(E)-4-Nitrobenzaldehyde oxime

In the title compound, C7H6N2O3, the planes containing the CNO and ONO atoms subtend dihedral angles of 5.47 (5) and 8.31 (5)°, respectively, with the benzene ring. In the crystal structure, intermolecular O—H⋯N hydrogen bonds link the molecules into centrosymmetric dimers with an R 2 2(6) graph-set motif.

Data collection: APEX2 (Bruker, 2008); cell refinement: SAINT (Bruker, 2008); data reduction: SAINT; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: X-SEED (Barbour, 2001); software used to prepare material for publication: publCIF (Westrip, 2010 (Jokanovic et al., 2009, Marrs et al., 2006. Oximes can act as useful protecting groups (Greene et al., 1999) and have served for the protection of carbonyl groups in the syntheses of erythromycin derivatives and perhydrohistrionicotoxin (Shinada et al., 1995). Oximes are also used for the purification and characterization of carbonyl compounds. As part of our interest in the study of oxime derivatives, we report here the crystal structure of the title compound (I). A depiction of the molecule is given in Fig. 1. In the crystal structure of the title compound, molecules are connected via intermolecular O-H···N hydrogen bonds (see Table 1 and Fig. 2) to form two-dimensional dimers. The oxime group has an E configuration [C3-C7-N2-O3 = 179.1 (2)°] and the planes containing the CNO and ONO atoms subtend dihedral angles of 5.47 (5)° and 8.31 (5)° with the phenyl C(1-6) ring. which is less than that reported for similar structures (Xing & Ding et al., 2007;. Each molecule is connected to a symmetry-related molecule through an inversion center by O-H···N hydrogen bonds, building an R 2 2 (6) graph-set motif in Fig. 2 (Etter et al., 1990;Bernstein et al., 1995;).

Experimental
To a warm solution of 4-nitrobenzaldehyde (0.907 g , 0.005 mol) in 25 ml e thanol, hydroxylamine hydrochloride (0.417 g, 0.006 mol) and sodium acetate trihydrate (2.04 g, 0.015 mol) were added and the mixture was heated under reflux until completion of the reaction. The concentrated reaction mixture was cooled down and water was added. The precipitated oxime was separated by filtration, washed with excess of water and dried. The crude product was recrystallized from ethanol to get the title compound (I).

Refinement
All H atoms were placed in calculated position and treated as riding on their parent atoms with C-H = 0.93Å or O-H = 0.82Å with U iso (H) = 1.2U eq (C) or 1.5U eq (O) for the hydroxyl H atom.

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 > σ(F 2 ) is used only for calculating Rfactors(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.
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )