Amorphous coordination polymers and metal-organic frameworks (MOFs) have attracted much attention owing to their various functionalities. Here, we demonstrate the tunable water adsorption behavior of a series of amorphous cyanide-bridged MOFs with different metals (M[Ni(CN)]: ; M = Mn, Fe, and Co). All three compounds adsorb up to six water molecules at a certain vapor pressure () and undergo conversion to crystalline Hofmann-type MOFs, M(HO)[Ni(CN)]·4HO (; M = Mn, Fe, and Co). The of , , and for water adsorption is / = 0.4, 0.6, and 0.9, respectively. Although the amorphous nature of these materials prevented structural elucidation using X-ray crystallography techniques, the local-scale structure around the N-coordinated M centers was analyzed using L-, K-edge X-ray absorption fine structure, and magnetic measurements. Upon hydration, the coordination geometry of these metal centers changed from tetrahedral to octahedral, resulting in significant reorganization of the MOF local structure. On the other hand, Ni[Ni(CN)] () containing square-planar Ni centers did not undergo significant structural transformation and therefore abruptly adsorbed HO in the low-pressure region. We could thus define how changes in the bond lengths and coordination geometry are related to the adsorption properties of amorphous MOF systems.

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http://dx.doi.org/10.1021/acs.inorgchem.0c03742DOI Listing

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