Linker Substituents Control the Thermodynamic Stability in Metal–Organic Frameworks

We report the first systematic experimental
and theoretical study
of the relationship between the linker functionalization and the thermodynamic
stability of metal–organic frameworks (MOFs) using a model
set of eight isostructural zeolitic imidazolate frameworks (ZIFs)
based on 2-substituted imidazolate linkers. The frameworks exhibit
a significant (30 kJ·mol–1) variation in the
enthalpy of formation depending on the choice of substituent, which
is accompanied by only a small change in molar volume. These energetics
were readily reproduced by density functional theory (DFT) calculations.
We show that these variations in the enthalpy of MOF formation are
in linear correlation to the readily accessible properties of the
linker substituent, such as the Hammett σ-constant or electrostatic
surface potential. These results provide the first quantifiable relationship
between the MOF thermodynamics and the linker structure, suggesting
a route to design and tune MOF stability.