Structural Variations in the Dithiadiazolyl Radicals p-ROC6F4CNSSN (R = Me, Et, nPr, nBu): A Case Study of Reversible and Irreversible Phase Transitions in p-EtOC6F4CNSSN

The 4'-alkoxy-tetrafluorophenyl dithiadiazolyls, ROC6F4CNSSN [R = Me (1), Et (2), nPr (3), nBu(4)] all adopt cis-oid dimers in the solid state. The methoxy derivative 1 adopts a p-stacked AA'AA' motif, whereas propoxy (3) and butoxy (4) derivatives exhibit an AA'BB' stacking. The ethoxy derivative (2) is polymorphic. The a-phase (2a) adopts an AA'BB' motif comparable with 3 and 4, whereas 2ß and 2y are reminiscent of 1 but combine a mixture of both monomers and dimers in the solid state. The structure of 2ß exhibits Z' = 6 with two dimers and two monomers in the asymmetric unit but undergoes a thermally induced phase transition upon cooling below -25 °C to form 2y (Z' = 14) with six dimers and two monomers in the asymmetric unit. The transition is associated with both rotation and translation of the dithiadiazolyl ring. Detailed differential scanning calorimetry and variable temperature powder X-ray diffraction studies coupled with SQUID magnetometry have been used to show that 2a converts irreversibly to 2ß upon heating and that 2ß and 2¿; interconvert through a reversible phase transition with a small thermal hysteresis in its magnetic response