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

The 4′-alkoxy-tetrafluorophenyl dithiadiazolyls, ROC6F4CNSSN [R = Me (1), Et (2), Pr-n (3), Bu-n(4)] all adopt cis-oid dimers in the solid state. The methoxy derivative 1 adopts a pi-stacked AA’AA’ motif, whereas propoxy (3) and butoxy (4) derivatives exhibit an AA’BB’ stacking. The ethoxy derivative (2) is polymorphic. The alpha-phase (2 alpha) adopts an AA’BB’ motif comparable with 3 and 4, whereas 2 beta and 2 gamma are reminiscent of 1 but combine a mixture of both monomers and dimers in the solid state. The structure of 2 beta exhibits Z’ = 6 with two dimers and two monomers in the asymmetric unit but undergoes a thermally induced phase transition upon cooling below 25 degrees C to form 27 (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 2 alpha converts irreversibly to 2 beta upon heating and that 2 beta and 2 gamma interconvert through a reversible phase transition with a small thermal hysteresis in its magnetic response.