Reversible Switching from Antiferro- to Ferromagnetic Behavior by Solvent-Mediated, Thermally-Induced Phase Transitions in a Trimorphic MOF-Based Magnetic Sponge System

Hydrothermal reactions of copper­(II) acetate, tetrazolate-5-carboxylate (tzc), and the neutral N-donor spacer ligand 1,3-di­(4-pyridyl)­propane (dpp) lead in a single reaction vial to the simultaneous formation of three different single-crystalline solvates [Cu­(tzc)­(dpp)]n·0.5C6H14·0.5H2O (1), [Cu­(tzc)­(dpp)]n·4.5H2O (2), and [Cu­(tzc)­(dpp)]n·1.25C6H14 (3). All three structures were characterized by single crystal X-ray diffraction. None of these solvates can be prepared as phase-pure bulk materials, but reaction conditions similar to those used for single crystal synthesis yield a phase-pure polycrystalline bulk material of an additional forth solvate phase [Cu­(tzc)­(dpp)]n·2H2O (4). Investigations of its thermal properties by in situ temperature-dependent synchrotron-based powder diffraction experiments have shown interesting phase transitions upon heating in a helium stream. Initially, the precursor dihydrate 4 transforms to an anhydrous phase [Cu­(tzc)­(dpp)]n (6I) via the intermediate monohydrate phase [Cu­(tzc)­(dpp)]n·H2O (5). Upon further heating, phase 6I transforms to a new anhydrous polymorph 6II, which transforms upon cooling to a further new phase 6III. Thermogravimetric measurements performed in tandem with differential scanning calorimetry as well as infrared spectroscopic investigations are in agreement with these findings. The de/resolvation behavior is accompanied by a dramatic change in their magnetic properties: The dihydrate phase shows antiferromagnetic exchange interactions, whereas ferromagnetic properties are observed for the trimorphic anhydrate system. This magnetic sponge-like behavior can be reversibly cycled upon de/resolvation of the material.以乙酸铜(II)、四唑-5-羧酸根（tetrazolate-5-carboxylate，tzc）与中性氮供体桥联配体1,3-二(4-吡啶基)丙烷（1,3-di(4-pyridyl)propane，dpp）进行水热反应，在单个反应瓶中可同时得到三种不同的单晶溶剂化物：[Cu(tzc)(dpp)]ₙ·0.5C₆H₁₄·0.5H₂O (1)、[Cu(tzc)(dpp)]ₙ·4.5H₂O (2) 与 [Cu(tzc)(dpp)]ₙ·1.25C₆H₁₄ (3)。所有三种结构均通过单晶X射线衍射（single crystal X-ray diffraction）完成表征。上述三类溶剂化物均无法制备为纯相块体材料，但采用与单晶合成相近的反应条件，可得到第四种溶剂化物相[Cu(tzc)(dpp)]ₙ·2H₂O (4)的纯相多晶块体材料。通过基于同步辐射的原位变温粉末衍射（synchrotron-based powder diffraction）实验对其热性能展开研究，结果显示在氦气流中加热时出现了有趣的相变过程：初始时，二水合物前驱体4经由中间相一水合物[Cu(tzc)(dpp)]ₙ·H₂O (5) 转化为无水相[Cu(tzc)(dpp)]ₙ (6I)；继续加热后，相6I转变为新的无水多晶型物6II，该多晶型物经冷却后进一步转化为另一新相6III。同步开展的热重-差示扫描量热（thermogravimetric measurements coupled with differential scanning calorimetry）联合测试以及红外光谱表征结果均与上述发现相符。该材料的脱/溶剂化过程伴随着磁性能的显著变化：二水合物相表现出反铁磁交换相互作用，而三态无水体系则呈现铁磁特性。这种类磁海绵行为可通过材料的脱/溶剂化过程实现可逆循环。