Two coordination compounds of SnCl2 with 4-methylpyridine N-oxide

Single crystals of SnCl2·MePyNO, 1, and SnCl2·2MePyNO, 2, have been synthesized and structurally characterized by X-ray diffraction to investigate the structural changes of MePyNO as a result of its different coordination modes and the variation of the coordination geometry at the tin(II) atoms.

Here we report the crystal structures of two complexes of MePyNO with tin in oxidation state +II having different compositions, viz. SnCl 2 ÁMePyNO, 1, and SnCl 2 Á2MePyNO, 2. Both compounds were obtained simultaneously in the same micro-scale experiment from SnCl 2 and MePyNO in excess using N,N-dimethylformamide as solvent. As reactions were performed on reaction plates we were able to inspect the progress of the reaction by microscopy, which allowed us to observe the intermediate compound formation as well as to study the crystal growth. No scaling-up experiments were performed but 1 has previously been mentioned in the literature with respect to its elemental analysis, X-ray-powder diffraction and IR data (Kauffman et al., 1977), giving hints of a low symmetric crystal system and coordination number of three for tin. Mö ssbauer investigations have been performed by Ichiba et al. (1978).

Structural commentary
Compound 1 crystallizes in the monoclinic space group P2 1 /c, and 2 in the orthorhombic space group Pbcn, each with one formula unit in the asymmetric unit and all atoms in general positions. In both compounds, the bivalent tin atoms adopt a seesaw coordination, which results from a 2 -coordination mode of the MePyNO-molecule in 1, giving rise to a one-dimensional coordination polymer along the c axis ( Fig. 1) while there are two crystallographically different MePyNO molecules in 2, resulting in a molecular structure (Fig. 2).
Irrespective of the controversial discussion on the hybridization ability of atomic orbitals in the case of the heavier p-block elements (Kutzelnigg, 1984), the formation of fourelectron three-center bonds (Rundle, 1963), and on the functionality of the so-called 5s lone electron pair (Dé nes et al., 2013) in hypervalent (Musher, 1969)   The asymmetric unit of SnCl 2 Á2MePyNO, 2, with the atom-numbering scheme; with exception of the hydrogen atoms (which are shown as spheres with arbitrary radius) all atoms are drawn with displacement ellipsoids at the 40% probability level.

Figure 1
The asymmetric unit of SnCl 2 ÁMePyNO, 1, with the atom-numbering scheme; with the exception of the hydrogen atoms (which are shown as spheres with arbitrary radius) all atoms are drawn with displacement ellipsoids at the 40% probability level; longer Sn-O bonds expanding the coordination sphere of the tin(II) atom from three, trigonalpyramidal, to four, seesaw, are drawn as dashed sticks. the fourfold coordination sphere around the tin(II) atoms of 1 and 2 can be expressed very well in terms of the VSEPR concept (Gillespie & Hargittai, 1991): its seesaw (ss) coordination results from two equatorially bonded chlorine atoms and two more electronegative and therefore axially located oxygen atoms of the Lewis base, MePyNO.
Axes of the seesaws are bent [161.40 (6) , 1; 169.66 (6) , 2] towards the chlorine atoms properly due to electronic repulsion of the axial bonds through the 5s free-electron pairs. The corresponding Sn-O bonds are strongly different in both compounds, but differences are more expressed in

Supramolecular features
A common feature of many tin(II) compounds is the nonspherical ligand distribution around the divalent tin atom for which the term 'hemidirected' has been introduced (Shimoni-Livny et al., 1998). The resulting void in the hemidirected coordination sphere often gives rise to additional more or less weak intermolecular (and intramolecular if appropriate Lewis base donor atoms are sterically available) interactions with interesting supramolecular features. In case of 1, the formation of a one-dimensional coordination polymer via the 2 -Oatom of the MePyNO molecule can be interpreted in terms of such supramolecular interactions: in this particular case, the hemidirected coordination sphere of a molecular, trigonalpyramidal SnCl 2 ÁMePyNO complex is partially filled through the oxygen atom of a MePyNO molecule of a neighboring building unit. The resulting coordination polymer forms a zigzag chain as all atoms are situated off the crystallographic glide plane at x, 1/4, z (Fig. 3). Between the zigzag chains no further Lewis base/Lewis acid interactions below 3.5 Å are observed, but within the chains a very weak [3.460 (1) Å ] attractive interaction is found between Cl2 and Sn1 of two neighboring building units (Fig. 3).
In case of 2 the tin atom of the SnCl 2 Á2MePyNO molecules shows a similar hemidirected coordination sphere. In the solid state, neighboring molecules form dimers via attractive but very weak [3.225 (2) Å ] Sn-O interactions. Both molecules of these dimers are related to each other via a crystallographic twofold rotation axis (Fig. 4) Ball-and-stick model of the one-dimensional coordination polymer of 1 viewed parallel to the glide plane (blue line); symmetry codes used to generate equivalent atoms: ( 0 ) x, 3 2 À y, 1 2 + z; ('') x, 3 2 À y, À 1 2 + z.

Figure 4
Ball-and-stick model of the dimeric aggregates found in the crystal structure of 2 looking down the crystallographic twofold rotation axis marked in red; additional Sn-O distances are indicated by dashed sticks in gray [symmetry codes used to generate equivalent atoms marked 0 : 1 À x, y, 3 2 À z.] aggregates (Fig. 5), no further intermolecular interactions could be observed below 3.5 Å .

Synthesis and crystallization
Both complexes are formed side by side on a reaction plate in the same micro-scale experiment when small amounts (about Table 1 Experimental details.

catena-Poly[[dichloridotin(II)]-µ 2 -(4-methylpyridine N-oxide)-κ 2 O:O] (1)
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.

Dichloridobis(4-methylpyridine N-oxide-κO)tin(II) (2)
Crystal data where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.001 Δρ max = 0.81 e Å −3 Δρ min = −0.33 e Å −3 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.