Secondary bonding in dimethylbis(morpholine-4-carbodithioato-κ2 S,S′)tin(IV): crystal structure and Hirshfeld surface analysis

In (CH3)2Sn[S2CN(CH2CH2)2O]2, a skew-trapezoidal bipyramidal coordination geometry based on a C2S4 donor set is found. Secondary Sn⋯S interactions lead to centrosymmetric dimeric aggregates in the crystal.

The title compound, [Sn(CH 3 ) 2 (C 5 H 8 NOS 2 ) 2 ], has the Sn IV atom bound by two methyl groups which lie over the weaker Sn-S bonds formed by two asymmetrically chelating dithiocarbamate ligands so that the coordination geometry is skew-trapezoidal bipyramidal. The most prominent feature of the molecular packing are secondary SnÁ Á ÁS interactions [SnÁ Á ÁS = 3.5654 (7) Å ] that lead to centrosymmetric dimers. These are connected into a threedimensional architecture via methylene-C-HÁ Á ÁS and methyl-C-HÁ Á ÁO(morpholino) interactions. The SnÁ Á ÁS interactions are clearly evident in the Hirshfeld surface analysis of the title compound along with a number of other intermolecular contacts.

Chemical context
Both binary tin and organotin dithiocarbamates, R n Sn(S 2 CNRR 0 ) m for n + m = 4, are well known to exhibit potential biological properties, e.g. anti-cancer (Ferreira et al., 2014), anti-fungal (Yu et al., 2014) and anti-microbial (Ferreira et al., 2012), as well to serve as useful molecular precursors for the generation of 'SnS' nanomaterials (Kevin et al., 2015). The structural chemistry of this class of compound has also attracted considerable interest over the years owing to the occurrence of significant structural diversity observed in seemingly closely related compounds (Tiekink, 2008). As a case in point and related to the title compound, [Sn(CH 3 ) 2 (C 5 H 8 NOS 2 ) 2 ] (I), reported herein, are the variations in molecular structure observed for the diorganotin bis(dithiocarbamate)s as discussed in the recent literature (Muthalib et al., 2014;Mohamad et al., 2016Mohamad et al., , 2017. These R 2 Sn(S 2 CNRR') 2 structures are known to adopt four distinct coordination geometries with the majority being skew-trapezoidal bipyramidal or octahedral, each based on C 2 S 4 donor sets. Fewer examples are known for five-coordinate, trigonalbipyramidal species, e.g. (t-Bu) 2 Sn(S 2 CNMe 2 ) 2 in which one dithiocarbamate ligand is monodentate (Kim et al., 1987), and seven-coordinate, pentagonal-bipyramidal, e.g. [MeOC( O)CH 2 CH 2 ] 2 Sn(S 2 CNMe) 2 where the carbonyl-O atom of one Sn-bound organic substituent is also coordinating the tin atom (Ng et al., 1989). This last example is of interest as ISSN 2056-9890 it demonstrates tin may in fact increase its coordination number by additional interactions. When additional interactions of this type occur intermolecularly, they are termed secondary bonding or tetrel bonding as a Group IV element, tin, is involved (Alcock, 1972;Marín-Luna et al., 2016;Tiekink, 2017). Generally, secondary interactions do not occur for R 2 Sn(S 2 CNRR') 2 structures as the strong chelating ability of the dithiocarbamate ligand reduces the Lewis acidity of the tin atom. However, in (I) such secondary SnÁ Á ÁS interactions do in fact occur. In a continuation of work in this area, herein the synthesis and crystal and molecular structures of (I) are described as well as an analysis of the Hirshfeld surface with a particular emphasis on investigating the role of the secondary SnÁ Á ÁS interaction.

