Isoreticular Ni₈‑Dipyrazolate Frameworks for Reversible Low-Humidity Water Vapor Sorption

Water vapor sorption in metal-organic frameworks (MOFs) is governed by a delicate interplay between framework topology, pore dimension, and distribution of water-binding sites. Isoreticular MOFs provide a powerful platform to disentangle these factors, yet direct comparisons that isolate the role of the metal node remain rare. Herein, we report a systematic study of water vapor adsorption in a family of fcu-type Ni₈-dipyrazolate frameworks, BUT-191-BUT-194 (BUT = Beijing University of Technology), constructed from a series of dipyrazolate ligands incorporating various heteroaromatic cores, including pyridyl, pyrimidyl, pyrazinyl, and pyridazinyl. All four Ni₈ frameworks exhibit exceptional chemical stability under harsh aqueous conditions. Water vapor sorption measurements at 298 K reveal pronounced S-shaped isotherms with steep uptake below ∼32% relative humidity (RH), with a maximum water uptake of up to 0.59 g g^-1. In contrast to previously reported isoreticular Co₈-dipyrazolate frameworks, no detectable redox transformation of the metal node is observed in the Ni₈ analogues during water sorption, enabling fully reversible adsorption-desorption behavior with excellent cycling stability. Particle size control experiments further reveal that hysteresis can be effectively suppressed without sacrificing the adsorption capacity, highlighting the role of mass transport in tuning water sorption reversibility. These findings provide fundamental insights into metal-node-dependent water sorption mechanisms in robust azolate MOFs, offering practical guidelines for the rational design of recyclable water vapor sorbents.