3,4-Bis(4-nitrophenyl)-1,2,5-oxadiazole 2-oxide

The title compound, C14H8N4O6, a new 1,2,5-oxadiazole N-oxide derivative, was formed by dimerization of 4-nitrobenzaldehyde oxime. The compound crystallizes with two independent molecules per asymmetric unit. The N-oxide O atom is disordered over two sites in each molecule; site occupancy factors are 0.57/0.43 and 0.5/0.5. The mean planes through the two benzene rings are inclined to the planar 1,2,3-oxadiazole ring by 25.03 (11) and 41.64 (11)° in one molecule, and 22.58 (11) and 42.66 (11)° in the other molecule, the smaller angle being for the ring on the oxide side of the oxadiazole ring in each case. In the crystal structure, the individual molecules form centrosymmetric dimers linked via C—H⋯O hydrogen bonds. The dimers of one molecule are then linked to those of the other molecule via C—H⋯O hydrogen bonds, forming a three-dimensional network.

The title compound, C 14 H 8 N 4 O 6 , a new 1,2,5-oxadiazole Noxide derivative, was formed by dimerization of 4-nitrobenzaldehyde oxime. The compound crystallizes with two independent molecules per asymmetric unit. The N-oxide O atom is disordered over two sites in each molecule; site occupancy factors are 0.57/0.43 and 0.5/0.5. The mean planes through the two benzene rings are inclined to the planar 1,2,3-oxadiazole ring by 25.03 (11) and 41.64 (11) in one molecule, and 22.58 (11) and 42.66 (11) in the other molecule, the smaller angle being for the ring on the oxide side of the oxadiazole ring in each case. In the crystal structure, the individual molecules form centrosymmetric dimers linked via C-HÁ Á ÁO hydrogen bonds. The dimers of one molecule are then linked to those of the other molecule via C-HÁ Á ÁO hydrogen bonds, forming a three-dimensional network.

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Acta Cryst. (2008). E64, o511 [ doi:10.1107/S1600536807066640 ] 3,4-Bis(4-nitrophenyl)-1,2,5-oxadiazole 2-oxide G. Alhouari, A. Kerbal, N. B. Larbi, T. B. Hadda, M. Daoudi and H. Stoeckli-Evans Comment In the course of our research aimed at the synthesis of new efficient antitubercular agents containing simple pharmacophore sites of the type X-C-C-Y we turned our attention to the spiro-isoxazolines which posses a rigid (O=C-C-O) pharmacophore. These compounds display interesting biological properties, such as herbicidal, plant-growth regulatory and antitumor activities (Howe & Shelton, 1990;Smietana et al., 1999). The preparation of the spiro-isoxazolines, in which we are interested, normally involves the reaction of a nitriloxyde [(E)-4-nitrobenzaldehyde oxime, (I)] with an isothiochromanone in a solution of hydrogen peroxyde (Kerbal et al., 1990). We have noted many times the formation of a by-product during this reaction. Finally this compound has been isolated and examined crystallographically. It was found to be a new 1,2,5oxadiazole N-oxide derivative, (II).
The molecular structure of compound (II) is shown in Fig. 1. The compound crystallizes with two independent molecules (1 & 2) per asymmetric unit. The 1,2,5-oxadiazole units are disordered with two alternative positions for the N-oxide O-atom [atom O1a/O1b in molecule 1, and atom O21a/O21b in molecule 2]. There are some short intramolecular C···O contacts in the two molecules involving the disordered atoms; O1b and neighbouring C-atoms C2 and C3, and atom O21b with atom C22. A search of the Cambridge Crystallographic Data Base (Version 1.8, last update May 2007: Allen, 2002) indicates that such short interactions are not unusual. The 1,2,5-oxadiazole ring is planar [to within 0.008 and 0.009 Å, in molecules 1 and 2, respectively] and the bond distances and angles are similar to those in the diphenyl analoque 3,4-Diphenylfurazan N-oxide, (III) [Sillitoe & Harding, 1978)]. They do not indicate the presence of delocalized electron density as in the dichlorophenyl analoque 4,5-bis(2,6-Dichlorophenyl)-1-oxide-2-oxa-1,3-diazole, (IV) [Easton et al., 1995] or a D-mannose-derived furoxan [Baker et al., 2002]. The C?N bonds being significantly shorter than the C-C or O-N bonds. The remainder of the bond distances in (II) are within normal limits (Allen et al., 1987). The best planes through the phenyl rings are inclined to the best plane through the 1,2,5-oxadiazole ring by 25.03 (11) and 41.64 (11)° in molecule 1, and 22.58 (11) and 42.66 (11)°i n molecule 2. This is quite different to the situation in (III), where the same dihedral angles are 16.7 and 59.6°, or in (IV), where the same dihedral angles are 63.1 (3) and 65.6 (5)°.
In the crystal structure of (II) the individual molecules are linked to their symmetry related molecule via C-H···O hydrogen bonds to form centrosynmetric dimers. These dimers are in turn linked by other C-H···O hydrogen bonds to form a three-dimensional network. Details of the hydrogen bonding are given in Table 1 and Fig. 2.
The formation of compound (II) is similar to that described by Baker et al. (2002), who have studied in detail the synthesis and X-ray structure of 3,4-dipyranosyl-1,2,5-oxadiazole 2-oxide. Similarly we found that the reaction of 4-nitrobenzaldehyde oxime with pure NaOCl in CHCl 3 , but never CH 2 Cl 2 , gives an almost quantitative yield of (II) (95%), on simply stirring at room temperature for 16 h.

supplementary materials sup-2 Experimental
The reaction of 4-nitrobenzaldehyde oxime with pure NaOCl, in a 2:1 molar ratio, in CHCl 3 (but never CH 2 Cl 2 ) gives an almost quantitative yield of (II) (95%), on stirring at room temperature for 16 h. Yellow block-like crystals suitable for X-ray analysis were obtained by slow evaporation of an ethanol solution of (II).

Refinement
The N-oxide O-atom is disordered over two sites in each molecule (1 & 2); the occupancies were finally fixed at O1a/O1b = 0.57/0.43 and O31a/O31b = 0.5/0.5. The hydrogen atoms could all be located from difference Fourier maps. They were included in calculated positons and treated as riding atoms with C-H distances = 0.95 Å and U iso (H) = 1.2U eq (parent C-atom). Fig. 1. Molecular structure of the two independent molecules (1 and 2) of compound (II), showing the crystallographic atom-numbering scheme and displacement ellipsoids drawn at the 50% probability level. The disordered N-oxide O-atoms, O1B and O21B, bonded to atoms N2 and N22, respectively, are drawn with red and white checkered patterned ellipses. The hydrogen atoms have been omitted for clarity. 3,4-Bis(4-nitrophenyl)-1,2,5-oxadiazole 2-oxide