Structural commentary
The Sn IV atom in the title compound (I), Fig. 1, adopts one of the common coordination geometries found for R 2 Sn(S 2 CNRR') 2 molecules, i.e. skew-trapezoidal bipyramidal rather than octahedral (Tiekink, 2008). This arises as the chelating dithiocarbamate ligands have asymmetric Sn-S bond lengths, Table 1. The values of Á(Sn-S) = [d(Sn-S long ) À d(Sn-S short ] for the S1-and S3-dithiocarbamate ligands are approximately the same at 0.35 Å , but the comparable bonds formed by the S3-dithiocarbamate ligand are systematically longer than those formed by the S1-dithiocarbamate ligand by approximately 0.02 Å , Table 1. The asymmetry in the Sn-S bond lengths is reflected in the disparity in the associated C-S bond lengths with the sulfur atom forming the longer Sn-S bond being involved in the significantly shorter, by approximately 0.05 Å , C-S bond, Table 1. Consistent with the skewtrapezoidal bipyramidal geometry about the Sn IV atom, the Sn-bound methyl substituents are directed over the longer Sn-S bonds and define an angle of 148.24 (11) at the tin atom. The angle subtended at the tin atom by the strongly bound sulfur atoms of 85.878 (19) is significantly less than that formed by the weakly bound sulfur atoms, i.e. 143.066 (18) , and is largely responsible for the formation of the skew-trapezoidal plane about the tin atom.

Supramolecular features
An interesting feature of the molecular packing in (I) is the formation of a supramolecular dimer sustained by SnÁ Á ÁS secondary interactions, as shown in Fig. 2a, where two long edges of the translationally displaced trapezoidal planes approach each other to form the interactions. Here, SnÁ Á ÁS4 i is 3.5654 (7) Å , which is approximately 0.4 Å shorter than the sum of the van der Waals radii of Sn and S of 3.97 Å (Bondi, 1964); symmetry operation (i): 1 À x, 1 À y, 1 À z. Connections between the dimeric aggregates are of the type methylene-C-HÁ Á ÁS and methyl-C-HÁ Á ÁO(morpholino),    The molecular structure of (I), showing the atom-labelling scheme and displacement ellipsoids at the 50% probability level.
interactions combine to generate a three-dimensional architecture, Fig. 2b.

Hirshfeld surface analysis
The Hirshfeld surfaces calculated on the structure of (I) also provide insight into the supramolecular association through secondary SnÁ Á ÁS, SÁ Á ÁS and other contacts, and was performed as per recent publications on related organotin dithiocarbamate structures (Mohamad et al., 2017(Mohamad et al., , 2016. The broad, bright-red spots appearing near the Sn and S4 atoms on the Hirshfeld surfaces mapped over d norm in Fig. 3a indicate the formation of the supramolecular dimer through secondary SnÁ Á ÁS contacts. On the Hirshfeld surface mapped over elec-trostatic potential in Fig. 4, these interactions are represented by the blue and red regions around these atoms, respectively. The faint-red spot appearing between the above bright-red spots near the S4 atom indicates the short inter-atomic SÁ Á ÁS contact, Table 3, between S4 atoms lying on diagonally opposite vertices of a parallelogram formed by symmetryrelated Sn and S4 atoms, Fig. 5a. The pair of bright-red spots appearing near the methyl-H12C and morpholine-O1 atoms in  Table 4.
In the fingerprint plot delineated into HÁ Á ÁH contacts, Fig. 6b, the points forming the single short peak at d e + d i < 2.4 Å are indicative of the short inter-atomic HÁ Á ÁH contact listed in Table 3. The involvement of S1 in the C-HÁ Á ÁS interaction Two views of the Hirshfeld surface for (I) plotted over d norm in the range À0.050 to 1.780 au.
Contact distance symmetry operation 2.36 1 2 + x, 3 2 À y, 1 2 + z and other sulfur atoms in short inter-atomic SÁ Á ÁH/HÁ Á ÁS contacts,     a pair of thin line segments, Fig. 6f, and a triangle, Fig. 6g, respectively, having minimum d e + d i distances at around 3.5 Å and 3.6 Å , respectively. The 1.1% contribution from NÁ Á ÁH/ HÁ Á ÁN contacts, Fig. 6h, to the Hirshfeld surface reflects an insignificant influence upon the molecular packing as the interatomic separations are greater than the sum of the respective van der Waals radii.

