Understanding Hydration in CPO‐27 Metal‐Organic Frameworks: Strong Impact of the Chemical Nature of the Metal (Cu, Zn)

CPO‐27 is a metal‐organic framework (MOF) with coordinatively unsaturated metal centers (open metal sites). It is therefore an ideal host material for small guest molecules, including water. This opens up numerous possible applications, such as proton conduction, humidity sensing, water harvesting, or adsorption‐driven heat pumps. For all of these applications, profound knowledge of the adsorption and desorption of water in the micropores is mandatory. The hydration and water structure in CPO‐27‐M (M = Zn or Cu) is investigated using water vapor sorption, Fourier transform infrared (FTIR) spectroscopy, density functional theory (DFT) calculations, and molecular dynamics simulation. In the pores of CPO‐27‐Zn, water binds as a ligand to the Zn center. Additional water molecules are stepwise incorporated at defined positions, forming a network of H‐bonds with the framework and with each other. In CPO‐27‐Cu, hydration proceeds by an entirely different mechanism. Here, water does not coordinate to the metal center, but only forms H‐bonds with the framework; pore filling occurs mostly in a single step, with the open metal site remaining unoccupied. Water in the pores forms clusters with extensive intra‐cluster H‐bonding.