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

Abstract

The 4′-alkoxy-tetrafluorophenyl dithiadiazolyls, ROC6F4CNSSN [R = Me (1), Et (2), n Pr (3), n Bu­(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 °C 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 dithiadiazolyl ring. Detailed differential scanning calorimetry and variable temperature powder X-ray diffraction studies coupled with SQUID magnetometry have been used to show that 2α converts irreversibly to 2β upon heating and that 2β and 2γ interconvert through a reversible phase transition with a small thermal hysteresis in its magnetic response.

Molecular Electrostatic Potential as a Predictor of Supramolecular Synthons in Non‐Hydrogen‐Bonded Systems: Application to Heavier p‐Block Systems

“…This antiparallel stacking is also seen in compounds 2 and 7 , and as mentioned above probably arises due to the wedge‐like shape of the dimer pair. This has also recently been observed in π‐stacked derivatives of ( p ‐ROC 6 F 4 CNSSN) 2 . The antiparallel arrangement of dimers results in intra‐stack S ··· F 3 C contacts ranging from 3.09–3.23 Å.…”

“…The sheets formed by interlocking pinwheels stack along the c ‐axis with an inversion of molecular orientation every four molecules along the stacking direction (Figure c) suppressing formation of a conventional DTDA π‐stack with eclipsed radicals. This inversion of orientation probably occurs due to the non‐coplanarity of the DTDA rings; the packing of such “wedge‐shaped” dimers has recently been discussed by Beldjoudi et al The flipping of molecular orientation results in four S‐centred pinwheels (i.e. two sets of dimers) stacked on top of four CF 3 ‐centered pinwheels in a repeating pattern along the c ‐axis (Figure c).…”

Structural, Magnetic, and Optical Studies of the Polymorphic 9′-Anthracenyl Dithiadiazolyl Radical

“…Preuss recently reported a hysteretic phase transition in a benzimidazolyl-functionalized DTDA radical associated with breakdown of the π*−π* dimer and stepped temperature dependence of the susceptibility similarly associated with breakdown of DTDA pancake dimers in La­(hfac) 3 (pyDTDA) 2 complexes . Awaga and co-workers examined stepwise breakdown of pancake dimers in biphenyl-functionalized bis­(DTDA) radicals and other studies have revealed structural transitions in p -EtOC 6 F 4 CNSSN leading to step changes in magnetic response . In all these previous reports, the magnetic response is linked to the abrupt or progressive breakdown in the long-distance bonding interactions between DTDA radicals.…”

“…Here the close similarity in structures (edge-to-face π–π interactions coupled with SN-IV contacts) supports the reversible nature of these displacive polymorphic transformations. Indeed it has been noted previously in dithiazolyl (DTA) radical chemistry that the reversible nature of such transformations requires only small atomic displacements. , Such reversible phase transitions between different polymorphs in DTDA chemistry are extremely rare. ,,, The observation of two single-crystal-to-single-crystal transitions between paramagnetic phases associated with 3β ↔ 3γ ↔ 3δ is almost unprecedented in DTDA chemistry. The successive breakdown in pancake bonding interactions in Preuss’ Ln­(hfac) 3 (boaDTDA) 2 complex, perhaps provides the closest comparison (also two transitions) but a much more pronounced change in magnetic response, since the latter is based around cleavage of the enthalpically favorable π*−π* pancake bond, switching from an S = 0 state to two S = 1/2 spins …”

Two-Electron Multicenter Bonding (‘Pancake Bonding’) in Dimers of 5,6-Dichloro-2,3-dicyanosemiquinone (DDQ) Radical Anions

“…During the past decade, pancake bonding has been used in crystal engineering − and materials design; ,,,− however, its application requires a deeper understanding of its nature. The nature of 2e/mc bonding has been studied mainly by theoretical methods. − A thorough experimental study of pancake bonding has barely commenced, with a handful X-ray charge density works on dithiadiazolyl , and semiquinone radicals , published recently.…”