Di-n-butylbis[N-(2-methoxyethyl)-N-methyldithiocarbamato-κ2 S,S′]tin(IV): crystal structure and Hirshfeld surface analysis

A skew trapezoidal bipyramidal coordination geometry based on a C2S4 donor set is found in the structure of (C6H5)2Sn[S2CN(Me)CH2CH2OMe]2, with the SnIV atom lying on a mirror plane.

The complete molecule of the title compound, [Sn(C 4 H 9 ) 2 (C 5 H 10 NOS 2 ) 2 ], is generated by a crystallographic mirror plane, with the Sn IV atom and the two inner methylene C atoms of the butyl ligands lying on the mirror plane; statistical disorder is noted in the two terminal ethyl groups, which deviate from mirror symmetry. The dithiocarbamate ligand coordinates to the metal atom in an asymmetric mode with the resulting C 2 S 4 donor set defining a skew trapezoidal bipyramidal geometry; the n-butyl groups are disposed to lie over the longer Sn-S bonds. Supramolecular chains aligned along the a-axis direction and sustained by methylene-C-HÁ Á ÁS(weakly coordinating) interactions feature in the molecular packing. A Hirshfeld surface analysis reveals the dominance of HÁ Á ÁH contacts in the crystal.

Chemical context
The structural chemistry of molecules with the general formula R 2 Sn(S 2 CNRR 0 ) 2 is diverse with coordination geometries ranging from five, as in trigonal bipyramid (t-Bu) 2 -Sn(S 2 CNMe 2 ) 2 (Kim et al., 1987), to seven, as in pentagonal bipyramidal [MeOC( O)CH 2 CH 2 ] 2 Sn(S 2 CNMe) 2 (Ng et al., 1989). However, the overwhelming majority of structures are comprised of a six-coordinate Sn IV atom, being based on either skew trapezoidal bipyramidal or octahedral coordination geometries (Tiekink, 2008). In the former, the dithiocarbamate ligands are coordinating in an asymmetric mode and lie in a plane, with the Sn-bound organic substituents orientated over the weaker Sn-S bonds. In the octahedral molecules, the Sn-bound substituents occupy mutually cispositions. As a general observation, compounds with Snbound aryl groups are octahedral and those with Sn-bound alkyl groups are skew trapezoidal bipyramidal. However, the capricious nature of the ultimate structure adopted in the solid state is nicely illustrated in a recent study whereby Ph 2 Sn[S 2 CN(CH 2 CH 2 OMe)Me] 2 , with a dithiocarbamate ligand with dissimilar substituents, was found to be octahedral but, Ph 2 Sn[S 2 CN(CH 2 CH 2 OMe) 2 ] 2 , with the dithiocarbamate ligand having similar substituents, was skew trapezoidal bipyramidal . The structural interest notwithstanding, organotin dithiocarbamates have potential biological applications, with recent ISSN 2056-9890 investigations focusing upon biocidal activities, e.g. anti-fungal (Yu et al., 2014) and anti-bacterial (Ferreira et al., 2012), and, especially, as anti-cancer agents (Ferreira et al., 2014;Kadu et al., 2015), the focus of our interest (Khan et al., 2014(Khan et al., , 2015. During the course of the latter studies, crystals of the title compound, n-Bu 2 Sn[S 2 CN(CH 2 CH 2 OMe)Me] 2 , (I), became available. Herein, the crystal and molecular structures of (I) are described along with a detailed analysis of the molecular packing via an analysis of the Hirshfeld surface.

Structural commentary
The asymmetric unit of (I) comprises half a molecule being located on a crystallographic mirror plane with the Sn atom along with the two inner C atoms of the n-butyl groups lying on the plane, Fig. 1. The dithiocarbamate ligand coordinates the Sn atom in an asymmetric fashion with the Á(Sn-S), i.e. the difference between the Sn-S long and Sn-S short distances, being ca 0.39 Å , Table 1. This asymmetry is reflected in the associated C-S bond lengths with the short Sn-S bond being correlated with a long C-S bond length, Table 1. The coordination environment is completed by two -C atoms of the n-butyl groups. The four S atoms are co-planar and define a skewed trapezoidal plane, and the -C atoms are disposed over the weaker Sn-S bonds so that the C 2 S 4 donor set defines a skew trapezoidal bipyramidal geometry.  Hydrogen-bond geometry (Å , ).

