Magnetic Modulation and Cation-Exchange in a Series of Isostructural (4,8)-Connected Metal–Organic Frameworks with Butterfly-like [M4(OH)2(RCO2)8] Building Units

A series of isostructural metal-carboxylate frameworks [Me2NH2]­[M2(bptc)­(μ3-OH)­(H2O)2] (H4bptc = 3,3′,4,4′-biphenyltetracarboxylic acid, M = Co, Co0.83Ni0.17, Co0.55Ni0.45, Co0.13Ni0.87, and Ni for 1–5, respectively) have been synthesized, which feature (4,8)-connected scu topological anionic frameworks constructed by a butterfly-like [M4(OH)2(RCO2)8] cluster and have three-dimensional (3D) channels filled by in situ generated [Me2NH2]+ cations. Flame atomic absorption spectroscopy (FAAS), scanning electron microscope energy-disperse X-ray spectroscopy (SEM-EDS), infrared spectroscopy (IR), Raman spectroscopy, and powder X-ray diffraction (PXRD) measurements reveal that 1 can selectively exchange alkali metal cations. Magnetic properties display that monometallic 1 and 5 are characteristic of a antiferromagnet and a ferromagnet, respectively. Incorporation of Co and Ni into the system produced heterometallic compounds 2, 3, and 4. Compound 2 with a very low ratio of Ni to Co shows antiferromagnetic behavior similar to 1, while compounds 3 and 4 with a large ratio of Ni to Co are characteristic of ferrimagnetic-like behavior. This work demonstrates that magnetic tuning from antiferromagnetic, ferrimagnetic-like, to ferromagnetic behaviors can be achieved by heterometallic substitution in isostructural magnetic frameworks.

一系列同构金属羧酸骨架[Me₂NH₂][M₂(bptc)(μ₃-OH)(H₂O)₂]（其中H₄bptc=3,3′,4,4′-联苯四甲酸，M分别为Co、Co₀.₈₃Ni₀.₁₇、Co₀.₅₅Ni₀.₄₅、Co₀.₁₃Ni₀.₈₇和Ni，对应样品1至5）已被成功合成。该类骨架具有由类蝴蝶状[M₄(OH)₂(RCO₂)₈]簇构筑的(4,8)-连接scu拓扑阴离子骨架，并拥有被原位生成的[Me₂NH₂]+阳离子填充的三维（3D）孔道。通过火焰原子吸收光谱法（Flame Atomic Absorption Spectroscopy, FAAS）、扫描电子显微镜能谱（Scanning Electron Microscope Energy-Dispersive X-ray Spectroscopy, SEM-EDS）、红外光谱（IR）、拉曼光谱以及粉末X射线衍射（Powder X-ray Diffraction, PXRD）测试证实，样品1可实现碱金属阳离子的选择性交换。磁学性能表征结果显示，单金属样品1和5分别表现出反铁磁体和铁磁体的磁学特性。将Co和Ni引入该骨架体系，得到杂金属化合物2、3和4：镍钴比例极低的化合物2展现出与样品1类似的反铁磁行为，而镍钴比例较高的化合物3和4则呈现类亚铁磁行为。本研究表明，通过在同构磁性骨架中进行杂金属取代，可实现从反铁磁、类亚铁磁到铁磁行为的磁学性能调控。