Poly[diacetonitrile[μ3-difluoro(oxalato)borato]sodium]

The title compound, [Na(C2BF2O4)(CH3CN)2]n, forms infinite two-dimensional layers running parallel to (010). The layers lie across crystallographic mirror planes at y = 1/4 and 3/4. The Na, B and two F atoms reside on these mirror planes. The Na+ cations are six-coordinate. Two equatorial coordination positions are occupied by acetonitrile molecules. The other two equatorial coordination sites are occupied by the chelating O atoms from the difluoro(oxalato)borate anion (DFOB−). The axial coordination sites are occupied by two F atoms from two different DFOB− anions.

The title compound, [Na(C 2 BF 2 O 4 )(CH 3 CN) 2 ] n , forms infinite two-dimensional layers running parallel to (010). The layers lie across crystallographic mirror planes at y = 1/4 and 3/4. The Na, B and two F atoms reside on these mirror planes. The Na + cations are six-coordinate. Two equatorial coordination positions are occupied by acetonitrile molecules. The other two equatorial coordination sites are occupied by the chelating O atoms from the difluoro(oxalato)borate anion (DFOB À ). The axial coordination sites are occupied by two F atoms from two different DFOB À anions.

Related literature
For the electrochemical properties of the DFOB À anion, see: Zhang (2007); Chen et al. (2007); Fu et al. (2010). For ionic liquids based on the DFOB À anion, see: Schreiner et al. (2009). For the benefits of ionic liquid additives in Li + ion batteries, see: Kim et al. (2010) Data collection: APEX2 (Bruker, 2009); cell refinement: SAINT (Bruker, 2009); data reduction: SAINT; program(s) used to solve structure: SHELXS (Sheldrick, 2008); program(s) used to refine structure: SHELXTL (Sheldrick, 2008); molecular graphics: ORTEP-3 (Farrugia, 1997); software used to prepare material for publication: cif2tables.py (Boyle, 2008). and high thermal stability. Recently, a new class of ILs based upon the DFOBanion has been reported (Schreiner et al., 2009). These ILs may contribute extensively to the solid-electrolyte interface (SEI) formation in Li-ion batteries (Zhang, 2007;Chen et al., 2007;Fu et al., 2010), especially when used as an additive (Kim et al., 2010;Sugimoto et al., 2009;Moosbauer et al., 2010). The synthesis of ILs based on the DFOBanion has thus far been a multi-step process involving the synthesis of tetrafluoroborate BF 4 --based ILs, which are then reacted to displace two F atoms with an oxalate moiety to form DFOBanions. The title compound, sodium difluoro(oxalato)borate (NaDFOB), can be used as an alternative reagent for the synthesis of DFOB --based ILs by reacting it directly with bromide salts with organic cations producing the DFOB --based ILs and NaBr. Therefore, the title compound may become an important reagent for use in the synthesis of ILs for Li-ion battery electrolytes.
The Na + cation in the title structure, which resides on a crystallographic mirror plane, is coordinated by two carbonyl O atoms from a single DFOBanion, two F atoms from two distinct DFOBanions and two N atoms from two acetonitrile molecules (Fig. 1). The pseudo-octahedral structure is packed in the crystal structure such that Z = 4 (Fig. 2), forming two dimensional layers in which acetonitrile molecules form the exterior of the layer (Fig. 3). The shortest C···C contact between the acetonitrile exteriors of the layers is 3.675 Å.

Experimental
NaDFOB was synthesized by the direct reaction of excess boron trifluoride diethyl etherate (BF 3 -ether) with sodium oxalate, both used as-received from Sigma-Aldrich. The resulting salt was extracted with anhydrous acetonitrile (Sigma-Aldrich) by dissolving the NaDFOB and filtering off the NaF solid byproduct. NaDFOB was allowed to slow crystallize at -20°C forming colorless crystals suitable for X-ray analysis.

Refinement
The structure was solved by direct methods using the XS program. All non-hydrogen atoms were obtained from the initial solution. The hydrogen atoms were introduced at idealized positions and were allowed to ride on the parent carbon atom.
The CH 3 orientation and the C-H distance were allowed to vary during the refinement. The structural model was fit to the data using full matrix least-squares based on F 2 . The calculated structure factors included corrections for anomalous dispersion from the usual tabulation. The structure was refined using the XL program from SHELXTL, graphic plots were produced using the ORTEP-3 crystallographic program suite.  Fig. 1. Asymmetric unit of (AN) 2 :NaDFOB reflected across its mirror plane with naming/ numbering scheme. Thermal ellipsoids are at 50% probability (Na-purple, O-red, F-green, Btan, C-grey, N-blue).

Poly[diacetonitrile[µ 3 -difluoro(oxalato)borato]sodium]
Crystal data 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 Rfactors(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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Na1 0.53868 (