Postsynthetic tuning of hydrophilicity in pyrazolate MOFs to modulate water adsorption properties

Abstract

Metal–organic frameworks (MOFs) have attracted interest as adsorbents in water-based adsorption heat pumps owing to their potential for increased water loading capacities and structural and functional tunability versus traditionally used materials such as zeolites and silica. Although pyrazolate-based MOFs exhibit exceptional hydrolytic stability, the water adsorption characteristics of this class of frameworks have remained unexplored in this context. In this report, we describe the modular synthesis of novel dipyrazole ligands containing naphthalenediimide cores functionalized with –H (H₂NDI–H), –NHEt (H₂NDI–NHEt), or –SEt (H₂NDI–SEt) groups. Reaction of these ligands with Zn(NO₃)₂ afforded an isostructural series of MOFs, Zn(NDI–X), featuring infinite chains of tetrahedral Zn²⁺ ions bridged by pyrazolate groups and ∼16 Å-wide channels with functionalized naphthalenediimide linkers lining the channel surface. The Type V water adsorption isotherms measured for these materials show water uptake in the 40–50% relative humidity range, suggesting hydrophobic channel interiors. Postsynthetic oxidation of Zn(NDI–SEt) with dimethyldioxirane was used to generate ethyl sulfoxide and ethyl sulfone groups, thereby rendering the channels more hydrophilic, as evidenced by shifts in water uptake to the 30–40% relative humidity range. Such tunability in water adsorption characteristics may find utility in the design of new adsorbents for adsorption-based heat transfer processes. An original MATLAB script, MOF-FIT, which allows for visual modeling of breathing and other structural deformations in MOFs is also presented.