Bis[S-benzyl 3-(furan-2-ylmethylidene)dithiocarbazato-κ2 N 3,S]copper(II): crystal structure and Hirshfeld surface analysis

The title CuII dithiocarbazate complex features a square-planar trans-N2S2 donor set for the metal atom (site symmetry ). Supramolecular layers parallel to (02) are found in the crystal, being sustained by π–π(furyl) and C—H⋯π interactions.


Chemical context
Dithiocarbazates, derived from sulfur-nitrogen donor ligands were first reviewed in the 1970s (Ali & Livingstone, 1974). These Schiff base molecules are readily prepared from the reaction of primary amines with aldehydes or ketones and are potentially multidentate ligands for metals (Ali et al., 2005;Mokhtaruddin et al., 2017). Schiff bases display significant biological and pharmacological activities that can be tuned by incorporating different types of substituents through the condensation reaction (How et al., 2008;Low et al., 2016). Transition-metal complexes containing Schiff base ligands have also been intensively studied because of their simple routes of synthesis, the variety of their structural geometries and, particularly pertinent, as small chemical changes often produce wide variations in their bioactivities (Mirza et al., 2014;Zangrando et al., 2015;Lima et al., 2018). Recently, a copper(II) dithiocarbazate complex containing a Schiff base derived from S-hexyldithiocarbazate and 4-methylbenzaldehyde was reported to have excellent anti-bacterial activity against Escherichia coli . More recently, investigators have reported the potent biological activity of a copper(II) complex that contained a tridentate Schiff base derived from S-benzyldithiocarbazate and 2-hydroxy-5-(phenyldiazenyl)benzaldehyde against a human cervical cancer line (HeLa) (Kongot et al., 2019). The copper(II) complex had comparable biological activities as the well-known anti-cancer drug cisplatin against the tested cells (Kongot et al., 2019). As part of on-going studies in the structural chemistry and potential bioactivity of copper(II) complexes containing dithiocarbazate Schiff base ligands, herein the synthesis of the title copper(II) complex, (I), its single crystal X-ray diffraction analysis and a detailed study of supramolecular association by an analysis of calculated Hirshfeld surfaces and computation chemistry are described.

Structural commentary
The molecular structure of (I), Fig. 1, has the Cu II atom located on a crystallographic centre of inversion and coordinated by two chelating dithiocarbazate anions, each via the thiolate-S and imine-N atoms ( Table 1). The resulting trans-N 2 S 2 donor set defines a distorted square-planar geometry: the major distortion from the ideal angles subtended at the copper atom is the acute S1-Cu-N2 chelate angle of 85.83 (6) . The conformation about the endocylic imine bond is Z, as a result of chelation, whereas the exocyclic imine bond has an E conformation.
The bidentate mode of the coordination of the dithiocarbazate ligand leads to the formation of five-membered CuN 2 CS chelate rings. While the r.m.s. deviation for the five atoms is relatively small at 0.0453 Å , suggesting a near planar ring, a better description for the conformation is that of an envelope with the copper atom being the flap atom. In this description, the r.m.s. deviation of the S1, N1, N2 and N3 atoms of the ring is 0.0002 Å , with the Cu atom lying 0.199 (3) Å out of the plane. The dihedral angle between the best plane through the chelate ring and the 2-furyl ring is 5.33 (18) indicating an essentially co-planar relationship. By contrast, the dihedral between the chelate and phenyl rings is 86.75 (7) , indicative of an orthogonal relationship. Finally, the dihedral angle between the peripheral organic rings is 81.42 (9) .
The structure of the acid form of the anion in (I) is available for comparison (Shan et al., 2008). Referring to the data in Table 1, significant changes in key bond lengths have occurred upon deprotonation and coordination of the molecule to Cu II in (I). Thus, the C1-S1 [1.669 (2) Å for the acid], N1-N2 [1.381 (2) Å ] and C9-N2 [1.280 (3) Å ] bond lengths have all elongated in (I), Table 1, while the C1-N1 bond length has shortened [1.336 (3) Å ]. Significant changes in the angles subtended at the quaternary C1 atom are also noted, in particular for the S1-C1-S2 angle which has narrowed by ca 10 in (I) from 124.76 (12) in the acid with concomitant widening of the S2-C1-N1 angle by ca 5 , changes consistent with the reorganization of -electron density from the C1-S1 to C1-N1 bonds in (I).

