Crystal structure of 4,10-dimethoxy-13-methyl-6H,12H-6,12-epiminodibenzo[b,f][1,5]dioxocine

The Schiff base molecule is transformed into a substituted dibenzoimino[1,5]dioxocin compound featuring a folded butterfly-like conformation with a dihedral angle of 84.72 (7)° between the benzene rings.

The title compound, C 17 H 17 NO 4 , lacks crystallographic symmetry with one molecule per asymmetric unit. The molecule exists in a folded butterfly-like conformation; the benzene rings form a dihedral angle of 84.72 (7) . The central eight-membered imino-bridged dioxocin ring adopts a twisted-boat conformation. In the crystal, inversion-related molecules are linked by pairs of weak C-HÁ Á ÁO hydrogen bonds, forming double-stranded chains parallel to the a axis.

Chemical context
Trö ger's base and its structural analogues are characterized by two flat, usually aromatic and identical, pincers interlocked in an almost perpendicular fashion (Dolenský et al., 2012). Both the chirality and the conformational rigidity of their central diazocine, dioxocin or dithiocin skeletons are the reasons why these cleft-shaped molecules have been of interest in molecular recognition (Hardouin-Lerouge et al., 2011), as chiral solvating agents (Wilen et al., 1991), and in the field of asymmetric synthesis (Minder et al., 1995).
Over the last few years, we have been exploring the chemistry of transition metal complexes of Schiff base ligands with the aim of preparing heterometallic polynuclear compounds with diverse potential advantages (Chygorin et al., 2012;Nesterova et al., 2013). The Schiff base ligand 2-methoxy-6-iminomethylphenol (HL) with various connectivity modes has been successfully used as a multidentate linker between several metal centres by our group and others (Meally et al., 2010;Sydoruk et al., 2013). The HL ligand is usually obtained by the standard method of condensation of the substituted salicylaldehyde with an aqueous solution of methylamine in methanol (Meally et al., 2010). In the present work, we used a mixture of 2-hydroxy-3-methoxy-benzaldehyde and methylamine hydrochloride to react with a zinc salt in an attempt to synthesize a Zn complex with the HL ligand (see Scheme). The resulting Schiff base apparently underwent self-condensation to form the substituted dibenzoimino[1,5]dioxocin, 4,10-dimethoxy-13-methyl-6H,12H-6,12-epiminodibenzo[b,f][1,5]dioxocine, (I), the crystal structure of which is presented here. A close analogue of the title compound was reported to result from 2-(N-methyliminomethyl)phenol, a liquid product of a similar condensation of salicylaldehyde and methylamine, after a few months storage in mild conditions (Filarowski et al., 1998). A tentative mechanism for the formation of the [1,5]iminodioxocin ring in the reaction between an aromatic aldehyde and a primary amine was given by Mandal et al. (2006).

Supramolecular features
In the crystal, double-stranded chains of inversion-related molecules linked by pairs of weak C-HÁ Á ÁO hydrogen bonds (Table 1) propagate in the a-axis direction (Fig. 2). Adjacent hydrogen-bonded chains are arranged in a parallel fashion to ensure efficient crystal packing of the clefts. Surprisingly, neitherstacking [the shortest centroid-centroid distance (offset) = 3.96 Å ] nor C-HÁ Á Á interactions (the shortest HÁ Á Ácentroid distance = 3.34 Å ) play a significant role in formation of the crystal structure of (I).

Figure 2
Crystal packing of (I), showing the parallel arrangement of doublestranded hydrogen-bonded chains of the dibenzoimino[1,5]dioxocin molecules along the a-axis direction. Intermolecular hydrogen bonds are shown as blue dashed lines.

Figure 1
The molecular structure of the title compound, showing the atomnumbering scheme. Non-H atoms are shown with displacement ellipsoids at the 50% probability level.
nature/size/position of the substituents and widening of the folding angle can be established due to the small number of compounds involved. While an example of [1,5]iminodioxocin bridgehead N-atom coordination to a metal atom (copper) has been reported (refcode XESBON; Mandal et al., 2006), the Zn atom did not demonstrate the ability to coordinate the ligand (I) in the present study.

Synthesis and crystallization
2-Hydroxy-3-methoxy-benzaldehyde (0.23 g, 1.5 mmol) and methylamine hydrochloride (0.10 g, 1.5 mmol) were added to methanol (5 ml) and stirred magnetically for 10 min. Zn(CH 3 COO) 2 Á2H 2 O (0.11 g, 0.5 mmol) dissolved in 5 ml dimethylformamide was added to the yellow solution of the Schiff base formed in situ, and the resulting deep-yellow solution was stirred at room temperature for an hour. The addition of N(Et) 3 (1 ml) produced a light precipitate which was filtered off. The solution, which was kept cold (283-285 K), changed colour from yellow to brown. It was diluted twice with methanol (4 ml) since it was thickening. Brown plate-like crystals of the title compound formed over two months after successive addition of Pr i OH (4 ml) in two portions. . The IR spectrum of powdered (I) in the range 4000-400 cm À1 shows all characteristic functional groups peaks: (CH) due to aromatic C-H and alkyl -C-H stretching above and below 3000, respectively, the aromatic rings vibrations in the 1600-1400 region, (CO) and (CN) at 1300-1000 and aromatic CH bending in the 900-600 cm À1 region (see Supporting information).