Modulation of Water Vapor Sorption by Pore Engineering in Isostructural Square Lattice Topology Coordination Networks

We report a crystal-engineering study conducted upon
a platform
of three mixed-linker square lattice (sql) coordination
networks of general formula [Zn(Ria)(bphy)] [bphy = 1,2-bis(pyridin-4-yl)hydrazine,
H2Ria = 5-position-substituted isophthalic acid, and R
= –Br, –NO2, and –OH; compounds 1–3]. Analysis of single-crystal X-ray
diffraction data of 1–2 and the simulated
crystal structure of 3 revealed that 1–3 are isomorphous and sustained by bilayers of sql networks linked by hydrogen bonds. Although similar pore shapes
and sizes exist in 1–3, distinct
isotherm shapes (linear and S shape) and uptakes (2.4, 11.6, and 13.3
wt %, respectively) were observed. Ab initio calculations indicated
that the distinct water sorption properties can be attributed to the
R groups, which offer a range of hydrophilicity. Calculations indicated
that the significantly lower experimental uptake in compound 1 can be attributed to a constricted channel. The calculated
water-binding sites provide insights into how adsorbed water molecules
bond to the pore walls, with the strongest interactions, water–hydroxyl
hydrogen bonding, observed for 3. Overall, this study
reveals how pore engineering can result in large variations in water
sorption properties in an isomorphous family of rigid porous coordination
networks.