A new polymorph of physcion

Abstract

The structure of the title compound, 7-methoxy-2-methyl-4,5-dihydroxyanthracene-9,10-dione, C(16)H(12)O(5), was originally reported by Ulický et al. [Acta Cryst. (1991). C47, 1879-1881] in the space group P2(1)2(1)2(1) [polymorph (Io)]. The new polymorph, (Im), crystallizes in the space group P2(1)/c. The molecular structures are closely similar, with both -OH groups forming intramolecular hydrogen bonds to one of the neighbouring quinone O atoms, thus slightly lengthening this C=O bond; the pattern of C-C bond lengths in the ring system is consistent with some contribution from a resonance form with a negative charge at the hydrogen-bonded quinone O atom and an aromatic region around its neighbouring C atoms. The packing of (Im) is simpler than the extensively crosslinked pattern of (Io), with molecular tapes connected by classical (but three-centre) and `weak' hydrogen bonds, parallel to [201].