Crystal structure of 2-bromo-1,4-dihydroxy-9,10-anthraquinone

In an attempt to brominate 1,4-dipropoxy-9,10-anthraquinone, a mixture of products, including the title compound, C14H7BrO4, was obtained. The molecule is essentially planar (r.m.s. deviation = 0.029 Å) and two intramolecular O—H⋯O hydrogen bonds occur. In the crystal, the molecules are linked by weak C—H⋯O hydrogen bonds, Br⋯O contacts [3.240 (5) Å], and π–π stacking interactions [shortest centroid–centroid separation = 3.562 (4) Å], generating a three-dimensional network.

Supporting information for this paper is available from the IUCr electronic archives (Reference: HB7282).
As shown in Figure 2, in the crystal, molecules are linked by C-H···O hydrogen bonds (Table 1) and Br···O contacts [Br1···O3 i = 3.240 (5) Å; symmetry code: (i) x -1/2, -y + 1, z], whose value is shorter than the sum of van der Waals radii of bromine and oxygen atoms. The molecules are π-stacked along the a axis with an interplanar distance of 3.450 Å.

S2. Experimental
A mixture of 1,4-dipropoxy-9,10-anthraquinone (653 mg, 2.01 mmol), iron powder (50 mg, 0.89 mmol), bromine (0.40 g, 2.5 mmol) in acetic acid (20 ml) was stirred at 80 °C under air. The reaction was quenched with an aqueous solution of Na 2 SO 3 . Then the reaction products were precipitated. After filtration, the residue was subjected to column chromatography on silica gel using CH 2 Cl 2 -hexane as the eluent to afford the title compound (18 mg, 2.8% yield) as a red solid. Red platelets were obtained by slow evaporation of a CH 2 Cl 2 solution.

S3. Refinement
All the aromatic H atoms were positioned geometrically and refined using a riding model with C-H = 0.95 Å and with U iso (H) = 1.2U eq (C). The H atoms of the OH groups were located in a difference Fourier map and freely refined [O1-H1 The molecular structure of (I), showing 50% probability displacement ellipsoids. The intramolecular hydrogen bonds are drawn by dashed lines.

Figure 2
The crystal packing of (I), showing short contacts of selected C-H···O and Br···O interactions by blue lines.    (10)