Planar packing of tetrachlorodicyanobenzene isomers

Ortho-, meta- and para-tetrachlorodicyanobenzene (TBD, tetrachlorobenzenedicarbonitrile) form 1/1 complexes with hexamethylbenzene (HMB). Seven polymorphic forms of the crystals, one of o-TDB, two of m-TDB and four of p-TBD, have been found. All of the complexes involve layers of two-dimensional arrays, and in six of the complexes the layers are homogeneous, alternate layers being entirely TDB or entirely HMB. The layers can be described in terms of an hexagonal tiling and in five of the homogeneous layers the TDB are ordered so that one CN and two Cl come together at each corner of the tiles. In the sixth the m-TDB is disordered, but a plausible arrangement can still be found with one CN and two Cl coming together in the same fashion.