Crystal structure of (E)-4-benzylidene-6-phenyl-1,2,3,4,7,8,9,10-octahydrophenanthridine

The title compound was synthesized using a novel one-pot method under mild conditions and fully characterized using NMR, ESI–MS and SXRD. The supramolecular structure of the title compound is defined by a combination of C—H⋯N and π–π interactions.

The preparation of the title compound, C 26 H 25 N, was achieved by the condensation of an ethanolic mixture of benzaldehyde, cyclohexanone and ammonium acetate in a 2:1:1 molar ratio. There are two crystallographically independent molecules in the asymmetric unit. The two cyclohexyl rings adopt an anti-envelope conformation with the benzyl moiety adopting a cis conformation with respect to the nitrogen atom of the phenanthridine segment. In the crystal, molecules are linked through C-HÁ Á ÁN interactions into hydrogen-bonded chains that are further arranged into distinct layers by weak offsetinteractions.

Chemical context
The preparation of piperidine derivatives via the Mannich reaction is well documented (Noller & Baliah, 1948). Further, the condensation of a ketone with -methylene groups, with an aldehyde in the presence of ammonium acetate results in the formation of the required piperidone derivatives through the Mannich reaction (Karthikeyan et al., 2009;Al-Jeboori et al., 2009). However, the formation of unpredicted phenanthridine derivatives as a second product with piperidone upon using a range of cyclic ketones has also been mentioned (Karthikeyan et al., 2009). Phenanthridine derivatives are an important class of heterocyclic nitrogen-based compounds that form a range of natural products and biologically important molecules (Tumir et al., 2014). These compounds have found significant applications in different fields, including their potential applications in medicinal chemistry (Stevens et al., 2008) and in the fabrication of materials (Gerfaud et al., 2009). Therefore, researchers have been interested in the development of efficient and versatile methods for the synthesis of these materials (Bao et al., 2014;Xu et al., 2014). These compounds can be fabricated using a range of synthetic methods, including cyclization, that require harsh conditions and several preparation steps to obtain phenanthridines (Herrera et al., 2006). In this paper, the formation of a phenanthridine derivative was achieved via a one-pot reaction using cyclohexanone and benzaldehyde in an ethanolic solution of ammonium acetate. ISSN 2056-9890

Structural commentary
The asymmetric unit contains two crystallographically independent molecules, A and B, shown in Figs. 1 and 2, with no solvent molecules incorporated into the crystal lattice. Selected geometric parameters for the title compound are given in Table 1. All of the bond lengths and bond angles are within the normal range of analogous phenanthridine compounds (Helesbeux et al., 2011;Shabashov & Daugulis, 2007). In the structure, the cyclohexane rings adopt the antienvelope conformation. In molecule B one of these rings shows static disorder of the C91 and C92 atoms over two sets of sites. This was modelled as two positions with the site occupancies refined to give 81.7 (3)% occupancy for the major component and 18.3 (3)% for the minor component. Full refinement details are given in Section 5. In both of the crystallographically independent molecules, the phenyl and benzylidene groups are rotated out-of-plane with respect to the octahydrophenanthrine moieties: in molecule A the angle between the mean planes of the phenyl and pyridine rings is 46.92 (5) with the equivalent angle in molecule B of 53.43 (5) . The angle between the mean planes of the benzylidine and pyridine rings in molecule A is 48.53 (5) and the corresponding angle in molecule B is 41.37 (5) .

Supramolecular features
The crystal structure features a combination of weak hydrogen bonds and weak offsetinteractions. A weak C-HÁ Á ÁN contact is formed from the octahydrophenanthridine C6 position in molecule A to the N1 position in a B molecule (symmetry operation 1 + x, À1 + y, z), with an equivalent weak contact formed from the C109 position in molecule B to the N2 position of a neighbouring molecule A (symmetry operation 1 -x, 2 À y, z). Geometric parameters for these contacts are given in Table 2. The geometric parameters for these contacts are within the accepted range of DÁ Á ÁA distances for weak hydrogen bonds of 3.2-4.0 Å , the D-HÁ Á ÁA angles being slightly more linear than the expected values of 90-150 (Gilli, 2002). These interactions lead to the formation of Atom arrangement and numbering scheme for molecule B, with displacement ellipsoids drawn at the 50% probability level.

