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

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 o f 5,6-dichlorobenzfurazan 1-oxide (CBF; systematic name 5,6-dichloro-2,1,3- benzoxadiazole 1-oxide) is isomorphous with CPA with disordered CBF molecul es being pseudo-isosteric with the CPA molecule. Polymorph C of CBF has sim ilar unit-cell dimensions but the arrangement of the molecules in the cell is different. 4,5-Dibromophthalic anhydride occurs in space group P (1) ove r bar in a structure derived from the CPA structure. The four structures al l 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 thr ee the molecules are disordered about twofold or pseudo-twofold axes. In al l three the molecules form chains involving similar Cl ... O 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-H ... O, C-H ... N and Cl ... Cl contacts, but with different stacking arrangements bet ween the sheets. In polymorph B the chains are associated in face-to-face p airs with the same face-to-face overlap as in A, at an average distance of 3.42 Angstrom compared to an average distance of 3.38 Angstrom in A (cf: Ta ble 8); there are no two-dimensional sheets.