Crystal structure of 9,9′-{(1E,1′E)-[1,4-phenylenebis(azanylylidene)]bis(methanylylidene)}bis(2,3,6,7-tetrahydro-1H,5H-pyrido[3,2,1-ij]quinolin-8-ol)

The whole molecule of the title compound is generated by inversion symmetry; the central benzene ring being situated about a crystallographic inversion center. The aromatic ring of the julolidine moiety is inclined to the central benzene ring by 33.70 (12)°, and the conformation about the C=N bonds is E. There are two intramolecular O—H⋯N hydrogen bonds in the molecule, generating S(6) ring motifs.

The whole molecule of the title compound, C 32 H 34 N 2 O 2 , is generated by inversion symmetry; the central benzene ring being situated about the crystallographic inversion center. The aromatic ring of the julolidine moiety is inclined to the central benzene ring by 33.70 (12) . There are two intramolecular O-HÁ Á ÁN hydrogen bonds in the molecule, generating S(6) ring motifs. The conformation about the C N bonds is E. The fused non-aromatic rings of the julolidine moiety adopt half-chair conformations. In the crystal, adjacent molecules are linked by pairs of C-HÁ Á Á interactions, forming a ladder-like structure propagating along the a-axis direction.

Chemical context
8-Hydroxyjulolidine-9-carboxaldehyde is a well-known chromophore used in fluorescence chemosensors; chemosensors with the julolidine moiety are usually soluble in aqueous solutions (Narayanaswamy & Govindaraju, 2012;Maity et al., 2011;Na et al., 2013;Noh et al., 2013). Compounds containing the julolidine group display chromogenic naked-eye detection of copper, zinc, iron, and aluminium ions as well as fluoride ions (Choi et al., 2015;Wang et al., 2013a,b;Kim et al., 2015;Jo et al., 2015). There are many reports in the literature on 8-hydroxyjulolidine-9-carboxaldehyde-based Schiff bases and their applications as sensors for metal ions (Park et al., 2014;Lee et al., 2014;Kim et al., 2016). Intramolecular C-HÁ Á ÁN hydrogen bonds have been observed in a julolidine-derived structure (Barbero et al., 2012). Julolidine dyes exhibiting excited-state intramolecular proton transfer (Nano et al., 2015) and julolidine ring-containing compounds are also fluorescent probes for the measurement of cell-membrane viscosity. The present work is a part of an ongoing structural study of Schiff bases and their utilization in the synthesis of new organic and polynuclear coordination compounds (Faizi & Sen 2014;Faizi et al., 2016). Recently Choi et al. (2016) have reported on a new chemosensor, similar to the title compound, which is a fluorescent chemosensor for the selective detection of Zn 2+ in aqueous solution. This was synthesized by a condensation reaction of 8-hydroxyjulolidine-9-carboxaldehyde with 2-(aminomethyl)benzeneamine in ethanol at room temperature. We report herein on the synthesis and crystal structure of the title julolidine derivative.

Structural commentary
The molecular structure of the title compound is illustrated in Fig. 1. The whole molecule of the title compound is generated by crystallographic inversion symmetry. The conformation about the azomethine C4 N1 bond [1.285 (3) Å ] is E. The C3-N1-C4-C5 torsion angle is 172.9 (2) . The molecule is non-planar, with the dihedral angle between the central benzene ring and the aromatic ring of the julolidine moiety being 33.70 (12) . Depending on the tautomers, two types of intramolecular hydrogen bonds are observed in Schiff bases: O-HÁ Á ÁN in phenol-imine and N-HÁ Á ÁO in keto-amine tautomers. The present analysis shows that the title compound exists in the phenol-imine form (Fig. 1). It exhibits two intramolecular O1-H1AÁ Á ÁN1 [d(NÁ Á ÁO) 2.579 (3) Å ] hydrogen bonds, which generate S(6) ring motifs ( Fig. 1 and Table 1).

Supramolecular features
In the crystal, adjacent molecules are linked by a pair of C-HÁ Á Á interactions (Table 1 and Fig. 2), forming a ladder-like structure propagating along the a-axis direction (Fig. 3).

Database survey
There are very few examples of similar compounds in the literature and, to the best of our knowledge, the new fluor-escent chemosensor for the selective detection of Zn 2+ in aqueous solution, mentioned in the Chemical context section  has not been characterized crystallographically. A search of the Cambridge Structural Database (CSD, Version 5.37, update May 2016; Groom et al., 2016) gave 120 hits for the julolidine moiety. Of these, six have an OH group in position 8, and four also have a C N group in position 1. Of the latter, one compound, viz. 9-{[(4-chlorophenyl)imino]methyl}-1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5H-pyrido[3,2,1-ij]quinolin-8-ol (CSD refcode: IGALUZ; Kantar et al., 2013), resembles the title compound and also exists in the phenol-imine form with an intramolecular O-HÁ Á ÁN hydrogen bond.

Synthesis and crystallization
An ethanolic solution of 8-hydroxyjulolidine-9-carboxaldehyde (100 mg, 0.46 mmol) was added to p-phenylenediamine (25 mg, 0.23 mmol) in absolute ethanol (3 ml). Two drops of HCl were added to the reaction solution and it was stirred for 30 min at room temperature. The resulting yellow precipitate was recovered by filtration, washed several times with small portions of ice-cold EtOH and then with diethyl ether to give 199 mg (85%) of the title compound. Crystals suitable for X-ray diffraction analysis were obtained within three days by slow evaporation of a solution in methanol.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2 The molecular structure of the title compound, with atom labelling. Displacement ellipsoids are drawn at the 40% probability level. Unlabelled atoms are generated by the symmetry operation Àx, Ày + 1, -z. The intramolecular O-HÁ Á ÁN hydrogen bonds (see Table 1) are shown as dashed lines. Table 1 Hydrogen-bond geometry (Å , ).

Figure 2
A view of the C-HÁ Á Á interactions, shown as dashed lines (see Table 1), in the crystal of the title compound. A view along the a axis of the crystal packing of the title compound.

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.