Database survey
The Cambridge Crystallographic Database (Groom et al., 2016) contains over 110 molecules of the general formula R 2 Sn(S 2 CNRR') 2 . Of these, 12 feature secondary SnÁ Á ÁS interactions which, with (I), means approximately 10% of all R 2 Sn(S 2 CNRR') 2 structures have SnÁ Á ÁS secondary interactions. Selected geometric details for the 13 structures are collated in Table 5. The SnÁ Á ÁS interactions assemble molecules in their crystals into three distinct structural motifs. The common motif, A, is a dimeric aggregate disposed about a centre of inversion, as is in (I), and is found in the majority of crystals, i.e. nine. This motif is illustrated in Fig. 7a for (PhCH 2 ) 2 Sn(S 2 CNEt 2 ) 2 (Yin et al., 2003). A second zerodimensional motif, B, is also known and is readily related to A.
In the structure of Me 2 Sn(S 2 CN(Et)CH 2 C 6 H 4 N-4) 2 (Barba et al., 2012), two independent molecules comprise the asymmetric unit. One of these self-assembles about a centre of inversion as for motif A. The nitrogen atom of each pendent 4pyridyl group of the dimeric aggregate thus assembled interacts with the tin atom of the second independent molecule via a SnÁ Á ÁN interaction to form the four-molecule aggregate shown in Fig. 7b. The final three molecules are binuclear owing to the presence of bis(dithiocarbamate) ligands and self-assemble into supramolecular chains. In {Me 2 SnS 2 CN (CH 2 Ph)CH 2 (1,3-C 6 H 3 )CH 2 (PhCH 2 )NCS 2 SnMe 2 } 2 (Santacruz-Juá rez et al., 2008), the molecule is situated about a centre of inversion and each tin atom forms an SnÁ Á ÁS contact to generate a linear, supramolecular chain, motif C, Fig. 7c. A variation is seen in the crystal of Me 2 SnS 2 CN(CH 2 CH 2 -i-Pr)CH 2 (1,3-C 6 H 3 )CH 2 (PhCH 2 )NCS 2 SnMe 2 } 2 , where there are two independent, centrosymmetric molecules in the asymmetric unit. Here, the resulting supramolecular chain is twisted (Santacruz-Juá rez et al., 2008) and is assigned as motif C 0 . The common feature of all motifs listed in Table 5 is that it is one of the weakly bound sulfur atoms that forms the   (19) 3.7050 (17) Notes: (a) piperazine mono-solvate; (b) two molecules in the asymmetric unit; (c) SnÁ Á ÁN secondary interaction; (d) the binuclear molecule is located about a centre of inversion; (e) CDCl 3 di-solvate per binuclear entity; (f) two molecules in the asymmetric unit with each being located about a centre of inversion.
secondary SnÁ Á ÁS interaction. Further, the tin-bound groups are relatively sterically unencumbered, allowing for the close approach of sulfur donors to the tin atoms. There are no geometric correlations. However, reflecting the weak nature of these interactions, the sulfur atom forming the SnÁ Á ÁS contact does not necessarily form the weaker of the Sn-S long interactions in each molecule. The range of SnÁ Á ÁS distances spans nearly 0.5 Å but, again, no correlations between these distances and the S long -Sn-S long angles is apparent, i.e. it might be expected that the shorter SnÁ Á ÁS interactions would result in wider S long -Sn-S long angles.

Synthesis and crystallization
All chemicals and solvents were used as purchased without purification, and all reactions were carried out under ambient conditions. The melting point was determined using an Electrothermal digital melting point apparatus and was uncorrected. The IR spectrum for (I) was obtained on a Perkin Elmer Spectrum 400 FT Mid-IR/Far-IR spectrophotometer in the range 4000 to 400 cm À1 . The 1 H NMR spectrum was recorded at room temperature in CDCl 3 solution on a Jeol ECA 400 MHz FT-NMR spectrometer. Sodium morpholinedithiocarbamate (prepared from the reaction between carbon disulfide and morpholine (Merck) in the presence of sodium hydroxide; 1.0 mmol, 0.185 g) in methanol (20 ml) was added to dimethyltin dichloride (Merck, 1.0 mmol, 0.219 g) in methanol (10 ml). The resulting mixture was stirred and refluxed for 2 h. The filtrate was evaporated until an off-white precipitate was obtained. The precipitate was recrystallized from methanol solution by slow evaporation to yield colourless prisms. Yield

Dimethylbis(morpholine-4-carbodithioato-κ 2 S,S′)tin(IV)
Crystal data [Sn(CH 3  where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.001 Δρ max = 0.45 e Å −3 Δρ min = −0.50 e Å −3 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.
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )