(N,N-Diisopropyldithiocarbamato)triphenyltin(IV): crystal structure, Hirshfeld surface analysis and computational study

The tin coordination geometry in (C6H5)3Sn[S2CN(i-Pr)2] is based on a tetrahedron but the geometry of the C3S donor set is distorted by the close proximity of the second thione-S atom. In the crystal, weak C—H⋯C interactions link molecules into centrosymmetric dimers.

The crystal and molecular structures of the title triorganotin dithiocarbamate, [Sn(C 6 H 5 ) 3 (C 7 H 14 NS 2 )], are described. The molecular geometry about the metal atom is highly distorted being based on a C 3 S tetrahedron as the dithiocarbamate ligand is asymmetrically chelating to the tin centre. The close approach of the second thione-S atom [SnÁ Á ÁS = 2.9264 (4) Å ] is largely responsible for the distortion. The molecular packing is almost devoid of directional interactions with only weak phenyl-C-HÁ Á ÁC(phenyl) interactions, leading to centrosymmetric dimeric aggregates, being noted. An analysis of the calculated Hirshfeld surface points to the significance of HÁ Á ÁH contacts, which contribute 66.6% of all contacts to the surface, with CÁ Á ÁH/HÁ Á ÁC [26.8%] and SÁ Á ÁH/HÁ Á ÁH [6.6%] contacts making up the balance.

Chemical context
Organotin(IV) compounds have long been investigated as potential anti-cancer agents (Gielen & Tiekink, 2005) and studies in this area continue. Further, organotin compounds have received much attention owing to their potential therapeutic potential as anti-fungal, anti-bacterial, anti-malarial and schizonticidal agents (Khan et al., 2014). Metal dithiocarbamates have also encouraged much interest in the context of chemotherapeutic agents (Hogarth, 2012) and these include organotin(IV) dithiocarbamate compounds (Tiekink, 2008;Adeyemi & Onwudiwe, 2018). In view of the wide-range of applications/potential of organotin(IV) dithiocarbamate compounds and in continuation of on-going studies in this area (Khan et al., 2015;Mohamad et al. 2016Mohamad et al. , 2017Mohamad et al. , 2018, the title compound, Ph 3 Sn[S 2 CN(i-Pr) 2 ], (I), was synthesized and characterized spectroscopically. Herein, the crystal and molecular structures of (I) are described along with a detailed analysis of the molecular packing via the calculated Hirshfeld surfaces and computational chemistry study. ISSN 2056-9890

Structural commentary
The tin atom in (I), Fig. 1, is coordinated by an asymmetrically bound dithiocarbamate ligand and three ipso-carbon atoms of the phenyl groups ( Table 1). The disparity in the Sn-S separations, i.e. Á(Sn-S) = [(Sn-S l ) À (Sn-S s )] = 0.45 Å (l = long, s = short), is rather great suggesting that the SnÁ Á ÁS2 interaction is weak. This is supported in the pattern of C-S bond lengths with that involving the more tightly bound S1 atom being nearly 0.06 Å longer than the equivalent bond with the weakly bound S2 atom. Despite this, a clear influence of the S2 atom is noted on the Sn-C bond lengths with the Sn-C31 bond length being significantly longer than the other Sn-C bonds, Table 1. The S2-Sn-C31 bond angle is 158.41 (4) and is suggestive of a trans-influence exerted by the S2 atom; there is no other trans angle about the tin atom. If the coordination geometry is considered as tetrahedral, the range of tetrahedral angles is 93.24 (4) , for S1-Sn-C31, to 119.87 (6) , for C11-Sn-C21. The range of angles assuming a five-coordinate, C 3 S 2 , geometry is 65.260 (11) , for the S1-Sn-S2 chelate angle to the aforementioned 158.41 (4) . A descriptor for assigning coordination geometries to fivecoordinate species is (Addison et al., 1984). In the case of (I), this computes to 0.64, which indicates a geometry somewhat closer to an ideal trigonal bipyramid ( = 1.0) than to an ideal square pyramid ( = 0.0). Also included in Table 1 are the C-N bond lengths, which show C1-N1 to be significantly shorter than the C2-N1 and C5-N1 bond lengths, an observation consistent with a significant contribution of the 2À S 2 C N + (i-Pr) 2 canonical form to the overall electronic structure of the dithiocarbamate ligand.

