Pore Size Engineering of MOFs by Pore Edge Reaction: Tetrazine Click and Hydrogen Adsorption in Theory and Experiment

Precise control over
the porosity of metal–organic frameworks
(MOFs) is crucial to optimize their properties and leverage their
inherent tunability. However, there are ongoing challenges in pore
size engineering for each MOF platform such as preserving crystallinity
and morphology and facilitating reliable theoretical predictions throughout
a series of modulated structures. Among postsynthetic strategies,
mainly covalent functionalization appears to simultaneously preserve
structural integrity and enable accurate theoretical predictions.
Here, we present a MOF platform [M2(RCOO)4(H2O)2], JUK-21(M), M = Cu or Zn, containing a tetrazine-based
tetracarboxylate linker, which we covalently functionalize using the
inverse electron-demand Diels–Alder reaction (iEDDA) and five
dienophiles of various bulkiness, yielding a series of JUK-21(Cu)-x MOFs. In addition to experiments, the iEDDA reactivity
is assessed by applying a charge distribution susceptibility analysis,
including Fukui functions, hardness, and relevant donor/acceptor orbitals.
Comprehensive theoretical and experimental insights into the adsorption
of nitrogen and hydrogen by JUK-21(Cu)-x enable rationalization
of the observed isotherms and show the isosteric heat of hydrogen
adsorption as a highly sensitive parameter to validate the modification
efficiency. Our findings indicate to what extent the pore size of
MOFs affects the adsorption properties and highlight potential pitfalls
that arise even with the precise covalent functionalization of MOFs.