Crystal structure and C—H⋯F hydrogen bonding in the fluorinated bis-benzoxazine: 3,3′-(ethane-1,2-diyl)bis(6-fluoro-3,4-dihydro-2H-1,3-benzoxazine)

The packing of the title benzoxazine derivative features C—H⋯F hydrogen bonds, which form a sheet structure further linked via weak C—H⋯π hydrogen bonds.

The title fluorinated bisbenzoxazine, C 18 H 18 F 2 N 2 O 2 , crystallizes with one halfmolecule in the asymmetric unit, which is completed by inversion symmetry. The fused oxazine ring adopts an approximately half-chair conformation. The two benzoxazine rings are oriented anti to one another around the central C-C bond. The dominant intermolecular interaction in the crystal structure is a C-HÁ Á ÁF hydrogen bond between the F atoms and the axial H atoms of the OCH 2 N methylene group in the oxazine rings of neighbouring molecules. C-HÁ Á Á contacts further stabilize the crystal packing.

Chemical context
Even though benzoxazines have been known for more than 60 years, a cursory look at the literature cited in relation to the polybenzoxazines in recent years reveals increasing interest in polybenzoxazine chemistry (Demir et al., 2013). Mannich condensation of a phenol and a primary amine with formaldehyde is perhaps the best synthetic route widely employed for the preparation of a variety of benzoxazine monomers. Mono-functional benzoxazines with one oxazine ring yield linear polymers, while bi-and polyfunctional benzoxazines produce cross-linked polymers. As a result, many kinds of benzoxazine monomers, including both mono-benzoxazines and bis-benzoxazines, have been synthesized. For composite applications, bifunctional benzoxazines are important as they produce fillers with good adhesion properties that in turn give high modulus composite materials (Santhosh-Kumar & Reghunadhan-Nair, 2014).
Much work in our group has been directed at the synthesis of a wide variety of bis-benzoxazines from ethylendiamine, formaldehyde and phenols in the molar ratio of 1:4:2 using a conventional method and solvent-free conditions (Rivera et al., 1989). Recently, we have also investigated the crystal structures of several bis-benzoxazines namely 3,3 0 -(ethane-1,2-diyl)bis(6-substituted-3,4-dihydro-2H-1,3-benzoxazine) ISSN 2056-9890 derivatives (Rivera et al., 2010(Rivera et al., , 2011(Rivera et al., , 2012a. These were prepared to determine whether replacement of the substituents at the para position of the phenol affects the molecular conformation and possible supramolecular aggregation. In this context, the title compound is a model for studying nonconventional molecular interactions where the halogen atom may act as a hydrogen-bond acceptor. Although debate has surrounded the role of fluorine as a hydrogen-bond (HB) acceptor, the presence of such weak molecular interactions in the solid state has been the subject of both theoretical and spectroscopic studies (Dalvit & Vulpetti, 2016). However, to the best of our knowledge, there are few examples of X-ray studies. On the other hand, polymers containing fluorinated aromatic systems often exhibit exceptional thermal stability and show good water-repellent properties (Su & Chang, 2003). Therefore we report herein the crystal structure of 3,3 0 -(ethane-1,2-diyl)bis(6-fluoro-3,4-dihydro-2H-1,3-benzoxazine) (I), which is a very good candidate as a monomer for the investigation of the polymerization of fluorine-containing bis-benzoxazine monomers.

Structural commentary
The molecular structure of the title compound is illustrated in Fig. 1. The asymmetric unit contains one-half of the formula unit; a centre of inversion located at the mid-point of the central C1-C1 i bond generates the other half of the bisbenzoxazine compound [symmetry code: (i) 1 À x, 1 À y, 1 À z]. Bond lengths in the benzoxazine moiety in (I) are within normal ranges and are comparable to those found in related structures (Rivera et al., 2012a(Rivera et al., ,b, 2011Chen & Wu, 2007).

Synthesis and crystallization
The title compound was synthesized according to the literature procedure (Rivera et al.,1989), and single crystals were obtained by slow evaporation from an ethyl acetate/benzene 1:3 solvent mixture at room temperature.  Table 1 Hydrogen-bond geometry (Å , ).

Figure 2
Packing diagram for title compound, viewed along the b axis. C-HÁ Á ÁF and C-FÁ Á Á contacts are drawn as dashed lines.

Figure 1
The molecular structure of the title compound. Displacement ellipsoids are drawn at the 50% probability level. Atoms labelled with the suffix A are generated using the symmetry operator (Àx + 1, Ày + 1, Àz + 1).

Refinement details
Crystal data, data collection and structure refinement details are summarized in Table 2. All H atoms were located in difference electron-density maps. C-bound H atoms were fixed geometrically (C-H = 0.95 or 0.99 Å ) and refined using a riding-model approximation, with U iso (H) set to 1.2U eq of the parent atom.  SHELXL2014 (Sheldrick, 2015); molecular graphics: XP in SHELXTL-Plus (Sheldrick, 2008); software used to prepare material for publication: SHELXL2014 (Sheldrick, 2015).

3,3′-(Ethane-1,2-diyl)bis(6-fluoro-3,4-dihydro-2H-1,3-benzoxazine)
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.