Crystal structure of 4,4′-dibromo-2′,5′-dimethoxy-[1,1′-biphenyl]-2,5-dione (BrHBQBr)

In the crystal, molecules pack in a centrosymmetric fashion and interact via a mixture of weak π–π stacking interactions, weak C—H⋯O hydrogen bonding, and Br⋯Br halogen bonding. This induces a geometry quite different than that predicted by theory.


Chemical context
Biphenyl derivatives have recently been investigated as conductors for single molecule electronic systems (Venkataraman et al., 2006). Researchers have shown that as the equilibrium twist angle between the two rings increases, conduction through the molecule decreases as cos 2 (). This effect is rationalized as a loss of overlap between two systems. Interrupting conjugation is a prerequisite for the design of unimolecular rectifiers (Aviram & Ratner, 1974). Biphenyl derivatives with one electron-rich and one electrondeficient ring may be able to bias the direction of electron flow through the molecule, thus acting as a molecular diode. To this end we propose a dimethoxybenzene-quinone structure ('hemibiquinone', HBQ) as a potential unimolecular device. The asymmetric biphenyl structure should allow for high conductivity through each of the rings, while the dihedral angle between the two rings decreases orbital overlap and allows for partial isolation of the electron-rich donor and electron-poor acceptor moieties.
Two HBQ structures have been previously reported by Taylor et al. (2007) and Zeng & Becker (2004). The molecule described herein is unique in that it possesses bromine substituents on each ring (Fig. 1). The distal halogens allow for high synthetic versatility: these groups can be elaborated sequentially with functional groups to allow deposition in a predictable manner onto a variety of substrates. Originally this ISSN 2056-9890 molecule was proposed by Love et al. (2009) as an impurity in the synthesis of 4,4 0 -dibromodiquinone; however, characterization of this compound was not reported. We have developed a selective synthesis for this hemibiquinone that is scalable to gram quantities.

Structural commentary
Because of the crucial role that the twist angle between the rings plays in the electronic properties of the molecule, the determination of the C12-C7-C4-C5 torsion angle is the key observation in this structural analysis. This angle measures À110.9 (4) in the crystal structure. DFT (B3LYP-DGDZVP) calculations performed on the target molecule in the gas phase predict an angle of À38.54 . This significant discrepancy is probably due to packing interactions in the solid phase.
Substituents on the HBQ system behave as expected. The C-Br bond distances reflect the natures of the electrondeficient quinone and electron-rich dimethoxybenzene rings: the C1-Br1 bond distance is 1.872 (5) Å , while the C10-Br2 bond is 1.897 (4) Å . Thus Br1 has a slightly strongerdonating character into the quinone moiety, strengthening the bond relative to the C10-Br2 bond of the dimethoxybenzene ring. The methoxy substituents are nearly coplanar to the benzene ring, with a C12-C11-O4-C14 torsion angle of 1.5 (6) and a C9-C8-O3-C13 torsion angle of À4.4 (5) . The methyl portions of each of these groups point away from the sterically restricting groups ortho to these positions. Finally, the quinone ring is slightly buckled (r.m.s. deviation = 0.064 Å ), probably due to supramolecular packing effects.

Supramolecular features
Each molecule is surrounded by eight neighboring molecules, which interact through hydrogen bonding, halogen bonding, andinteractions (Figs. 2 and 3). The strongest interactions appear to be between functional groups on the quinone ring of one molecule with those on the dimethoxybenzene ring of another. These include especially short but non-directional C-HÁ Á ÁO hydrogen bonds (Table 1) between the quinone carbonyl groups and dimethoxybenzene ring hydrogen atoms of two neighbors. Interactions between like parts of neighboring molecules include edge-to-edge stacking of quinone rings with quinone rings, dimethoxybenzene rings with dimethoxybenzene rings, and dimeric hydrogen bonding between methoxy groups. Quinone rings on adjacent molecules along the c axis show some face-to-face -stacking.

Figure 1
The molecular structure of the title compound, showing displacement ellipsoids at the 50% probability level.

Figure 2
The unit-cell packing of the title compound, viewed down the b axis.

Figure 3
Packing diagram showing the stacking of parallel halogen-bonded chains. The view is down the a axis.
Along the a axis, the benzene rings 'nestle' closely to one another in an antiparallel geometry, where one quinone points up and the layer behind it points down. Within the cb plane, the benzene rings are coplanar; hydrogen atoms from C14 on one molecule project closely to O3 on the adjacent molecule and vice versa for a hydrogen atom attached to C13 to the adjacent O4 (Fig. 2). Symmetric C-HÁ Á Á short contacts exist between pairs of C13-H13CÁ Á Ádimethoxybenzene (Table 1).
Molecules are aligned linearly in a head-to-tail manner where the bromine atoms participate in BrÁ Á ÁBr halogen bonding (Fig. 3). As discussed above, Br1 is electron deficient with respect to Br2, and a distinct halogen bond forms along the molecular x-axis (the C7-C4 biphenyl bond). The Br1Á Á ÁBr2 separation is 3.4204 (8) Å , with almost linear C1-Br1Á Á ÁBr2 and C10-Br2Á Á ÁBr1 angles of 178.2 (4) and 170.9 (4) , respectively. Equivalent rings from molecules packed along this axis are parallel to one another; the quinone and benzene rings aligned coplanar to the corresponding ring in the next molecule.

Synthesis and crystallization
Cerium(IV) ammonium nitrate (0.956 g, 1.75 mmol, 1.75 eq) was dissolved in 30 ml of H 2 O. A solution of 2-bromo-1,4dimethoxybenzene (0.253 g, 1.17 mmol) in 25 ml of acetonitrile was quickly added with vigorous stirring. After three hours, the product had precipitated as a grey-green powder. The precipitate was filtered, washed with water, and dried. The crude product was purified using flash chromatography (silica gel, chloroform), yielding 0.0959 g of the desired product (20.3%). Crystals were obtained by slow evaporation of a solution in chloroform.

Refinement
Hydrogen atoms were placed in calculated positions, and their coordinates and displacement parameters were constrained to ride on the carrier atom [C-H = 0.98 Å and U iso (H) = 1.5U eq (C) for methyl H atoms, C-H = 0.95 Å and U iso (H) = 1.5U eq (C) for other H atoms]. Hydrogen atoms on methyl groups were refined with a riding rotating model. Crystal data, data collection and structure refinement details are summarized in Table 2  Computer programs: APEX2 and SAINT (Bruker, 2010), SHELXS97 and SHELXTL (Sheldrick, 2008) and SHELXL2014 (Sheldrick, 2015). Table 1 Hydrogen-bond geometry (Å , ).

Special details
Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.