Bis[N-(2-hydroxyethyl)-N-isopropyldithiocarbamato-κ2 S,S′](piperazine-κN)cadmium: crystal structure and Hirshfeld surface analysis

A distorted square-pyramidal CdNS4 coordination geometry is found in {Cd[S2CN(iPr)CH2CH2OH]2[HN(CH2CH2)2NH]}. The packing features supramolecular layers sustained by O—H⋯O, O—H⋯N and N—H⋯O hydrogen bonding.


Structural commentary
The molecular structure of the title compound, {Cd[S 2 CN( i Pr)CH 2 CH 2 OH] 2 [HN(CH 2 CH 2 ) 2 NH]}, Fig. 1, comprises a penta-coordinated cadmium atom, being chelated by two dithiocarbamate ligands and connected to a piperazine-N atom, the latter ligand having a chair conformation. The coordination geometry is best described as being distorted square pyramidal with the nitrogen atom in the apical position. This description is quantified by the value of , i.e. 0.18, which is closer to the value of 0.0 for an ideal square-pyramidal geometry cf. 1.0 for an ideal trigonal bipyramid (Addison et al., 1984). The r.m.s. deviation of the four sulfur atoms is 0.1023 Å , and the cadmium atom lies 0.6570 (4) Å above the plane in the direction of the N3 atom. The distortions from the ideal geometry are related, in part, to the acute chelate angles subtended by the chelating ligands, i.e. 68-69 , and the range of N axial -Cd-S basal angles is 98-116 , Table 1. The dithiocarbamate ligands are coordinating in a slightly asymmetric manner with the difference between the short and long Cd-S bond lengths being ca 0.1 Å for the S1-containing ligand and ca 0.2 Å for the S3-containing ligand, Table 1. The almost symmetric mode of coordination of the dithiocarbamate ligands is reflected in the near equivalence of the associated C-S bond lengths, Table 1, and is consistent with significant delocalization of -electron density over each fourmembered chelate ring.

Analysis of the Hirshfeld surfaces
The program Crystal Explorer (Wolff et al., 2012) was used to generate Hirshfeld surfaces mapped over d norm , d e and electrostatic potential for the title compound. The electrostatic potentials were calculated using TONTO (Spackman et al., 2008;Jayatilaka et al., 2005) and were mapped on Hirshfeld surfaces using the STO-3G basis set at the Hartree-Fock level of theory over a range AE0.14 au. The contact distances d i and d e from the Hirshfeld surface to the nearest atom inside and outside, respectively, enable the analysis of the intermolecular interactions through the mapping of d norm . The combination of d e and d i in the form of a two-dimensional fingerprint plot (Rohl et al., 2008) provides a summary of intermolecular contacts in the crystal. The relative contributions from various contacts to the Hirshfeld surfaces are tabulated in Table 3. The hydrogen-bonding network generated in the crystal through hydroxyl groups located at the edges, piperazine nitrogen-H at the apex and sulfur atoms on the vertices of the distorted square-pyramidal polyhedron can be visualized

Figure 3
A view of the unit-cell contents of the title compound shown in projection down the c axis, whereby the supramolecular layers, illustrated in Fig. 2, stack along the b axis. One layer is highlighted in space-filling mode.

Figure 4
Two views of the Hirshfeld surface mapped over d norm . The contact points (red) are labelled to indicate the atoms participating in the intermolecular interactions.
using Hirshfeld surface analysis. The bright-red spots on the Hirshfeld surface mapped over d norm , Fig. 4, with labels H1O, H2O and H3N, on the surface represent donors for potential hydrogen bonds, Table 2; the corresponding acceptors on the surfaces appear as bright-red spots at O2, N4 and O1, respectively. The Hirshfeld surface mapped over the electrostatic potential, Fig. 5, represents donors with positive potential (blue regions) and the acceptors with negative potential (red). In addition, the negative potential around the sulfur atoms appear as light-red clouds and the positive potential around piperazine as a light-blue cloud in Fig Hirshfeld surface mapped for a reference molecule over d norm showing hydrogen bonds with neighbouring molecules.

Figure 5
Two views of the Hirshfeld surface mapped over the electrostatic potential with positive and negative potential indicated in blue and red, respectively.

