Spectroscopic, crystallographic, and Hirshfeld surface characterization of nine-membered-ring-containing 9-methoxy-3,4,5,6-tetrahydro-1H-benzo[b]azonine-2,7-dione and its parent tetrahydrocarbazole

9-Methoxy-3,4,5,6-tetrahydro-1H-benzo[b]azonine-2,7-dione and 6-methoxy-1,2,3,4-tetrahydrocarbazole represent the structures of a benzoazonine that contains a nine-membered ring and its parent tetrahydrocarbazole. The molecules of the former pack together via strong amide N—H⋯O hydrogen bonding and weak C—H⋯O interactions, whereas the parent tetrahydrocarbazole packs with C/N—H⋯π interactions, as visualized by Hirshfeld surface characterization.

calculated with CrystalExplorer21 was mapped over d norm in the range from À 0.5838 to 1.1871 a.u.(Spackman et al., 2021).
The brightest red spot on the surface indicates the N1-H1� � �O1 i hydrogen bond, the second most intense spot corresponds to the shorter C5-H5B� � �O2 ii interaction, with a hydrogen-acceptor distance of 2.41 A ˚and a D-H� � �A angle of 138 � , while the least intense spot corresponds to the longer C13-H13B� � �O2 iii interaction at a distance of 2.60 A ˚and with a D-H� � �A angle of 174 � (Fig. 4 and Table 1).The twodimensional fingerprint plots (Fig. 5) reveal that the most important interatomic contacts, summing to 97.3%, are H� � �H The molecules of (II) pack with a herringbone motif (Fig. 6).Although (II) contains an acidic proton, the structure does not exhibit conventional hydrogen bonding, nor any meaningful intermolecular C-H� � �O/N contacts.However, the Hirshfeld surface calculated with CrystalExplorer21, mapped over d norm in the range from À 0.2999 to 1.3163 a.u.Hirshfeld surface of 6-methoxy-1,2,3,4-tetrahydrocarbazole, (II), mapped over d norm , showing via dashed lines the N1-H1� � �� i and C11-H11A� � �� i interactions (left), and the surface mapped over the shape-index property.[Symmetry code: (i) À x + 3 2 , y + 1 2 , À z + 3 2 .](Spackman et al., 2021), reveals that the molecules interact via pairwise N-H� � �� and C-H� � �� interactions (Fig. 7).The brighter red spot on the top left of the surface indicates the N-H� � �� interaction N1-H1� � �Cg1 i (Table 2), which is directed towards the C7-C12 ring on a neighboring molecule, in an offset fashion from the centroid towards C11, with the shortest contact to the ring being C11� � �H1 at a distance of 2.51 A ˚.The less intense red spot on the top right of the surface indicates the longer C-H� � �� ineraction C11-H11A� � �Cg2 i (Table 2), which is directed towards the carbazole ring on a neighboring molecule, in an offset fashion from the centroid towards C1, with a C1� � �H11A distance of 2.65 A ˚.The Hirshfeld surface for (II) mapped over the shape-index property further confirms the blue bump shapes of the N/ C-H� � �� donors on top and the red valleys of the acceptors on the face (Fig. 7) (Tan et al., 2019).The two-dimensional fiingerprint plots (

Database survey
A search for compounds similar to compound (I) in the Cambridge Structural Database (Groom et al., 2016) found a single structure (CSD refcode COMBEO) which contains the nine-membered ring with an additional acetamide-containing group bridging the 3-and 5-position methylene C atoms of the title compound (Baranova et al., 2012).The additional bridging group in COMBEO positions the amide carbonyl and N-H groups cis to one another, with an O-C-N-H torsion angle of 7.37 � , allowing for the formation of an R 2 2 (8) graph-set centrosymmetric hydrogen-bonding dimer, whereas in compound (I), they are oriented trans, with an O-C-N-H torsion angle of 170.69 � , which precludes hydrogen bonding via a similar dimer, and (I) forms a one-dimensional hydrogenbonding chain.

Synthesis and crystallization
In a fashion similar to that reported previously in the literature (Kumar et al., 2014), equimolar amounts of (p-methoxyphenyl)hydrazine hydrochloride (10 mmol, 1.746 g) and cyclohexanone (10 mmol, 1.04 ml) were added to a roundbottomed flask along with 10 mol% antimony trioxide as a catalyst (0.001 mol, 0.291 g) in methanol solvent (40 ml).The resulting mixture was refluxed in a mineral oil bath at 338 K overnight.The reaction mixture was then cooled to room temperature and quenched slowly with 10 ml of water and 10 ml of saturated sodium bicarbonate.The aqueous layer was then extracted with ethyl acetate (3 � 30 ml).The combined organic layer was dried overnight with anhydrous MgSO 4 , filtered, and evaporated under reduced pressure, yielding 740 mg (37%) of (II).The 1 H NMR data matched those reported previously in the literature.After isolating the tetrahydrocarbazole, the remaining aqueous methanol was set aside in a refrigerator for several days, from which a batch of faint-yellow crystalline material was collected and found by X-ray crystallography, as well as NMR and IR spectroscopy, mass spectrometry, and elemental analysis, to be the ninemembered-ring compound 9-methoxy-3,4,5,6-tetrahydro-1Hbenzo[b]azonine-2,7-dione, (I), formed by the oxidative cleavage of the indole carbon-carbon double bond of the parent tetrahydrocarbazole 6-methoxy-1,2,3,4-tetrahydrocarbazole, (II).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. H atoms on C atoms were included in calculated positions and refined using a riding model, with C-H = 0.95 A ˚and U iso (H) = 1.2U eq (C) for aryl H atoms, C-H = 0.98 A ˚and U iso (H) = 1.5U eq (C) for methyl H atoms, and C-H = 0.99 A ˚and U iso (H) = 1.2U eq (C) for methylene H atoms.The positions of the amide H atom in (I) and the amine H atom in (II) were found in difference maps and refined semi-freely using a distance restraint of N-H = 0.88 A ˚and U iso (H) = 1.2U eq (N).Computer programs: APEX2 (Bruker, 2013), SAINT (Bruker, 2013), SHELXT2018 (Sheldrick, 2015a), SHELXL2017 (Sheldrick, 2015b), SHELXTL2014 (Sheldrick, 2008), OLEX2 (Dolomanov et al., 2009), and Mercury (Macrae et al., 2020).

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

Table 3
Bruker, 2013)details.Experiments were carried out at 125 K with Mo K� radiation using a Bruker APEXII CCD diffractometer.Absorption was corrected for by multi-scan methods (SADABS;Bruker, 2013).Refinement was with 1 restraint.H atoms were treated by a mixture of independent and constrained refinement.