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

摘要

The bisdithiazolyl radical 1a is dimorphic, existing in two distinct molecular and crystal modifications. The a-phase crystallizes in the tetragonal space group P42_1m and consists of π-stacked radicals, tightly clustered about 4 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 x measurements confirm that α-1a behaves as a Curie-Weiss paramagnet; the low-temperature variations in x can be modeled in terms of a 1D Heisenberg chain of weakly coupled AFM S = 1/2 centers. The dimeric phase β-a is essentially diamagnetic up to 380 K. Above this temperature there is a sharp hysteretic (T↑= 380 K, T↓ = 375 K) increase in x and xT. 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 σRT 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 σRT 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 σRT 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 a-bond in α-1a to heat and pressure is compared to that of related dimers possessing S…Se-Se…S σ-bonds.