Slow Magnetic Relaxation Induced by a Large Transverse Zero-Field Splitting in a Mn^IIRe^IV(CN)₂ Single-Chain Magnet

The model compounds (NBu4)2[ReCl4(CN)2] (1), (DMF)4ZnReCl4(CN)2 (2), and [(PY5Me2)2Mn2ReCl4(CN)2]­(PF6)2 (3) have been synthesized to probe the origin of the magnetic anisotropy barrier in the one-dimensional coordination solid (DMF)4MnReCl4(CN)2 (4). High-field electron paramagnetic resonance spectroscopy reveals the presence of an easy-plane anisotropy (D > 0) with a significant transverse component, E, in compounds 1–3. These findings indicate that the onset of one-dimensional spin correlations within the chain compound 4 leads to a suppression of quantum tunneling of the magnetization within the easy plane, resulting in magnetic bistability and slow relaxation behavior. Within this picture, it is the transverse E term associated with the ReIV centers that determines the easy axis and the anisotropy energy scale associated with the relaxation barrier. The results demonstrate for the first time that slow magnetic relaxation can be achieved through optimization of the transverse anisotropy associated with magnetic ions that possess easy-plane anisotropy, thus providing a new direction in the design of single-molecule and single-chain magnets.本研究合成了模型化合物(NBu4)2[ReCl4(CN)2]（化合物1）、(DMF)4ZnReCl4(CN)2（化合物2）以及[(PY5Me2)2Mn2ReCl4(CN)2](PF6)2（化合物3），旨在探究一维配位固体(DMF)4MnReCl4(CN)2（化合物4）的磁各向异性垒起源。高场电子顺磁共振谱学（High-field electron paramagnetic resonance spectroscopy）研究表明，化合物1至3均存在易平面各向异性（easy-plane anisotropy，D>0），且伴有显著的横向分量E。上述结果显示，链状化合物4的链内一维自旋关联形成后，会抑制易平面内的磁化量子隧穿（quantum tunneling of the magnetization），进而产生磁双稳态（magnetic bistability）与慢弛豫行为（slow relaxation behavior）。据此可知，与四价铼（ReIV）中心相关的横向E项，决定了易轴方向以及与弛豫垒（relaxation barrier）相关的各向异性能级尺度（anisotropy energy scale）。本研究首次证实，可通过优化具有易平面各向异性的磁性离子的横向各向异性（transverse anisotropy），实现慢磁弛豫，从而为单分子磁体与单链磁体（single-molecule and single-chain magnets）的设计提供了全新方向。