Crystal structure of 4′-bromo-2,5-dihydroxy-2′,5′-dimethoxy-[1,1′-biphenyl]-3,4-dicarbonitrile

In the crystal of the title substituted hemibiquinone derivative, the ring systems interact through an intramolecular O—H⋯Omethoxy hydrogen bond, which induces a geometry quite different from those in previously reported hemibiquinone structures. The molecules associate through an intermolecular O—H⋯Nnitrile hydrogen bond and are interlinked through very weak C—H⋯N hydrogen bonds.

In the crystal of the title substituted hemibiquinone derivative, C 16 H 11 BrN 2 O 4 or [BrHBQH 2 (CN) 2 ], the substituted benzene rings are rotated about the central C-C bond, forming a dihedral angle of 53.59 (7) . The ring systems interact through an intramolecular O-HÁ Á ÁO methoxy hydrogen bond, which induces a geometry quite different from those in previously reported hemibiquinone structures. In the crystal, the molecules associate through an intermolecular O-HÁ Á ÁN nitrile hydrogen bond, forming chains which extend along [100] and are interlinked through very weak C-HÁ Á ÁN hydrogen bonds, giving a overall two-dimensional structure lying parallel to (010).

Chemical context
Recently, a new class of molecules (hemibiquinones, HBQs) has been reported as potential molecular rectifiers (Meany et al., 2015). Biphenyl derivatives have garnered great attention as conductors of electricity (Venkataraman et al., 2006). The symmetric nature of the biphenyl and polyphenyl derivatives studied so far allows for reasonable conduction through the orbitals. Biphenyl derivatives with one electron-rich and one electron-deficient ring may be able to preferentially bias the direction of electron flow through the molecule, thus acting as a molecular diode. The donor-bridge-acceptor model has long been accepted as a basis for the design of molecular rectifiers (Aviram & Ratner, 1974). The asymmetric biphenyl structure should allow for 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. The efficiency of conduction through a given molecule can be tuned depending on the torsion angle between the two rings.
As one of the series of molecules made for testing rectification through HBQs, the title compound, C 16 H 11 BrN 2 O 4 , [BrHBQH 2 (CN) 2 ], (I) has been isolated as an intermediate in the preparation of an HBQ derivative which can self-assemble on a gold surface. We have developed a selective synthesis for ISSN 2056-9890 this reduced hemibiquinone derivative that is scalable to gram quantities. Molecule (I) is predicted not to act as a molecular diode itself because both rings act as donor moieties. The oxidation of the hydroquinone ring of (I) would produce a potential rectifier.
Dicyano-functionalized hydroquinones are known for their ability to form hydrogen-bonded networks (Reddy et al., 1996) and charge-transfer complexes (Bock et al., 1996), sometimes both at once (Ghorai & Mani, 2014). They have also been used as rigid ligands in coordination polymers (Kuroda-Sowa et al., 1997). However, there are no crystal structures in which a dicyano-functionalized hydroquinone moiety has been appended to another aromatic ring. The present study affords an opportunity to investigate the mutual effects of these two functionalized ring systems on both the geometry of the molecule and its intermolecular interactions.

Structural commentary
In the title compound ( Fig. 1), the benzene rings are twisted out of a common plane, forming a dihedral angle of 53.59 (7) , which appears to optimize the 2.7576 (18) Å O3-HÁ Á ÁO2 intramolecular hydrogen bond (Table 1). The rings are essentially planar although the O3-H group, which participates in the intramolecular hydrogen bond, is displaced slightly out of the plane. Also, the rings are not co-axial with the C4-C7 bond that bridges them. This can be seen in torsion angles involving three carbon atoms from one ring and the bridging carbon atom from the other, which deviate from linearity by ca 5 [C2-C3-C4-C7 = 173.88 (14) , C6-C5-C4-C7 = À175.45 (14) , C4-C7-C8-C9 = 174.94 (13) , C4-C7-C12-C11 = À175.62 (13) ]. This bending of the molecule about its long axis may also be due to hydrogen bonding as it causes the methoxy group to approach the OH group more closely. The aromatic C-C bonds of both rings have a narrow range of distances [from 1.387 (2) to 1.412 (2) Å ]. The C-C, C-O, C-N, and C N distances for the molecule are similar to the corresponding distances in 2,3,5,6-tetracyanohydroquinone (Bock et al., 1993). The C-C bond distances around the bromodimethoxybenzene ring are close to those in the other hemibiquinone molecules containing this ring (Meany et al., 2015(Meany et al., , 2016. The C9-C10 bond in (I) [1.408 (2) Å ] is longer than the corresponding C1-C6 bond in BrHBQBr (1.334 Å ; Meany et al., 2015). The stronger polarization of (I) relative to the starting material should weaken the bond through repulsive effects. The Br1-C1 bond is slightly shorter in (I) [1.885 (1) Å ] compared to the starting material [1.898 (4) Å ] as well, also suggesting decreased electron density on the dimethoxybenzene ring due to increased polarization. The calculated dipole (B3LYP-DGDZVP) of BrHBQBr is only 1.33 D, compared to 6.17 D for (I).
As in the other reported hemibiquinone molecules (Meany et al., 2015), we seek to use and compare the inter-ring torsion angles in the crystals as a guide compared to gas-phase calculated values. The intramolecular hydrogen bond from the C8 phenol to the O2 methoxy group causes a greater torsion angle than that in the starting HBQ (Meany et al., 2015). In (I), the C5-C4-C7-C8 torsion angle is À126.5 (2) , compared to À110.9 (5) in HBQ. DFT (B3LYP-DGDZVP) calculations performed on the target molecule in the gas phase predict an angle of 48.85 . This significant discrepancy is due to packing interactions in the solid phase as well as the additional hydrogen bond. The hydrogen bond is indicated in Fig. 1, while the relative orientations of the rings can be seen in Fig The molecular structure of (I), showing the atom-numbering scheme. Displacement ellipsoids are displayed at the 50% probability level. The intramolecular O3-HÁ Á ÁO2 hydrogen bond is shown as a dashed line. Table 1 Hydrogen-bond geometry (Å , ).

Figure 2
Ball-and-stick plot of (I), viewed down the C4-C7 bond.

Supramolecular features
Each molecule makes short (less than the sum of the van der Waals radii) contacts to six neighboring molecules (Fig. 3). As in previously reported HBQ structures, rings of like identity are all aligned in parallel planes. All short contacts are associated with Lewis acid-base interactions of some kind, and for each interaction there is one neighboring molecule that acts as a donor and second that acts as an acceptor. Two central molecules in the unit cell stack antiparallel to one another, the quinone rings shifted off-center from one another in the a-axis direction. Both nitrile groups are involved in intermolecular hydrogen-bonding interactions, the first one (O4-HÁ Á ÁN1) strong, the second one (C2-HÁ Á ÁN2) weaker but still highly directional. For details, see Table 1. These interactions link molecules along the crystallographic a-and b-axis directions, respectively, forming sheets parallel to (010) (Fig. 4). The quinone rings are aligned parallel to the bc plane diagonal.
The remaining two molecules in the unit cell are oriented orthogonally to the central molecules. These molecules are antiparallel to each other, where the dimethoxybenzene rings stack with those of the central pair. Slightly repulsiveinteractions between molecules along b and stacking along c can be seen in Fig. 5. Intercentroid distances for the rings are longer than expected for closeinteractions at 4.107 (1) Å . However, since the rings are slightly offset from one another, this is not the correct centroid to use. Instead, a close 3.598 (1) Å -interaction between two intermolecular C9-C10 centroids exists. A centroid calculated for the C7-C8-  Hydrogen-bonded sheets along ab. Dashed green lines indicate short contacts. Axes are color coded: red = a axis, green = b axis and blue = c axis.

Figure 5
Unit-cell packing of (I), viewed along the a axis. Short contacts show the long ring stacking along the c axis.
C9-C11-C12 ring sits 3.574 (1) Å from a centroid for N1-C15-C9-C10-C16-N2, which may be explained by the electron-donating character of the hydroquinone as compared to the dinitrile substituents. The planes of the dimethoxybenzene rings are oriented parallel to the short diagonal of the ac plane.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. Hydroxyl hydrogen atoms were located from the difference map and their coordinates were refined while the thermal parameters were constrained to ride on the carrier atom with U iso = 1.5U eq (O). Hydrogen atoms bonded to carbon were placed in calculated positions with C-H = 0.93 Å (aromatic) or 0.96 Å (methyl) and their coordinates and thermal parameters were constrained to ride on the carrier atom, with U iso = 1.5U eq (aromatic C) or 1.5U eq (methyl C).  Data collection: APEX2 (Bruker, 2010); cell refinement: SAINT (Bruker, 2010); data reduction: SAINT (Bruker, 2010); program(s) used to solve structure: SHELXS97 (Sheldrick 2008); program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015); molecular graphics: SHELXTL (Sheldrick, 2008); software used to prepare material for publication: SHELXTL (Sheldrick, 2008).

4′-Bromo-2,5-dihydroxy-2′,5′-dimethoxy-[1,1′-biphenyl]-3,4-dicarbonitrile
Crystal data 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.