Crystal structure of 2-azaniumyl-3-bromo-6-oxo-5,6-dihydropyrido[1,2-a]quinoxalin-11-ium dibromide

The title salt, C12H10BrN3O2+·2Br−, was synthesized from the reaction of N 1,N 4-bis(pyridin-2-ylmethylidene)benzene-1,4-diamine and bromine in a methanol solution. All non-H atoms of the 2-azaniumyl-3-bromo-6-oxo-5,6-dihydropyrido[1,2-a]quinoxalin-11-ium cation are nearly coplanar, the maximum deviation being 0.114 (4) Å. In the crystal, the cations and anions are linked through N—H⋯Br hydrogen bonds and weak C—H⋯Br interactions, forming a three-dimensional supramolecular architecture. A short Br⋯Br contact [3.3088 (9) Å] is observed in the crystal.


S1. Comment
Quinoxalines are an important class of heterocyclic compounds, some of which are found to be useful as fluorophores, dyes, and antibiotics (Duffy et al., 2002;Gazit et al., 1996). Many drug candidates bearing quinoxaline core structures are in clinical trials in antiviral, anticancer and CNS (central nervous system) therapeutic areas (Harmenberg et al., 1991;Naylor et al., 1993). The present work is a part of an ongoing structural study of Schiff bases and their utilization in the synthesis of previously unknown organic and polynuclear coordination compounds (Faizi & Sen, 2014;Moroz et al., 2012), and we report here synthesis and structure of 2-azaniumyl-3-bromo-6-oxo-5,6-dihydropyrido[1,2a]quinoxalin-11-ium bromide (ABODQ). There are very few examples similar to title compound have been reported in the literature (Eiden & Peter, 1966).
The title compound was synthesized from the reaction of two equimolar amounts of molecular bromine and pyridine derivative Schiff base N1,N4-bis(pyridine-2-ylmethylene) benzene-1,4-diamine (BPYBD). The cyclization occurs by oxidation of BPYBD, reduction of molecular bromine and finally hydrolysis of the imine bond which creates the dication at two of the nitrogen atoms in the quinoxaline ring system.
In the structure of the title bromide salt, the dication is essentially planar with a longer C10-N3 distance of 1.45 (3) Å, compared to the usual C aro -N amine single bond distance of 1.43 (3) Å. This might be due to the electron withdrawing effect of positively charged pyridine, which increased 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).
All H ammonium atoms and H pyrazine N-H group are involved in hydrogen bonds with two different bromide ions, and each anion accepts hydrogen bonds from three different cations. No intermolecular π-π interations are evident in the hydrocarbon layer in title compound.

Figure 1
The molecular conformation and atom-numbering scheme for the title compound, with non-H atoms drawn as 40% probability displacement ellipsoids. where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 1.23 e Å −3 Δρ min = −0.88 e Å −3 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.