Figure 1
The molecular structure of (I) showing the atom-labelling scheme and displacement ellipsoids at the 70% probability level. Unlabelled atoms are related by the symmetry operation 1 À x, 1 À y, 1 À z.
Cg1 is the centroid of the (C3-C8) ring. literature, especially for sterically unencumbered squareplanar complexes and can impart significant energies of stabilization to the molecular packing (Malenov et al. 2017;Tiekink, 2017). In the present case, these interactions link molecules along the b-axis direction. Links between the chains to form layers are of the type phenyl-C-HÁ Á Á(phenyl), Table 2. A view of the unit-cell contents is shown in Fig. 2 Details of the weak intermolecular contacts connecting layers are given in the analysis of the calculated Hirshfeld surfaces below.

Analysis of the Hirshfeld surfaces
The analysis of the Hirshfeld surfaces calculated for (I) was conducted as per literature precedents (Tan et al., 2019) employing Crystal Explorer (Turner et al., 2017). The assumption of the intermolecular C-HÁ Á Á contact in the crystal of (I) is justified through the diminutive red spots near the phenyl-C4 and H5 atoms on the Hirshfeld surfaces mapped over d norm in Fig. 3. The short interatomic HÁ Á ÁH contact, involving phenyl H8 atoms and occurring between layers, and the CÁ Á ÁC contact, between the methylene-C9 and furyl-C11 atoms, are also evident as the faint-red spots near the respective atoms in Fig. 3. On the Hirshfeld surfaces mapped over electrostatic potential in Fig. 4, the donors and acceptors of intermolecular C-HÁ Á Á contacts, Table 2, are viewed as blue bumps and light-red concave regions, respectively. Also, the short interatomic SÁ Á ÁH/HÁ Á ÁS contacts, which are electrostatic in nature, Table 3, show red and blue regions about the respective atoms. The environment around a reference molecule within the Hirshfeld surface mapped with the shape-index property is illustrated in Fig. 5, and highlights the C-HÁ Á Á/Á Á ÁH-C contacts. The overall two-dimensional fingerprint plot, Fig. 6(a), and those delineated into HÁ Á ÁH, CÁ Á ÁH/HÁ Á ÁC, SÁ Á ÁH/HÁ Á ÁS and CÁ Á ÁC contacts are illustrated in Fig. 6(b)-(e), respectively; the percentage contribution from all the identified interatomic contacts to the Hirshfeld surface are summarized quantitatively in Table 4.

Figure 3
A view of the Hirshfeld surface for (I) mapped over d norm in the range À0.080 to +1.213 arbitrary units.

Figure 4
A view of the Hirshfeld surface for (I) mapped over the electrostatic potential in the range À0.036 to + 0.034 atomic units.
The conical tip appearing at d e + d i $2.1 Å in the fingerprint plot delineated into HÁ Á ÁH contacts in Fig. 6(b), represents the short inter-layer HÁ Á ÁH contact involving phenyl-H8 atoms, Table 3. The presence of the C-HÁ Á Á interaction is evident through the short interatomic CÁ Á ÁH/HÁ Á ÁC contact characterized as the pair of forceps-like tips at d e + d i $2.7 Å in the respective delineated fingerprint plot of Fig. 6(c) and Table 3. In the fingerprint plot delineated into SÁ Á ÁH/HÁ Á ÁS contacts, Fig. 6(d), the short interatomic contact involving the S-benzyl atoms, Table 3, appear as the pair of forceps-like tips at d e + d i < 3.0 Å , i.e. at the sum of van der Waals radii. The distribution of points in the fingerprint plot delineated into CÁ Á ÁC contacts, Fig. 6(e), forming triangular tip at d e + d i $3.3 Å is due to the presence of such short interatomic contacts summarized in Table 3. The presence of intermolecularstacking between chelate and furyl rings results in the small but significant percentage contribution from the participating atoms, as listed in Table 4. The small contributions from the other remaining interatomic contacts summarized in Table 4 have a negligible effect on the packing.

