Flexible sorption and transformation behavior in a microporous metal-organic framework

Crystals of the metal-organic framework material Ni2(4,4‘-bipyridine)3(NO3)4 (A) have been grown by reaction of Ni(NO3)2·6H2O and 4,4‘-bipyridine in methanol solution. Single-crystal X-ray diffraction experiments show that the ladder structure of the framework is maintained after desolvation of the material, resulting in the production of a porous solid stable to 215(4) °C. Powder X-ray diffraction has been employed to confirm the bulk purity and temperature stability of this material. The crystal structure indicates that the pore window has an area of 12.3 Å2. However, sorption experiments show these windows will admit toluene, which has a minimum cross-sectional area of 26.6 Å2, with no significant change in the structure. Monte Carlo docking calculations show that toluene can be accommodated within the large pores of the structure. Exposure of the related microporous material Ni2(4,4‘-bipyridine)3(NO3)4·2C2H5OH (B) to methanol vapor causes a guest-driven solid-state transformation to A which is observed using powder X-ray diffraction. This structural rearrangement proceeds directly from crystalline B to crystalline A and is complete in less than 1 day. Mechanisms for the transformation are proposed which require breaking of at least one in six of the covalent bonds that confer rigidity on the framework