Crystal structure of (3E,5E)-3,5-bis[4-(diethylazaniumyl)benzylidene]-1-methyl-4-oxopiperidin-1-ium trichloride dihydrate: a potential biophotonic material

A stable oxopiperidinium trication salt was synthesized. In the crystal, N—H⋯Cl, C—H⋯Cl and C—H⋯O hydrogen bonds link cations and anions into a three-dimensional network.

In the trication of the title hydrated molecular salt, C 28 H 40 N 3 O 3+ Á3Cl À Á2H 2 O, the central heterocyclic ring adopts a sofa conformation, with the exocyclic N-C bond in an equatorial orientation. The dihedral angles between the planar part of this heterocyclic ring and the two almost flat side-chain fragments, which include the aromatic ring and bridging atoms, are 28.8 (1) and 41.1 (1) . Both diethylazaniumyl substituents have a tetrahedral geometry, while the dihedral angles between the above-mentioned flat part of the aryl fragments and the imaginary planes drawn through atoms C-N-C of the diethylazaniumyl substituents are 86.3 (2) and 80.4 (1) , respectively. In the crystal, N-HÁ Á ÁCl hydrogen bonds link the cations and anions into [100] chains. The chains are cross-linked by numerous C-HÁ Á ÁO and C-HÁ Á ÁCl interactions, generating a three-dimensional network. One of the chloride ions is disordered over two adjacent positions in a 0.895 (4):0.105 (4) ratio.

Chemical context
In a continuation of our work on the synthesis and structural investigations of non-linear optical organic compounds with two-photon absorption properties and potential biophotonic materials (Nesterov et al., 2003(Nesterov et al., , 2007Nesterov et al., 2011a,b;Sarkisov et al., 2005), we determined the crystal structure of the title compound. This compound belongs to a group that has shown anticancer activity (Jia et al., 1988;Dimmock et al., 2001). It may also find application as an agent for locating cancer cells with two-photon excited fluorescence and as a potential agent for a photodynamic treatment of cancer (Nesterov et al., 2003;Sarkisov et al., 2005).

Structural commentary
The structure of the trication with chloride anions is illustrated in Fig. 1. There are also two water molecules of crystallization. ISSN 2056-9890 The central heterocycle adopts a sofa conformation: atom N1 lies À0.732 (3) Å out of the central C 5 plane [planar within 0.027 (2) Å ]. The dihedral angles between the flat part of the heterocycle (atoms C2, C3, C4, C5, and C6) and the two almost planar fragments that include the phenyl-ring and the bridging atoms are 28.7 (1) and 41.1 (1) for (C7-C13) and (C18-C24), respectively. Such non-planarity might partly be caused by the presence of short intramolecular contacts H2ABÁ Á ÁH24A and H6ABÁ Á ÁH13A with distances 2.18 and 2.14 Å , respectively, which are shorter than the doubled van der Waals radius of the H atom (Rowland & Taylor, 1996). The mutual orientations of both aryl substituents relative to the flat part of the diethylazaniumyl groups (N2, C14, C16 and N3, C25, C27) are almost orthogonal [dihedral angles of 86.3 (2) and 80.4 (1) , respectively]. This is in contrast to the starting material where such angles are close to zero and the substituents participate in conjugated systems with the respective aromatic rings (Nesterov et al., 2003).

Figure 2
Projection of the crystal packing of the title compound along the c axis. Dashed lines denote strong intermolecular N-HÁ Á ÁCl hydrogen bonds. Water molecules have been omitted for clarity.

Database survey
A search in the Cambridge Structural Database (Groom & Allen, 2014) for structures of piperidone with the amino substituents revealed eight hits with two salt structures of the oxopiperidinium iodide (Jia et al., 1989;Nesterov et al., 2007). Among these, there is a starting compound in which both diethylamino substituents participate in a conjugation with aromatic rings (Nesterov et al., 2003).

Synthesis and crystallization
The starting compound (3E,5E)-3,5-bis[4-(diethylamino)benzylidene[À1-methyl-4-piperidone was obtained according to a literature procedure (Nesterov et al., 2003). The relatively stable colorless crystals of the investigated salt were obtained by slow evaporation of the solution of the above piperidone from a mixture of ethanol and hydrochloric acid over several days.

Refinement
Crystal data, data collection, and structure refinement details are summarized in Table 2. All C-bound H-atoms were placed in idealized positions and allowed to ride on their parent atom: C-H = 0.95, 0.99 and 0.98 Å for CH, CH 2 and CH 3 H atoms, respectively, with U iso (H) = k Â U eq (C), where k = 1.2 for CH and CH 2 and 1.5 for CH 3 H atoms. All N-bound H atoms were located using difference Fourier maps, but in the final refinement their distances were constrained at 0.90 Å (DFIX). H atoms of the two water molecules were not localized properly, since they appeared to be disordered over several positions. These H atoms were therefore removed from the refinement, but they were still included in the resulting chemical formula. Atom Cl3 is disordered over two positions in a 0.895 (4):0.105 (4) ratio. Data collection: APEX2 (Bruker, 2007); cell refinement: SAINT (Bruker, 2007); data reduction: SAINT (Bruker, 2007); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: SHELXTL (Sheldrick, 2008); software used to prepare material for publication: SHELXTL (Sheldrick, 2008). 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.