Monoclinic polymorph of chlorido(dimethyl sulfoxide-κO)triphenyltin(IV)

A new polymorph of [Sn(C6H5)3Cl(C2H6OS)] has been characterized, which crystallizes in space group P21 with Z′ = 2, while the previously reported phase was in space group P212121 with Z′ = 1.

The crystal structure of the title tin complex, [Sn(C 6 H 5 ) 3 Cl(C 2 H 6 OS)], (I), has been reported with one molecule in the asymmetric unit in an orthorhombic cell [Kumar et al. (2009). Acta Cryst. E65, m1602-m1603]. While using SnPh 3 Cl as a starting material for a reaction for which the products were recrystallized over a very long time (six months) from dimethyl sulfoxide (DMSO), a new polymorph was obtained for (I), with two independent molecules in the asymmetric unit of a monoclinic cell. The coordination geometry of the Sn centres remains unchanged, with the Cl À ion and the DMSO molecule in the apical positions and the phenyl C atoms in the equatorial positions of a trigonal bipyramid. The main difference between the polymorphs is the relative orientation of the phenyl rings in the equatorial plane, reflecting a degree of free rotation of these groups about their Sn-C bonds. In the crystal, molecules are linked into [010] chains mediated by weak C-HÁ Á ÁO interactions.

Chemical context
The Dakar research group and others worldwide have been focusing for a long time on the study of interactions of ammonium salts of oxyacids with metallic halides, to obtain adducts and complexes in which the oxyanion behaves as a ligand through its O atoms (Diassé -Sarr & Diop, 2011;Pouye et al., 2014;Toure et al., 2016;Sarr et al., 2016;Ng & Hook, 1999). The main advantage of this general strategy is the high solubility of the ammonium salts in common organic solvents, which facilitates the development of traditional synthetic methods in solution. The well-known flip side is that separation and purification procedures are almost always necessary, and that such syntheses are not in line with the principles of Green Chemistry, since solvent is an intrinsic waste. However, from time to time, when the recrystallization is the method of purification, as-yet undiscovered polymorphs of unreacted materials, products or by-products, are emerging. In such instances, the involved chemistry may be of little interest, while the chemical crystallography of the unexpected polymorph(s) may be of significant interest, even in borderline cases like the disappearing polymorphs (Bučar et al., 2015). Actually, the propensity of a given molecule to crystallize in various polymorphic forms is still difficult to predict (Price, 2009), and, for example, Ostwald's 'law of stages' that states it is the least stable polymorph that crystallizes first, is of limited interest for concrete crystallizations (Threlfall, 2003). The current situation is thus that a significant number of new polymorphs are still obtained serendipitously, using a tech-nique that could be coined as crystallization by oblivion. The herein reported title compound, (I), a new monoclinic polymorph of a frequently used starting material in tin chemistry, was obtained in this way: in one of our research programs, we have initiated the study of the interactions between [CH 3 NH 2 (CH 2 ) 2 NH 2 CH 3 ]SO 4 and SnPh 3 Cl in a mixture of CH 2 Cl 2 and dimethyl sulfoxide (DMSO) as solvent. One of the products obtained in an attempt of crystallization carried out over a very long time was the adduct obtained by addition of DMSO to the starting material SnPh 3 Cl, to form [SnPh 3 Cl(DMSO)]. The crystal structure of this compound has been reported previously, in space group P2 1 2 1 2 1 (Kumar et al., 2009;CSD refcode: RUGYOI, Groom et al., 2016). In that case, crystals were obtained by dissolving SnPh 3 Cl in hot DMSO, affording fine colourless crystals by solvent evaporation over three days.