Figure 1
Atom arrangement and numbering scheme for molecule A, with displacement ellipsoids drawn at the 50% probability level. Table 2 Hydrogen-bond geometry (Å , ). chains consisting of alternating A and B molecules oriented along the a-axis direction. These chains propagate along the baxis, with neighbouring chains offset from each other along the a axis to allow intercalation of the phenyl and benzyl aromatic rings of neighbouring groups, as shown in Fig 3, forming layers. These layers further stack along the c-axis with the orientation of the layers inverted with respect to the layer above and below, as shown in Fig. 4. The structure is further stabilized by along the b-axis stabilized by weak offsetstacking interactions between the benzylidine rings of B molecules in adjacent layers where the aromatic groups are oriented towards each other (symmetry operation for second B molecule 1 À x, Ày, 1 À z) with a centroid-centroid distance of 3.9853 (14) Å and shift distance of 2.285 (3) Å .

Database survey
Version 5   Packing arrangement of the structure viewed along the crystallographic a axis with the c axis parallel to the long axis of the paper. Theinteractions occur between the benzyl rings that lie between the second and third rows of molecules The labels of the axes should be larger.
linear than the average, indicating a non-trivial role in determining the supramolecular structure.

Hirschfeld surface analysis
Fingerprint analysis of the intermolecular interactions by the generation of Hirschfeld surfaces using CrystalExplorer (Spackman & McKinnon, 2002) reveals that the two types of molecules have similar intermolecular contact patterns. Selected fingerprint plots corresponding to the complete intermolecular contact surface and HÁ Á ÁH, HÁ Á ÁC and HÁ Á ÁN contacts are shown in Fig. 5. The percentage contributions of each contact type to the overall interaction environment are tabulated in Table 3. In both cases, the major contribution is from HÁ Á ÁH contacts, accounting for 66.9% of the surface area in molecule A and 64.8% in molecule B. It is notable that, in addition to making the largest contribution to the intermolecular contact surfaces, the HÁ Á ÁH contacts account for the closest intermolecular contact in the case of both molecules, between cyclohexyl hydrogen atoms on a molecule A and B (H91BÁ Á ÁH10X). The direction of these contacts runs parallel to the axis of the C-HÁ Á ÁN contacts between molecules on neighbouring hydrogen-bonded chains and appears to result from the intercalation of these chains. As these contacts are not associated with either of the major attractive interactions (A-B C-HÁ Á ÁN hydrogen bonds or B-Bstacking), it is probable that this contact arises solely from the packing arrangement required to maximize the number and strength of these favourable interactions.

Synthesis and crystallization
The title compound was isolated from the reaction mixture using a flash column chromatography and as follows: A solution of benzaldehyde (4.02 mL, 0.038 mol), ammonium acetate (1 g, 0.019 mol) and cyclohexanone (2 mL, 0.019 mol) in ethanol (20 mL) was heated to reflux for 2 h. The obtained residue was purified from the crude product by flash chromatography with an eluent mixture of 33% ethyl acetate in hexane, m.p. = 467-469 K, yield: 42%. Colourless crystals suitable for X-ray single crystal analysis were obtained by slow evaporation of a methanol solution of the compound.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 4. Hydrogen atoms were positioned geometrically (C-H = 0.95-0.99 Å ) and refined using a riding model with U iso (H)= 1.2U eq (C). Disorder at C90/C91/C92/C93 was modelled by splitting the component atoms across two positions and refining the occupancy using FVAR to 82% for C90A-C93A and 12% for C90B-C93B. 1,2 distances were restrained using SADI and ADPs for C90A/C90B and C93A/ C93B constrained using EADP commands. General numbering pattern for NMR spectra of the title compound.   -6-phenyl-1,2,3,4,7,8,9,10- 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. Positional disorder at C90-C91-C92-C93 modelled by splitting the component atoms across two positions and refining occupamcy using FVAR to 82% for C90A-C93A and 12% for C90B-C93B. C90A/C90B and C93A/C93B. 1,2 distances were restrained using SADI and ADPs for C90A/C90B and C93A/C93B constrained using EADP commands.