Utilization of σ‑Holes on Sulfur and Halogen Atoms for Supramolecular Cation···Anion Interactions in Bilayer Ni(dmit)₂ Anion Radical Salts

We prepared novel Ni­(dmit)2 anion radical salts with ethyl-4-halothiazolium cations (Et-4XT, with X denoting the halogen: I, Br, or Cl), (Et-4IT)­[Ni­(dmit)2]2 (1), (Et-4BrT)­[Ni­(dmit)2]2 (2), and (Et-4ClT)2[Ni­(dmit)2]5 (3). Single-crystal X-ray diffraction analysis of 1–3 indicates that, unlike the halogen atoms that have only one σ-hole each, the cations’ sulfur atoms each have two σ-holes that lie approximately along the extensions of the C–S bonds. The presence of the σ-holes is supported by electrostatic potential of the cations calculated based on the density functional theory method. In the crystals of 1–3, these σ-holes interact with lone pairs on the terminal thioketone moieties in the Ni­(dmit)2 anion radicals to form electrostatic σ-hole bonds (halogen bonds and chalcogen bonds). This results in supramolecular cation···anion networks. Crystal and electronic structure analyses, and electrical and magnetic measurements reveal that the salts 1 and 2 are isostructural bilayer Mott systems, in which two crystallographically different Mott-insulating anion layers coexist in one crystal. The unusual magnetic properties, including the ferromagnetic anomalies of 1 and 2, are consistent with one of the anion layers forming an antiferromagnetic short-range ordering (SRO) state and the other layer forming a ferromagnetic SRO state. The spin-polarization of the Ni­(dmit)2 anion radical was shown to influence significantly the observed ferromagnetic interactions, while the antiferromagnetic interactions resulted from π–π overlapping in the anions. The competition between these two interactions dominates the low-temperature magnetic properties of the present bilayer Mott systems. This study reveals that noncovalent intermolecular interactions mediated by σ-holes are influential in preparing novel crystal and electronic structures and that they have the potential to allow the development of materials with unusual physical properties.本研究制备了一系列以乙基-4-卤代噻唑鎓阳离子（Et-4XT，其中X代表卤素原子：I、Br或Cl）为抗衡阳离子的新型Ni(dmit)2阴离子自由基盐，具体包括(Et-4IT)[Ni(dmit)2]2（样品1）、(Et-4BrT)[Ni(dmit)2]2（样品2）以及(Et-4ClT)2[Ni(dmit)2]5（样品3）。对样品1至3的单晶X射线衍射分析表明，与每个卤素原子仅拥有一个σ空穴（σ-hole）不同，该类阳离子的硫原子各具有两个近似沿C-S键延伸方向分布的σ空穴。通过密度泛函理论（density functional theory, DFT）方法计算得到的阳离子静电势，证实了此类σ空穴的真实存在。在样品1至3的晶体结构中，这些σ空穴可与Ni(dmit)2阴离子自由基末端硫酮基团上的孤对电子发生相互作用，形成静电σ空穴键（包括卤键与硫族键），由此构建出超分子阳离子-阴离子网络结构。晶体结构、电子结构分析结合电学与磁学测试结果表明，样品1与2为同构的双层莫特系统，单颗晶体中同时存在两种晶体学特征不同的莫特绝缘阴离子层。样品1与2展现出的反常磁学性质（包括铁磁异常行为），与其中一层阴离子层形成反铁磁短程有序（SRO）状态、另一层形成铁磁短程有序状态的理论预测相符。研究表明，Ni(dmit)2阴离子自由基的自旋极化效应对观测到的铁磁相互作用具有显著调控作用，而反铁磁相互作用则源于阴离子间的π-π堆积重叠。这两种相互作用之间的竞争主导了该类双层莫特系统的低温磁学行为。本研究证实，由σ空穴介导的非共价分子间相互作用，对新型晶体与电子结构的构筑具有重要调控作用，同时具备助力开发具有反常物理性质材料的应用潜力。