Anomalous halogen bonds in the crystal structures of 1,2,3-tribromo-5-nitrobenzene and 1,3-dibromo-2-iodo-5-nitrobenzene

Two halogenated nitrobenzene derivatives have been characterized. The substitution of a Br substituent by an I atom modifies the network of halogen bonds, and gives rise to the formation of non-classical Brδ+⋯Iδ- bonds.

The title trihalogenated nitrobenzene derivatives, C 6 H 2 Br 3 NO 2 and C 6 H 2 Br 2 INO 2 , crystallize in triclinic and monoclinic cells, respectively, with two molecules per asymmetric unit in each case. The asymmetric unit of the tribromo compound features a polarized Br + Á Á ÁBrintermolecular halogen bond. After substitution of the Br atom in the para position with respect to the nitro group, the network of XÁ Á ÁX halogen contacts is reorganized. Two intermolecular polarized halogen bonds are then observed, which present the uncommon polarization Br + Á Á ÁI -: the more electronegative site (Br) behaves as a donor and the less electronegative site (I) as an acceptor for the charge transfer.

Chemical context
Within the large class of non-covalent interactions studied in chemical crystallography, halogen bonds are of special interest in crystal engineering. The stabilizing interaction between a halogen atom and a Lewis base, XÁ Á ÁB, shares many aspects with classical hydrogen bonds, but is more directional. On the other hand, halogen contacts XÁ Á ÁX are more difficult to conceptualize (Wang et al., 2014), for instance because the charge transfer in the BrÁ Á ÁBr contact is not as obvious as in hydrogen bonds. Evidence supporting the importance of this topic is the recent organization of an international meeting dedicated to halogen bonding (Erdelyi, 2014).
In this context, we are engaged in the synthesis and structural characterization of a series of halogen-substituted nitrobenzenes. The present communication describes two closely related compounds in the series, which differ only by the halogen atom substituting at the ring position para to the nitro group. Despite the small chemical modification, the resulting crystal structures are very different, as a consequence of a different network of halogen bonds. ISSN 2056-9890

Structural commentary
Both compounds crystallize with two molecules in the asymmetric unit, but in different space groups. The tribromo derivative, (I, Fig. 1), is a P1 crystal isomorphous to the chloro analogue (Bhar et al., 1995), although the unit-cell parameters are significantly larger for (I) compared to the chloro compound: the cell volume is increased by more than 7%. In the present work, we retained the Niggli reduced triclinic cell (a < b < c), while Bhar et al. used a non-reduced cell. Moreover, the asymmetric unit content was defined in order to emphasize the strongest BrÁ Á ÁBr bond in (I). The bromo-iodo derivative (II, Fig. 2) crystallizes in the monoclinic system and, in that case, the standard setting was used for space group P2 1 /c.
The important feature in these halogenated molecules is rather the possibility of steric repulsion between vicinal halogen atoms, which is related to the reduction of endocyclic angles. Regarding this point, it is worth reading the Acta E article about 1,2,3-triiodobenzene (Novak & Li, 2007). As in polyiodo derivatives, intramolecular steric crowding between the halogen atoms in (I) and (II) is offset by benzene ring distortion. As a consequence, the C1-C2-C3 and equivalent C11-C12-C13 angles are systematically less than 120 : 116.2 (11) and 118.8 (13) in (I); 118.1 (12) and 117.3 (13) in (II). Again, the nitro group has little influence on intramolecular halogenÁ Á Áhalogen contacts. For instance, in 1,3dibromo-2-iodobenzene, the C1-C2-C3 angle is 118.0 (Schmidbaur et al., 2004), very close to that observed in (II), which presents the same halogen substitution.
The 5-nitro substituent is almost conjugated with the benzene nucleus in (I): the dihedral angle between the NO 2 plane and the benzene ring is 6(2) and 1(2) for each independent molecule. For (II), twisting of the NO 2 groups is more significant, with dihedral angles of 10 (1) and 7(1) . This near planar conformation is identical to that observed for 1,2,3trichloro-5-nitrobenzene (Bhar et al., 1995), but contrasts with the twisted conformation observed in perhalogenated nitrobenzene derivatives: pentachloronitrobenzene (twist angle of NO 2 : 62 ; Tanaka  The asymmetric unit of (I), with displacement ellipsoids at the 30% probability level. The dashed bond connecting the independent molecules is a type-II halogen bond.

Figure 2
The asymmetric unit of (II), with displacement ellipsoids at the 30% probability level. The dashed bonds connecting the independent molecules are halogen contacts. 2011). It thus seems clear that twisting of the nitro group with respect to the benzene ring in nitrobenzene derivatives is a direct consequence of intramolecular crowding with ortho substituents. For 1,2,3-halogenated-5-nitrobenzenes such as (I) and (II), a planar conformation should be expected as a rule.

