Bis(4-methoxychalcone 4-ethylthiosemicarbazonato-κ2 N 1,S)zinc(II): crystal structure and Hirshfeld surface analysis

The title thiosemicarbazone compound features tetrahedrally coordinated ZnII atoms within N2S2 donor sets because of the presence of chelating thiosemicarbazone anions. Supramolecular chains are found in the crystal owing to the formation of thioamide-N—H⋯S(thione) hydrogen bonds and eight-membered thioamide {⋯HNCS}2 synthons.


Chemical context
With potentially five different substituents, thiosemicarbazone derivatives, R 1 R 2 C N-N(R 3 )-C( S)NR 4 R 5 for R 1-5 = H/alkyl/aryl, are numerous and multi-functional. Their preparation is often facile, being formed from the condensation reaction between an aldehyde (or a ketone) with the amine group of a thiosemicarbazide precursor. In the same way, the diversity in ligand construction ensures a rich coordination chemistry (Lobana et al., 2009). A primary motivation for investigating metal complexes of thiosemicarbazones and related derivatives rests with their putative biological activity (Espíndola et al., 2015;Pelivan, et al., 2016;Low et al., 2016;Bisceglie et al., 2018). Thus, promising activity has been exhibited by various metal complexes against a range of diseases (Dilworth & Hueting, 2012). In the context of the present report, it is noteworthy that Zn II thiosemicarbazone complexes have been explored as therapeutics for the treatment of cancer (Afrasiabi et al., 2003), viral diseases (Garoufis et al., 2009) and bacterial infections (Quiroga & Ranninger, 2004). Such considerations motivate our interest in this class of compound (Yusof et al., 2015). Herein, in ISSN 2056-9890 continuation of our structural studies of Zn II thiosemicarbazones (Tan et al., 2017), the X-ray crystal structure of the title compound, (I), is described along with an analysis of its Hirshfeld surfaces in order to gain more information on the mode of association between molecules in the molecular packing.

Structural commentary
The molecular structure of (I), Fig. 1, sees the Zn II atom coordinated by two chelating thiosemicarbazone anions, each via the thiolate-S and imine-N atoms, Table 1. The resulting N 2 S 2 donor set defines a distorted tetrahedral geometry, with the range of angles subtended at the zinc atom being an acute 87.29 (9) for the S1-Zn-N3 chelate angle to 127.92 (4) for S1-Zn-S2. The assignment of four-coordinate geometries can be quantified by comparing the calculated value of 4 , in this case 0.74, with the ideal values for an ideal tetrahedron, i.e. 1.00, and perfect square-planar geometry, i.e. 0.00 (Yang et al., 2007), indicating a distorted tetrahedral geometry in (I). The configuration about each of the endocyclic imine bonds is Z, because of the dictates of chelation. By contrast, each of the exocyclic imine C N bonds is E, as are the configurations about the ethylene bonds, Table 1.
The mode of the coordination of the thiosemicarbazone ligands leads to the formation of five-membered ZnSCN 2 chelate rings, and these adopt different conformations. Whereas, the (Zn,S1,C1,N2,N3) ring is almost planar (r.m.s. deviation = 0.0325 Å ), the (Zn,S2,C20,N5,N6) chelate ring is best described as an envelope with the Zn atom lying 0.205 (5) Å out of the plane of the remaining four atoms (r.m.s. deviation = 0.0011 Å ). The dihedral angle between the mean planes through the chelate rings is 79.68 (8) . To a first approximation, the thiosemicarbazone ligands comprise two planar regions. Thus, the non-hydrogen, non-phenyl atoms of the atoms of the S1-ligand define one plane (r.m.s. deviation = 0.1910 Å ), which forms a dihedral angle of 54.53 (8) with the (C14-C19) ring, consistent with a near perpendicular relationship. The comparable values for the S2-ligand are 0.2800 Å and 75.09 (11) , respectively.

Supramolecular features
The most prominent feature of the molecular packing is the formation of supramolecular chains along the c-axis direction sustained by eight-membered thioamide {Á Á ÁHNCS} 2 synthons, Fig. 2a

Figure 1
The molecular structure of (I) showing the atom-labelling scheme and displacement ellipsoids at the 70% probability level.

Analysis of the Hirshfeld surfaces
The Hirshfeld surfaces calculated for (I) were performed in accord with recent work on a related complex (Tan et al., 2017) and provide more insight into the intermolecular interactions occurring in the crystal. The donors and acceptors of the intermolecular N-HÁ Á ÁS hydrogen bonds are viewed as bright-red spots, labelled as '1' and '2' in Fig. 3a, and the intermolecular C-HÁ Á ÁO contacts appear as tiny red spots with label '3' in Fig. 3b on the Hirshfeld surface mapped over d norm . The faint-red spots near the H3B, H11, H28 and C6 sites represent significant short interatomic HÁ Á ÁH and CÁ Á ÁH/ HÁ Á ÁC contacts, Fig. 3 and Table 3. The structure features two intramolecular C-HÁ Á Á(chelate) contacts, i.e. between ethylene-C5-H and the (Zn,S2,C20,N5,N6) ring and between ethylene-C24-H and the (Zn,S1,C1,N2,N3) ring, Two views of Hirshfeld surface mapped over d norm for (I) in the range À0.152 to +1.534 au. Table 3 Summary of short inter-atomic contacts (Å ) in (I).
Contact Distance Symmetry operation

