Crystal structure of bis(2-methyl-1H-imidazol-3-ium) μ-oxalato-bis[n-butyltrichloridostannate(IV)]

The SnIV atom in the centrosymmetric anion of the title salt is coordinated in a distorted octahedral fashion by two O atoms of the bridging oxalate moiety, a C atom of the butyl chain and three Cl atoms. The bis(2-methyl-1H-imidazol-3-ium) cation forms hydrogen bonds with Cl and oxalate O atoms yielding [001] chains.

The Sn IV atom in the centrosymmetric anion of the title salt, (C 4 H 7 N 2 ) 2 [Sn 2 (C 4 H 9 ) 2 (C 2 O 4 )Cl 6 ], is coordinated in a distorted octahedral mode by two O atoms of a bridging oxalate moiety, three Cl atoms and a C atom of an n-butyl group. The latter is disordered over two sets of sites in a 0.66:0.33 occupancy ratio. N-HÁ Á ÁO and N-HÁ Á ÁCl hydrogen bonds involving the 2-methylimidazolium cation and neighbouring anions result in the formation of chains extending parallel to [001].

Structural commentary
The distannate anion, [Sn 2 (C 4 H 9 ) 2 (C 2 O 4 )Cl 6 ] 2À , is located about a center of symmetry and thus only one half of the molecule is present in the asymmetric unit (Fig. 1). The full molecule consists of a central oxalate anion bridging two SnBuCl 3 moieties (Fig. 2) similar to the binuclear stannate(IV) anion reported for (Et 4 N) 2 [C 2 O 4 (SnCl 4 ) 2 ] (Skapski et al., 1974). In addition to the bis-chelating and bridging oxalate oxygen atoms, the octahedral coordination sphere is completed by three chlorine atoms and the C atom of a disordered n-butyl group (Fig. 1). The C-O distances (Table 1) are consistent with an almost perfect delocalization within the oxalate anion, as expected for a centrosymmetric bis-chelation. The Sn-C length is consistent with previously reported values (Table 1; Diop et al., 2013). The Sn-Cl distances (Table 1) are also comparable with those in related compounds, e.g. in (Bu 4 N)[SnBuCl 4 ]  (Sow et al., 2013)  . The equatorial Sn-Cl1 bond that is coplanar with the oxalate anion is considerably shorter than the Sn-Cl2 and Sn-Cl3 bonds that are oriented axially (Fig. 2, Table 1). The Sn-O1 and Sn-O2 bond lengths are fully consistent with previously characterized examples (Sow et al., 2013;Gueye et al., 2014;Sarr et al., 2015). Distortions from an ideal octahedral coordination environment are reflected in the bond angles about the Sn IV atom (Table 1). Notably, the O1-Sn-O2 angle is less than 90 and the axial Cl2-Sn-Cl3 bond angle deviates considerably from an ideal of 180 .
One methyl-2-imidazolium counter-cation is also present in the asymmetric unit. As expected, the lengths of the C-N and C7-C8 bonds indicate -delocalization in this cation (Table 1).

Supramolecular features
The imidazolium cation bridges two neighbouring [Sn 2 (C 4 H 9 ) 2 (C 2 O 4 )Cl 6 ] 2À anions through N-HÁ Á ÁO and N-HÁ Á ÁCl hydrogen bonds, leading to the formation of chains extending parallel to [001] (Fig. 3, Table 2) whereby pairs of the cations are involved in this bridging motif, each alternating across the inversion center located between the cations. The chains are connected by additional C-HÁ Á ÁCl hydrogen bonds, giving a layer structure parallel to (100). The full anion and two counter-cations in the title compound. Displacement ellipsoids are drawn at the 50% probability level. Only the major part of the disordered n-butyl chain is shown. [Symmetry code: (A) Àx + 1, Ày + 1, Àz + 1.] Table 1 Selected geometric parameters (Å , ).

Figure 3
The packing of the molecular components in a view approximately along [010]. N-HÁ Á ÁO and N-HÁ Á ÁCl hydrogen bonds are shown as dashed lines. Displacement ellipsoids are drawn at the 50% probability level.

Figure 1
The asymmetric unit of the title compound. Displacement ellipsoids are drawn at the 50% probability level. Disordered parts of the n-butyl chain are shown.

Database survey
A search of the Cambridge Structural Database (Version 5.37 with one update; Groom et al., 2016) returned 51 different structures containing 2-methyl-1H-imidazol-3-ium cations and hundreds of those containing bis-chelating oxalate anions. Those of particular relevance to the title structure have been detailed above.

Synthesis and crystallization
were obtained by mixing equimolar amounts of 2-methylimidazole with oxalic acid in water, followed by forced evaporation of the solvent at 333 K. A molar 2:1 mixture of (L) with SnBuCl 3 in acetonitrile was allowed to react. Crystals of the title compound suitable for structural examination were obtained after slow evaporation of acetonitrile at room temperature.

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. Refinement. Disorder in the n-butyl chain was modeled over two sites. Occupancies were initially refined and subsequently set to 0.66667:0.33333. Carbon atoms were refined with anisotropic atomic displacement parameters and the disordered carbon atoms were restrained to have similar displacement parameters.