Flexible metal–organic frameworks (MOFs) undergo cooperative structural transformations in response to external stimuli, yet the crystallographic origins of these transformations remain poorly understood. In particular, it is unclear how subtle changes in linker topology influence the propagation of structural strain through otherwise closely related framework architectures. Here, we present a crystallographic investigation of a family of isoreticular Zn–triazolate pillared-layer MOFs incorporating three-dimensional aliphatic dicarboxylate linkers based on cubane (cdc), bicyclo[1.1.1]pentane (pdc), and bicyclo[2.2.2]octane (boct).1–2
The three frameworks exhibit markedly different structural responses despite sharing the same Zn–triazolate sheet topology. Variable-temperature powder X-ray diffraction reveals distinct framework behaviour across the series, ranging from gradual lattice evolution to abrupt cooperative structural transformations and irreversible pore closure. Complementary variable-temperature single-crystal X-ray diffraction follows the evolution of the solvent-filled frameworks and identifies temperature-dependent symmetry changes, anisotropic lattice expansion, and changes in the Zn coordination environment associated with desolvation. High-pressure in situ CO₂ powder diffraction further demonstrates fundamentally different guest-induced responses, with the cubane framework exhibiting reversible phase transitions, while the boct analogue undergoes continuous lattice distortion without the formation of distinct intermediate phases.
To understand the structural origin of these behaviours, machine-learning interatomic potential calculations and elastic tensor analysis were performed. These calculations reveal that framework deformation is governed primarily by hinging of the Zn–triazolate sheets rather than large distortions of the organic pillars themselves. Instead, linker topology dictates how the strain generated by this hinging is accommodated throughout the framework. The boct linker behaves as a mechanically compliant element that dissipates strain through local deformation, the cubane linker provides balanced mechanical coupling that enables reversible cooperative breathing, while the shorter bicyclo[1.1.1]pentane linker suppresses linker shear and redirects strain into Zn-node distortion, producing a kinetically trapped contracted phase.
By combining variable-temperature single-crystal and powder diffraction, in situ gas-loading diffraction, and complementary computational modelling, this work establishes structure–mechanics relationships within a family of flexible Zn–triazolate frameworks. More broadly, the results demonstrate that subtle variations in linker topology provide a powerful strategy for programming cooperative structural behaviour in crystalline porous materials and illustrate the central role of crystallography in revealing the mechanisms underpinning framework flexibility.