Dynamic structural flexibility of Fe-MOF-5 evidenced by 57Fe Mössbauer spectroscopy
Temperature-dependent 57Fe Mössbauer spectra were collected on FexZn4−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.