Figure 7
Two-dimensional fingerprint plots: (a) overall, and delineated into contributions from different contacts: confirm the contribution of these pairs of atoms in the comparatively weak intermolecular C-HÁ Á ÁS interactions. The pale-red spot near the S2 atom indicates an additional reinforcement to the two-dimensional framework through a non-bonded SÁ Á ÁS contact. The overall two-dimensional fingerprint plot, Fig. 7a, and those delineated into HÁ Á ÁH, SÁ Á ÁH/HÁ Á ÁS, OÁ Á ÁH/HÁ Á ÁO, CÁ Á ÁH/HÁ Á ÁC, NÁ Á ÁH/HÁ Á ÁN and SÁ Á ÁS interactions are illustrated in Fig. 7b-g, respectively. The greatest contribution to the overall Hirshfeld surface, i.e. 67.5%, is due to HÁ Á ÁH contacts and is reflected in Fig. 4b as widely scattered points with a high concentration in the middle region, shown in green. The contribution from the SÁ Á ÁH/HÁ Á ÁS contacts, corresponding to C-HÁ Á ÁS interactions, is represented by the pair of short spikes in the outer region, at d e + d i $ 2.8 Å , Fig. 7c. In the plots delineated into OÁ Á ÁH/HÁ Á ÁO and NÁ Á ÁH/ HÁ Á ÁN contacts, Fig. 7d and f, the pairs of adjacent peaks have almost same lengths near d e + d i $ 1.8 Å , and clearly indicate the significance of intermolecular hydrogen bonds associated with them in the molecular packing. There is only a very small contribution from CÁ Á ÁH/HÁ Á ÁC contacts, i.e. 3.2% (Fig. 7d), and there is no contribution from CÁ Á ÁC contacts in the structure as the result of the absence of C-HÁ Á Á andstacking interactions. Finally, the presence of SÁ Á ÁS contacts can also be viewed in the delineated fingerprint plot, Fig. 7g, by the density of points in the d e , d i region around 1.7-2.4 Å as a broken line segment.
The identified intermolecular interactions were further evaluated by an analysis of the enrichment ratios (ER) that give a quantitative measure of the likelihood of specific intermolecular interactions to occur based on a Hirshfeld surface analysis (Jelsch et al., 2014); ratios are given in Table 4.
A total of 83.2% of the Hirshfeld surface involves hydrogen atoms and of this, non-bonded HÁ Á ÁH contacts account for 67.5% of the contacts, which is close to the value of 69.2%, being the value calculated for random contacts so that the corresponding ER value is 0.98, i.e. near unity and in accord with earlier published results (Jelsch et al., 2014). The sulfur atoms comprise 9.7% of the surface and SÁ Á ÁH/HÁ Á ÁS contacts provide an overall 17.4% contribution to the surface resulting in an ER of 1.1 which is in the expected range, i.e. 1.0-1.5, for C-HÁ Á ÁS interactions. The ER value of 1.2 corresponding to OÁ Á ÁH/HÁ Á ÁO contacts indicate these show a high propensity to form even though the relative contribution to the overall surface, i.e. 7.9%, is small as is the 4.0% exposure to the surface provided by the hydroxyl oxygen atoms. The other contributions to the surface, i.e. NÁ Á ÁH/HÁ Á ÁN and CÁ Á ÁH/ HÁ Á ÁC, are small and the ER values are not particularly informative although being > 1, indicate a propensity to form as discussed above in Supramolecular features.

Database survey
The structural chemistry of cadmium dithiocarbamates where the ligands have been functionalized with one or two hydroxyethyl groups has received some attention in recent years owing to the constant stream of unexpected crystallization outcomes. As mentioned in the Chemical context, [Cd(S 2 CNR 2 ) 2 ] 2 compounds are usually binuclear (Tiekink, 2003). However, recent studies of Cd[S 2 CN( i Pr)CH 2 CH 2 OH] 2 (Tan et al., 2013(Tan et al., , 2016 have revealed solvent-dependent and solvent-independent supramolecular molecular isomers, e.g. crystallization from ethanol produced two species [Cd[S 2 CN( i Pr)CH 2 CH 2 OH] 2 ÁEtOH] x for x = 2 and, unprecedented, n (Tan et al., 2016), with the kinetic, polymeric (x = n) form transforming in solution to the thermodynamic, binuclear form (x = 2). Other recrystallization conditions led to decomposition of the dithiocarbamate ligands and subsequent formation of a co-crystal and some salts. This behaviour, along with the unexpected structure of {Cd[S 2 CN( i Pr)CH 2 -CH 2 OH] 2 (4-pyridinealdazine    (Ishak et al., 2014) compounds of these ligands display promising potential as anti-cancer agents, and that some gold compounds also exhibit exciting anti-microbial activity (Sim et al., 2014). The monodentate mode of coordination of the piperazine ligand in the title compound is quite rare, and has not been observed in the structural chemistry of cadmium. However, crystallographically confirmed examples of a monodentate coordination mode for piperazine have been seen in five structures, e.g. as in centrosymmetric, all-trans CoCl 2 (piperazine) 2 (MeOH) 2 (Suen et al., 2004).

Synthesis and crystallization
The reagents Cd[S 2 CN( i Pr)CH 2 CH 2 OH] 2 (Tan et al., 2013;206 mg, 0.44 mmol) and piperazine (Sigma-Aldrich; 76 mg, 0.43 mmol) were dissolved in chloroform (15 ml) and acetonitrile (5 ml), respectively. The latter solution was added dropwise into the chloroform solution and the resulting mixture was stirred for 1 h at room temperature.

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 = 0.98-1.00 Å ) and were included in the refinement in the riding-model approximation, with U iso (H) set to 1.2U eq (C). The oxygen-and nitrogenbound H atoms were located in a difference Fourier map but were refined with a distance restraints of O-H = 0.84AE0.01 Å and N-H = 0.88AE0.01 Å , and with U iso (H) set to 1.5U eq (O) and 1.2U eq (N).  (Sheldrick, 2008); program(s) used to refine structure: SHELXL2014/7 (Sheldrick, 2015); molecular graphics: ORTEP-3 for Windows (Farrugia, 2012) and

Bis[N-(2-hydroxyethyl)-N-isopropyldithiocarbamato-κ 2 S,S′](piperazine-κN)cadmium
Crystal data [Cd(C 6  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.