2,9-Dimethyl-6H,13H-5:12,7:14-dimethanodibenzo[d,i][1,3,6,8]tetraazecine

In the title structure, C18H20N4, the aromatic rings are almost orthogonal [81.6 (2)°]. The molecule has symmetry 2 since it is situated on a crystallographic twofold axis. There are only weak intermolecular interactions present in the structure, notably C—H⋯π-electron ring interactions. The 1H and 13C NMR spectra are in accordance with the X-ray structure analysis.

In the title structure, C 18 H 20 N 4 , the aromatic rings are almost orthogonal [81.6 (2) ]. The molecule has symmetry 2 since it is situated on a crystallographic twofold axis. There are only weak intermolecular interactions present in the structure, notably C-HÁ Á Á-electron ring interactions. The 1 H and 13 C NMR spectra are in accordance with the X-ray structure analysis.
Financial support of this research from the Division de Investigació n sede Bogotá (DIB), and from the Departamento de Química, Universidad Nacional de Colombia, is gratefully acknowledged. DG-S thanks COLCIENCIAS for a fellowship.
Supplementary data and figures for this paper are available from the IUCr electronic archives (Reference: FB2167). For many years, cyclic aminals (gem-diamine) have attracted intense attention because of their intriguing molecular structures, many of which have been determined by the X-ray crystallography (Murray-Rust, 1974;Murray-Rust & Ridell, 1975;Murray-Rust & Smith, 1975;Glister et al., 2005) and/or by 1 H-and 13 C-NMR spectroscopy (Kuznetsov et al., 2007). In the course of the research of the reactivity of aminals we have synthesized crystals of the title compound that contains a cyclic aminal, i. e. 2,9-dimethyl-6H,13H-5:12,7:
There are only weak intermolecular interactions present in the structure, notably C-H···π-electron ring interactions (Tab.

Experimental
A solution of 4-methyl-1,2-diaminebenzene (100 mg, 0.82 mmol) in water (8 ml) and methanol (2 ml) was added dropwise at 278 K to 5 ml of 37% aqueous formaldehyde while stirring it. The reaction mixture was removed from the cooling bath and allowed to warm to room temperature while still stirring it. After stirring at room temperature for 1 h the resultant precipitate was filtered off, washed with water, dried in vacuum and recrystallized from 2-propanol to give the title compound with 65% yield. The melting point of the title structure is 465 K. The melting point was determined visually using glass capillary tube with an Electrothermal melting point apparatus, model 9100, accuracy ±0.5 K, manufacturer: Electrothermal Thermo Scientific.
The NMR spectra were acquired at room temperature on a Bruker AMX 400 Advance spectrometer. 1

Refinement
All the H atoms were discernible in the difference electron density maps. However, the H atoms were constrained by the riding model approximation: C-H methyl =0.96 Å; C-H aryl =0.93 Å; U iso H methyl =1.5U eq C methyl ; U iso H aryl =1.2U eq C aryl .
In the absence of significant anomalous scattering effects 667 Friedel pairs have been merged.
supplementary materials sup-2 Figures   Fig. 1. A view of the title molecule with the displacement ellipsoids shown at the 50% probability level with the atomic labelling scheme. The symmetry-related atoms by a crystallographic two-fold axis are indicated by "i". 2,9-Dimethyl-6H, 13H-5:12,7:14-dimethanodibenzo[d,i]

Special details
Experimental. The temperature was set with accuracy ±2 K.
Geometry. All s.u.'s (except the s.u. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The 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 > 2σ(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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Occ. (