Crystal structure of μ-oxalato-κ2 O 1:O 2-bis[(dimethyl sulfoxide-κO)triphenyltin(IV)]

The structure of the studied adduct consists of two dimethyl sulfoxide molecules coordinating the [C2O4(SnPh3)2] core via their oxygen atoms. The Sn atoms display a trans trigonal–bipyramidal [SnC3O2] arrangement, and the oxalate dianion behaves as a bidentate bridging ligand.

In the previously reported [C 2 O 4 (SnPh 3 ) 2 ] complex [Diop et al. (2003). Appl. Organomet. Chem. 17,[881][882], the Sn IV atoms are able to formally complete their coordination by addition of dimethyl sulfoxide (DMSO) molecules provided by the reaction medium, affording the title complex, [Sn 2 (C 6 H 5 ) 6 (C 2 O 4 )(C 2 H 6 OS) 2 ]. The Sn IV atoms are then pentacoordinated, with a common trans trigonal-bipyramidal arrangement. The asymmetric unit contains one half-molecule, which is completed by inversion symmetry in space group type C2/c. The inversion centre is placed at the mid-point of the central bis-monodentate oxalate dianion, C 2 O 4 2À , which bridges the [(SnPh 3 )(DMSO)] moieties. The molecule crystallizes as a disordered system, with two phenyl rings disordered by rotation about their Sn-C bonds, while the DMSO molecule is split over two positions due to a tetrahedral inversion at the S atom. All disordered parts were refined with occupancies fixed of 0.5.

Chemical context
One of the values of Sn IV coordination chemistry is related to the ambiguous valency of this main element, for which a plethora of tetra-and pentacoordinated compounds have been described. This makes a difference with C and Si compounds, based almost exclusively on tetravalent nodes, with very few cases of hypervalency. For Sn mononuclear compounds, a survey of the current Cambridge database (CSD V5.38 updated February 2017; Groom et al., 2016) shows that coordination number four is more represented than coordination number five, with distributions of 63 and 37%, respectively, for the ca 4700 structures deposited to date. Stable compounds with a coordination number of four for the Sn IV atom are thus attractive starting materials for the chemistry of Sn IV complexes with a coordination number of five, including polynuclear species, which have no equivalent with the other elements of group 14. Triphenyltin chloride, SnPh 3 Cl, is one of these well used molecules, with the additional advantage that the Cl atom may behave as a leaving group, while the SnPh 3 fragment is a stable core structure.
The here reported dinuclear compound is a continuation of previous works carried out by the Dakar group about the synthesis of Sn 2 complexes using the oxalate dianion as a bridging ligand. The simplest member of this family is [C 2 O 4 (SnPh 3 ) 2 ], where both Sn sites exhibit coordination number four (Diop et al., 2003 (Diop et al., 1997;Gueye et al., 2011;Sow et al., 2012). In this dynamic, we now report a new complex synthesized using a mixture of dimethyl sulfoxide (DMSO) and methanol as solvent. The former component of this mixture is clearly a more stabilizing ligand for Sn atoms, resulting in the crystallization of the title compound, {C 2 O 4 [(SnPh 3 )(DMSO)] 2 }. Interestingly, the complex [(DMSO)SnPh 3 ] is known (Kumar et al., 2009), but was not detected in this reaction, indicating that the oxalatebridged species is probably formed prior to solvent coordination.

Structural commentary
As expected, the oxalate dianion behaves as a bis-monodentate 2 -bridging ligand for two [SnPh 3 (DMSO)] moieties. The resulting neutral dinuclear complex is situated on a crystallographic inversion centre, placed at the midpoint of the C-C bond of the oxalate bridge ( Fig. 1). Although that symmetry is consistent with the molecular symmetry, the molecule is strongly disordered: two of the three phenyl rings in the asymmetric unit are disordered over two positions by rotation about their Sn-C bonds, and the DMSO molecule is also disordered over two positions, as a consequence of an inversion at the tetrahedral S atom.
The Sn IV atom is pentacoordinated in a common trans trigonal-bipyramidal manner, the phenyl groups being in equatorial positions, while the coordinating O atoms from the oxalate and DMSO ligands occupy the apical sites. The three Sn-C bonds are similar in length to those already reported for related complexes including the SnPh 3 fragment (Sow et al., 2012;Gueye et al., 2011) Ng et al., 1994). This tight bonding character for the bridging oxalate may be related to its planar conformation, imposed by symmetry. The staggered arrangement for the six phenyl rings, also imposed by symmetry, avoids any steric hindrance in the complex. The apical DMSO molecule has an Sn-O bond length of 2.354 (6)-2.403 (6) Å , reflecting a less pronounced coordination strength.

Supramolecular features
These discrete binuclear molecules interact through van der Waals forces in the crystal, and no strong interactions are observed. The carbonyl groups of the oxalate dianion, C1-O1 and C1 O2, are the unique potential acceptor groups for hydrogen bonding, and indeed, weak intermolecular C-HÁ Á ÁO contacts are formed (Table 1): two molecules related by a glide plane are oriented almost perpendicular, in such a way that methyl groups of the terminal DMSO ligands in one molecule form C-HÁ Á ÁO contacts with the oxalate bridge of the other molecule (Fig. 2). These contacts are favoured by the disorder affecting the DMSO ligands, and allow to pack the complexes densely in the crystal, even in the absence of anycontacts between the phenyl rings.  Table 1 Hydrogen-bond geometry (Å , ). Symmetry codes: (i) x; Ày þ 1; z À 1 2 ; (ii) Àx þ 3 2 ; y þ 1 2 ; Àz þ 3 2 .

Figure 1
The molecular structure of the title complex, with displacement ellipsoids at the 30% probability level. For phenyl rings C8-C13 and C14-C19 and for the DMSO molecule, only disordered part A (occupancy 0.5) is represented, and all H atoms are omitted. Unlabelled atoms are generated by the symmetry operation ( 3 2 À x, 1 oxalate bridge dominates. The oxalate dianion is, however, known for having a very rich coordination behaviour, and the 2 -2 -O,O 0 coordination mode observed in the title compound is not the most common. Limiting the survey to Sn compounds, the chelating bis-bidentate bridging mode is more represented (i.e. polynuclear complexes including the 2 - In that case, the conformation of the bridge is invariably planar, while the 2 -2 -O,O 0 bridge may be planar or twisted.

Synthesis and crystallization
[CH 3 NH 2 (CH 2 ) 2 NH 2 CH 3 ]C 2 O 4 (L) was obtained as a powder, on mixing the diamine CH 3 NH(CH 2 ) 2 NHCH 3 with C 2 O 4 H 2 Á-2H 2 O in a 1:1 ratio (v/v) in water, and allowing the water to evaporate at 333 K. When 0.10 g (0.26 mmol) of SnPh 3 Cl in 15 ml of a 1:1 ratio (v/v) DMSO/methanol mixture was reacted with 0.06 g (0.26 mmol) of L, a clear solution was obtained. Slow solvent evaporation over two weeks afforded a powder, which was collected. This powder dissolved in acetonitrile gave a slightly cloudy solution, which was quickly filtered off. The resulting clear solution, when allowed to evaporate slowly, afforded, six months after, colourless crystals of the title complex suitable for X-ray diffraction.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. The molecular structure is strongly disordered. Three different data sets were collected for three Part of the crystal structure viewed down reciprocal axis a*. For disordered phenyl rings, only one orientation is retained, while both disordered parts for the DMSO molecules are represented, in green and magenta (parts A and B, respectively). Intermolecular C-HÁ Á ÁO contacts listed in Table 1 are represented for the central molecule as blue dashed lines. different crystals, on different diffractometers; however, all gave the same disordered structure. In the asymmetric unit, two of the three phenyl rings are disordered over two positions: rings C8-C13 and C14-C19 were split over sites A and B. Attempts to refine site occupancies for the disordered parts resulted in free variables converging to values very close to 1/2 [maximum deviation for DMSO: 0.477 (5) and 0.523 (5)] with no clear improvement for the involved displacement parameters.
The four rings were restrained to be flat with standard deviation of 0.02 Å 3 , and the C atoms in a given ring were restrained to have the same anisotropic components, within a standard deviation of 0.04 Å 2 . Finally, A and B rings for each disordered phenyl group were restrained to have the same geometry (standard deviations: 0.02 Å for C . . . C bond lengths and 0.04 Å for 1,3-distances). The DMSO molecule is also disordered over two positions, labelled A and B, with occupancies fixed to 0.5. These parts were refined freely.  (Sheldrick, 2008) and Mercury (Macrae et al., 2008); software used to prepare material for publication: SHELXL2016 (Sheldrick, 2015b).