Crystal structure of 5,7,12,14-tetrahydro-5,14:7,12-bis([1,2]benzeno)pentacene-6,13-dione

5,7,12,14-Tetrahydro-5,14:7,12-bis([1,2]benzeno)pentacene-6,13-dione, used as a precursor in the synthesis of polymers of intrinsic microporosity (PIM) membranes, recrystallizes from DMF.

The lattice of 5,7,12,14-tetrahydro-5,14:7,12-bis([1,2]benzeno)pentacene-6,13dione, C 34 H 20 O 2 , at 173 K has triclinic (P1) symmetry and crystallizes with four independent half-molecules in the asymmetric unit. Each molecule is generated from a C 17 H 10 O substructure through an inversion center at the centroid of the central quinone ring, generating a wide H-shaped molecule, with a dihedral angle between the mean planes of the terminal benzene rings in each of the two symmetry-related pairs over the four molecules of 68.6 (1) (A), 65.5 (4) (B), 62.3 (9) (C), and 65.8 (8) (D), an average of 65.6 (1) . This compound has applications in gas-separation membranes constructed from polymers of intrinsic microporosity (PIM). The title compound is a product of a double Diels-Alder reaction between anthracene and p-benzoquinone followed by dehydrogenation. It has also been characterized by cyclic voltammetry and rotating disc electrode polarography, FT-IR, high resolution mass spectrometry, elemental analysis, and 1 H NMR.

Chemical context
Pentiptycene and its derivatives are members of the iptycene family (Hart et al., 1981). They possess a rigid, bulky, aromatic, three-dimensional scaffold which makes them suitable for specific applications in porous material construction (Yang & Swager, 1998a), fluorescent polymers, chemical sensing (Yang & Swager, 1998b) and molecular machines (Sun et al., 2010). The first iptycene derivative was reported 85 years ago (Clar, 1931). Pentiptycene, first prepared by Theilacker et al. (1960), is readily available from inexpensive materials and is made by Clar synthesis, which involves a Diels-Alder cycloaddition between a polycyclic diene and a benzoquinone followed by chloranil-induced dehydrogenation. Pentiptycene quinone is a precursor for pentiptycene-6,13-diol, which is subsequently used as a principal reactant for polymer synthesis. Gong & Zhang (2011) synthesized poly(arylene ether sulfone)s to fabricate highly conductive polymer electrolyte membranes for high-temperature and low-humidity conditions. Pentiptycene-based diamines have been used in the preparation of polyimides with controlled molecular cavities, for application in gas separation membranes (Luo et al., 2015(Luo et al., , 2016.
The dihedral angle between the mean planes of the terminal benzene rings in each of the symmetry-related sets over the four molecules is (the complement of) 68.6 (1) (A), 65.5 (4) (B), 62.3 (9) (C) and 65.8 (8) (D), an average of 65.6 (1) . The three six-membered carbon rings fused between the benzene rings and the central quinone ring in each of the four molecules adopt a boat conformation (Table 1). No classical hydrogen bonds are observed.
The central quinone moiety and H-shaped nature of the title compound make it very similar to its hydroquinone analogue (Nozari et al., 2016) which crystallized in a monoclinic (P2 1 /n) space group with a solvent DMF molecule that generated O-HÁ Á ÁO hydrogen bonds and weak C-HÁ Á ÁO intermolecular interactions in the crystal lattice. The average lengths of the C O bonds in the title molecule are shorter than the C-OH bond in the hydroquinone, 1.219 (2) vs 1.3665 (16) Å , respectively. The average lengths of the C1-C2 and C2-C3 bonds in the central symmetry-generated quinone rings of the four molecules are respectively 1.478 (1) and 1.344 (8) Å , while in the hydroquinone analogue they are 1.395 (2) and 1.394 (2) Å . The average angle of the C1-C2-C3 group of the central core moiety of the four title quinone molecules is 122.58 (16) , whereas for the hydroquinone analogue it is 117.31 (12) . The oxidative conversion of the hydroquinone to the quinone inevitably breaks the central ring's aromaticity and localizes the remaining bondingelectrons into the C O and flanking (C2A-C3A) bonds. This phenomenon is typified by the comparison of a known hydroquinone (also with hydrogen-bonded OH groups; Barnes et al. 1990) with a closely related quinone (Gautrot et al., 2006). In the former case, the C-O single bonds are about 1.38 Å , while the ring C-C bonds are of like length. For the quinone, the C O bonds are typically 1.22 Å , the four C-C bonds adjacent to C1A range from 1.48 to 1.50 Å , and the two C-C bonds flanking those in turn are 1.40 to 1.41 Å . In the hydroquinone, the hydrogen bonds must nonetheless somewhat influence these bond lengths. In the quinone molecule, only weakring interactions provide little if any influence toward the bonding motifs within the molecule (Fig. 1).

Figure 1
The structure of molecule A, C 34 H 20 O 2 , one of four independent molecules (A, B, C, and D) in the unit cell, showing the atom-labeling scheme with 30% probability ellipsoids. H atoms are rendered as spheres of arbitrary radius. An inversion center (1 À x, 1 À y, 1 À z) at the centroid of the central quinone ring generates the complete molecule from a C 17 H 10 O substructure. Table 1 Packing parameters (Å , ) for six-molecule carbon rings in molecules A, B, C, and D.

