Consecutive Single-Crystal-to-Single-Crystal Isomerization of Novel Octamolybdate Anions within a Microporous Hybrid Framework with Robust Water Sorption Properties

The 3D hybrid framework[{Cu(cyclam)}(3)(k-Mo8O27)]center dot 14H(2)O (1) (cyclam = 1,4,8,11-tetraazacyclotetradecane) undergoes sequential single-crystal-to-single-crystal transformations upon heating to afford two different anhydrous phases (2 a and 3a). These transitions modify the framework dimensionality and enable the isomerization of kappa-octamolybdate (kappa-Mo-8) anions into lambda (2 a) and mu (3 a) forms through metal migration. Hydration of 3a involves condensation of one water molecule to the cluster to afford the gamma-Mo-8 isomer in 4, which dehydrates back into 3a through the 6a intermediate. In contrast, 2a reversibly hydrates to form 5, exhibiting the same Mo-8 cluster as that of 1. It is remarkable that three of the Mo-8 clusters (kappa, lambda and mu) are new and that up to three different microporous phases can be isolated from 1 (2 a, 3a, and 6a). Water vapor sorption analyses show high recyclability and the highest uptake values for POM-based systems. The isotherms display an abrupt step at low humidity level desirable for humidity control devices or water harvesting in drylands.