Crystal structure of 4′-bromo-2′,5′-dimethoxy-2,5-dioxo-[1,1′-biphenyl]-3,4-dicarbonitrile [BrHBQ(CN)2] benzene hemisolvate

The geometry of the title hemibiquinone is different from previous examples and may be correlated with the weak interactions in the crystal.


Chemical context
A new class of molecules, dubbed hemibiquinones (HBQs), has been developed and reported as potential molecular rectifiers. Biphenyl derivatives have garnered great attention to themselves as conductors of electricity (Venkataraman et al., 2006). Thus, control over the molecular equilibrium geometry, and therefore the overlap of the orbitals, allows for control over the governing electrical characteristics. The efficiency of conduction through a given molecule is dependent on the torsion angle between adjacent electrophores.
The title hemibiquinone (HBQ) molecule, 4 0 -bromo-2 0 ,5 0dimethoxy-2,5-dioxo-[1,1 0 -biphenyl]-3,4-dicarbonitrile 1 (Fig. 1), has been isolated as a molecule that will self-assemble on a gold surface as a potential unimolecular rectifier. HBQ 1 is predicted to act as a molecular diode due to the linking of the electron-rich 4 0 -bromo-2 0 ,5 0 -dimethoxybenzene donor with an electron-poor 3,4-dicarbonitrile quinone acceptor. This follows the scheme outlined by Aviram & Ranter (1974), where an electron-rich donor and an electron-poor acceptor are covalently bonded through an isolating saturated bridge. In HBQs, the predicted dihedral twist away from coplanarity of the two rings would decrease orbital overlap and allow for partial isolation of the donor and acceptor moieties. We have developed a selective synthesis for this hemibiquinone derivative that is scalable to gram quantities. ISSN 2056-9890

Structural commentary
As in the other reported HBQ molecules (Meany et al., 2015), we seek to use and compare the inter-ring torsion angles in the crystals as a guide against gas-phase calculated values. However, crystallized benzene solvent molecules in the crystal structure prevent us from drawing direct conclusions about the geometry. Packing effects distort the biphenyl molecule out of plane in the opposite direction as the hydroquinone starting material [BrHBQH 2 (CN) 2 ], Fig. 2. The C-C biphenyl bond [1.482 (2) Å ] in 1 is comparable to that in the hydroquinone [1.481 (2) Å ]. In this molecule, the C5-C4-C7-C8 torsion is 125.3 (2) , compared to the hydroquinone torsion angle of À126.50 , (Meany, 2015). DFT (B3LYP-DGDZVP) calculations performed on the target molecule in the gas phase predict an angle of À39.71 . This significant discrepancy is due to packing interactions in the solid phase, especially from benzene. Finally, the quinone ring is slightly buckled, likely due to supramolecular packing effects.
As in other structures, the methoxy groups are aligned nearly in-plane with the benzene ring, C2-C3-O2-C13 being bent out of plane by 2.9 (3) and C5-C6-O1-C14 bent out of plane by À7.0 (3) . The methoxy group bond angle C3-O2-C13 is measured at 117.4 (1) , and C6-O1-C14 is measured at 117.3 (1) . The methyl portions of each of these groups point away from the sterically restricting groups ortho to these positions, typical for this class.

Supramolecular features
The molecule packs in space group P1 with two HBQ molecules and one solvent molecule, the latter completed by a crystallographic inversion centre. The molecules align antiparallel to one another in the unit cell, primarily along the c crystallographic axis. The quinone rings are mostly parallel to the ac plane and sandwich, in a 2:1 ratio, a benzene solvent molecule. The plane of the dimethoxybenzene ring aligns with the diagonal of the ab plane.
Analysis of the short contacts shows an off-center donoracceptor-typeinteraction between benzene and the HBQ molecule. Fig3. 3 and 4 show the great extent of -overlap between benzene and HBQ 1. It is readily apparent that the benzene ring, rather than being centered between the quinone rings exactly, is actually slightly off-center. Instead, the electron density of the benzene is centered over the slightly electropositive C9-C10 bond.
Each HBQ molecule interacts with a total of three benzene molecules by short contacts. As mentioned above, one molecule of benzene is sandwiched between two quinone rings. Additionally, the 3-substituted nitrile group accepts a C-HÁ Á ÁN hydrogen bond from a solvent molecule (HÁ Á ÁN = 2.81 Å , C-HÁ Á ÁN = 147 ). The third benzene molecule exhibits short contacts to the 4 0 -bromine atom on the opposite end of the molecule, where H17 and H18 link to Br1 almost symmetrically (H17Á Á ÁBr1 = 3.05 Å , C17-H17Á Á ÁBr1 = 127 ; H18Á Á ÁBr1 = 3.04 Å , C18-H178Á Á ÁBr1 = 128 ). Since the benzene molecule -stacks parallel to the quinones, the benzene molecule is oriented in the same direction relative to Molecular overlay of 1 (blue) with the reduced precursor hydroquinone (BrHBQH 2 (CN) 2 , red). Viewed along the plane of the dimethoxybenzene ring and the C-C biphenyl bond (left), and parallel to the plane of the dimethoxybenzene ring (right). The overlay is meant to show the divergent geometry between the precursor and the title molecule, based on the different solid-state interactions.

