Experimental and Theoretical Evaluation of the Stability of True MOF Polymorphs Explains Their Mechanochemical Interconversions

We
provide the first combined experimental and theoretical evaluation
of how differences in ligand structure and framework topology affect
the relative stabilities of isocompositional (i.e., true polymorph)
metal–organic frameworks (MOFs). We used solution calorimetry
and periodic DFT calculations to analyze the thermodynamics of two
families of topologically distinct polymorphs of zinc zeolitic imidazolate
frameworks (ZIFs) based on 2-methyl- and 2-ethylimidazolate linkers,
demonstrating a correlation between measured thermodynamic stability
and density, and a pronounced effect of the ligand substituent on
their stability. The results show that mechanochemical syntheses and
transformations of ZIFs are consistent with Ostwald’s rule
of stages and proceed toward thermodynamically increasingly stable,
more dense phases.