Redetermination of 1,3,6,8-tetraazatricyclo[4.4.1.13,8]dodecane

The structure of the title compound, C8H16N4, which consists of four fused seven-membered rings, has been redetermined at 173 K. This redetermination corrects the orientation of two H atoms, which were located at unrealistic positions in the original room-temperature study [Murray-Rust (1974 ▶). J. Chem. Soc. Perkin Trans. 2, pp. 1136–1141]. The complete molecule is generated by -42m symmetry, with one quarter of a molecule [one N atom (site symmetry m), two C atoms (one with site symmetry m and the other with site symmetry 2) and two H atoms] in the asymmetric unit. No directional interactions beyond van der Waals contacts are apparent in the crystal structure.

The structure of the title compound, C 8 H 16 N 4 , which consists of four fused seven-membered rings, has been redetermined at 173 K. This redetermination corrects the orientation of two H atoms, which were located at unrealistic positions in the original room-temperature study [Murray-Rust (1974). J. Chem. Soc. Perkin Trans. 2, pp. 1136-1141. The complete molecule is generated by 42m symmetry, with one quarter of a molecule [one N atom (site symmetry m), two C atoms (one with site symmetry m and the other with site symmetry 2) and two H atoms] in the asymmetric unit. No directional interactions beyond van der Waals contacts are apparent in the crystal structure.

Related literature
For the original synthesis procedure, see: Bischoff (1898). For the previous determination of the structure, see: Murray-Rust (1974). For crystal structures containing the title compound as a fragment, see: Rivera et al. (2007); Glister et al. (2005). For a description of the Cambridge Crystallographic Database, see: Allen et al. (2002). Data collection: X-AREA (Stoe & Cie, 2001); cell refinement: X-AREA; data reduction: X-AREA; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: XP in SHELXTL (Sheldrick, 2008); software used to prepare material for publication: SHELXL97. The crystal structure of the title compound, a fully saturated cage-like molecule, was first reported (Murray-Rust, 1974) at room temperature. This heterocyclic system is of considerable conformational interest. While by now the structure of the molecule would seem well established, the position of some hydrogen atoms is rather strange (Fig. 1). Both H atoms bonded to C1 almost lie in a common plane with the two N atoms attached to C1 (r.m.s. deviation for all five atoms 0.050 Å). The dihedral angle between the NC 2 plane and the CH 2 plane is 10.7°. However, the CH 2 plane should be more or less perpendicular to the CN 2 plane.

Redetermination
Our interest in the title compound prompted the present re-investigation carrying out the data collection at 173 (2) K which provides more regular positions for the H atoms (Fig. 2).
The re-determination of the crystal structure is consistent with the observation that TATD belongs to the D 2d point group as pointed out Murray-Rust. The only significant difference is the localization of the H atoms. The largest difference between the two determinations pertains to the C-C distance of the ethylene bridge. In the original study this distance was determined as 1.534 (8) Å, whereas it is 1.477 (8) Å in this study. Taking the displacement ellipsoid which is elongated perpendicular to the N-C-C-N moiety into account, the shortened bond length might be due to a slight disorder of C2.
Concerning the H atoms, the orientation of the methylene group connecting two N atoms in particular is corrected: in the structure of Murray-Rust the orientation of the H atoms is such that both C-H bonds are in the same plane with the N-C bonds, with a dihedral angle of 10.7° between the NC 2 plane and the CH 2 plane. In the redetermination, the dihedral angle between these planes is exactly 90°.
The title molecule is composed of four seven-membered rings which have exactly the same conformation due to the molecular symmetry. The conformation can be described as a chair. A search in the Cambridge Crystallographic Database (CSD, Version 5.35 of 2013, plus two updates; Allen 2002) yielded three structures containing the title compound as a fragment, namely the structure determination of Murray-Rust (1974), the title compound as a co-crystal with hydroquinone (Rivera et al., 2007), the title compound substituted with a cyclohexane ring at the C-C bonds (Glister et al., 2005). The conformation of the seven-membered rings is a chair in all cases which is not surprising since the the molecule is rigid.
Concerning the crystal packing, molecules of the title compound are located at the origin and at the centre of the unit cell. Thus, the crystal packing can be regarded as two sets of layers. The molecules in neighbouring layers are displaced by the symmetry operation 1/2 + x, 1/2 + y, 1/2 + z (Fig. 3).

Refinement
All H atoms were located in a difference map. Nevertheless, they were geometrically placed and refined using a riding model, with C-H = 0.99 Å and with U iso (H) = 1.2U eq (C). The absolute structure was indeterminate in the present refinement.