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.