Supramolecular features
The geometric parameters characterizing the identified intermolecular interaction operating in the crystal of (I) are collated in Table 2. A phenyl-C-HÁ Á ÁC(phenyl) contact less than the sum of the sum of the Waals radii (Spek, 2009) is noted to occur between centrosymmetrically related molecules, Fig. 2(a). This is an example of a localized C-HÁ Á Á contact whereby the hydrogen atom directed towards a single carbon atom of the ring as opposed to a delocalized interaction where the hydrogen atom (or halide atom or lone-pair of electrons) is directed towards the centroid of the ring (Schollmeyer et al., 2008;Tiekink, 2017  Symmetry code: (i) Àx þ 1; Ày; Àz.

Figure 2
Molecular packing in the crystal of (I): (a) supramolecular dimer sustained by localized phenyl-C-HÁ Á ÁC(phenyl) interactions shown as purple dashed lines and (b) a view of the unit-cell contents in projection down the a axis. One column of dimeric aggregates is highlighted in space-filling mode.

Figure 1
The molecular structure of (I) showing the atom-labelling scheme and displacement ellipsoids at the 70% probability level. The long Sn1Á Á ÁS2 contact is indicated by a double-dashed line.

Hirshfeld surface analysis
The Hirshfeld surface calculations for (I) were performed employing Crystal Explorer 17 (Turner et al., 2017) and recently published protocols (Tan et al., 2019). In the absence of classical hydrogen bonds, the influence of the localized C-HÁ Á Á interaction, Table 2, as well as interatomic HÁ Á ÁH and CÁ Á ÁH/HÁ Á ÁC contacts, Table 3, upon the molecular packing are evident as the diminutive-red spots near the participating carbon and hydrogen atoms on the Hirshfeld surfaces mapped over d norm in Fig. 3. It is also noted that with the exception of the methyl-H7A atom, all of the specified interatomic contacts only involve the carbon and hydrogen atoms of the coordinated phenyl rings, Table 3. On the Hirshfeld surface mapped over electrostatic potential in Fig. 4, the light-blue and faint-red regions, corresponding to positive and negative electrostatic potential, respectively, occur about the di-iso-propyl and triphenyltin groups, respectively. As reported recently (Pinto et al., 2019), in addition to analysing the nature and strength of intermolecular interactions among molecules, the analyses of Hirshfeld surfaces can also provide useful insight into metal-ligand/donor atom interactions in coordination compounds. Thus, the distance from the surface to the nearest external (d e ) and internal (d i ) nuclei, the shape-index (S) and the curvedness (C) can also be plotted. Accordingly, Fig. 5 illustrates the Hirshfeld surfaces for the tin atom coordinated by dithiocarbamate ligand as well as by the three phenyl groups. The close proximity of the dithiocarbamate-S1 and phenyl-C11, C21 and C31 atoms to the tin centre are characterized as bright-red regions perpendicular to bond directions on the Hirshfeld surfaces mapped over d e , Fig. 5(a), and d norm , Fig. 5(b), whereas the comparatively weak Sn-S2 interaction appears as the faintred region. The longer Sn-C31 bond compared to other two Sn-C bonds, Table 1, is also characterized from these Hirshfeld surfaces through the curvature of the red region. The Sn-S1 and Sn-C bonds result in the large red regions on the shape-index mapping in Fig. 5(c) compared to a small red region for the Sn-S2 bond. On the Hirshfeld surfaces mapped over curvedness in Fig. 5(d), the strength of the tinligand bonds are characterized as the yellow areas separated by green regions. The coordination bonds for tin are also rationalized in the fingerprint plot taking into account only the Hirshfeld surface about the metal atom, Fig. 6. The distribution of green points having upper short spike at d e + d i $2.4 Å and the lower, long red spike at d e + d i $2.1 Å are the result of the Sn-S and Sn-C bonds, respectively. This asymmetric distribution of points about the diagonal lacking homogeneity in colouration is due to the distorted coordination geometry about the tin atom.  Table 3 Summary of short interatomic contacts (Å ) in (I).
The interatomic distances are calculated in Crystal Explorer 17 (Turner et al., 2017) whereby the X-H bond lengths are adjusted to their neutron values.

Contact
Distance Symmetry operation Two views of Hirshfeld surface for (I) mapped over d norm in the range À0.085 to +1.355 (arbitrary units), highlighting short interatomic HÁ Á ÁH and CÁ Á ÁH/HÁ Á ÁC contacts as diminutive red spots near the respective atoms.

