules, most notably, the Zn1-containing molecule is less spherical than the Zn2-containing molecule.

Supramolecular features
The most prominent feature of the molecular packing is the formation of an eight-membered heterosynthon, {Á Á ÁHNCNÁ Á ÁHNCS}, mediated by amine-N-HÁ Á ÁN(imine)   A comparison of some physical properties of the independent molecules comprising the asymmetric unit of (I).
In the present case, the inter-centroid separation between rings is 3.6873 (11) Å and the angle between rings is 7.89 (9) ; symmetry operation: Àx, 1 À y, 1 À z. Additional interactions between chains are of the type phenyl-C-HÁ Á Á(phenyl) involving residues of the Zn1-containing molecule exclusively, Table 4. The result of the identified intermolecular interactions is the formation of a three-dimensional architecture, Fig. 3c.

Analysis of the Hirshfeld surfaces
The Hirshfeld surface calculations of (I), and for each of the Zn1-and Zn2-molecules, were performed according to a recent publication on related dithiocarbamate ligands (Jotani et al., 2016). From the views of the Hirshfeld surfaces mapped over d norm in Fig. 4a and e, the bright-red spots near the amine-H1N, H7N, H10N, imime-N2 and thiolate-S2 and S3 atoms indicate their participation in N-HÁ Á ÁN and N-HÁ Á ÁS bonds between the two independent molecules. In the views of the Hirshfeld surfaces mapped over electrostatic potential for the Zn1-molecule in Fig. 4b and c, and for the Zn2-molecule in Fig. 4f and g, the hydrogen-bond donors and acceptors are represented by blue and red regions, respectively. Greater insight into intermolecular interactions in the crystal can be obtained by modifying the mapping range for d norm , as shown in Fig. 4d and h, which reveals additional characteristic spots on the surface. A pair of red spots near amine-HN4 and near phenyl-C7 and C8 in Fig. 4d indicate the presence of short inter-atomic CÁ Á ÁH/HÁ Á ÁC contacts in the crystal, see Table 5 for data. The tiny, faint-red spots present near the amine-N1 and N7, phenyl-C32, C66 and C77, thiolate-S3, ethene-C5 and H6 atoms reflect the short inter-atomic CÁ Á ÁN, CÁ Á ÁS and CÁ Á ÁH contacts, Table 5. The comparatively weak C-HÁ Á ÁS interaction influential between the atoms of the independent molecules is represented by faint-red spots near atoms H11 and S3 in Fig. 4a and e, respectively. The immediate environments about the Zn1-and Zn2-molecules within shapeindex-mapped Hirshfeld surfaces highlighting hydrogenbonding and C-HÁ Á Á interactions are illustrated in Fig   Hydrogen-bond geometry (Å , ).
The short inter-atomic HÁ Á ÁH contacts for Zn1-and Zn2molecules, Table 5, results in the peak at d e + d i $2.2 Å , appearing broader for the former and narrower for the latter molecule in Fig. 6b. In the fingerprint plot delineated into SÁ Á ÁH/HÁ Á ÁS contacts, Fig. 6c, the distinct distribution of the points such as the well separated donor-acceptor regions for the Zn1-molecule and the adjoining regions for the Zn2molecule are entirely consistent with the different patterns of contacts formed by these. A pair of thin spikes at d e + d i $2.7 Å in the respective fingerprint plots in the donor and acceptor regions for the Zn1-and Zn2-molecules represents the N-HÁ Á ÁS hydrogen bond linking the two independent molecules. This pair of spikes disappears in the plot for the overall system. Another N-HÁ Á ÁS hydrogen bond is recognized in the plots as differently shaped donor-acceptor regions of the Zn1-and Zn2-molecules with their tips at d e + d i $2.6 Å . As the contribution from SÁ Á ÁH/HÁ Á ÁS contacts to the Hirshfeld surfaces of the Zn1-and Zn2-molecules involves N-HÁ Á ÁS hydrogen bonds and comparatively weak C-HÁ Á ÁS interactions, the percentage contribution from these contacts to the Hirshfeld surface of the overall system is reduced to 8.5% due to disappearance of points corresponding to interlinking N-HÁ Á ÁS hydrogen bond. In Fig. 6d, a pair of spikes at d e + d i $2.1 Å in the acceptor and donor regions of the Zn1and Zn2-molecules, respectively, results from the linking N-HÁ Á ÁN hydrogen bond between the independent molecules; the spikes disappear in the plot for the overall system. The greater contribution, i.e. 24.1%, from CÁ Á ÁH/HÁ Á ÁC contacts to the Hirshfeld surface for the Zn1-molecule cf. 17.3% for the Zn2-molecule is due to the greater involvement of atoms of the Zn1-molecule in C-HÁ Á Á interactions and short inter-atomic CÁ Á ÁH/HÁ Á ÁC contacts, Table 5. In the fingerprint plot delineated into CÁ Á ÁH/HÁ Á ÁC contacts for the Zn1 molecule, Fig. 6e, a pair of forceps-like tips at d e + d i $2.6 Å represent a short inter-atomic CÁ Á ÁH contact formed between the phenyl-C7 and amino-H7N atoms,  Table 6 Percentage contributions of inter-atomic contacts to the Hirshfeld surfaces for the Zn1-molecule, Zn2-molecule and (I).
Contact distance symmetry operation 2.87 Àx, 2 À y, 1 À z d e + d i $2.8 Å reflect the short inter-atomic CÁ Á ÁH contact between the phenyl-C51 and -H70 atoms, with the other short contacts merged within the plot. In Fig. 6f, the short interatomic CÁ Á ÁN contacts between atoms of the Zn1-and Zn2molecules, Table 5, appear as a pair of short spikes with their tips at d e + d i $3.2 Å . The small contributions from CÁ Á ÁC contacts for the Zn1-and Zn2-molecules and for the overall system, Fig. 6g, suggests little impact on the molecular packing.
The presence of a short inter-atomic CÁ Á ÁS contact between the thiolate-S3 and phenyl-C66 atoms is evident from the typical H-shaped plot in Fig. 7a and makes a contribution of 0.6% to the Hirshfeld surface of the Zn2-molecule. In the fingerprint plot delineated into ZnÁ Á ÁH/HÁ Á ÁZn contacts, Fig. 7b, the tips at d e + d i < 3.45 Å with the shape of a folded sheet with a low density of points indicate the short contacts between these atoms. The presence ofstacking between chelate ring Zn2/S4/C55/N11/N12 and phenyl (C61-C66) rings of Zn2-molecules is evident from the presence of short interatomic ZnÁ Á ÁC/CÁ Á ÁZn contacts, Table 5. In the fingerprint plot delineated into ZnÁ Á ÁC/CÁ Á ÁZn contacts, Fig. 7c, these contacts are reflected by a pair of points with an S-shaped distribution at around d e + d i $1.9 to 2.1 Å . Thisstacking is also apparent from the small but effective contributions from ZnÁ Á ÁC/CÁ Á ÁZn and CÁ Á ÁN/NÁ Á ÁC contacts to the Hirshfeld surface of the Zn2-molecule, Table 6.

