Redetermination of the crystal structure of dimethylbis[2,4-pentanedionato(1−)-κ2 O 2,O 4]tin(IV)

The current redetermination confirms the previous structure report, but with considerably higher precision and accuracy.

The redetermination of the title compound, [Sn(CH 3 ) 2 (C 5 H 7 O 2 ) 2 ] or SnMe 2 (acac) 2 , from CCD data recorded at 100 K basically confirms the previous study based on integrated film data recorded at room temperature [Miller & Schlemper (1972). Inorg. Chem. 12, 677-681], but reveals a remarkable shrinkage of the a axis [7.12 (1) > 6.7694 (4) Å ]. The molecule belongs to point group C i with the Sn IV atom on a centre of inversion. The Sn IV atom shows a slightly distorted octahedral coordination sphere with the methyl groups in trans positions and a Sn-C bond length of 2.115 (2) Å which may serve as a standard value for an Sn-CH 3 bond of an octahedrally coordinated Sn IV atom. The Sn-O bonds involving the two carbonyl groups of the acetylacetonate ligand are of equal length [2.180 (1) and 2.183 (1) Å ], as are the C O [1.273 (1) and 1.274 (1) Å ] and C-C bond lengths [1.393 (2) and 1.400 (2) Å ]. The acetylacetonate ligand deviates considerably from planarity, with a dihedral angle of 5.57 (9) between the least-squares planes of the two acetone moieties. The four O atoms of the two symmetry-related acetylacetonate ligands are arranged in a nearly quadratic rectangle. Weak C-HÁ Á ÁO interactions consolidate the crystal packing.

Chemical context
The crystal structure of the title compound, [Sn(CH 3 ) 2 (C 5 H 7 O 2 ) 2 ] or SnMe 2 (acac) 2 , was determined in the early 1970s at room temperature by visual estimation of film data and refined to a final conventional R value of 0.079 (Miller & Schlemper, 1972). All bond lengths and angles of the original study seem chemically reasonable but accuracy suffers from the limited precision of that kind of data collection. As SnMe 2 (acac) 2 serves as a reference for all diorganotin(IV) diacetylacetonates and bis-1,3-diketonates in general, we decided to redetermine its structure from CCD data recorded at low temperature. Moreover, the title compound is an excellent candidate for the determination of the Sn-C Me bond length as another reference in case the Sn IV atom is in a well-defined octahedral coordination. The precise measurement of this Sn-C distance therefore should supplement the observations of Britton (2006) who found a significant change in Sn-C bond lengths depending on the organic moiety attached to an Sn IV atom.

Structural commentary
The redetermination of the crystal structure of the title compound confirms the former results obtained by Miller & Schlemper (1973) with respect to the chosen space group (P2 1 /n) and the constitution of the asymmetric unit comprising half a formula unit with the Sn atom at a crystallographic centre of inversion [Wyckhoff symbol: b]. As we performed the X-ray measurement at 100 K, the unit-cell volume is somewhat smaller in comparison with the original roomtemperature data which is mainly caused by a considerable change of the a axis from 7.12 (1) to 6.7694 (4) Å while changes of all other lattice parameters [b original = 13.87 (2), c original = 7.69 (1) Å , original = 104.7 (2) ] show a normal temperature-dependent shrinkage.

Figure 2
Twisting of the acetylacetonate ligand at atom C2 with respect to the least-squares planes (green dashed lines) O1/C1/C2/C5 and O2/C3/C2/C4 in a view parallel to these planes. Non-H atoms are shown as displacement ellipsoids at the 50% probability level.

Figure 3
Orientation of the acetylacetate ligand (least-squares plane through both carbonyl groups) in relation to the plane defined by the four O atoms coordinating to the Sn IV atom. The view is parallel to these planes (green lines). Non-H atoms are shown as displacement ellipsoids at the 50% probability level.

Figure 1
The molecular structure of the title compound, showing the atom-labeling scheme of the asymmetric unit. Displacement ellipsoids are drawn at the 50% probability level.

Synthesis and crystallization
The synthesis of the title compound by refluxing a suspension of dimethyltin oxide, Me 2 SnO, in acetylacetone, acacH, for several hours followed the procedure and experimental details described by McGrady & Tobias (1965). Single crystals for X-ray diffraction were grown from toluene solution. A suitable single crystal was selected under a polarization microscope and mounted on a 50 m MicroMesh MiTeGen Micromount TM using FOMBLIN Y perfluoropolyether (LVAC 16/6, Aldrich). The crystals are stable in air.

Refinement details
Crystal data, data collection and structure refinement details are summarized in Predominant OÁ Á ÁH-C contacts (blue dotted lines) of O atoms with the methyl H atoms of the acetylacetonate groups of neighbouring molecules. The central molecule is drawn in space-filling mode, while neighbouring molecules are drawn in the stick-model mode visualizing the delocalized system of the acetylacetonate ligands.

Dimethylbis[2,4-pentanedionato(1-)-κ 2 O 2 ,O 4 ]tin(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.