Crystal structure of 2,2′-(ethane-1,2-diyl)bis(2,3-dihydro-1H-naphtho[1,2-e][1,3]oxazine): a prospective raw material for polybenzoxazines

The molecular and crystal structures of the centrosymmetric naphthoxazine derivative is reported. In the absence of hydrogen-bonding and π–π stacking interactions, the crystal structure is stabilized by short C—H⋯π contacts.

In the title compound, C 26 H 24 N 2 O 2 , the oxazine moiety is fused to a naphthalene ring system. The asymmetric unit consists of one half of the molecule, which lies about an inversion centre. The C atoms of the ethylene spacer group adopt an antiperiplanar arrangement. The oxazine ring adopts a half-chair conformation. In the crystal, supramolecular chains running along the b axis are formed via short C-HÁ Á Á contacts. The crystal studied was a non-merohedral twin with a fractional contribution of 0.168 (2) of the minor twin component.

Chemical context
The oxazine moiety is well known as a building block for highperformance phenolic resins, which are of great interest in industry due to their superior mechanical and physical properties together with unusually high thermal resistance (Kiskan & Yagci, 2005). Recently, because of their high flexibility in molecular design and performance-to-cost ratio, these monomers have gained attention for the preparation of cured thermosetting resins (Song et al., 2014;Yeganeh & Jangi, 2010). Benzoxazines and naphthoxazines, originally proposed by Holly & Cope (1944) and subsequently elaborated by Burke and co-workers (Burke et al., 1952), are obtained by Mannich-type condensation-cyclization reactions of phenols or naphthols with formaldehyde and primary amines in a 1:2:1 ratio (Deck et al., 2014). Interest in the synthesis of polybenzoxazines and polynaphthoxazines has greatly increased during the past few years as they have a great deal of molecular design flexibility compared to ordinary phenolics (Yildirim et al., 2006). The title bisnapthoxazine, 2,2 0 -(ethane-1,2-diyl)bis(2,3-dihydro-1H-naphtho[1,2-e][1,3]oxazine), C 26 H 24 N 2 O 2 , was prepared by condensation of 2-naphthol with ethylenediamine and formaldehyde in a 2:1:4 molar ratio at room temperature for 15 min in methanol solution. Evaporation at room temperature afforded the title compound in 73% yield after recrystallization.
The aromatic C-C bonds of naphthalene ring system have a narrow range of distances [from 1.365 (5) to 1.431 (4) Å ]. The central C5-C10 bond at 1.415 (4) Å is, however, shorter by 0.014 Å than those in related structures (Yang et al., 2007;Rivera et al., 2015). The N1-C1 and O1-C1 bond lengths are normal and comparable to the corresponding values observed in these related structures.

Supramolecular features
In the crystal, the packing of the title compound is dominated by short contacts (Table 1), as indicated by a PLATON (Spek, 2009) analysis. These contacts result from short C12-H12BÁ Á ÁC2 and C12-H12BÁ Á ÁC3 separations, which at 2.75 Å are both 0.15 Å shorter than the sum of the van der Waals radii, while the C-HÁ Á ÁCg1 contact to the mid-point of the C2--C3 bond is even shorter at approximately 2.65 Å . These contacts are also much shorter than the C-HÁ Á ÁCg2 contact to the centroid of the C2-C4/C11/C12 ring (Fig. 2). The molecules are by these short C-HÁ Á Á contacts linked into chains propagating along the b-axis direction (Fig. 3). Table 1 Selected short-contact geometry (Å , ).
Cg1 is the mid-point of the C2-C3 bond and Cg2 is the centroid of the C2-C4/ C11/C12 ring.

Figure 2
Possible C-HÁ Á Á contacts, shown as dotted green lines, between molecules of (I). Bond mid-points and ring centroids are shown as colored spheres.

Figure 3
Crystal packing of (I), showing C-HÁ Á Á(C,C) short contacts that result in chains propagating along the b-axis direction.

Figure 1
The molecular structure of the title compound, with displacement ellipsoids drawn at the 50% probability level. Atoms labelled with the suffix A are generated using the symmetry operator (1 À x, 1 À y, 1 À z).

Database survey
The title compound is the first example of two naphthooxazine moieties linked by an ethylene bridge.

Synthesis and crystallization
The title compound was prepared as described by Rivera et al. (2006). Crystals were obtained by slow evaporation of the reaction solution at ambient temperature and were isolated from the solution before complete evaporation of the solvent mixture.

Refinement details
Crystal data, data collection and structure refinement details are summarized in Table 2. All H atoms were located in the difference electron-density map. 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. The crystal was a non-merohedral twin with a fractional contribution of 0.168 (2) of the minor twin component.  SHELXL2016 (Sheldrick, 2015b); molecular graphics: XP in SHELXTL-Plus (Sheldrick, 2008); software used to prepare material for publication: SHELXL2016 (Sheldrick, 2015b) and publCIF (Westrip, 2010).

2,2′-(Ethane-1,2-diyl)bis(2,3-dihydro-1H-naphtho[1,2-e][1,3]oxazine)
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.