A Hybrid Ultramicroporous Material Family With Metal-Controlled Dynamic Behavior

Hybrid ultramicroporous materials (HUMs) are comprised of combinations of inorganic and organic linker ligands as exemplified by the prototypal pcu topology variant [Zn(pyrazine)₂(SiF₆)]_n, SIFSIX-3-Zn, and its recently reported first-generation derivatives [M(3,3'-bipyridine)₂(SiF₆)]_n (M = Cu, Zn). Herein, we report the synthesis, structure, and sorption properties of a second-generation family of HUMs of general formula [M(3,3'-bipyridine)₂(NbOF₅)]_n, NbOFFIVE-3,3'-bpy-M, M = Co, Ni, Cu, Zn. The four HUMs are structurally characterized by single crystal x-ray diffraction, revealing the expected pcu topology of the isostructural as-synthesized phases. All variants initially crystallized in large-pore (lp) phases with water molecules occupying 1D channels. Upon dehydration, Cu and Zn transformed from water-loaded lp phases into narrow-pore (np) guest-free phases, whereas Co and Ni retained their lp structures after desorption. Water vapor sorption isotherms (300 K) revealed stepped isotherms corresponding to structural transformations (np→lp) for Cu and Zn, and rare examples of induced-fit (lp→np→lp) from guest-free lp to np to water-loaded lp phases for Co and Ni. The NbOFFIVE-3,3'-bpy-M family exhibits uncommon examples of metal-dependent dynamic behavior in HUMs.