An Investigation of Halogen Bonding as a Structure-Directing Interaction in Dithiadiazolyl Radicals.

The preparation and characterization of the halo-functionalized dithiadiazolyl radicals p-XC6F4CNSSN (X = Br (1) or I (2)) are described. Compound 1 is trimorphic. The previously reported phase 1 alpha (Z ‘ = 1) comprises monomeric radicals, whereas 1 beta comprises a mixture of one cis-oid pi*-pi* dimer and one monomer (Z’ = 3), and 1 gamma exhibits a single cis-oid dimer (Z ‘ = 2) in the asymmetric unit. We have only been able to isolate a single polymorph of 2, isomorphous with 1 alpha. Both the bromo and iodo groups in 1 and 2 promote sigma-hole type interactions of the type C-X center dot center dot center dot N (X = Br, I), reflecting the increasing strength of this interaction for the heavier halo-derivatives. An analysis of the intermolecular forces is made using dispersion corrected density functional theory (DFT) (UM06-2X-D3/LACV3P*) and compared to a unified pair potential model (UNI) embodied in the crystallographic software Mercury. While there is a correlation between DFT and UNI force-field models, there are some discrepancies, although both reveal that a number of intermolecular contacts beyond the sum of the van der Waals radii are significant (>5 kJ mol(-1)). A natural bond order analysis of the intermolecular interactions reveals lone pair donation from the heterocyclic N atom to C-X or S-S sigma* orbitals contributes to these intermolecular interactions with relative energies in the order C-I > SN-II > C-Br > SN-III. The magnetism of 2 reveals a broad maximum in chi around 20 K indicative of short-range antiferromagnetic interactions. These are supported by DFT calculations that reveal a set of three significant exchange interactions which propagate in two dimensions.