Structural commentary
Instead of the known orthorhombic structure of the title compound, we crystallized a monoclinic polymorph, in space group P2 1 , with two molecules in the asymmetric unit (Fig. 1).
The independent molecules display different conformations, as a consequence of a degree of free rotation of the phenyl groups about their Sn-C bonds. An overlay between both molecules gives deviations as high as 1.7 Å , and the rotation of one phenyl group is obvious (Fig. 1, inset). This conformational flexibility seems to be the reason why the compound has at least two stable polymorphs, even if the trigonal-bipyramidal geometry for the Sn centre is retained. The relative orientation of the phenyl rings in the observed conformers may be estimated using the dihedral angles formed by the rings in each molecule. These angles span a large range, from 28.3 (4) to 87.2 ( Table 1). As a consequence, the orientation of the DMSO molecule with respect to the SnPh 3 core is also variable. In the orthorhombic phase, the S-Me groups of DMSO are staggered with the Sn-C bonds; in the new monoclinic phase, one complex displays a similar conformation, while in the other the S-Me groups are eclipsed with the Sn-C bonds (Fig. 2). The resulting simulated powder diffraction patterns for each polymorph are, as expected, also very different (Fig. 2).
With such contrasting features for the dimorphic phases of [SnPh 3 Cl(DMSO)], obtained basically from DMSO solutions using short and long evaporation times, one could expect the apparition of other phases under different conditions of crystallization, for example by varying the solvent or the temperature of crystallization.

Supramolecular features
In the extended structure of the orthorhombic phase, one methyl group in DMSO forms weak C-HÁ Á ÁCl and C-HÁ Á Á interactions, and molecules related by the 2 1 screw axis in the [010] direction featureinteractions between two phenyl rings, separated by 3.934 (3) Å (Kumar et al., 2009). In the monoclinic form, molecules related through the 2 1 axis in space group P2 1 no longer forminteractions. The supramolecular structure of (I) is based rather on weak C-HÁ Á ÁCl contacts involving, as in the first polymorph, the methyl groups of the DMSO molecule as donor, with HÁ Á ÁCl separations Acta Cryst. (2018). E74, 163-166 research communications Table 1 Relative orientation ( ) of the phenyl rings in the three conformers of the title molecule.

Figure 1
The asymmetric unit for the new monoclinic phase of the title compound, with displacement ellipsoids at the 30% probability level. The inset is a fit between independent molecules, based on all non-H atoms (Macrae et al., 2008), evidencing the rotation of one phenyl ring.
ranging from 2.82 to 2.94 Å . The resulting supramolecular one-dimensional structure is a zigzag chain of alternating Sn1 and Sn2 independent molecules, running along the screw axis ( Fig. 3). The absence of other stabilizing intermolecular contacts may suggest a less thermodynamically stable crystal, compared to the orthorhombic crystal obtained by fast crys-tallization, in contradiction with Ostwald's rule (Threlfall, 2003). However, the crystal structures are in agreement with the calculated densities for both polymorphs: 1.562 g cm À3 for the orthorhombic form and 1.514 g cm À3 for the less stable monoclinic form reported here.

Database survey
According to the CSD (V5.39; Groom et al., 2016) Part of the crystal structure of the title polymorph, showing the supramolecular network formed along the screw axis 2 1 in space group P2 1 . Dashed bonds represent C-HÁ Á ÁCl intermolecular contacts.
[Symmetry codes:  in a 1:1 ratio. Slow evaporation of the resulting solution at 300 K gave after six weeks a yellowish viscous liquid supposed to be [CH 3 NH 2 (CH 2 ) 2 NH 2 CH 3 ]SO 4 (L). When L (0.024 g, 0.130 mmol) dissolved in 50 ml of a 1:1 water/ethanol mixture was reacted with SnPh 3 Cl (0.100 g, 0.260 mmol) dissolved in a 1:1 dichloromethane/methanol mixture (50 ml), a slightly cloudy solution was obtained and filtered. The filtrate, when submitted to a slow solvent evaporation at 300 K over three days, produced a powder, which was redissolved in DMSO. Slow solvent evaporation at 300 K over six months afforded colourless blocks of (I) suitable for X-ray diffraction.