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.