Figure 4
Two views of Hirshfeld surface mapped over the electrostatic potential (the red and blue regions represent negative and positive electrostatic potentials, respectively) in the range À0.032 to +0.035 atomic units.

Figure 5
The Hirshfeld surfaces of the tin centre in (I) highlighting the coordination by the dithiocarbamate ligand (left-hand images) and the phenyl rings (right-hand images) mapped over (a) the distance, d e , external to the surface in the range À0.981 to 2.436 Å , (b) d norm in the range À0.890 to +1.135 Å , (c) the shape-index (S) from À1.0 to +1.0 (arbitrary units) and (d) curvedness (C) from À4.0 to +0.4 (arbitrary units).
It is clear from the the calculation of the overall twodimensional fingerprint plot for (I), Fig. 7(a), that the plot is asymmetric about the (d e , d i ) diagonal in the longer distance regions and have contributions only from the interatomic contacts involving carbon, hydrogen and sulfur atoms, Table 4. The two-dimensional fingerprint plots delineated into HÁ Á ÁH, CÁ Á ÁH/HÁ Á ÁC, CÁ Á ÁC and SÁ Á ÁH/HÁ Á ÁS contacts are shown in Fig. 7(b)-(d), respectively. In the fingerprint plot delineated into HÁ Á ÁH contacts in Fig. 7(b), the presence of the short interatomic HÁ Á ÁH interaction involving the methyl-H7A and phenyl-H12 atoms is evident as the pair of short overlapping peaks at d e + d i $2.1 Å with the other short interatomic HÁ Á ÁH contact (Table 3) merged within the plot. The intermolecular C-HÁ Á ÁC interactions describing the localized C-HÁ Á Á contacts are evidenced by a pronounced pair of characteristic wings around (d e , d i ) $(1.2 Å , 1.8Å ) and $(1.8 Å , 1.2 Å ) in the fingerprint plot delineated into CÁ Á ÁH/HÁ Á ÁC contacts shown in Fig. 7(c). The other short interatomic CÁ Á ÁH contacts summarized in Table 3 appear as the pair of forceps-like tips at d e + d i $2.7 Å . The fingerprint plot delineated into SÁ Á ÁH/ HÁ Á ÁS contacts in Fig. 7(d) indicate that sulfur atoms are nearly at van der Waals separation from the symmetry-related hydrogen atoms.

Computational chemistry
The pairwise interaction energies between the molecules within the crystal were calculated using Crystal Explorer 17 (Turner et al., 2017) and summing up the four energy components: 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 of theory. The strength and the nature of intermolecular interactions in terms of their energies are summarized in Table 5 Table 5 Summary of interaction energies (kJ mol À1 ) calculated for (I).

Figure 6
The fingerprint plot taking into account only the Hirshfeld surface about the tin atom.  to the energies in the absence of classical hydrogen (electrostatic) bonds. Among the short interatomic contacts listed in Table 5, the intermolecular phenyl-C-H36Á Á ÁC12 contact combined with the methyl-H7AÁ Á ÁH12(phenyl) interaction, occurring between the same pair of symmetry-related molecules, gives rise to the maximum total energy of interaction, compared to the other interactions, which have almost the same energy values. The graphical representation of the magnitudes of intermolecular energies in Fig. 8, i.e. energy frameworks, relies on a red, green and blue colour code scheme, reflecting the E ele , E disp and E tot components, respectively. For the direct comparison of magnitudes of interaction energies, their magnitudes were adjusted to same scale factor of 30 with a cutoff value of 3 kJ mol À1 within 2 Â 2 Â 2 unit cells. It is clear from Fig. 8 that the green cylinders joining the centroids of molecular pairs highlighting the dispersion components make a significant contribution to the supramolecular architecture in the crystal.

