Hysteretic Spin Crossover between a Bisdithiazolyl Radical and Its Hypervalent σ-Dimer

Abstract

The bisdithiazolyl radical 1a is dimorphic, existing in two distinct molecular and crystal modifications. The α-phase crystallizes in the tetragonal space group P4̅2(1)m and consists of π-stacked radicals, tightly clustered about 4̅ points and running parallel to c. The β-phase belongs to the monoclinic space group P2(1)/c and, at ambient temperature and pressure, is composed of π-stacked dimers in which the radicals are linked laterally by hypervalent four-center six-electron S···S-S···S σ-bonds. Variable-temperature magnetic susceptibility χ measurements confirm that α-1a behaves as a Curie-Weiss paramagnet; the low-temperature variations in χ can be modeled in terms of a 1D Heisenberg chain of weakly coupled AFM S = (1)/(2) centers. The dimeric phase β-1a is essentially diamagnetic up to 380 K. Above this temperature there is a sharp hysteretic (T↑= 380 K, T↓ = 375 K) increase in χ and χT. Powder X-ray diffraction analysis of β-1a at 393 K has established that the phase transition corresponds to a dimer-to-radical conversion in which the hypervalent S···S-S···S σ-bond is cleaved. Variable-temperature and -pressure conductivity measurements indicate that α-1a behaves as a Mott insulator, but the ambient-temperature conductivity σ(RT) increases from near 10(-7) S cm(-1) at 0.5 GPa to near 10(-4) S cm(-1) at 5 GPa. The value of σ(RT) for β-1a (near 10(-4) S cm(-1) at 0.5 GPa) initially decreases with pressure as the phase change takes place, but beyond 1.5 GPa this trend reverses, and σ(RT) increases in a manner which parallels the behavior of α-1a. These changes in conductivity of β-1a are interpreted in terms of a pressure-induced dimer-to-radical phase change. High-pressure, ambient-temperature powder diffraction analysis of β-1a confirms such a transition between 0.65 and 0.98 GPa and establishes that the structural change involves rupture of the dimer in a manner akin to that observed at high temperature and ambient pressure. The response of the S···S-S···S σ-bond in β-1a to heat and pressure is compared to that of related dimers possessing S···Se-Se···S σ-bonds.

The key role of vibrational entropy in the phase transitions of dithiazolyl-based bistable magnetic materials

“…Most of the bistable compounds so far reported are based on transition metal ions [7][8][9] , but recent work 10 strongly suggests that organic radicals can also be at the forefront in the design and fabrication of the next generation of sensors and devices. Indeed, thiazyl- 11,12 , spiro-biphenalenyl- 13 and nitroxide-based radicals 14,15 have furnished striking examples of metal-free bistable materials whose phase transitions are accompanied by changes in the magnetic, optical and/or electrical response.…”

“…Some of these magnetic phase transitions are associated with a hysteretic behaviour 11,12,[16][17][18][19] (thus giving rise to a region of bistability), while other transitions occur without hysteresis 20,21 . In addition to these intriguing solid state phase transitions, many other members of the key family of thiazyl radicals show remarkable conductivity and magnetic properties 16,[22][23][24][25] .…”

Spin Transition and Exchange Interaction: Janus Visions of Supramolecular Spin Coupling between Face‐to‐Face Verdazyl Radicals

“…[10] In these compounds, there is a wide variety of interactions between the radicals, ranging from s-type covalent bonds to weaker intermolecular interactions. [11][12][13] Accordingly, the interactions between the spins of the unpaired electrons give rise to various magnetic behaviors. One way to favor strong p-p interactions between the verdazyl moieties is to introduce an intramolecular constraint to force stacking.…”

Current Applications of Computational Chemistry in JACS—Molecules, Mechanisms, and Materials

“…18 In another study of how solid-state structure affects the magnetic properties of radicals, Oakley and co-workers describe the results of DFT calculations and experiments on the two crystalline forms of a bisthiazoyl radical. 19 In yet another investigation of an organometallic reaction mechanism, Poli and Dub investigate the mechanism for the Pt-catalyzed hydroamination of ethylene by aniline. 20 Solomon and co-workers report the results of a computational investigation of electron transport through π-stacked molecules.…”