Single-Crystal-to-Single-Crystal Cluster Transformation in a Microporous Molybdoarsenate(V)-Metalorganic Framework.

The hybrid compound [Cu(cyclam)(HO)][{Cu(cyclam)}{-HAsMoO(HO)}]·9HO () (cyclam = 1,4,8,11-tetraazacyclotetradecane) was synthesized in aqueous solution by reacting the {Cu(cyclam)} complex with a mixture of heptamolybdate and an arsenate(V) source. Crystal packing of exhibits a supramolecular open-framework built of discrete covalent molybdoarsenate/metalorganic units and additional [Cu(cyclam)(HO)] cations, the stacking of which generates squarelike channels parallel to the axis with an approximate cross section of 10 × 11 Å where all the hydration water molecules are hosted. Thermal evacuation of solvent molecules yields a new anhydrous crystalline phase, but compound does not preserve its single-crystalline nature upon heating. However, when crystals are dehydrated under vacuum, they undergo a structural transformation that proceeds via a single-crystal-to-single-crystal pathway, leading to the anhydrous phase [{Cu(cyclam)}(-HAsMoO)] (). Total dehydration results in important modifications within the inorganic cluster skeleton which reveals an unprecedented solid-state to isomerization of the polyoxoanion. This transition also involves changes in the Cu bonding scheme that lead to covalent cluster/metalorganic layers by retaining the open-framework nature of . Compound adsorbs ambient moisture upon air exposure, but it does not revert back to , and the hydrated phase [{Cu(cyclam)}(-HAsMoO)]·6HO () is obtained instead. Structural variations between and are reflected in electron paramagnetic resonance spectroscopy measurements, and the permanent microporosity of provides interesting functionalities to the system such as the selective adsorption of gaseous CO over N.