Enhancing Water Stability of Metal–Organic Frameworks via Phosphonate Monoester Linkers

A new porous metal–organic framework (MOF), barium
tetraethyl-1,3,6,8-pyrenetetraphosphonate
(CALF-25), which contains a new phosphonate monoester ligand, was
synthesized through a hydrothermal method. The MOF is a three-dimensional
structure containing 4.6 Å × 3.9 Å rectangular one-dimensional
pores lined with the ethyl ester groups from the ligand. The presence
of the ethyl ester groups makes the pores hydrophobic in nature, as
determined by the low heats of adsorption of CH4, CO2, and H2O (14.5, 23.9, and 45 kJ mol–1, respectively) despite the polar and acidic barium phosphonate ester
backbone. The ethyl ester groups within the pores also protect CALF-25
from decomposition by water vapor, with crystallinity and porosity
being retained after exposure to harsh humid conditions (90% relative
humidity at 353 K). The use of phosphonate esters as linkers for the
construction of MOFs provides a method to protect hydrolytically susceptible
coordination backbones through kinetic blocking.