Polymorphism and pseudosymmetry of 10,10'-oxybis(9-thia-10-hydro-10-boraanthracene).

We have encountered two polymorphs of the title compound, C24H16B2OS2, both of which display almost the same unit-cell parameters. Compound (I) crystallizes in the non-centrosymmetric space group P21 with four mol-ecules in the asymmetric unit. These mol-ecules are related by pseudosymmetry. As a result, the space group looks like P21/c, but the structure cannot be refined successfully in that space group. Compound (II) on the other hand crystallizes in the centrosymmetric space group P21/c with only two mol-ecules in the asymmetric unit. The crystals studied for (I) and (II) were both non-merohedral twins.