Thermally-Triggered Crystal Dynamics and Permanent Porosity in the First Heptatungstate-Metalorganic...

The hybrid compound [{Cu(cyclam)}₃ (W₇ O₂₄ )]⋅15.5 H₂ O (1) (cyclam=1,4,8,11-tetraaza-cyclotetradecane) was synthesized by reacting the {Cu(cyclam)}²⁺ complex with a tungstate source in water at pH 8. Compound 1 exhibits an unprecedented three-dimensional covalent structure built of heptatungstate clusters linked through metalorganic complexes in a POMOF-like framework that displays water-filled channels. This dynamic architecture undergoes two sequential single-crystal-to-single-crystal transformations upon thermal evacuation of water molecules to result in the partially dehydrated [{Cu(cyclam)}₃ (W₇ O₂₄ )]⋅12 H₂ O (2) and anhydrous [Cu(cyclam)]_0.5 [{Cu(cyclam)}_2.5 (W₇ O₂₄ )] (3) crystalline phases. These transitions are associated with cluster rotations and modifications in the Cu^II coordination geometries, which reduce the dimensionality of the original lattice to layered systems but preserving the porous nature. Phase 3 reverts to 2 upon exposure to ambient moisture, whereas the transition between 1 and 2 proved to be irreversible. The permanent microporosity of 3 was confirmed by gas sorption measurements (N₂ , CO₂ ), which reveal a system of parallel channels made of wide cavities connected through narrow necks that limit the adsorption process. This observation is in good agreement with Grand Canonical Monte Carlo simulations.