Figure 4
Two views of Hirshfeld surface mapped over the electrostatic potential for (I) in the range AE 0.051 au highlighting intramolecular C-HÁ Á Á(chelate) interactions as black dotted lines.
which are viewed as blue and red regions assigned to positive and negative potentials, respectively, on the Hirshfeld surfaces mapped over electrostatic potential and are highlighted in Fig. 4a. The donors and acceptors of the intermolecular N-HÁ Á ÁS and C-HÁ Á ÁO interactions are also viewed as blue and red regions about respective atoms in the images of Fig. 4. The C-HÁ Á Á interactions involving imine-phenyl and methoxybenzene rings are evident in short interatomic CÁ Á ÁH/HÁ Á ÁC contacts, Table 3. The views of Hirshfeld surfaces about a reference molecule mapped over the electrostatic potential highlighting short interatomic HÁ Á ÁH and CÁ Á ÁH/HÁ Á ÁC contacts and that mapped within the shape-index property highlighting C-HÁ Á Á/Á Á ÁH-C contacts are illustrated in Fig. 5a and b, respectively. The overall two dimensional fingerprint plot for ( Table 4 Percentage contributions of inter-atomic contacts to the Hirshfeld surface for (I).

Contact
Percentage contribution

Figure 5
Views of Hirshfeld surface about reference molecule of (I) mapped (a) over the electrostatic potential highlighting short interatomic HÁ Á ÁH and CÁ Á ÁH/HÁ Á ÁC contacts by red and yellow dashed lines, respectively, and (b) with the shape-index property highlighting C-HÁ Á Á/Á Á ÁH-C contacts involving imine-phenyl and methoxy-benzene rings by red and black dashed lines, respectively.  due to a short interatomic HÁ Á ÁH contact (Table 3) and the two pairs of spikes about this central spike, at d e + d i $ 2.6 Å , indicate the intermolecular C-HÁ Á ÁO and N-HÁ Á ÁS interactions, Fig. 6c,d. The points related to short interatomic OÁ Á ÁH/HÁ Á ÁO contacts listed in Table 3 are merged within the respective plot of Fig. 6e. The CÁ Á ÁH/HÁ Á ÁC contacts provide the second greatest contribution to the Hirshfeld surface, Table 4. This is due to the combined effect of short interatomic CÁ Á ÁH/HÁ Á ÁC contacts (Table 3) in addition to C-HÁ Á Á contacts, summarized in Table 2. The most significant short atomic C6Á Á ÁH28 contact is evident from a pair of short peaks at d e + d i $ 2.7 Å in the fingerprint plot delineated into CÁ Á ÁH/ HÁ Á ÁC contacts, Fig. 6c. The short interatomic contact between the Zn II atom and imine-phenyl-C18 and H18 atoms, Table 3, and the contribution of 0.6% from ZnÁ Á ÁH/HÁ Á ÁZn and ZnÁ Á ÁC/CÁ Á ÁZn contacts to the Hirshfeld surface, Table 4, reflect the presence of intermolecular C-HÁ Á Á(chelate) interactions in the crystal. The (chelate)-(benzene) contacts described in the Supramolecular features section (x3) are also reflected from the small but important contribution from CÁ Á ÁN/NÁ Á ÁC and CÁ Á ÁS/SÁ Á ÁC contacts, Table 4, to the Hirshfeld surface of (I).

Database survey
The most relevant structure available for comparison is that of the recently described bis(N 0 -{(E)-[(2E)-1,3-diphenylprop-2-en-1-ylidene]-amino}-N-ethylcarbamimidothioato-2 N 0 ,S)zinc(II) molecule, which differs from (I) in that there are no additional substituents in the phenyl ring appended at the ethylene bond (Tan et al., 2017). Similar tetrahedral N 2 S 2 coordination geometries are found with values of 4 of 0.70 and 0.74 for the two independent molecules comprising the asymmetric unit. Indeed, in the publication describing this structure (Tan et al., 2017), it was mentioned there are nine structures in the literature conforming to the general formula Zn[SC(NHR) NN CR 0 R 00 ] 2 and all structures adopt the same basic structural motif as described herein for (I).

Synthesis and crystallization
Analytical grade reagents were used as procured and without further purification. 4-Ethyl-3-thiosemicarbazide (1.1919 g, 0.01 mol) and 4-methoxychalcone (2.3828 g, 0.01 mol) were dissolved separately in hot absolute ethanol (30 ml) and mixed while stirring. About five drops of concentrated hydrochloric acid were added to the mixture to catalyse the reaction. The reaction mixture was heated and stirred for about 20 min, and stirring was continued for another 30 min at room temperature. The resulting yellow precipitate, 4-methoxychalcone-4ethyl-3-thiosemicarbazone, was filtered off, washed with cold absolute ethanol and dried in vacuo after which it was used without further purification.    (Farrugia, 2012) and DIAMOND (Brandenburg, 2006); software used to prepare material for publication: publCIF (Westrip, 2010).

Bis[(N-ethyl-N′-{(Z)-[(2E)-3-(4-methoxyphenyl)-1-phenylprop-2-en-1ylidene]amino}carbamimidoyl)sulfanido]zinc(II)
Crystal data [Zn(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. Refinement. The maximum and minimum residual electron density peaks of 1.10 and 0.59 eÅ -3 , respectively, were located 1.04 Å and 0.71 Å from the Zn atom.