Crystal structure and DFT study of 8-hydroxy-1,2,3,5,6,7-hexahydropyrido[3,2,1-ij]quinoline-9-carbaldehyde

In the title compound, the hydroxy group forms an intramolecular hydrogen bond to the aldehyde O atom, generating an S(6) ring motif. The fused non-aromatic rings of the julolidine moiety adopt envelope conformations. Geometrical parameters, determined using X-ray diffraction techniques, are compared with those calculated by density functional theory (DFT), using the B3LYP/6–311 G(d,p) level of theory.


Chemical context
Julolidine is chemically an aniline derivative with two N-alkyl substituents forming rings back to the aromatic ring; the fused rings lock the nitrogen lone-pair of electrons into conjugation with the aromatic ring leading to unusual reactivity. The presence of the julolidine ring system in some molecules makes them useful for chromogenic naked-eye detection of copper, zinc, iron and aluminium ions as well as fluoride ions (Wang et al., 2013;Choi et al., 2015;Kim et al., 2015;Jo et al., 2015). Julolidine dyes exhibit excited-state intramolecular proton transfer (Nano et al., 2015). Compounds containing lulolidine rings are also used as fluorescent probes for the measurement of cell-membrane viscosity. Julolidine-based materials are also used as red emitters in OLEDs when linked to dicyanomethylpyran modules (Lee, et al., 2012). The julolidine unit plays an important role as it has strong electronicdonating properties for chelating (Nano, et al., 2013). Julolidine malononitrile acts as a 'push-pull' molecule with large hyperpolarizability and is used as a model system for understanding the non-linear optical properties of molecules (Mennucci et al., 2009).
There are many reports in the literature on julolidine-based Schiff bases and their applications as sensors for metal ions (Park et al., 2014;Lee et al., 2014;Kim et al., 2016). The present work is a part of an ongoing structural study of Schiff bases based on the julolidine ring system (Faizi et al., 2016(Faizi et al., , 2017. We report here the crystal structure and DFT computational calculation of the title julolidine compound (I). The results of calculations by density functional theory (DFT) on (I) carried out at the B3LYP/6-311 G(d,p) level are compared with the experimentally determined molecular structure in the solid state.

Structural commentary
The molecular structure of the title compound (I) is shown in Fig. 1. The -conjugated system is nearly planar, with a 2.5 (1) twist between the aromatic and aldehyde groups. The julolidine ring system comprises three fused rings and one locked nitrogen atom. The C1-O1 and C3-O2 bond lengths are of 1.231 (3) and 1.345 (3) Å , respectively, indicate double-and single-bond character for these bonds. The two fused nonaromatic rings of the julolidine moiety adopt slightly distorted envelope conformations with atoms C9 and C12 displaced from the plane through the remaining ring atoms by 0.654 (2) and 0.648 (2) Å , respectively. The intramolecular O2-H2Á Á ÁO1 hydrogen bond forms an S(6) ring motif ( Fig. 1 and Table 1) between the phenol and aldehyde groups. Such an intramolecular hydrogen bond is common in salicylaldehyde derivatives, and the metrical parameters are comparable to those for related structures such as hydroxybenzaldehyde (Kirchner et al., 2011).

Supramolecular features
In the crystal, molecules are linked by C-HÁ Á ÁO hydrogen bonds, forming an A-B--A-B-A-B arrangement through the inversion centre and propagating along the c-axis direction (see Fig. 2 and Table 1). There are no other significant intermolecular contacts present in the molecule.

DFT study
The DFT quantum-chemical calculations were performed at the B3LYP/6-311 G(d,p) level (Becke, 1993;Lee et al., 1988) as implemented in GAUSSIAN09 (Frisch et al., 2009). DFT structure optimization of (I) was performed starting from the X-ray geometry and the values compared with experimental values (see Table 2). From these results we can conclude that basis set 6-311 G(d,p) is well suited in its approach to the experimental data.
The DFT study of (I) shows that the HOMO and LUMO are localized in the plane extending from the whole julolidine ring to the salicylaldehyde ring. The electron distribution of the HOMO-1, HOMO, LUMO and the LUMO+1 energy levels are shown in Fig The molecular structure of the title compound, with the atom labelling. Displacement ellipsoids are drawn at the 30% probability level. The intramolecular O-HÁ Á ÁO hydrogen bond is shown as a dashed line (see Table 1). Table 1 Hydrogen-bond geometry (Å , ). Symmetry codes: (i) x; y þ 1; z; (ii) Àx þ 1; Ày þ 1; Àz þ 1.

Figure 2
A view of the A-B-A-B-A-B arrangement in the crystal structure of the title compound. The hydrogen bonds are shown as dashed lines (see Table 1). For clarity, only the H atoms involved in hydrogen bonding have been included. The packing structure exhibits R 2 contain both and character whereas HOMO-1 is dominated by -orbital density. The LUMO is mainly composed of density while LUMO+1 has both and electronic density. The HOMO-LUMO gap was found to be 0.154 a.u. and the frontier molecular orbital energies, E HOMO and E LUMO were f À0.19624 and À0.04201 a.u., respectively.

Database survey
A search of the Cambridge Structural Database (CSD, Version 5.37, update May 2016; Groom et al., 2016) gave 121 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. The very similar compound 2-[(2,3,6,7-tetrahydro-1H,5Hbenzo[ij]-quinolizin-9-yl)methylene]propanedinitrile (II) reported by Liang et al. (2009) has the aldehydic group in (I) replaced by dicyanovinyl groups and the hydroxy group replaced by hydrogen. The N1-C5 bond length [1.381 (2) Å ] in the title compound is longer than in (II) [1.365 (3) Å ] due to conjugation with dicyanovinyl group. In the julolidine-1,6dione compound reported by Wu et al. (2007), the N atom of the julolidine moiety lies approximately in the plane of the benzene ring with a deviation of 0.023 (2) Å , similar to that in title compound [0.043 (2) Å ], as might be expected for the maximum conjugation normally found for N-atom substituents on benzene rings.

8-Hydroxy-1,2,3,5,6,7-hexahydropyrido[3,2,1-ij]quinoline-9-carbaldehyde
Crystal data 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.
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )