Lithium difluoro(oxalato)borate tetramethylene sulfone disolvate

The title compound, Li+·C2BF2O4 −·2C4H8O2S, is a dimeric species, which resides across a crystallographic inversion center. The dimers form eight-membered rings containing two Li+ cations, which are joined by O2S sulfone linkages. The Li+ cations are ligated by four O atoms from the anions and solvent molecules, forming a pseudo-tetrahedral geometry. The exocyclic coordination sites are occupied by O atoms from the oxalate group of the difluoro(oxalato)borate anion and an additional tetramethylene sulfone ligand.

The title compound, Li + ÁC 2 BF 2 O 4 À Á2C 4 H 8 O 2 S, is a dimeric species, which resides across a crystallographic inversion center. The dimers form eight-membered rings containing two Li + cations, which are joined by O 2 S sulfone linkages. The Li + cations are ligated by four O atoms from the anions and solvent molecules, forming a pseudo-tetrahedral geometry. The exocyclic coordination sites are occupied by O atoms from the oxalate group of the difluoro(oxalato)borate anion and an additional tetramethylene sulfone ligand.

Comment
Solvate crystal structures provide invaluable information for understanding the ionic association tendency and manner in which anions and solvent molecules coordinate Li + cations. Understanding the solid-state behavior provides insight into the various solvates that may exist in liquid solvent-lithium salt electrolytes utilized in state-of-the-art Li-ion batteries. The physiochemical and electrochemical properties of both tetramethylene sulfone (TMS) and lithium difluoro(oxalato)borate (LiDFOB) have attracted much attention recently for non-aqueous secondary battery applications.
The Li + cation in the title structure, which resides across a crystallographic inversion center, is coordinated by two sulfonyl O atoms from TMS and a carbonyl O atom from the DFOBanion ( Fig. 1). An eight member dimer ring structure is formed from this coordination by linking two Li + cations through their coordination by TMS molecules coordinated to both Li + cations with each cation coordinated by a different sulfonyl oxygen (Fig. 2). The eight membered rings are packed in the crystal structure in layers such that Z = 2 (Fig. 3).

Experimental
LiDFOB was synthesized by the direct reaction of excess boron trifluoride diethyl etherate (BF 3 -ether) with lithium oxalate (oxalic acid dilithium salt), both used as-received from Sigma-Aldrich, and extracted/recrystallized from dimethyl carbonate (DMC). The DMC-LiDFOB solvate was vacuum dried at 378 K for 48 h, yielding a high purity salt. TMS (Sigma-Aldrich, >99.8%) was used as-received. A solution was made by dissolving LiDFOB (1.566 mmol) in TMS (6.264 mmol) at 353 K. The solution was allowed to slowly cool to room temperature. Colorless crystals formed suitable for X-ray analysis on standing.

Refinement
The structure was solved by direct methods using the SIR92 program. All non-hydrogen atoms were obtained from the initial solution. The hydrogen atoms were introduced at idealized positions and were allowed to refine isotropically. 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 (Sheldrick, 2008). Graphic plots were produced using the ORTEP-3 program.