Precise Control of Water Adsorption Behavior in Solid Solutions of Amorphous Coordination Polymers

Metal-organic frameworks (MOFs) and coordination polymers (CPs) with flexible coordination networks can exhibit rapid adsorption accompanied by structural changes at a specific pressure called the gate-opening pressure (P_go), making them well-suited for various water-uptake-based applications. Herein, we demonstrated the precise modulation of the water uptake behavior in the amorphous coordination polymers (Am-CPs), Co_xM_(1-x)[Ni(CN)₄] (Co_xM_{( 1 -x)}; M = Mn, Fe), via a solid solution approach. All amorphous solid solutions of Co_xM_{( 1 -x)} showed abrupt water adsorption within the relative pressure range of P/P₀ = 0.5-0.9. This phenomenon was associated with an amorphous-to-crystalline transformation into the 2D Hofmann-type analog of Co_xM_(1-x)(H₂O)₂[Ni(CN)₄]·4H₂O (Co_xM_{( 1 -x)}-H₂O). Systematic changes in P_go for Co_xM_{( 1 -x)} were controlled by the type and ratio of the central metals, which were revealed using a combination of L_2,3- and K-edge X-ray absorption fine structure (XAFS) techniques. Slight differences in the bond energies of bonds in the vicinity of the water-induced amorphous-to-crystalline transformation. This transformation involves a change in the coordination geometry, from tetrahedral to octahedral, through the rearrangement of CN-Co(M) bonds via cleavage/reformation, giving rise to the desirable water uptake characteristics.