6,11-Dihydroxynaphthacene-5,12-dione

The molecule of the title compound, C18H10O4, is centrosymmetric and planar. A long phenolic O—H bond is observed [1.19 (9) Å], which is involved in an intramolecular hydrogen bond between the phenolic and quinonoid O atoms. The molecules pack in a herringbone pattern and are linked to each other via intermolecular C—H⋯O hydrogen bonds (2.73–2.77 Å).

The molecule of the title compound, C 18 H 10 O 4 , is centrosymmetric and planar. A long phenolic O-H bond is observed [1.19 (9) Å ], which is involved in an intramolecular hydrogen bond between the phenolic and quinonoid O atoms. The molecules pack in a herringbone pattern and are linked to each other via intermolecular C-HÁ Á ÁO hydrogen bonds (2.73-2.77 Å ).
The molecule of (I) is centrosymmetric and planar, with an r.m.s deviation of 0.0098 Å from the least-squares plane for the fitted non-H atoms. The molecular structure is similar to that of naphthazarin. The symmetry of the carbon skeleton of (I) is close to D 2 h ( Table 1). The long phenolic O2-H2 bond [1.19 (9) Å] constitutes an intramolecular hydrogen bond between the phenolic O2 and quinonoid O1 atoms (Gilli et al., 1989;Bertolasi et al., 1991;Gilli et al., 1993;Steiner & Saenger, 1994). These facts imply that the structure of (I) can be interpreted not as 1,4-nor 1,5-quinone but as resonance between two zwitterion contributors, as shown in Scheme 1 (Cradwick & Hall, 1971;Shiau et al., 1980).
In the crystal structure, the molecules form a herringbone-type stacking along the b axis, where the distance between the molecular planes is 3.42 Å (Fig. 2). The packing mode is similar to that characteristic of aromatic hydrocarbon atoms.

Experimental
The title compound (I) was commercially available. Red crystals of (I) suitable for X-ray analysis were grown from a chloroform solution.

Refinement
The H atom bonded to the phenolic O atom was located in a difference map and refined isotropically. Other H atoms were positioned geometrically refined using a riding model with C-H = 0.95 Å and with U iso (H) = 1.2U eq (C). Fig. 1. The molecular structure of (I), with atom labels and 50% probability displacement ellipsoids for non-H atoms and H atoms are shown as small spheres of arbitrary radii [symmetry code: (i) -x, -y, -z + 1]. Fig. 2. The packing diagram of (I), viewed along the a axis. Fig. 3. The resonance structure of (I).