Supramolecular features
In the crystal, there are four independent quinone molecules oriented in different directions in the lattice. Despite the variation in orientation of the quinones with respect to one another, there are prominent arrays of the molecules along the a-axis direction of the lattice (Figs. 2 and 3). The dihedral angles between the mean planes of the quinone rings, which emphasize the different orientations of the molecules, range from 46 to 90 . While the hydrogen bonding found for the hydroquinone is presumably a major lattice-structuring influence, we propose that the absence of such interactions for the quinone leads to a lattice geometry dominated by close packing of these exaggeratedly shaped quinone molecules, and indeed the quinone crystal is more dense (1.338 g cm À3 ) than hydroquinone (1.264 g cm À3 ). The crystal packing is influenced by weakintermolecular interactions involving the benzene rings from a flap of the V-shaped terminus of each of the molecules B [C5BÁ Á ÁC10B(1 À x, 1 À y, 1 À z) = 3.8375 (12) Å , ] and molecules C [C5CÁ Á ÁC10C(Àx, 2 À y, 2 À z) = 3.9342 (12) Å ]. Additional weak C-HÁ Á Á intermolecular interactions also contribute to the packing stability (Table 2).

Electrochemistry
The quinone-hydroquinone system is a prototype organic redox system; Q + e À Ð Q ÁÀ , Q ÁÀ + e À Ð Q 2À . These systems have been studied electrochemically since the 1920s (Fieser, 1928). Cyclic voltammetry (CV) and rotating disc electrode (RDE) polarography were performed at 298 K on 1 mM quinone in DMF with 0.1 M tetrabutylammonium hexafluoridophosphate (TBAPF 6 ) as the supporting electrolyte, at scan rates ranging from 50 to 10000 mV s À1 for CV, and 1200 to 3200 r.p.m. for the RDE. Experiments were run on a BASi-Epsilon instrument using a three-electrode cell incorporating a non-aqueous reference electrode (APE) (Pavlishchuk & Addison, 2000) and a 3 mm diameter Pt disc working electrode (Figs. 4 and 5). The first reduction to Q ÁÀ (E 1/2 a ) was found by CV to b À0.741 (2) V, while formation of Q 2À (E 1/2 b ) was seen in the rotating disc polarogram at about À1.53 V; the RDE results also demonstrate unequivocally the reductive nature of these processes. The first reduction is reversible, with ÁE p close to 59 mV, but the second reduction is complicated [similar outcomes have previously been observed for quinones in DMF solutions (Jeong et al., 2000)]. The E 1/2 values are within the range reported for quinone systems in the literature with E 1/2 a ranging from À0.72 to À1.37 V and E 1/2 b from À1.18 to À1.90 V vs AgCl/Ag (Bauscher & Mä ntele 1992). View of the crystal packing along the a-axis direction. The molecules are color-coded as green (A), yellow (B), blue (C), and red (D). All four types of molecules are arrayed along the a-axis direction, though none of the quinone planes is oriented simply parallel or perpendicular to the a axis. The A and D molecules also form arrays along the b-axis direction more discernibly than other directions in the lattice.

Figure 3
Crystal packing of the four independent molecules (A, B, C, and D) viewed along along the c axis. Table 2 Weak C-HÁ Á Á intermolecular interactions (Å , ).
compound is estimated to be 5.4 Â 10 À06 cm 2 s À1 in DMF, corresponding to a D value of 4.7 Â 10 À08 g cm s À2 , consistent with the n = 1 assignment.

Database survey
X-ray structures for some hydroquinone derivatives of the corresponding quinone compound have been reported. We recently described the undecorated hydroquinone (Nozari et al., 2016). Bis(trimethylsilylethynyl)pentiptycene was reported by Yang & Swager (1998b), while a long-chain ether and an arylsulfonyl diamide derivative were reported by Yang et al. (2000a,b). The hydroquinone triflate ester was reported by Zyryanov et al. (2008), and a 4 0 -carboxybenzyl ether derivative by Crane et al. (2013).

Synthesis and crystallization
The title pentiptycene quinone was prepared using a double Diels-Alder reaction between anthracene and p-benzoquinone (Fig. 6). The procedure reported by Cao et al. (2009) was followed. For this synthesis, 7.12 g (40 mmol) of anthracene and 2.16 g (20 mmol) of p-benzoquinone were added to glacial acetic acid (250 mL), followed by addition of 9.84 g (40 mmol) of chloranil. The mixture was refluxed for 18 h, following which the solution was allowed to cool to room temperature. The resulting dark-yellow solid was filtered off, washed with diethyl ether, and vacuum desiccated, yielding the crude product (

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. All of the H atoms were refined using a riding-model approximation with C-H = 0.95 Å or 1.0 Å . Isotropic displacement parameters for these atoms were set to 1.2U eq of the parent atom.

Figure 4
Cyclic voltammogram for reduction of 1 mM quinone versus the APE in DMF containing 0.1 M TBAPF 6 as the supporting electrolyte, at a scan rate of 100 mV s À1 . The APE potential is 340 mV more positive than that of the AgCl/Ag electrode (Pavlishchuk & Addison, 2000).

Figure 5
Rotating disc electrode polarogram for reduction of 1 mM quinone versus the APE in DMF containing 0.1 M TBAPF 6 as the supporting electrolyte at a rotation rate of 2400 r.p.m. The APE potential is 340 mV more positive than the AgCl/Ag electrode (Pavlishchuk & Addison, 2000).   (4). 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.