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 4ʹ-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 π-stacked AA’AA’ motif whereas propoxy (3) and butoxy (4) derivatives exhibit an AA’BB’ stacking. The ethoxy derivative (2) is polymorphic. The α-phase (2α) adopts an AA’BB’ motif comparable with 3 and 4, whereas 2β and 2γ are reminiscent of 1 but combine a mixture of both monomers and dimers in the solid state. The structure of 2β exhibits Z’ = 6 with two dimers and two monomers in the asymmetric unit but undergoes a thermally-induced phase transition upon cooling below -25 oC to form 2γ (Z’ = 14) with six dimers and two monomers in the asymmetric unit. The transition is associated with both rotation and translation of the dithiadiazol-yl ring. Detailed DSC and variable temperature PXRD studies coupled with SQUID magnetometry have been used to show that 2α converts irreversibly to 2β upon heating and that 2β and 2γ interconvert t...