Crystal structure of the 1:2 co-crystal of 1,3,6,8-tetraazatricyclo[4.3.1.13,8]undecane (TATU) and 4-chlorophenol (1/2)

The components of the ternary co-crystalline adduct are linked by intermolecular O–H⋯N hydrogen bonds.

In the title compound, C 7 H 14 N 4 Á2C 6 H 5 ClO, which crystallized with two crystallographically independent 4-chlorophenol molecules and one 1, 3,6,8tetraazatricyclo[4.3.1.1 3,8 ]undecane (TATU) molecule in the asymmetric unit, the independent components are linked by two O-HÁ Á ÁN hydrogen bonds. The hydrogen-bond acceptor sites are two non-equivalent N atoms from the aminal cage structure, and the tricyclic system distorts by changing the C-N bond lengths. In the crystal, these hydrogen-bonded aggregates are linked into chains along the c axis by C-HÁ Á ÁN hydrogen bonds. The crystal structure also features C-HÁ Á Á contacts.
TATU, a small tricyclic aminal cage, is an interesting option for studying hydrogen-bonding situations as it has four nitrogen atoms as potential hydrogen-bond acceptors. These N atoms have two different environments, N1 and N2 from the ethylene fragment (NCH 2 CH 2 N) and N3 and N4 from the 1,1-gem-diaminic units. These present two discrete options for hydrogen bonding to the aminal cage. With different types of phenols, the preference for a particular hydrogen-bondinteraction site depends strongly upon the lone-pair orbital hybridization of the nitrogen atom (Rivera et al., 2007). Studies on phenol complexes with tertiary amines in the solid state show that the proton transfer depends not only on the ÁpKa (pKa amine À pKa acid) value but also on steric and packing effects (Majerz & Sawka-Dobrowolska, 1996). In the structure found for the three-component aggregates observed here, both types of nitrogen atom mentioned above are involved in hydrogen bonding with N1 and N3 acting as hydrogen-bond acceptors. The reaction to produce the cocrystal occurs efficiently in the solid state by grinding a mixture of finely powdered TATU and 4-chlorophenol at room temperature; there are no by-products, and the work-up procedure is easy.

Structural commentary
The molecular structure of the title compound is illustrated in Fig. 1. The asymmetric unit comprises two crystallographically independent 4-chlorophenol molecules and one 1, 3,6,8-tetraazatricyclo[4.3.1.1 3,8 ]undecane (TATU) molecule. The phenols are linked to the aminal cage by two O-HÁ Á ÁN hydrogen bonds (Table 1), forming 2:1 hydrogen-bonded aggregates. This is similar to the situation observed in the structure of the 2:1 co-crystal of 4-nitrophenol and TATU (Rivera et al., 2015a) which also crystallizes in the P2 1 /c space group and has two different types of N atom acting as the hydrogen-bond acceptors. The measured dimensions of the aminal cage structure in the present adduct are similar to the corresponding values in this related structure. The observed N-CH 2 bond lengths are longer than those found in a co-crystal formed between TATU and hydoquinone (Rivera et al., 2007). This is presumably related to the formation of strong hydrogen bonds by the N1 and N3 hydrogen atoms.
A comparison of the O-HÁ Á ÁN hydrogen bonds in the title compound with those found for the nitrophenyl analogue (Rivera, et al., 2015a) reveal that both NÁ Á ÁO distances are significantly longer in the current structure, suggesting that the hydrogen bonds may be somewhat weaker.

Supramolecular features
In the crystal of title compound, O1-H1Á Á ÁN1 and C15-H15Á Á ÁN2 hydrogen bonds form columns of TATU molecules and O1 chlorophenol molecules along the c axis, Fig. 2. The columns are linked by O2-H2Á Á ÁN3 hydrogen bonds on one side and C2-H2AÁ Á ÁCg8 contacts on the other (Cg8 is the centroid of the C11-C16 ring).

Figure 2
The crystal packing of the title compound, showing the chain that extends along the c-axis direction. C-HÁ Á ÁN and O-HÁ Á ÁN hydrogen bonds are drawn as dashed lines The molecular structure of the title compound. Displacement ellipsoids are drawn at the 50% probability level. H atoms bonded to C atoms are omitted for clarity. Hydrogen bonds are drawn as dashed lines. monitored by TLC. Crystals suitable for X-ray diffraction were obtained from a methanol solution upon slow evaporation of the solvent at room temperature (72% yield, m.p. = 334-336 K)

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. All H atoms were located in difference electron-density maps. The hydroxyl H atoms were refined freely, while C-bound H atoms were fixed geometrically (C-H = 0.95, 0.98 or 0.99 Å ) and refined using a riding model, with U iso (H) values set at 1.2U eq of the parent atom (1.5 for methyl groups).
Acta Cryst. (2016). E72, 1648-1650 research communications  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.