1,4-Di-n-heptyloxy-2,5-dinitrobenzene

The complete molecule of the title compound, C20H32N2O6, is generated by crystallographic inversion symmetry. The two mutually trans nitro substituents are hence in fully eclipsed conformation and also twisted by 43.2 (2)° with respect to the phenyl ring plane. The benzene-connected portions of the alkoxy substituents lie almost coplanar with the ring [C—O—C—C torsion angle = 2.0 (2)°]. In the crystal, weak C—H⋯O interactions link the molecules.


Comment
The title compound, (I), is the minor product formed from the nitration of 1,4-di(n-heptoxy)benzene and was synthesized as a precursor to derivatized "salen-like" ligands for co-ordination to transition metals. Although (I) is commercially available, apparently a synthetic method has not been reported previously. Our synthesis involves a standard nitration procedure (Hammershøj et al., 2006) and produces a mixture of the 2,3 and 2,5 structural isomers in a ca 2:1 ratio as indicated by the 1 H NMR spectrum of the crude material. The isomeric ratio produced in such reactions is clearly quite variable. For example, nitration of 1,4-dimethoxybenzene by a very similar method, but with heating at 373 K for 1 h produced the 2,3 isomer in 90% yield after recrystallization (Hammershøj et al., 2006). Similar results were reported previously (Flader et al., 2000;Fisher et al., 1975), while nitration of 1,4-di(n-butoxy)benzene in a mixture of nitric and acetic acids gives the 2,3 and 2,5 isomers in a 4:1 ratio (Kawai et al., 1959;Baker et al., 2008).
Having structural confirmation for (I), the two isomers are also distinguished by significant differences in their 1 H NMR spectra, especially a high field shift of 0.36 p.p.m. for the singlet assigned to the two phenyl protons on moving from 2,5 to 2,3-isomer. This change can be attributed to an increased extent of shielding when these protons are located meta rather than ortho to the nitro substituents. The two isomers also show significantly different melting points and electronic absorption spectra. Compound (I) melts at a temperature ca 70 K higher than that observed for its 2,3-isomer, indicating that the forces holding together the crystal lattice are considerably stronger for (I). A similarly large difference in melting points has also been reported for the corresponding n-butoxy compounds (Kawai et al., 1959).
Both isomers show relatively intense near UV absorption bands that are responsible for their observed colours. These bands are attributable to π→ π* intramolecular charge-transfer (ICT) excitations from the HOMO primarily localized on the electron-rich heptoxy groups to the LUMO localized on the electron-deficient nitro units. The stronger yellow colour of (I) when compared with its 2,3-isomer is due to the ICT band maximum being lower in energy by ca 940 cm -1 , with an approximately doubled molar extinction coefficient, producing more extensive tailing of the absorption into the visible region. Clearly, both the HOMO-LUMO energy gap and the extent of overlap between these orbitals are affected significantly by isomerization.
Compound (I) readily forms large and high-quality, yellow block-shaped crystals upon slow evaporation of a n-hexane/ ethyl acetate solution. Its structure ( Fig. 1) resembles that reported previously for the compound 2-(n-heptoxy)-5-methoxy-3,6-dinitrobenzaldehyde (Voss et al., 2003), with generally similar geometric parameters. In both compounds, the two mutually trans nitro substituents are twisted with respect to the phenyl ring plane. However, in (I) these groups are fully eclipsed, since they are related by inversion, each with a O2-N1-C1-C2 torsion angle of 43.2 (2)°, while their mutual orientation is staggered in the previously published structure, with corresponding angles of 39.3 (5) and 87.5 (4)°. Another difference between these two structures is the relative orientations of their alkoxy substituents. In (I), for the inversion-related alkoxy groups C4-O3-C2-C3, the torsion angles are very small (2.0 (2)°), but in 2-(n-heptoxy)-5-methoxy-3,6-dinitrobenzal-supplementary materials sup-2 dehyde, the C-O-C-C angles are quite different, being 1.0 (5)° for the methoxy substituent, while the OCH 2 unit of the heptoxy group is almost perpendicular to the phenyl ring, with a C-O-C-C torsion angle of 86.9 (4)°.

Experimental
Synthesis of 1,4-di(n-heptoxy)benzene. A solution of hydroquinone (5.00 g, 0.045 mol), 1-bromo-n-heptane (17.9 g, 0.100 mol) and K 2 CO 3 (25.1 g, 0.182 mol) in DMF (100 ml) was heated at reflux for 3 h. The resulting brown solution was poured into cold water and the brown precipitate filtered off, washed with cold water and recrystallized from ethanol to give a colourless solid (yield 7.22 g, 52%).