Department of Chemistry, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada.
J Am Chem Soc. 2010 Nov 17;132(45):16212-24. doi: 10.1021/ja106768z. Epub 2010 Oct 21.
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.
双二硫杂茂自由基 1a 具有二态性，存在两种不同的分子和晶体修饰形式。α-相在四方晶系 P4̅2(1)m 中结晶，由紧密堆积的自由基组成，大约 4̅ 点聚集，并平行于 c 方向排列。β-相属于单斜晶系 P2(1)/c，在环境温度和压力下，由π-堆积的二聚体组成，其中自由基通过超价四中心六电子 S···S-S···S σ 键横向连接。变温磁化率 χ 测量证实 α-1a 表现为居里-外斯顺磁体；χ 的低温变化可以用弱耦合 AFM S = (1)/(2) 中心的 1D Heisenberg 链来建模。二聚体相 β-1a 在 380 K 以下基本上是抗磁性的。高于此温度，χ 和 χT 会发生急剧的滞后（T↑= 380 K，T↓ = 375 K）增加。在 393 K 下对 β-1a 的粉末 X 射线衍射分析表明，相转变对应于超价 S···S-S···S σ 键的断裂，从而导致二聚体到自由基的转化。变温变压电导率测量表明，α-1a 表现为莫特绝缘体，但环境温度电导率 σ(RT)从 0.5 GPa 时的近 10(-7) S cm(-1)增加到 5 GPa 时的近 10(-4) S cm(-1)。β-1a 的 σ(RT)值（0.5 GPa 时近 10(-4) S cm(-1)）在相变发生时最初随压力降低而降低，但超过 1.5 GPa 后，这种趋势发生逆转，σ(RT)以与 α-1a 相似的方式增加。β-1a 电导率的这些变化可以用压力诱导的二聚体到自由基的相变来解释。β-1a 的高压、环境温度粉末衍射分析证实了在 0.65 到 0.98 GPa 之间的这种转变，并确定了结构变化涉及二聚体的断裂，类似于在高温和环境压力下观察到的断裂。β-1a 中 S···S-S···S σ 键对热和压力的响应与具有 S···Se-Se···S σ 键的相关二聚体进行了比较。
