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

The bisdithiazolyl radical 1a is dimorphic, existing in two distinct molecular and crystal modifications. The α-phase crystallizes in the tetragonal space group P4̅2₁m 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 P2₁/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.