Coordination Geometry Changes in Amorphous Cyanide-Bridged Metal–Organic Frameworks upon Water Adsorption

Abstract

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)₄]: <b>MNi</b>; M = Mn, Fe, and Co). All three compounds adsorb up to six water molecules at a certain vapor pressure (<i>P</i>_ads) and undergo conversion to crystalline Hofmann-type MOFs, M(H₂O)₂[Ni(CN)₄]·4H₂O (<b>MNi-H</b>_<b>2</b><b>O</b>; M = Mn, Fe, and Co). The <i>P</i>_ads of <b>MnNi</b>, <b>FeNi</b>, and <b>CoNi</b> for water adsorption is <i>P</i>/<i>P</i>₀ = 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 M²⁺ centers was analyzed using L_2,3-, 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)₄] (<b>NiNi</b>) containing square-planar Ni²⁺ centers did not undergo significant structural transformation and therefore abruptly adsorbed H₂O 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.