Structural consequences of benzidine dihydrochloride substitution in the solid state

The effect of symmetrical substitution of benzidine dihydrochloride at the ortho and/ or meta positions of the benzidine rings on the structure of the crystals formed is studied. We examined eleven structures, nine of which are new ones and two retrieved from the Cambridge Structural Database (CSD). The structures obtained were compared with those of pure benzidine dihydrochlorides. The organic cations aggregate in stacks forming molecular layers. When the hydrogen atoms in the parent compound are replaced, a looser crystal packing in the layers is realized due to repulsion of the substituted moieties. This effect is governed by organic cations which form either 1- D or mainly 2-D types of aggregates - some with extra water solvent. We employ linear and/ or area density parameters of cations in stacks or sheets in order to rationalise changes in packing of benzidine dihydrochloride derivatives due to substitution effects. The area density of charge decreases for all the structures because the substituents decrease the packing of the cations in the layers. On the other hand, for the linear density of charge, substitution is far less significant. In this case the relative orientation of cations in the stacks determines the distance between them. Both density parameters correlate with the inter-stack distance s and the angle tau describing the tilt of the N-N axis of a molecule with respect to the layer of cations. Two distinct outliers are observed-one denoting a monoprotonated cation and the other one for ions with a bulky methoxy substituent.