2,2-Difluoro-4-phenyl-1,3,2-dioxaborolo[4,5-c]chromen-5-ium-2-ide

In the crystal, the inversely oriented molecules of the title compound, C15H9BF2O3, form stacks along the a axis via π–π interactions between parallel phenylchromenium fragments. Linked by a network of C—H⋯F interactions, the stacks form layers in the ac plane that are dispersively stabilized in the crystal structure. Two F atoms bonded to the B atom are located in the plane perpendicular to the planar skeleton of the molecule made rigid by two intramolecular C—H⋯O interactions.

In the crystal, the inversely oriented molecules of the title compound, C 15 H 9 BF 2 O 3 , form stacks along the a axis viainteractions between parallel phenylchromenium fragments. Linked by a network of C-HÁ Á ÁF interactions, the stacks form layers in the ac plane that are dispersively stabilized in the crystal structure. Two F atoms bonded to the B atom are located in the plane perpendicular to the planar skeleton of the molecule made rigid by two intramolecular C-HÁ Á ÁO interactions.
Data collection: CrysAlis CCD (Oxford Diffraction, 2008); cell refinement: CrysAlis RED (Oxford Diffraction, 2008); data reduction: CrysAlis RED; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: ORTEP-3 (Farrugia, 1997); software used to prepare material for publication: SHELXL97 and PLATON (Spek, 2009  3-Hydroxy-2-phenyl-4H-chromene-4-one (flavonol) and its derivatives have been investigated for a long time owing to their unique spectral properties emerging from the excited state intramolecular proton transfer occurring in them (Sytnik et al., 1994). These compounds turned out to be convenient analytical probes because of their considerable ability to complex various chemical entities (molecules, ions) (Roshal et al., 1998;Roshal et al., 2003;Petković et al., 2010). The latter property was the reason for turning our attention to the possibility of applying of flavonol as an analytical spectral probe for boron compounds (Roshal et al., 2002). As part of these investigations we wanted to see how flavonol behaved in the presence of BF 3 . Thus we mixed both reagents in dichloromethane, expecting to obtain a molecular complex of flavonol and BF 3 .
The structure of the crystalline product that was actually separated is presented here. It appears that not complexation but condensation of the two reagents, accompanied by the release of HF, takes place (Kukharenko & Avramenko, 2001) and a molecule of a formally zwitterionic canonical structure was produced. Crystal structures of various flavonol complexes have so far been reported (Farina & Yamin, 1995;Belogh-Hergovich et al., 1999;Okabe et al., 2003;Kaizer et al., 2007), but none of them have contained boron.
The canonical structure of the title compound suggests that the phenylchromenium core of the molecule is aromatic. This is confirmed by analysis of the bond lengths and angles, as well as comparison of the structure determinated here with the structures of selected compounds containing flavonol units (Farina et al., 1995;Belogh-Hergovich et al., 1999;Okabe et al., 2003;Kaizer et al., 2007). Furthermore, the average deviation from planarity of the phenylchromenium core is 0.0215 (2) and that of the molecule's skeleton is 0.0373 (2). This implies that both the above-mentioned molecular fragments are planar and that two F atoms at the B atom are located in a plane perpendicular to the molecular one (the dihedral angle between the plane of the molecule's skeleton and the plane of B1-F1-F2 is 89.5 (1)°). Furthermore, two intramolecular C-H···O interactions (Table 1, Fig. 1) stiffen the phenylchromenium core, undoubtedly contributing to its planarity.
In the crystal structure, inversely oriented molecules form stacks along the a axis via π-π interactions between parallel phenylchromenium fragments (  Figs. 2 and 3). The above-mentioned layers are dispersively stabilized in the crystal lattice.

Experimental
The title compound was obtained during the reaction of 3-hydroxy-2-phenyl-4H-chromene-4-one (flavonol) with BF 3 (Roshal et al., 2002). Thus, boron trifluoride dissolved in anhydrous diethyl ether was added dropwise, with continuous stirring, to an equimolar amount of a saturated solution of flavonol in anhydrous dichloromethane. After evaporation of the solvents, the residue was recrystallized twice from N,N-dimethylformamide yielding yellow fluorescing crystals suitable for X-Ray investigations (m.p. 485 -487 K).
supplementary materials sup-2 Refinement H atoms were positioned geometrically, with C-H = 0.93 Å, and constrained to ride on their parent atoms with U iso (H) = 1.2U eq (C). Fig. 1. The molecular structure of the title compound showing the atom-labeling scheme. Displacement ellipsoids are drawn at the 25% probability level, and H atoms are shown as small spheres of arbitrary radius. Cg1, Cg2 and Cg3 denote the ring centroids. The C-H···O hydrogen bonds are represented by dashed lines. 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.
Cg1, Cg2 and Cg3 are the centroids of the O3/C7-C10/C15, C10-C15 and C1-C6 rings, respectively. CgI···CgJ is the distance between ring centroids. The dihedral angle is that between the planes of the rings I and J. CgI_Perp is the perpendicuar distance of CgI from ring J. CgI_Offset is the distance between CgI and the perpendicular projection of CgJ on ring I. Symmetry codes: (ii) -x + 1, -y, -z + 1; (iii) -x, -y, -z + 1.