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.

我们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：I、Br或Cl）为抗衡阳离子的新型Ni(dmit)2阴离子自由基盐：(Et-4IT)[Ni(dmit)2]2 (1)、(Et-4BrT)[Ni(dmit)2]2 (2)以及(Et-4ClT)2[Ni(dmit)2]5 (3)。对化合物1~3的单晶X射线衍射分析表明：与每个仅含一个σ空穴（σ-hole）的卤原子不同，该类阳离子的硫原子各拥有两个大致沿C-S键延伸方向分布的σ空穴。基于密度泛函理论（density functional theory，DFT）计算得到的阳离子静电势能，证实了这些σ空穴的存在。在1~3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（包括卤键与硫属键），由此构建出超分子阳离子-阴离子网络。晶体与电子结构分析、电学及磁学性能测试结果表明，化合物1与2为同构双层莫特（Mott）系统，单颗晶体中同时存在两种结晶学不等价的莫特绝缘阴离子层。二者所展现出的反常铁磁特性等非常规磁学行为，与其中一层阴离子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模型相符。研究表明，Ni(dmit)2阴离子自由基的自旋极化会显著影响观测到的铁磁相互作用，而反铁磁相互作用则源于阴离子间的π-π重叠。这两类相互作用之间的竞争主导了本研究中双层莫特系统的低温磁学特性。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，且有望用于开发具备非常规物理性能的功能材料。