Thermally‐Triggered Crystal Dynamics and Permanent Porosity in the First Heptatungstate‐Metalorganic Three‐Dimensional Hybrid Framework

• Published in: Chemistry : a European journal Vol. 23; no. 59; pp. 14962 - 14974
• Main Authors: Martín‐Caballero, Jagoba, Artetxe, Beñat, Reinoso, Santiago, San Felices, Leire, Castillo, Oscar, Beobide, Garikoitz, Vilas, José Luis, Gutiérrez‐Zorrilla, Juan M.
• Format: Journal Article
• Language: English
• Published: Germany Wiley Subscription Services, Inc 20.10.2017
• Subjects: Carbon dioxide; Channels; Chemistry; Computer simulation; Copper; Dehydration; gas sorption; Microporosity; Monte Carlo simulation; Neck; polyoxometalates; Porosity; SCSC transformations; Single crystals; supramolecular chemistry; Water chemistry; X-ray diffraction; SCSC transformations; X-ray diffraction; gas sorption; supramolecular chemistry; polyoxometalates
• ISSN: 0947-6539; 1521-3765; 1521-3765
• DOI: 10.1002/chem.201703585

The hybrid compound [{Cu(cyclam)}3(W7O24)]⋅15.5 H2O (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(W7O24)]⋅12 H2O (2) and anhydrous [Cu(cyclam)]0.5[{Cu(cyclam)}2.5(W7O24)] (3) crystalline phases. These transitions are associated with cluster rotations and modifications in the CuII 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. Framework transformation: A three‐dimensional covalent open framework built of copper(II)‐complexes of a macrocyclic tetraaza ligand and heptatungstate anions undergoes two sequential single‐crystal‐to‐single‐crystal transformations upon dehydration. The permanent porosity of the system has been confirmed by CO2 and N2 sorption experiments.