An Investigation of Halogen Bonding as a Structure-Directing Interaction in Dithiadiazolyl Radicals
Mitchell A. Nascimento, Elodie Heyer, Robert J. Less, Christopher M. Pask, Ana Arauzo, Javier Campo, Jeremy M. Rawson
Abstract
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 1α (Z′ = 1) comprises monomeric radicals, whereas 1β comprises a mixture of one cis-oid π*−π* dimer and one monomer (Z′ = 3), and 1γ 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 1α. Both the bromo and iodo groups in 1 and 2 promote sigma-hole type interactions of the type C–X···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 σ* 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 χ 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.