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 alpha-UWDM-6 and alpha-UWDM-7, and their structures were determined by single-crystal X-ray diffraction. Both MOFs undergo reversible phase transitions upon solvent removal to generate gamma-UWDM-6 and beta-UWDM-7. The dynamics of the 24-membered macrocycle ([24]crown-6) inside each MOF phase was studied using variable-temperature H-2 solid-state NMR spectroscopy. Interestingly, only solvated alpha-UWDM-7 demonstrates unrestricted rotation of the macrocycle inside the MOF pores; studies on alpha-UWDM-7 with various solvents showed that the dynamics of the wheel is essentially independent of the solvent.