Aquatrifluoridoboron–1,3-dioxolan-2-one (1/2)

Aquatrifluoridoboron and ethylene carbonate form a 1:2 co-crystal with a C=O⋯H—O—H⋯O=C hydrogen-bonding motif.

The crystal structure of the co-crystal of aquatrifluoridoboron with two ethylene carbonate (systematic name: 1,3-dioxolan-2-one) molecules, BF 3 H 2 OÁ2OC(OCH 2 ) 2 , was determined by low-temperature single-crystal X-ray diffraction. The co-crystal crystallizes in the orthorhombic space group P2 1 2 1 2 1 with four formula units per unit cell. The asymmetric unit consists of an aquatrifluoridoboron molecule and two ethylene carbonate molecules, connected by O-HÁ Á ÁO C hydrogen bonds. This crystal structure is an interesting example of a superacidic BF 3 H 2 O species co-crystallized with an organic carbonate.

Structure description
Adducts synthesized from boron trifluoride and various organic carbonates have been reported as potential functional electrolyte additives for secondary (rechargeable) lithium-ion batteries (Eisele et al., 2020), and have been shown to modify the electrode surfaces, resulting in reduced cell resistance and better capacity retention at high current rates. Recently, the use of BF 3 -based additives has been extended to divalent-metal batteries, namely calcium-ion batteries (Forero-Saboya et al., 2021;Bodin et al., 2023), where their decomposition into boron-crosslinked polymeric matrices in the passivation layer was found to be crucial for calcium plating and stripping. Such BF 3 adducts are moisture sensitive and readily hydrolyze to form BF 3 H 2 O (Simonov et al., 1996;Fonari et al., 1997). The title co-crystal formed from the boron trifluoride-ethylene carbonate (1/1) adduct, BF 3 ÁOC(OCH 2 ) 2 , upon exposure to moisture.
The BF 3 H 2 OÁ2OC(OCH 2 ) 2 co-crystal crystallizes in the orthorhombic Sohncke space group P2 1 2 1 2 1 with one aquatrifluoridoboron and two ethylene carbonate molecules in the asymmetric unit (Fig. 1). The two OC(OCH 2 ) 2 molecules have an essentially identical molecular shape (slightly twisted), which also agrees well with the crystal structure determination of 1,3-dioxolan-2-one (Atterberry & Bond, 2019 (Barthen & Frank, 2019), or adducts of BF 3 and organic carbonates (Bodin et al., 2023). The F-B-F angles [110.75 (12)-112.57 (12) ] are larger than the O-B-F angles, with the angle involving F1 [109.23 (11) ] being significantly larger than the other two angles [105.47 (11) and 106.41 (12) ]. The hydrogen atoms of the H 2 O moiety in the BF 3 H 2 O adduct are inclined toward the F1 atom, with the angle between the B-O bond and the plane defined by the water atoms being 128 (2) . The overall shape of the BF 3 moiety in BF 3 H 2 O in terms of bond lengths and angles is similar to that of the BF 4 À anion (Lozinšek, 2021).

Synthesis and crystallization
Single crystals of the BF 3 H 2 OÁ2OC(OCH 2 ) 2 co-crystal were discovered when a crystalline sample of the air-sensitive Table 1 Hydrogen-bond geometry (Å , ).

Figure 1
The asymmetric unit and the atom-labelling scheme of the BF 3 H 2 OÁ2OC(OCH 2 ) 2 co-crystal. Anisotropic displacement ellipsoids are drawn at the 50% probability level, hydrogen atoms are depicted as spheres of arbitrary radius, and hydrogen bonds are indicated by blue dashed lines.  BF 3 ÁOC(OCH 2 ) 2 adduct was examined under a protective cold nitrogen stream at about À50 C. The BF 3 ÁOC(OCH 2 ) 2 compound was synthesized from dry ethylene carbonate and BF 3 gas under anhydrous conditions, as described previously (Bodin et al., 2023). Platelet-shaped co-crystals of BF 3 H 2 OÁ2OC(OCH 2 ) 2 were located in a droplet at the tip of the aluminium trough (Veith & Bä rnighausen, 1974) of the low-temperature crystal mounting apparatus, which likely formed by an inadvertent introduction of a small amount of moisture. Selected crystals were mounted on the diffractometer employing a previously described procedure for mounting crystals at low temperatures . The crystals melted and turned into droplets when exposed to air at room temperature.

Refinement
Crystal data, data collection, and structure refinement details are summarized in Table 2. Positions and isotropic thermal displacement parameters of hydrogen atoms were freely refined (Cooper et al., 2010).

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.