Database survey
As indicated in a recent report (Mohamad et al., 2018), there are nearly 50 crystal structures available for molecules of the general formula Ph 3 Sn(S 2 CNRR 0 ) and, with a number of these having multiple molecules in the asymmetric unit, there are almost 60 independent molecules. These conform to the same structural motif. An analysis of the key geometric parameters defining the mode of coordination of the dithiocarbamate ligands showed that there were no systematic variations that could be correlated with the nature of the dithiocarbamate ligand, i.e. R/R 0 substituents. This observation is borne out by DFT calculations on different organotin systems that proved the influence of molecular packing on (non-systematic) geometric parameters, including metal-sulfur/halide bonds (Buntine et al., 1998a(Buntine et al., ,b, 1999. In terms of Ph 3 Sn(S 2 CNRR 0 ), the mean Sn-S s bond length is 2.47 Å (standard deviation = 0.013 Å ) and the average Sn-S l bond length is 3.04 Å (0.070 Å ). The Sn-S s and Sn-S l bond lengths in (I) both fall within 2 of their respective means.
The homogeneity in the molecular structures of Ph 3 Sn(S 2 CNRR 0 ) is quite remarkable. Structural diversity is well-established for the organotin dithiocarbamates (Tiekink, 2008;Muthalib et al., 2014) such as for molecules of the general formula R 00 2 Sn(S 2 CNRR 0 ) 2 , for which four distinct structural motifs are known (Zaldi et al., 2017). Also, the behaviour of triphenyltin dithiocarbamates contrasts the analogous chemistry of triphenyltin carboxylates (Tiekink, 1991). These are often monomeric (e.g. Basu Baul et al., 2001), as for (I), but, polymeric examples are known (e.g. Willem et al., 1997;Smyth & Tiekink, 2000). The polymeric structures occur when the carboxylate ligands are bidentate bridging, leading to trans-C 3 O 2 trigonal-bipyramidal coordination geometries for the tin atoms. This fundamental difference in structural chemistry arises as a result of the significant contribution of the 2À S 2 C N + RR 0 canonical form to the electronic structure of the dithiocarbamate anion, as discussed The energy frameworks calculated for (I) viewed down the c-axis direction showing the (a) electrostatic potential force, (b) dispersion force and (c) total energy. The energy frameworks were adjusted to the same scale factor of 30 with a cut-off value of 3 kJ mol À1 within 2 Â 2 Â 2 unit cells.
above. The formal negative charge on each sulfur atom makes this ligand a very efficient chelator which effectively reduces the Lewis acidity of the tin centre. Far from being a curiosity, such behaviour, i.e. dithiocarbamate ligands reducing the Lewis acidity of metal centres, when compared to related xanthate ( À S 2 COR) and dithiophopshate [( À S 2 P(OR) 2 ] ligands, leads to stark differences in coordination propensities in zinc-triad element 1,1-dithiolate compounds, as has been reviewed recently (Tiekink, 2018a,b).

Synthesis and crystallization
All chemicals and solvents were used as purchased without purification. The melting point was determined using an automated melting point apparatus (MPA 120 EZ-Melt). Carbon, hydrogen, nitrogen and sulfur analyses were performed on a Leco CHNS-932 Elemental Analyzer.
Di-iso-propylamine (Aldrich; 1.41 ml, 10 mmol) dissolved in ethanol (30 ml) was stirred under ice-bath conditions at 277 K for 20 mins. A 25% ammonia solution (1-2 ml) was added to provide basic conditions. Then, a cold ethanolic solution of carbon disulfide (0.60 ml, 10 mmol) was added dropwise into the solution followed by stirring for 2 h. After that, triphenyltin(IV) chloride (Merck; 3.85 g, 10 mmol) dissolved in ethanol (20-30 ml) was added dropwise into the solution followed by further stirring for 2 h. The precipitate that formed was filtered and washed a few times with cold ethanol to remove impurities. Finally, the colourless precipi-tate was dried in a desiccator. Recrystallization was carried out by dissolving the compound in a chloroform and ethanol mixture (1:1 v/v). This solution was allowed to slowly evaporate at room temperature yielding colourless slabs of (I). Yield: 47%, m. p.: 437.8-440.2 K. Elemental analysis: Calculated (%): C 57. 07,H 5.51,N 2.66,S 12.19. Found (%): C 57.39,H 5.31,N 2.48,S 11.26.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 6. Carbon-bound H atoms were placed in calculated positions (C-H = 0.95-1.00 Å ) and were included in the refinement in the riding model approximation, with U iso (H) set to 1.2-1.5U eq (C).  refine structure: SHELXL2014 (Sheldrick, 2015b); molecular graphics: ORTEP-3 for Windows (Farrugia, 2012) and DIAMOND (Brandenburg, 2006); software used to prepare material for publication: publCIF (Westrip, 2010).

(N,N-Diisopropyldithiocarbamato)triphenyltin(IV)
Crystal data [Sn(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.