Tuning the Adsorption-Induced Phase Change in the Flexible Metal–Organic Framework Co(bdp)

Metal–organic
frameworks that flex to undergo structural
phase changes upon gas adsorption are promising materials for gas
storage and separations, and achieving synthetic control over the
pressure at which these changes occur is crucial to the design of
such materials for specific applications. To this end, a new family
of materials based on the flexible metal–organic framework
Co­(bdp) (bdp2– = 1,4-benzenedipyrazolate) has been
prepared via the introduction of fluorine, deuterium, and methyl functional
groups on the bdp2– ligand, namely, Co­(F-bdp), Co­(p-F2-bdp), Co­(o-F2-bdp), Co­(D4-bdp), and Co­(p-Me2-bdp). These frameworks are isoreticular to the parent framework
and exhibit similar structural flexibility, transitioning from a low-porosity,
collapsed phase to high-porosity, expanded phases with increasing
gas pressure. Powder X-ray diffraction studies reveal that fluorination
of the aryl ring disrupts edge-to-face π–π interactions,
which work to stabilize the collapsed phase at low gas pressures,
while deuteration preserves these interactions and methylation strengthens
them. In agreement with these observations, high-pressure CH4 adsorption isotherms show that the pressure of the CH4-induced framework expansion can be systematically controlled by
ligand functionalization, as materials without edge-to-face interactions
in the collapsed phase expand at lower CH4 pressures, while
frameworks with strengthened edge-to-face interactions expand at higher
pressures. Importantly, this work puts forth a general design strategy
relevant to many other families of flexible metal–organic frameworks,
which will be a powerful tool in optimizing these phase-change materials
for industrial applications.