Crystal structure of (1Z,4Z)-2,4-dimethyl-3H-benzo[b][1,4]diazepine

The title compound is not planar due to the folding of the seven-membered ring. In the crystal, molecules are packed opposite each other to minimize the electronic repulsion but the long intermolecular distances indicate that no directional contacts are found.


Chemical context
(1Z,4Z)-2,4-Dimethyl-3H-benzo [b][1,4]diazepine, C 11 H 12 N 2 (Me, Me) (1), also called a 1,5-benzodiazepine, is a molecule situated at the crossroad of many avenues of chemistry. This compound is associated with the names of Douglas Lloyd and Donald R. Marshall of the University of St Andrews in Scotland (Gibson et al., 2002). These authors reported the synthesis of 1, determined that its tautomeric structure is 1 and not 1 0 0 0 , and also determined that the protonation of 1 yields the cation 1H + (Lloyd et al., 2002). For molecules such as 1H + they introduced the term 'quasi-aromatic' (Lloyd & Marshall, 1971), a term that has not survived the authors (Claramunt et al., 2013). The inversion barrier of the seven-membered ring of 1 was measured to be 48.9 kJ mol À1 (Mannschreck et al., 1967); our calculated value is 43.4 kJ mol À1 (Claramunt et al., 2013). [1,4]diazepines continue to be the subject of many studies, but with other substituents (Bonacorso et al., 1996;El-Azab, 2013;Aastha et al., 2013;Solan et al., 2014;Young et al., 2016). Of the different procedures existing in the literature to prepare compound 1, we used the reaction of acetylacetone with o-phenylenediamine using silica-supported sulfuric acid as catalyst under solvent-free conditions (Chen et al., 2009). In spite of what the authors described in the paper, the reaction was not complete at room temperature and it was necessary to heat up to 273 K to attain a quantitative yield of the product, which was purified by column chromatography on silica gel and crystallized from ethyl acetate/hexane solution presenting a melting point of 408 K. We report herein on its characterization by 1 H, 13 C and 15 N NMR in solution and solid state spectroscopy and, since its X-ray molecular structure is unknown, we decided to complete the panorama of compound 1 determining it. Note that the structures of the monomethyl compound (Me, H) and the parent compound (H, H) are unknown, as well as those of their salts.

Structural commentary
The title compound 1 crystallizes in the monoclinic space group P2 1 /c with one molecule in the asymmetric unit ( Fig. 1). As expected, the derivative is not planar due to the folding of the seven-membered ring. According to the electronic distribution for the two imine groups N1-C2 and N5-C4 [bond distances = 1.283 (3) and 1.281 (3) Å , respectively], atoms C2, C3 and C4 together with the two methyl groups of the diazepine ring deviate from the phenyl ring plane: atom C3 shows the largest displacement at 1.495 (1) Å while C2 and C4 are situated symmetrically at about 0.58 Å from it. The dihedral angle between the phenyl ring and C2/C3/C4 fragment of the diazepine ring is 87.8 (2) , giving rise to a boat conformation for the diazepine ring.
It is noteworthy that this is the only example found in the CSD database (Groom et al., 2016) of a neutral diazepine derivative. For this reason, this structure is compared with the reported cationic diazepines C 11 H 13 N 2 + ÁX À [X = PF 6 À (Blake et al., 1991), Cl À (Speakman et al., 1976;Svensson & Timby, 1981) and ZnI 4 2À (Orioli & Lip, 1974)] showing relevant structural differences. In the latter compounds, there is electronic delocalization in the N1/C2/C3/C4/N5 moiety that results in an almost planar geometry of this part of the sevenmembered ring. However, in the neutral species, the C3 atom keeps both hydrogen atoms in an sp 3 conformation, leading to localization of the double bonds between the nitrogen atoms and their adjacent carbon atoms, which induces a great deviation of this moiety from planarity (Fig. 2).

Supramolecular features
In the crystal, the molecules are packed opposite each other to minimize electronic repulsion but the long intermolecular distances indicate that no relevant contacts are found (Fig. 3). This feature differs from the salts previously mentioned, where the presence of the hydrogen atoms on the nitrogen atoms allows the formation of N-HÁ Á ÁX hydrogen bonds, leading to different supramolecular networks. The absence of these atoms in 1, along with the boat conformation described above, prevents the formation of any supramolecular structure.

Synthesis and crystallization
All chemicals cited in the synthetic procedures are commercial compounds. Melting points were determined by DSC and thermograms (sample size 0.002-0.004 g) were recorded with a scan rate of 5.0 K min À1 . Column chromatography was performed on silica gel 70-230 mesh. The NMR solution spectra were recorded on a 9.4 Tesla spectrometer 648 Nieto et al. Comparative views of the seven-membered rings in 1 (left) and 1H + (right) in the salt C 11 H 13 N 2 + ÁPF 6 À (Blake et al., 1991).
(400.13 MHz for 1 H, 100.62 MHz for 13 C and 40.54 MHz for 15 N) at 300 K with a 5 mm inverse detection H-X probe equipped with a z-gradient coil. Chemical shifts ( in p.p.m.) are given from internal solvents: CDCl 3 7.26 for 1 H and 77.0 for 13 C. Nitromethane was used for 15 N as external reference. CPMAS NMR spectra were obtained on a 9.4 Tesla spectrometer at 300 K (100.73 MHz for 13 C and 40.60 MHz for 15 N) using a 4 mm DVT probehead at spinning rates of 12 and 6 kHz, respectively. 13 C spectra were originally referenced to a glycine sample and then the chemical shifts were recalculated to the Me 4 Si (for the glycine carbonyl atom = 176.1 p.p.m.) and 15 N spectra to 15 NH 4 Cl and then converted to the nitromethane scale using the relationship: 15 N (nitromethane) = 15 N (ammonium chloride) À 338.1 p.p.m.. Samples were spun at the magic angle at rates of 25 kHz and the experiments were carried out at 300 K.

(1Z,4Z)-2,4-dimethyl-3H-benzo[b][1,4]diazepine
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. 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 > 2sigma(F 2 ) is used only for calculating R-factors(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.
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )