Modulation of Water Vapor Sorption by a Fourth-Generation Metal-Organic Material with a Rigid Framework and Self-Switching Pores

Hydrolytically stable adsorbents are needed for water vapor sorption related applications; however, design principles for porous materials with tunable water sorption behavior are not yet established. Here, we report that a platform of fourth-generation metal-organic materials (MOMs) with rigid frameworks and self-switching pores can adapt their pores to modulate water sorption. This platform is based upon the hydrolytically stable material CMOM-3S, which exhibits bnn topology and is composed of rod building blocks based upon S-mandelate ligands, 4,4-bipyridine ligands, and extraframework triflate anions. Isostructural variants of CMOM-3S were prepared using substituted R-mandelate ligands and exhibit diverse water vapor uptakes (20-67 cm³/g) and pore filling pressures (P/P₀, 0.55-0.75). [Co₂(R-4-Cl-man)₂(bpy)₃](OTf) (33R) is of particular interest because of its unusual isotherm. Insight into the different water sorption properties of the materials studied was gained from analysis of in situ vibrational spectra, which indicate self-switching pores via perturbation of extraframework triflate anions and mandelate linker ligands to generate distinctive water binding sites. Water vapor adsorption was studied using in situ differential spectra that reveal gradual singlet water occupancy followed by aggregation of water clusters in the channels upon increasing pressure. First-principles calculations identified the water binding sites and provide structural insight on how adsorbed water molecules affect the structures and the binding sites. Stronger triflate hydrogen bonding to the framework along with significant charge redistribution were determined for water binding in 33R. This study provides insight into a new class of fourth-generation (self-switching pores) MOM and the resulting effect upon water vapor sorption properties.