Leitch Alicea A, Yu Xueyang, Winter Stephen M, Secco Richard A, Dube Paul A, Oakley Richard T
Department of Chemistry, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada.
J Am Chem Soc. 2009 May 27;131(20):7112-25. doi: 10.1021/ja900853t.
The synthesis and solid-state characterization of the resonance-stabilized heterocyclic thia/selenazyl radicals 1a-4a is described. While all the radicals crystallize in undimerized slipped pi-stacked arrays, the four crystal structures do not constitute an isomorphous set; crystals of 1a and 3a belong to the orthorhombic space group P2(1)2(1)2(1), while those of 2a and 4a belong to the monoclinic space group P2(1)/n. The origin of the structural dichotomy can be traced back to the packing of the radicals in the P2(1)/n structure, which maximizes intermolecular Se-Se' contacts. There are marked differences in the transport properties of the two groups. Variable temperature conductivity measurements reveal high, but activated, conductivity for the monoclinic pair (2a/4a), with sigma(298 K) > 10(-3) S cm(-1). The application of physical pressure increases the conductivity of both compounds, with sigma(298 K) at 5 GPa reaching 0.5 S cm(-1) for 2a and 2 S cm(-1) for 4a. Variable-temperature magnetic susceptibility measurements indicate strong antiferromagnetic (AFM) coupling for the monoclinic pair 2a and 4a, the behavior of which has been modeled in terms of a molecular-field modified 1D Heisenberg chain of AFM coupled S = 1/2 centers. Extended Huckel theory band structure calculations and density functional theory first principles methods have been used to develop a qualitative understanding of the conductive and magnetic properties of radicals of the type 1-4 as a function of the degree and direction of slippage of the radical pi-stacks.
本文描述了共振稳定的杂环噻唑/硒唑基自由基1a - 4a的合成及固态表征。虽然所有自由基都以未二聚化的滑移π堆积阵列形式结晶，但这四种晶体结构并不构成同构集；1a和3a的晶体属于正交空间群P2(1)2(1)2(1)，而2a和4a的晶体属于单斜空间群P2(1)/n。结构二分法的起源可追溯到P2(1)/n结构中自由基的堆积方式，这种堆积方式使分子间Se - Se'接触最大化。两组的输运性质存在显著差异。变温电导率测量表明，单斜对（2a/4a）具有高但呈活化状态的电导率，σ(298 K) > 10(-3) S cm(-1)。施加物理压力会增加两种化合物的电导率，在5 GPa下，2a的σ(298 K)达到0.5 S cm(-1)，4a的σ(298 K)达到2 S cm(-1)。变温磁化率测量表明，单斜对2a和4a存在强反铁磁（AFM）耦合，其行为已根据分子场修正的AFM耦合S = 1/2中心的一维海森堡链进行了建模。已使用扩展休克尔理论能带结构计算和密度泛函理论第一性原理方法，来定性理解1 - 4型自由基的导电和磁性性质与自由基π堆积滑移程度和方向的函数关系。
