Solvent and Steric Influences on Rotational Dynamics in Porphyrinic Metal–Organic Frameworks with Mechanically Interlocked Pillars

Zinc­(II) metal–organic framework (MOF) materials were prepared using a porphyrin-based tetracarboxylate linker and pyridine-terminated [2]­rotaxane linkers as pillars. Two pillaring rotaxanes (R = H or CH₃ at pyridine 2-position) were used to create two MOFs α-UWDM-6 and α-UWDM-7, and their structures were determined by single-crystal X-ray diffraction. Both MOFs undergo reversible phase transitions upon solvent removal to generate γ-UWDM-6 and β-UWDM-7. The dynamics of the 24-membered macrocycle ([24]­crown-6) inside each MOF phase was studied using variable-temperature ²H solid-state NMR spectroscopy. Interestingly, only solvated α-UWDM-7 demonstrates unrestricted rotation of the macrocycle inside the MOF pores; studies on α-UWDM-7 with various solvents showed that the dynamics of the wheel is essentially independent of the solvent.