2-Bromo-3-nitrobenzaldehyde

The title compound, C7H4BrNO3, was isolated as a by-product while attempting to prepare a diselenide. There is a close intramolecular Br⋯O contact [2.984 (2) Å]. The molecules form loosely associated dimers held together by weak intermolecular Br⋯O interactions with the nitro O atoms [Br⋯O = 3.179 (3) Å]. As a result of these interactions, there is also a close Br⋯Br intermolecular contact [3.8714 (6) Å]. In addition, there are weak intermolecular C—H⋯O interactions. The combination of these interactions produces sheets which propagate in the (210) and (10) directions perpendicular to c.

The title compound, C 7 H 4 BrNO 3 , was isolated as a by-product while attempting to prepare a diselenide. There is a close intramolecular BrÁ Á ÁO contact [2.984 (2) Å ]. The molecules form loosely associated dimers held together by weak intermolecular BrÁ Á ÁO interactions with the nitro O atoms [BrÁ Á ÁO = 3.179 (3) Å ]. As a result of these interactions, there is also a close BrÁ Á ÁBr intermolecular contact [3.8714 (6) Å ]. In addition, there are weak intermolecular C-HÁ Á ÁO interactions. The combination of these interactions produces sheets which propagate in the (210) and (210) directions perpendicular to c.
HBS is grateful to the Department of Science and Technology (DST) for the award of a Ramanna Fellowship. VPS is grateful to IIT Bombay for the award of a teaching assistantship. RJB wishes to acknowledge the NSF-MRI program (grant No. CHE-0619278) for funds to purchase the diffractometer. The title compound 1, C 7 H 4 NO 3 Br, was isolated as a by-product while attempting to prepare diselenide 2 by reacting 2-bromo-3-nitrobenzylalcohol with disodium diselenide (Wirth & Fragale, 1997) as shown in scheme 1. Presumably, the formation of 1 takes place during column chromatography on silica gel where the alcohol function is oxidized to the aldehyde function. The preparation (but not the structure) of the title compound by different routes has been previously reported (Rahman & Scrowston, 1984;Sienkowska et al., 2000). In 1, with two withdrawing ortho groups present, the 2-position is highly susceptible to nucleophilic substitution by Na 2 Se 2 , Na 2 Te 2 , Na 2 Se to afford a series of novel chalcogen compounds (Singh et al. 2009). In this paper we report the structure of the precursor.

2-Bromo
The bond lengths and angles in the title compound are within the normal ranges for related compounds (Allen et al., 2002). When chalcogens (Se, Te) are present in the 2-position in place of bromine there is an intramolecular chalcogen (Se/Te···oxygen(aldehyde/nitro)) interaction (Singh et al. 2009). It was of interest to see whether the bromo analog will interact intramolecularly with the nitro/aldehyde donor groups. There is a close intramolecular Br···O contact of 2.984 (2) Å. The molecules form loosely associated dimers held together by weak intermolecular Br···O interactions with the nitro O atoms (Br···O 3.179 (3) Å, see Figure 1). Similar interactions have been previously reported (Jones & Lozano, 2004;Kruszynski, 2007;Pedireddi et al., 1992;Xie et al., 2009). As a result of these interactions there is also a close Br···Br intermolecular contact (3.8714 (6) Å) as has been commonly observed [42 examples found in a search of the Cambridge Structural Database (Allen, 2002)]. In addition there are weak intermolecular C-H···O interactions. Of the intermolecular interactions, only that between O3 and the aldehyde H is out of plane. As a result of this out-of-plane interaction the nitro group is twisted by 43.6 (4)° from the plane of the aromatic ring. The combination of these interactions produces sheets which propagate in the (2 1 0) and (-2 1 0) directions perpendicular to c as shown in Figure 2.

Experimental
The title compound 1, C 7 H 4 NO 3 Br, was isolated as a by-product while attempting to prepare diselenide 2 by reacting 2-bromo-3-nitrobenzylalcohol with disodium diselenide (Wirth & Fragale, 1997). Presumably, the formation of 1 takes place during column chromatography on silica gel where the alcohol function is oxidized to the aldehyde function. It has been prepared previously by a different routes (Rahman & Scrowston, 1984;Sienkowska et al. 2000).
Crystal suitable for X-ray diffraction were obtained from CH 2 Cl 2 /ethyl acetate.