Figure 1
The molecular structure of HBQ 1 showing the atom-numbering scheme. Ellipsoids are displayed at the 50% probability level. Hydrogen atoms are displayed as calculated.
the dimethoxybenzene ring. Although, in previous HBQ crystals the 4 and 4 0 groups show evidence of intermolecular halogen bonding, due to the excess electron density around the aryl bromine atoms and the nitrile groups, an attractive interaction is not possible, rather a slightly repulsive interaction is favored. Instead, the protons on C17 and C18 bifurcate to Br1 as an acceptor, forming slightly asymmetric hydrogen bonds between the dimethoxybenzene ring and the benzene solvent molecule. As discussed above, the quinone carbonyl groups are deflected from perfect planarity. In previous structures, methoxy oxygen atoms tended to deflect the carbonyl groups through repulsive effects. However, this structure contains some attractive intermolecular hydrogen bonding character, including the C14-H14BÁ Á ÁO4 contact, which is a moderate interaction at 2.57 Å and a bond angle of 157 . A second weaker interaction occurs between the C5-H5 dimethoxybenzene grouping and O4 (2.64 Å and 134 ). Projection of the O4 carbonyl atom to a neighboring quinoid proton H12 is also evident at a bond length of 2.65 Å and a C12-H12Á Á ÁO4 angle of 147 . There is a fourth and weakest interaction with O4, viz. C16 N1Á Á ÁO4 with an N1Á Á ÁO4 bond length of 3.159 (2) Å and a bond angle of 129.91 (1) . Two contacts are made with O3: a C2-H2Á Á ÁO3 (2.65 Å and 142 ) bond and weak -contacts [C10Á Á ÁO3 and C11Á Á ÁO3 = 3.251 (2) and 3.187 (2) Å , respectively]. Additionally, there is a short contact between C10 and C8, at 3.486 (3) Å . The N2 nitrile atom possibly accepts a very weak interaction from the methoxy C13 and H13A pair (2.82 Å and 97 ). There is a long C13Á Á ÁN2 [(3.101 (3) Å ] interaction as well. Even longer than those interactions, H13 also has a weak HÁ Á Á interaction with the dimethoxybenzene ring on an adjacent molecule (HÁ Á Á = 2.88 Å ). The packing is shown in Fig. 5.

Synthesis and crystallization
4 0 -Bromo-2,5-dihydroxy-2 0 ,5 0 -dimethoxy-[1,1 0 -biphenyl]-3,4dicarbonitrile (0.126 g, 0.337 mmol) was suspended in a mixture of 100 mL of H 2 O and 100 mL of benzene. FeCl 3 (0.340 g, 2.09 mmol) was added in one portion. The resulting mixture was capped and stirred overnight. The resulting phases were separated, and the organic phase was washed with water and dried over anhydrous Na 2 SO 4 . Evaporation of the solvent produced a crude product. The pure product was precipitated from a chloroform solution by addition of hexane, yielding 0.0460 g (36.7%). Black, block-shaped crystals of 1 were grown from chloroform solution with residual benzene at 296 K. 1  Packing diagram showing the off-centershort contacts of 1 with the intercalated solvent benzene as the asymmetric unit to only one HBQ. Weak hydrogen bonds from O2 project close to the benzene protons.

Figure 3
Packing diagram showing the off-centershort contacts of 1 with the intercalated solvent benzene. Weak hydrogen bonds from O2 project closely to the benzene protons.

Figure 5
Packing diagram showing the b directional -stacks and the weak c directional halogen bonds. Viewed along the a axis.