Effects of Halo-Substitution on 2′-Chloro-5′-halo-phenyl-1,2,3,5-dithiadiazolyl Radicals: A Crystallographic, Magnetic, and Electron Paramagnetic Resonance Case Study

Abstract

ABSTRACT:The syntheses and characterization of the aryl-substituted dithiadiazolyls, 2 -Cl-5 -X-C 6 H 3 CNSSN • [1 (X = F), 2 (X = Cl), 3 (X = Br), 4 (X = I)] are described. In all four cases the radicals adopt distorted stacks of π*-π* dimers with inter-stack S···X contacts. In 1 (monoclinic P2/c) S···Cl contacts are manifested through a non-crystallographic 3-fold axis forming supramolecular trimers whereas the inter-stack S…X contacts in 2 (triclinic P-1), 3 and 4 (which form an isostructural pair, orthorhombic Pna2 1 ) form supramolecular chains. While all the structures adopt π*-π* cis-oid dimer motifs, tuning the halogen modifies the intra-dimer S···S distance. Variable temperature SQUID magnetometry and X-band CW-EPR studies revealed the presence of a thermally accessible triplet state in all cases, with the singlet-triplet separation appearing in the order 1 > 2 > 3 > 4, consistent with a reduction in the overlap integral with increasing intra-dimer S···S separation. Variable temperature structural studies on both 2 and 4 reveal a structural evolution from a distorted π-stack motif towards a regular π-stacked array on warming. This is particularly pronounced in 2 where the intermolecular S…S separations along the stacking direction converge on a regular 3.6 Å spacing at ambient temperature.3

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

“…56,63 Such reversible phase transitions between different polymorphs in DTDA chemistry are extremely rare. 57,59,60,64 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 = ½ spins.…”

“…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 …”

Spin Delocalization, Polarization, and London Dispersion Forces Govern the Formation of Diradical Pimers

“…20,24−26 Examples of radicals that form σ-dimers include trityl radicals, 13,14 pyridyl radicals, 27 and aza-adamantyl nitroxyl radicals. 17 Radicals that form pimers include halo-phenyl dithiazolyl radicals, 19 planar corranulene-derived cation radicals, 16 viologen cation radicals, 18 π-radical anions such as tetracyanoquinodimethane (TCNQ), 11 and naphthalene diimide radical anions, 12 although many others are known. 28−30 To understand what structural and molecular parameters dictate whether a σor π-dimer is formed for that radical, a library of aryl dicyanomethyl radicals was synthesized and evaluated.…”

“…For example, phenalenyl radicals and triangulene radicals can form either σ- or π-dimers depending on the substitution. ,− Examples of radicals that form σ-dimers include trityl radicals, , pyridyl radicals, and aza-adamantyl nitroxyl radicals . Radicals that form pimers include halo-phenyl dithiazolyl radicals, planar corranulene-derived cation radicals, viologen cation radicals, π-radical anions such as tetracyanoquinodimethane (TCNQ), and naphthalene diimide radical anions, although many others are known. − …”

X-ray Absorption Near-Edge Structure Spectroscopy of a Stable 6-Oxoverdazyl Radical and Its Diamagnetic Precursor

“…Much of the activity in this field is driven by the desire to better understand the structure and electronic properties of stable radicals and to develop new methods for their preparation. − This research has already led to many practical applications of stable radicals and their polymers as components of polymer and flow batteries, − imaging probes and contrast agents, − active layers in resistive memory devices, − and catalysts for alcohol oxidation …”