Crystal structure of catena-poly[[aquadi-n-propyltin(IV)]-μ-oxalato]

The SnIV atom in the title compound shows a slightly distorted pentagonal–bipyramidal SnC2O5 coordination with the C atoms of the aliphatic chain in the axial positions.


Chemical context
In a previous paper (Reichelt & Reuter, 2014), we described the formation and structure of the first diorganotin(IV) oxalate (Ox), (R 2 Sn)Ox for R = t-butyl in the course of a systematical study on the reaction of diorganotin(IV) oxides with nitric acid (Reuter & Reichelt, 2014a,b). Applying similar reaction conditions to di-n-propyltin oxide resulted in the formation of the title compound as an unexpected side product. This diorganotin(IV) oxalate hydrate gives new insights into the structural chemistry of organotin(IV) oxalates.

Structural commentary
The asymmetric unit of the title compound comprises one half of the formula unit ( Fig. 1), consisting of an Sn IV atom lying on a twofold rotation axis, a water molecule with the O atom on the same rotation axis as the Sn atom, a bilateral chelating centrosymmetric oxalate anion and an n-propyl group attached to the Sn atom in general positions. Different from the unsubstituted t-butyl oxalate (Reichelt & Reuter, 2014), the Sn IV atom is sevenfold coordinated by two n-propyl groups, four oxygen atoms of two symmetry-related oxalate anions and one water molecule.
As a result of of symmetry, both Sn-C bond lengths are of equal length. At 2.127 (3) Å , they are considerably shorter than the Sn-C bond lengths of 2.186 (2) and 2.190 (2) Å in the di-t-butyl tin oxalate although the higher coordination number of the Sn atom in the hydrate compared with the Sn atom in the pure oxalate should result in longer bonds. This reflects the influence of the organic part (n-propyl versus t-butyl) on Sn-C bond length, as already mentioned by Britton (2006). The n-propyl group itself is well ordered as can be deduced from the aniostropic displacement parameters as well as from the C-C bond lengths of 1.521 (3) and 1.522 (4) Å , which are in good agreement with the values reported by Allen et al. (1987) for sp 3 -hybridized carbon atoms [1.513 (14) for -CH 2 -CH 3 , 1.524 (14) Å for -CH 2 -CH 2 -]. The corresponding bond angles are 117.0 (2) at C11 and 112.1 (2) at C12. All in all, this group adopts a nearly staggered conformation with an Sn1-C11-C12-C13 torsion angle of À174.3 (2) . Although both n-propyl groups attached to the Sn atom are related to each other by the twofold rotation axis, the bond angle is not exactly 180 because the Sn-C bond is not exactly perpendicular to this axis.
The two symmetry-related oxalate anions coordinate sideon to the Sn atom with only slightly different Sn-O bond lengths [Sn1-O2 = 2.290 (2) Å and Sn1-O1 = 2.365 (2) Å ]. This symmetrical coordination mode is in sharp contrast to the asymmetrical coordination mode of the oxalate anions in the anhydrous t-butyl compound [2.150 (1) to 2.4245 (1) Å ] and is also reflected in C-O bond lengths which are much more closer to each other [C-O = 1.248 (3)/1.254 (3) Å , Á = 0.006 Å ] than in the t-butyl compound [1.242 (1)/1.269 (1) Å , Á = 0.027 Å ] as an expression of more delocalized C O bonds. The oxalate ion itself is planar as it belongs to point group C i and exhibits a C-C bond length of 1.549 (4) Å , [1.545 (3) Å ], which is slightly longer than a normal bond between two sp 2 -hybridized C atoms. From the bilateral, sideon coordination mode of the oxalate anion to the organotin moieties, a one-dimensional coordination polymer parallel to [001] results ( Fig. 2).
It is remarkable that the sevenfold coordination of the Sn atom corresponds to a pentagonal bipyramid (Fig. 3). The axis formed by the two n-propyl groups is only slightly bent Ball-and-stick model of one formula unit in the crystal structure of the title compound with the atomic numbering scheme used. With exception of the H atoms, which are shown as spheres of arbitrary radius, all other atoms are drawn as displacement ellipsoids at the 50% probability level.

Figure 2
Stick-model showing a part of the one-dimensional coordination polymer. Colour code: Sn = bronze, O = red, C = dark grey, H = light grey.

Figure 3
Schematic representation of the pentagonal-bipyramidal coordination polyhedron around the Sn atom.
of 72 . These structural features are caused (i) by the distance of the chelating oxalate anion to the Sn atom, (ii) by the symmetrical position of the water molecule exactly between the two oxalate anions, and (iii) by a tilt of the plane of the oxalate anions relative to the least-squares plane through the atoms of the equatorial plane.

Supramolecular features
In the solid state, this coordination polymer is stabilized by hydrogen bonds (Table 1) between the water molecule of one chain as donor and the oxygen atom of the oxalate ion of neighboring chains as acceptor, and vice versa. As the plane of the water molecule coincides with the propagation plane of the coordination polymer, an almost planar, two-dimensional linkage of the chains results (Fig. 4). These planes are staggered one above the other with the n-propyl groups of one plane protruding into the shell of n-propyl groups of the neighboring plane (Fig. 5).

Figure 5
Perspective view of the crystal structure parallel to [001], looking down the chains of the one-dimensional coordination polymer. Table 1 Hydrogen-bond geometry (Å , ). Symmetry code: (i) x; y þ 1; z.

Refinement
All hydrogen atoms could be localized in difference Fourier syntheses. Those of the n-propyl group were idealized and refined at calculated positions riding on the carbon atoms with C-H distances of 0.99 Å (-CH 2 -) and 0.98 Å (-CH 3 ). Those of the water molecule were refined with respect to a common O-H distance of 0.96 Å and an H-O-H bond angle of 104.5 before they were fixed and allowed to ride on the corresponding oxygen atom. For the hydrogen atoms of the n-propyl group, a common isotropic displacement parameter was refined as well as one common isotropic displacement parameter for the hydrogen atoms of the water molecule. Experimental details are summarized in Table 2.

Special details
Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.