Effects of Halo-substitution on 2'-Chloro-5'-halo-phenyl-1,2,3,5-dithiadiazolyl Radicals: A Crystallographic, Magnetic and EPR Case Study

The syntheses and characterization of the aryl-substituted dithiadiazolyls, 2ʹ-Cl-5ʹ-\ud X-C6H3CNSSN• [1 (X = F), 2 (X = Cl), 3 (X = Br), 4 (X = I)] are described. In all four cases the\ud radicals adopt distorted stacks of π*-π* dimers with inter-stack S···X contacts. In 1 (monoclinic\ud P2/c) S···Cl contacts are manifested through a non-crystallographic 3-fold axis forming\ud supramolecular trimers whereas the inter-stack S…X contacts in 2 (triclinic P-1), 3 and 4 (which\ud form an isostructural pair, orthorhombic Pna21) form supramolecular chains. While all the\ud structures adopt π*-π* cis-oid dimer motifs, tuning the halogen modifies the intra-dimer S···S\ud distance. Variable temperature SQUID magnetometry and X-band CW-EPR studies revealed the\ud presence of a thermally accessible triplet state in all cases, with the singlet-triplet separation\ud appearing in the order 1 > 2 > 3 > 4, consistent with a reduction in the overlap integral with\ud increasing intra-dimer S···S separation. Variable temperature structural studies on both 2 and 4\ud reveal a structural evolution from a distorted π-stack motif towards a regular π-stacked array on\ud warming. This is particularly pronounced in 2 where the intermolecular S…S separations along\ud the stacking direction converge on a regular 3.6 Å spacing at ambient temperature.