Acta Crystallographica Section B

Structural Science

Volume 55, Part 4 (August 1999)

research papers

Acta Cryst. (1999). B55, 530-542 [ doi:10.1107/S0108768199000348 ]

Packing similarities of three isosteric molecules: 4,5-dichlorophthalic anhydride, 4,5-dibromophthalic anhydride and 5,6-dichlorobenzfurazan 1-oxide, including three polymorphs of 5,6-dichlorobenzfurazan 1-oxide

C. R. Ojala, W. H. Ojala, D. Britton and J. Z. Gougoutas

Abstract: 4,5-Dichlorophthalic anhydride (CPA) lies on a twofold axis in space group C2/c; the molecules pack as stacks of two-dimensional sheets. Polymorph A of 5,6-dichlorobenzfurazan 1-oxide (CBF; systematic name 5,6-dichloro-2,1,3-benzoxadiazole 1-oxide) is isomorphous with CPA with disordered CBF molecules being pseudo-isosteric with the CPA molecule. Polymorph C of CBF has similar unit-cell dimensions but the arrangement of the molecules in the cell is different. 4,5-Dibromophthalic anhydride occurs in space group P1 in a structure derived from the CPA structure. The four structures all have similar two-dimensional sheets of molecules, but differ, in addition to the disorder of the CBF molecules, in the stacking of successive sheets. There are three polymorphs of 5,6-dichlorobenzfurazan 1-oxide. In all three the molecules are disordered about twofold or pseudo-twofold axes. In all three the molecules form chains involving similar ClO contacts along the direction of the twofold axis. In polymorphs A and C these chains form virtually identical two-dimensional sheets with inter-chain C-HO, C-HN and ClCl contacts, but with different stacking arrangements between the sheets. In polymorph B the chains are associated in face-to-face pairs with the same face-to-face overlap as in A, at an average distance of 3.42 Å compared to an average distance of 3.38 Å in A (cf. Table 8); there are no two-dimensional sheets.