Database survey
An analysis of the Cambridge Crystallographic Database (Groom et al., 2016) indicates there are nine literature precedents for the structure of (I), i.e. of general formula Zn[SC(NHR) NN CR 0 R 00 ] 2 reflecting the interest in this class of compound. All of the structures resemble the molecular geometry described above for (I). The substituents at the hydrazone-C atom can be equivalent and alkyl, i.e. R 0 = R 00 = Me for the R = Ph compound (Tan et al., 2009), or aryl, i.e. R 0 = R 00 = Ph for the R = 3-FPh compound (Ferraz et al., 2012) or mixed alkyl/aryl, i.e. R 0 = Me and R 00 = Ph for the R = Ph compound (Wang et al., 2009); the latter structure has two molecules in the asymmetric unit. The R 0 and R 00 groups can be part of a ring, e.g. cyclohexyl in the structure with R = Me (Vikneswaran et al., 2016). In most examples, the N-bound group is aryl with the exceptions being the aforementioned structure and the cyclopentyl analogue (Vikneswaran et al., 2016). Clearly, there is immense scope for derivatization of these species which may assist in the optimization of their biological properties.

Synthesis and crystallization
Analytical grade reagents were used as procured without further purification. Equimolar quantities of 4-ethyl-3-thiosemicarbazide (1.1919 g, 0.01 mol) and 1,3-diphenylprop-2-en-1-one (2.0826 g, 0.01 mol) were dissolved in heated absolute ethanol (30 ml) separately and the mixtures were mixed with stirring. About five drops of concentrated hydrochloric acid were added to the mixture to catalyse the reaction. The reaction mixture was kept under heating and stirring for about 10 mins, followed by stirring for 1 h at room temperature. The resulting yellow precipitate was filtered off, washed with chilled absolute ethanol and dried in vacuo. The resulting precipitate, N-ethyl-N-(1,3-diphenyl-2-propen-1-one)thiosemicarbazide (0.3090, 0.01 mol), was used without further purification and was dissolved in heated absolute ethanol (50 ml). Zn(CH 3 COO) 2 Á2H 2 O (0.1098 g, 0.50 mmol) was dissolved separately in heated absolute ethanol (30 ml) and then added into an ethanolic N-ethyl-N-(1,3-diphenyl-2propen-1-one)thiosemicarbazide solution. The mixture was heated and stirred for about 10 mins, followed by stirring for 1 h at room temperature. The obtained yellow precipitate was filtered, washed with cold ethanol and dried in vacuo. Single crystals were grown at room temperature from the slow evaporation of a solution of dimethylformamide and acetonitrile (1:1 v/v 20 ml).
included in the refinement in the riding-model approximation, with U iso (H) set to 1.2-1.5U eq (C). The nitrogen-bound H atoms were located in a difference-Fourier map but were refined with a distance restraint of N-H = 0.88AE0.01 Å , and with U iso (H) set to 1.2U eq (N).  (Gans & Shalloway, 2001) and DIAMOND (Brandenburg, 2006); software used to prepare material for publication: publCIF (Westrip, 2010).

Bis(N′-{(E)-[(2E)-1,3-diphenylprop-2-en-1-ylidene]amino}-N-ethylcarbamimidothioato-κ 2 N′,S)zinc(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.