(3Z,3′Z)-3,3′-(Ethane-1,2-diylidene)bis[isobenzofuran-1(3H)-one]

The title compound, C18H10O4, has been isolated as an impurity in commercially available 6,11-dihydroxy-5,12-naphthacenedione. The title compound exhibits yellow fluorescence in the solid state. The molecule has crystallographic inversion symmetry and is planar, with an r.m.s. deviation of 0.031 (1) Å. The crystal structure is stabilized by C—H⋯O hydrogen bonds and π–π stacking interactions between 3-methyleneisobenzofuran-1(3H)-one units [interplanar distance 3.43 (1) Å].


Comment
Through the study of the crystallographic analysis and functionalities of 6,11-dihydroxy-5,12-naphthacenedione (Tomura et al., 2008;Ono et al., 2009), we found the existence of a yellow fluorescent compound as a by-product in the commercially available chemical reagent. After the separation of the by-product, the structure was determined to be the title compound, (I). Although the synthesis of (I) was already reported (Ji et al., 2006), the characterization has not been performed. In this paper, we report the separation, spectral data, and molecular and crystal structures of (I).
In the crystal structure, the molecules are linked to each other through intermolecular C-H···O interactions ( Table 1).
The molecular arrangements shown in Figs. 2 and 3 run stepwise along the b and c axes, respectively. Fig. 4 also exhibits a stepwise arrangement along the (021) direction with C-H···O contacts. Furthermore, the 3-methyleneisobenzofuran-1(3H)-one rings overlap each other to form a π-stacking structure with an interplanar distance of 3.43 Å, as shown in Fig. 5.

Experimental
The commercially available 6,11-dihydroxy-5,12-naphthacenedione (100 mg, Aldrich) was recrystallized from ethyl acetate (280 ml) to afford pure red crystals of 6,11-dihydroxy-5,12-naphthacenedione (70 mg). The filtrate containing the title compound, (I) was concentrated, and the residue (25 mg) was dissolved in toluene (100 ml). The solution was washed with 1 M NaOH solutions (100 ml) ten times over until the NaOH solution was colorless. The organic solution was dried over Na 2 SO 4 and concentrated to provide a yellow solid of (I) (ca 9% yield). Yellow prisms of the title compound, suitable for X-ray analysis were grown from a dichloromethane solution.

Refinement
All H atoms were placed in geometrically calculated positions, with C-H = 0.95 (aromatic) Å and U iso (H) = 1.2U eq (C) (aromatic), and refined using a riding model.  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.      Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating Rfactors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.