Highly Anisotropic Rhenium(IV) Complexes: New Examples of Mononuclear Single-Molecule Magnets

The rhenium­(IV) complex (NBu4)2[ReBr4(ox)] (1) (ox = oxalate and NBu4+ = tetra-n-butylammonium cation) has been prepared and its crystal structure determined by X-ray diffraction. The structure is made up of discrete [ReBr4(ox)]2– anions and bulky NBu4+ cations. Each [ReBr4(ox)]2– anion is surrounded by six NBu4+ cations, which preclude any significant intermolecular contact between the anionic entities, the shortest rhenium···rhenium distance being 9.373(1) Å. Variable temperature dc and ac magnetic susceptibility measurements and field-dependent magnetization experiments on polycrystalline samples of 1 reveal the occurrence of highly anisotropic magnetically isolated Re­(IV) centers (SRe = 3/2), which exhibit slow relaxation of the magnetization at very low temperatures in a dc field. Ac measurements conducted on a polycrystalline sample of the complex (NBu4)2[ReCl4(ox)] (2) [compound isostructural to 1 whose structure and dc magnetic susceptibility study were previously reported in Tomkiewicz, A.; Bartczak, T. J.; Kruszyński, R.; Mroziński, J. J. Mol. Struct. 2001, 595, 225] show a similar behavior, both complexes thus constituting new examples of mononuclear single-molecule magnets. High-frequency and -field electron paramagnetic resonance on polycrystalline samples of 1 and 2 and on single crystals of 2 allowed for the determination for the first time of the negative sign and confirmed a significant magnitude and rhombicity (E/D) of the zero-field splitting tensor of the [ReCl4(ox)]2– and [ReBr4(ox)]2– centers, originating from a combination of spin–orbit coupling and low molecular symmetry. D and E values of 1 and 2 were estimated through magnetization measurements and theoretically calculated through complete active space and density functional theory methodologies.本研究合成了铼(IV)配合物(NBu₄)₂[ReBr₄(ox)]（编号为1，其中ox代表草酸根（oxalate），NBu₄⁺代表四正丁基铵阳离子（tetra-n-butylammonium cation）），并通过X射线衍射（X-ray diffraction）测定了其晶体结构。该晶体结构由离散的[ReBr₄(ox)]²⁻阴离子与大体积的NBu₄⁺阳离子构成。每个[ReBr₄(ox)]²⁻阴离子被6个NBu₄⁺阳离子包围，这一结构阻断了阴离子实体间的有效分子间接触，铼原子间的最短距离为9.373(1) 埃（Å）。对化合物1的多晶样品开展变温直流（dc）与交流（ac）磁化率测量以及场相关磁化实验，结果表明体系中存在高度各向异性且磁孤立的铼(IV)中心（S_Re = 3/2），该中心在直流场下的极低温环境中表现出磁化慢弛豫行为。对配合物(NBu₄)₂[ReCl₄(ox)]（编号为2，其与化合物1同构，相关结构与直流磁化率研究此前已见于Tomkiewicz等人的报道：Tomkiewicz, A.; Bartczak, T. J.; Kruszyński, R.; Mroziński, J. J. Mol. Struct. 2001, 595, 225）的多晶样品进行交流测量，结果显示其行为与化合物1相似，因此两种配合物均为单核单分子磁体（mononuclear single-molecule magnets）的全新实例。通过对化合物1、2的多晶样品以及化合物2的单晶样品开展高频高场电子顺磁共振（high-frequency and -field electron paramagnetic resonance）测试，首次确定了[ReCl₄(ox)]²⁻与[ReBr₄(ox)]²⁻中心的零场分裂张量（zero-field splitting tensor）符号为负，并证实该张量具有显著的幅值与菱形度（E/D），这一特性源于自旋轨道耦合（spin–orbit coupling）与低分子对称性的共同作用。研究人员通过磁化测量估算了化合物1与2的D和E参数，并通过完全活性空间（complete active space）与密度泛函理论（density functional theory）方法完成了理论计算。