Computational chemistry
Utilizing Crystal Explorer (Turner et al., 2017), the pairwise interaction energies between the molecules within the crystal were calculated by summing up four energy component, namely electrostatic (E ele ), polarization (E pol ), dispersion (E dis ) and exchange-repulsion (E rep ). The energies were obtained using the wave function calculated at the HF/STO-3G level theory. The strength and nature of the intermolecular interactions are summarized quantitatively in Table 5. From the interaction energies calculated between the reference molecule and the symmetry-related molecule at x, À1 + y, z in Table 5, it is observed that the greatest energy value is due to the combined influence of CuÁ Á Áfuryl [CuÁ Á ÁCg(furyl) = 3.74 Å ], (chelate)-(furyl), CÁ Á ÁC and SÁ Á ÁH/HÁ Á ÁS interactions. Among these interactions, the short interatomic SÁ Á ÁH/HÁ Á ÁS contact contributes to the electrostatic component while the others to the dispersion component of the energies. Even though the inter-centroid distance between symmetry-related phenyl (C3-C8) rings are greater than 4.0 Å [CgÁ Á ÁCg i = 4.3102 (17) Å ; (i) À x, 2 À y, 1 À z] and the interatomic SÁ Á ÁH distance is greater than sum of their van der A view of the Hirshfeld surface with the shape-index property highlighting C-HÁ Á Á/Á Á ÁH-C contacts by black dotted lines.  Waal radii (S1Á Á ÁH11 ii = 3.11 Å ; x, 3 2 À y, À 1 2 + z), they possess greater interaction energies compared to intermolecular phenyl-C-HÁ Á Á(phenyl) and short interatomic HÁ Á ÁH contacts, as summarized in Table 5. The magnitudes of the intermolecular energies are represented graphically in the energy frameworks down the b-axis direction in Fig. 7. Here, the supramolecular architecture of crystals is viewed through the cylinders joining the centroids of molecular pairs by using red, green and blue colour codes for the components E ele , E disp and E tot , respectively; the radius of the cylinder is proportional to the magnitude of interaction energy. It is clearly evident from the energy frameworks shown in Fig. 7 that the major contribution to the intermolecular interactions is from the dispersion energy component in the absence of conventional hydrogen bonds in the crystal.

Database survey
The Cambridge Structural Database (Groom et al., 2016) contains just about 100 structures with the basic core found in (I). Manual sorting to identify ligands without additional donors as in (I), e.g. substituents carrying pyridyl or phenoxide, neutral molecules only and non-solvated structures yielded 24 analogues to (I) with deposited atomic coordinates. Eleven of these structures adopt the trans-N 2 S 2 square-planar geometry as in (I), while the remaining 13 structures adopt a flattened tetrahedral coordination geometry. The structural diversity exhibited by these complexes is emphasized by the binuclear species [Cu{SCS[(CH 2 ) 5 Me] NN CC 6 H 4 OMe-4} 2 ] 2 arising from intermolecular CuÁ Á ÁS interactions between centrosymmetrically related trans-N 2 S 2 square-planar geometries .

Synthesis and crystallization
Synthesis of the 2-furaldehyde Schiff base of S-benzyldithiocarbazate: S-Benzyldithiocarbazate (SBDTC) was synthesized following a procedure adapted from a previous report (Tarafder et al., 2001). The Schiff base was synthesized using a procedure adapted from the literature (Yusof et al., 2015) by reacting SBDTC (3.96 g, 0.02 mol) and an equimolar amount of 2-furaldehyde (1.92 g, 0.02 mmol) in hot ethanol (20 ml).

Figure 7
The energy frameworks viewed down the b-axis direction comprising (a) electrostatic potential force, (b) dispersion force and (c) total energy for a cluster about a reference molecule of (I). The energy frameworks were adjusted to the same scale factor of 50 with a cut-off value of 3 kJ mol À1 within 2 Â 2 Â 2 unit cells.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 6. 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 1.2U eq (C).

Bis[S-benzyl 3-(furan-2-ylmethylidene)dithiocarbazato-κ 2 N 3 ,S]copper(II)
Crystal data [Cu(C 13  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.