Crystal structure of 2-amino-1,3-dibromo-6-oxo-5,6-dihydropyrido[1,2-a]quinoxalin-11-ium bromide monohydrate

In the title compound, the 2-amino-1,3-dibromo-6-oxo-5,6-dihydropyrido[1,2-a]quinoxalin-11-ium cations are non-planar and are linked through centrosymmetric hydrogen-bonded cyclic Br2(H2O)2 anion–water units by N—H⋯Br, N—H⋯O and O—H⋯Br hydrogen bonds, forming one-dimensional ribbons, with the planes of the cations lying parallel to (100).

The title singly protonated monobromide monohydrate salt, C 12 H 8 Br 2 N 3 O + ÁBr À ÁH 2 O, was synthesized from the reaction the pyridine derivative Schiff base N1,N4-bis(pyridine-2-ylmethylene)benzene-1,4-diamine (BPYBD) with molecular bromine. The cyclization occurs by oxidation of BPYBD, ISSN 2056-9890 reduction of molecular bromine and finally hydrolysis of the imine bond which creates the charge at the pyridine nitrogen atom in the quinoxaline ring system. The structure is reported herein.

Structural commentary
The asymmetric unit of the title compound contains a discrete 2-amino-1,3-dibromo-6-oxo-5,6-dihydropyrido[1,2-a]quinoxalin-11-ium cation with a protonated pyridine moiety, and a bromide counter-anion and a water molecule of solvation ( Fig. 1). The cation is non-planar compared to the previously reported structure (Faizi et al., 2015). The mean plane of the pyridine ring forms a dihedral angle of 24.2 (4) with the benzene ring and 14.6 (4) with the pyrazine ring of the fused system while the dihedral angle between the pyrazine and the benzene ring is 11.5 (4) . A shorter C10-N3 distance of 1.367 (9) Å , compared to the usual aromatic C-N amine single bond distance of 1.43 (3) Å , might be due to the electronwithdrawing effect of the positively charged pyridine N atom, and the ortho-substituted bromine atom which decreases the C-N amine bond order. Other C-C and C-N bond distances are well within the limits expected for aromatic rings (Koner & Ray, 2008;Kanderal et al., 2005;Fritsky et al., 2006). Present also in the cations are intramolecular N3-HÁ Á ÁBr1 and N3-HÁ Á ÁBr2 interactions [3.048 (7), 3.006 (7) Å , respectively, Table 1].

Figure 2
The one-dimensional hydrogen-bonded ribbon structure, viewed along the a-axis direction. Inter-species interactions are shown as dashed lines.
ionic layers of anions and vice versa, forming an alternating hydrocarbon-ionic layer structure. No intermolecularinterations are evident in the hydrocarbon layer in the structure.

Synthesis and crystallization
Molecular bromine (440 mg, 144.0 mL, 2.80 mmol) was added to a methanolic solution (10 mL) of Schiff base, N1,N4-bis (pyridine-2-ylmethylene)benzene-1,4-diamine (BPYBD) (197 mg, 0.70 mmol). The color of the solution immediately changed from yellow to orange. The reaction mixture was stirred for 4 h at room temperature under a fume hood. The resulting yellow precipitate was recovered by filtration, washed several times with small portions of acetone and then with diethyl ether to give 200 mg (yield: 64%) of 2-amino-1,3dibromo-6-oxo-5,6-dihydropyrido[1,2-a]quinoxalin-11-ium bromide monohydrate (ADOQBM). The crystal of the title compound suitable for X-ray analysis was obtained within three days by slow evaporation of a solution of the compound in methanol.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. All N-bound H atoms were located in difference-Fourier maps and their positions were then held fixed. The isotropic displacement parameters were refined for these atoms. Aromatic H atoms were placed in calculated positions and treated as riding on their parent C atoms [C-H = 0.93 Å and U iso (H) = 1.2 or 1.5U eq (C)].

Computing details
Data collection: SMART (Bruker, 2003); cell refinement: SAINT (Bruker, 2003); data reduction: SAINT (Bruker, 2003); program(s) used to solve structure: SIR97 (Altomare et al., 1999); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: DIAMOND (Brandenberg & Putz, 2006); software used to prepare material for publication: DIAMOND (Brandenberg & Putz, 2006). where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 1.18 e Å −3 Δρ min = −1.16 e Å −3 Special details Experimental. The OH H-atom was located in difference Fourier map and refined with with U iso (H) = 1.2U eq (O). The Nand C-bound H-atoms were positioned geometrically and refined using a riding model: N-H = 0.86 Å and C-H = 0.93 Å with U iso (H) = 1.2U eq (parent atom). 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.