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 (x, 1 2 À y, z). Only one component of each of the disordered n-butyl groups is shown.

Supramolecular features
The only notable contacts identified in the molecular packing are methylene-C-HÁ Á ÁS(weakly coordinating) interactions that assemble molecules into linear supramolecular chains propagating along the a-axis direction, Fig. 2a and Table 2. The chains pack in the crystal with no specific interactions between them, Fig. 2b. In order to ascertain more information of the nature of interactions between molecules, the molecular packing and its Hirshfeld surface was analysed, as discussed in Hirshfeld surface analysis.

Hirshfeld surface analysis
The Hirshfeld surface analysis for (I) was performed as described recently for organotin dithiocarbamates (Mohamad, Awang, Kamaludin et al., 2016). From the views of the Hirshfeld surface mapped over d norm , in the range À0.298 to +1.346 au, in Fig. 3, the pairs of bright-red spots near hydrogen atoms H9C and H13B of the disordered methyl groups, i.e. deviating from mirror symmetry, indicate their participation in specific intermolecular HÁ Á ÁH interactions. In the crystal, these lead to a supramolecular chain along the c axis. The presence of this dihydrogen interaction, resulting from disparate charges on respective hydrogen atoms, can also be viewed by the different curvatures and electrostatic potentials around these atoms on the Hirshfeld surface mapped over the electrostatic potential in the range À0.082 to +0.163 au, Fig. 4. Two views of the Hirshfeld surfaces mapped over the electrostatic potential highlighting the disparate charge about the terminal hydrogen atoms (the red and blue regions represent negative and positive electrostatic potentials, respectively) for (I).

Figure 3
Two views of the Hirshfeld surface mapped over d norm for (I). The disorder component has been retained in the images.
In the fingerprint plot delineated into HÁ Á ÁH contacts, Fig. 6b, a long and distinctive spike at d e + d i $ 1.8 Å represents HÁ Á ÁH bonding described above, Table 4, i.e. between methyl-H9B and -H13B atoms. The major contribution from these contacts to the Hirshfeld surface, i.e. 74.5%, and the essentially same shape of overall and HÁ Á ÁH delineated fingerprint plots in the upper (d e , d i ) region, Fig. 6a and b, show the dominance of these interactions in the molecular packing. The peak in the plot corresponding to a second short interatomic HÁ Á ÁH contact, i.e. between methyl-H2B and methylene-H10A, Table 4, is diminished within the plot due to H9BÁ Á ÁH13B interaction. The dihydrogen HÁ Á ÁH bonding also results in short interatomic CÁ Á ÁH/HÁ Á ÁC contacts, Table 4, leading to a pair of short peaks at d e + d i $ 2.8 Å in the delineated fingerprint plot, Fig. 6c; the other interatomic short CÁ Á ÁH/HÁ Á ÁC contact is merged within the plot. The presence of the weak C-HÁ Á ÁS interactions, Table 2, is seen from the fingerprint plot corresponding to SÁ Á ÁH/HÁ Á ÁS contacts, Fig. 6d, and is evident as a pair of broad peaks at d e + d i $ 2.9 Å . The fingerprint plots delineated into OÁ Á ÁH/HÁ Á ÁO and NÁ Á ÁH/HÁ Á ÁN contacts, Fig. 6e and f, contribute in a minor fashion to the Hirshfeld surface and their characteristic points are longer than their respective van der Waals separations, i.e. longer than 2.72 and 2.75 Å , respectively, and hence it is likely they do not make any significant contribution to the molecular packing.
A comment on the relationship of the modelled disorder, the contribution of HÁ Á ÁH contacts to the Hirshfeld surface and the nature of the HÁ Á ÁH contacts is warranted. In the statistical disorder model for (I), it might be normally assumed   Table 4 Short interatomic contacts in (I).
Contact distance symmetry operation 2.96 1 + x, y, z

Figure 5
A view of the Hirshfeld surface mapped over d norm for a reference molecule in contact with nearest neighbouring molecules and highlighting intermolecular C-HÁ Á ÁS and HÁ Á ÁH interactions, shown as white and black dashed lines, respectively. (as done in Fig. 2b) that that H atoms adopt positions as far apart from each other as possible rather than participate in 'non-bonded steric repulsion' (Matta et al., 2003). In (I), this does not appear to the case but, rather is an example where HÁ Á ÁH contacts contribute to the stabilization of the molecular packing. In examples where dihydrogen HÁ Á ÁH contacts are formed intramolecularly, energies of stabilization up to 10 kcal mol À1 have been suggested (Matta et al., 2003).

Database survey
The interest in organotin dithiocarbamates is reflected in the relatively large number of crystal structures available in the crystallographic literature (Groom et al., 2016). An example of this interest is twenty structures conforming to the general formula n-Bu 2 Sn(S 2 CNRR') 2 . One structure, i.e. R = R 0 = i-Pr (Farina et al., 2000), conforms to crystallographic mm2 symmetry (implying disorder in the terminal residues), seven, i.e. R = Me, R 0 = n-Bu ( The first noteworthy comment to be made on the structures of n-Bu 2 Sn(S 2 CNRR 0 ) 2 is that they all conform to the same structural motif as adopted for (I). The Sn-S short bond lengths in these structures span a relatively narrow range of 2.51 to 2.55 Å and cluster around 2.53 Å . As might be anticipated, a wider range is exhibited by the Sn-S long bonds, i.e. 2.83 to 3.08 Å and these cluster around 2.96 Å . Given the range of Sn-S short bond lengths is 0.04 Å and that for Sn-S long is 0.25 Å , the observation that differences between the average values of Sn-S short and Sn-S long span a range of 0.43 Å indicates no specific correlations exist between Sn-S short and Sn-S long bond lengths. The S short -Sn-S short , S long -Sn-S long and C-Sn-C angles cluster around 83, 147 and 136 , respectively. However, these angles span ranges of 8 (range: 80 to 88 ), 10 (140 to 151 ) and 18 (127 to 145 ), respectively. The disparity in the S-Sn-S angles is as expected from the adopted coordination geometry. While, generally, the S long -Sn-S long angles are wider than the C-Sn-C angles, there are three exceptional structures, namely R = R 0 = Et (Vrá bel et al., 1992), R = Et and R 0 = Cy (Awang, Baba, Yamin et al., 2010) and R = benzyl and R = methylene-4-pyridyl (Gupta et al., 2015) have C-Sn-C which are marginally wider, by ca 1, than the S long -Sn-S long angles. The fact of non-systematic variations in the geometric parameters in organotin dithio-carbamates has been commented upon previously (Buntine et al., 1998;Muthalib et al., 2014).
The homogeneity in the n-Bu 2 Sn(S 2 CNRR 0 ) 2 structural motif does not translate to the diphenyl analogues, i.e. Ph 2 Sn(S 2 CNRR 0 ) 2 . Of the 19 structures conforming to this general formula, seven resemble the skew trapezoidal bipyramidal motif with the majority, i.e. twelve, having a cisdisposition of the tin-bound phenyl substituents. In this context, it is noteworthy that all structures of the general formula Sn(S 2 CNRR 0 ) 2 X 2 , where X = halide, are invariably cis-S 4 X 2 octahedral (Tiekink, 2008). Given the electronegativity of a phenyl group is intermediate between that of an alkyl group and a halide, it seems that there is a fine balance between adopting one structural motif over the other for Ph 2 Sn(S 2 CNRR 0 ) 2 compounds.

Synthesis and crystallization
(2-Methoxyethyl)methylamine (10 mmol) dissolved in ethanol (30 ml) was stirred in an ice bath (ca 277 K) for 30 min. 25% Ammonia solution (ca 2 ml) was added to make the solution basic. Then, a cold ethanol solution of carbon disulfide (10 mmol) was added to the solution followed by stirring for about 2 h. Next, di-n-butyltin(IV) dichloride (5 mmol), dissolved in ethanol (30 ml), was added to the solution which was further stirred for 2 h. The precipitate that formed was

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 5. Carbon-bound H atoms were placed in calculated positions (C-H = 0.98-0.99 Å ) and were included in the refinement in the riding model approximation, with U iso (H) set to 1.2-1.5U eq (C). The molecule has crystallographic mirror symmetry with the Sn atom and n-butyl-C atoms lying on the plane. The terminal CH 2 CH 3 residue of each n-butyl group is statistically disordered across this plane.

Di-n-butylbis[N-(2-methoxyethyl)-N-methyldithiocarbamato-κ 2 S,S′]tin(IV)
Crystal data [Sn(C 4  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.