Dynamic structural flexibility of Fe-MOF-5 evidenced by 57Fe Mössbauer spectroscopy

Abstract

Temperature-dependent ⁵⁷Fe Mössbauer spectra were collected on Fe_xZn_4−x(1,4-benzenedicarboxylate)₃ (Fe-MOF-5). When measured under an Ar atmosphere, the data at higher temperatures reveal thermal population of the lowest-lying electronic excited state, as expected for low symmetry tetrahedral ferrous ions. In the presence of N₂, however, the temperature dependence becomes exaggerated and the spectra cannot be fitted to a single species. A fluctuating electric field gradient at the Fe nuclei best explains these data and suggests dynamic structural distortions induced by weak interactions with N₂. This direct evidence of dynamic behaviour at MOF open metal sites is relevant for the use of MOF SBUs in catalysis, gas separation, and other applications that invoke similar phenomena.