Crystal structures of diaquadi-μ-hydroxido-tris[trimethyltin(IV)] diformatotrimethylstannate(IV) and di-μ-hydroxido-tris[trimethyltin(IV)] chloride monohydrate

The title compounds are partially condensed products of hydrolysed trimethyltin chloride. In the two structures, short cationic tristannatoxanes (C9H29O2Sn3) are bridged by a diformatotrimethyltin anion or a chloride anion.


Chemical context
Nowadays, there are many discussions about climate change and CO 2 emissions. Therefore, the activation of CO 2 plays an important role in today's research. It is already known that CO 2 is activated by electroreduction of different metals (Machunda et al., 2011). A selective method to transform CO 2 into formate uses nanostructured tin catalysts (Zhang et al., 2014). Compound 1 (Fig. 1) was formed from atmospheric CO 2 and thus can be regarded in the context of tin-mediated CO 2 activation. Compound 2 (Fig. 2) shows structural analogies and is also discussed herein. Structures 1 and 2 were obtained as byproducts from trapping reactions with trimethyltin chloride (Dä schlein et al., 2010;Unkelbach et al., 2012;Koller et al., 2015). ISSN 2056-9890

Structural commentary
In the crystal structures, no polymeric Sn-O structures were formed, as found in the trimethyltin hydroxide. The short trimethyltin hydroxide chain has a positive and the chloride or bisformatostannate a negative charge. In the structure of 1, both the cation and the anion are located about a twofold rotation axis whereas in that of 2 all atoms are on general positions. Owing to the presence of hydrogen bonds, there is a change to a smaller Sn-O-Sn angle relative to the polymeric trimethyltin hydroxide (Sn-O-Sn = 140 ; Anderson et al., 2011). In 1, the Sn1-O1-Sn2 angle is 135.44 (9) while in 2 it is 135.30 (17) . In the chloride structure 2, a change in two further angles is noticed. The O1-Sn1-Cl1 angle [177.58 (10) ] and the O2-Sn3-Cl1 0 angle [175.5 (12) ] decreases (compare Lerner et al., 2005). The water molecules exist in different situations in the two structures. In the formate structure 1, a water molecule coordinates directly to the Sn2 atom. In compound 2, the water is embedded in a hydrogen-bonded network between the negatively charged hydroxyl unit (O3Á Á ÁH2-O2) and the chloride anion.

Supramolecular features
As described, both structures are intermolecularly linked via hydrogen bonds. In structure 1 (Fig. 3 and Table 1), the formate anion is sterically too demanding to coordinate directly to the outer tin atom of the cationic chain. Therefore, the formate bridges four cationic tristannoxanes via hydrogenbonding interactions (O3Á Á ÁH2A--O2, O4Á Á ÁH2B--O2), thus forming a two-dimensional network. Additionally, hydrogen bonds between these sheets form a two-dimensional network along the bc plane (O4Á Á ÁH1-O1).

Figure 2
The molecular structure and atom numbering for compound 2, with displacement ellipsoids drawn at the 30% probability level. [Symmetry codes: (i) 1 2 + x, Ày, z; (ii) 3 2 À x, y, 1 2 + z; (iii) 3 2 À x, À1 + y, 1 2 + z.]   Thus, a three-dimensional network of hydrogen bridges is formed. The interactions between Sn-Cl differ due to steric repulsion of the C2 and C7 iii methyl groups. The van der Waals radius of a methyl group is 2 Å (Brown et al., 2009) and the distance between the two units is ca 3.9 Å .

Database survey
The basic building block, trimethyltin hydroxide, has been known for a long time and has been completely characterized (Kraus & Bullard, 1929;Okawara & Yasuda, 1964). Since then, studies using single crystal X-ray analysis have been made for the exact structure. A polymeric structure with eight units has been found, which has an angle of ca 140 for the Sn-O-Sn bond (Anderson et al., 2011). Tiekink (1986) succeeded in obtaining a bis(trimethyltin)carbonate, wherein the basic polymeric structure has been changed. Here, the trimethyltin units are linked via a carbonate. A dimeric structure including chloride as anion and water is also noted. The tin atoms are coordinated by the bridging Cl and HO substituents and angles of 133.2 (2) for Sn1-Cl1-Sn2 and 179.2 (2) for O1-Sn1-Cl1 were observed (Lerner et al., 2005).

Synthesis and crystallization
The two structures were obtained as byproducts from trapping reactions with trimethyltin chloride (Strohmann et al., 2006;Ott et al., 2008

Figure 4
Crystal packing of compound 2. H atoms not involved in hydrogen bonds have been omitted for clarity. Hydrogen bonds are drawn as black dashed lines (see Table 2).
conditions for a few months. By reaction with atmospheric moisture, partial hydrolysis occurred. In the case of compound 1, CO 2 was also activated by a tin-mediated reaction.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. H atoms involved in hydrogen bonding were located in a difference Fourier synthesis map and freely refined. All other H atoms were positioned geometrically and refined using a riding model: C-H = 0.98 Å with U iso (H) = 1.5U eq (Cmethyl). The CH 3 hydrogen atoms were allowed to rotate but not to tip. Due to point group symmetry 2 of both the cation and anion in 1, with the twofold rotation axis running through the respective central Sn atom and one of the methyl groups, the latter is equally disordered over two positions. For both compounds, data collection: APEX3 (Bruker, 2014); cell refinement: SAINT (Bruker, 2014); data reduction: SAINT (Bruker, 2014); program(s) used to solve structure: SHELXT (Sheldrick, 2015a); program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015b); molecular graphics: OLEX2 (Dolomanov et al., 2009); software used to prepare material for publication: OLEX2 (Dolomanov et al., 2009). (

1) Diaquadi-µ-hydroxido-tris[trimethyltin(IV)] diformatotrimethylstannate(IV)
Crystal data 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 )
x y z U iso */U eq Occ. (<1)  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.