Supramolecular features
The crystal structures are directed by intermolecular weak halogen bonds, also known as type-II interactions in the Desiraju classification scheme (Reddy et al., 2006). Such a bond is present in the asymmetric unit of (I), between Br2 and Br11 (Fig. 3). The type-II arrangement is characterized by angles 1 = C2-Br2Á Á ÁBr11 and 2 = C11-Br11Á Á ÁBr2, which should be close to 180 and 90 , respectively. For (I), observed angles are 1 = 165.2 (5) and 2 = 82.3 (5) . The crystal packing thus polarizes the involved halogen atoms, forming the halogen bond Br2 + Á Á ÁBr11 -. This dimolecular polar unit is connected via inversion centers to neighboring units in the cell, forming C-HÁ Á ÁBr hydrogen bonds, and OÁ Á ÁBr contacts. This packing motif induces secondary halogen-Á Á Áhalogen contacts, which are clearly unpolarized. These type-I interactions are characterized by angles 1 ' 2 (Table 1, entries 2 and 3) and display larger BrÁ Á ÁBr separations compared to the polarized halogen bond (entry 1), in which electrostatic forces bring the atoms into close contact.
The substitution of one Br atom by I, to form crystal (II), changes dramatically the packing structure, affording a more complex network of halogen contacts ( Fig. 4 and Table 2). Within the asymmetric unit, the type-II polarized contact is Br1Á Á ÁI12 ( Part of the crystal structure of (I), emphasizing the halogen bonds (dashed lines). The green molecules correspond to the asymmetric unit.
3.787 (2) 142.8 (4) 122.9 (4) I-unpolarized Br11Á Á ÁBr3 ii 3.858 (2) 143.9 (4) 124.4 (4) I-unpolarized polarized in the wrong way, Br + Á Á ÁI -. The opposite polarization was expected for this bond, due to the lower electronegativity and higher polarizability of iodine compared to bromine. The other significant contact observed in the asymmetric unit is a BrÁ Á ÁBr unpolarized contact. The network of halogen bonds is expanded in the [100] direction by Br11, which gives a bifurcated contact with I2 and Br3 (Table 2, entries 2 and 4). One contact is polarized, with the polarization, once again, oriented in the unexpected way, I2 -Á Á ÁBr11 + . These anomalous halogen bonds are not present in other mixed halogen derivatives. Indeed, in 1,3-dibromo-2iodobenzene (Schmidbaur et al., 2004), the iodine atom is not engaged in halogen bonding.
Synthesis of (I). A solution of 2,6-dibromo-4-nitroaniline (1.0 g, 3.38 mmol) in acetic acid (3 ml) was cooled to 278 K, and concentrated H 2 SO 4 (7 ml) was carefully added under stirring. While ensuring that the temperature was still below 278 K, NaNO 2 (0.708 g, 10.26 mmol) was added in one batch. The reaction was stirred at this temperature for 2 h to afford Synthetic scheme for (I) and (II).  (12).
Synthesis of (II). A solution of 2,6-dibromo-4-nitroaniline (1.0 g, 3.38 mmol) in acetic acid (3 ml) was cooled to 278 K in an ice-salt bath, and concentrated H 2 SO 4 (3 ml) was carefully added under stirring. While ensuring that the temperature was still below 278 K, NaNO 2 (0.242 g, 3.516 mmol) was added in one batch. The reaction was stirred at this temperature for 30 min to afford the diazonium salt. An aqueous solution (10 ml) of KI (5.635 g, 33.95 mmol) was prepared, and the diazotization solution previously prepared was added in one batch. The mixture was then further stirred for 1 h. The reaction was neutralized with NaOH, extracted with CH 2 Cl 2 (3 Â 30 ml), and concentrated under vacuum. The crude material was purified by flash chromatography (petroleum ether/CH 2 Cl 2 4/1, R f = 0.31) to give (II). Crystals were obtained by slow evaporation of an acetone/methanol/CH 2 Cl 2 solution (yield: 1.

Refinement
Crystal data, data collection and structure refinement details for (I) and (II) are summarized in Table 3. The absorption correction for (I) was challenging, and eventually carried out by applying DIFABS on the complete isotropic model (Walker & Stuart, 1983). In the case of (II), measured -scans were used. H atoms were refined as riding to their carrier C atoms, with C-H bond lengths fixed at 0.93 Å and with U iso (H) = 1.2U eq (carrier atom).