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.