Coordinative interaction between nitrogen oxides and iron–molybdenum POM Mo72Fe30

Abstract

The process of adsorption of nitrogen monoxide and dioxide by the giant Keplerate nanocluster Mo₇₂Fe₃₀ was studied in detail under ambient conditions and air/argon atmosphere. The obtained Raman and IR spectra showed that the coordination of NO_x to the Mo₇₂Fe₃₀ leads to the formation of nitrate ions by sharing the bridged or terminal oxygen in FeO₆ polyhedra with the adsorbed NO₂ molecules. In accordance with elemental analysis and X-ray photoelectron spectroscopy, the composition of the produced complex was found to be [POM–(NO₂)_x]·(NO₂)_y (where x = 6, y = 14 ± 3). The carried out thermal analysis revealed the significant influence of NO_x coordination in the release of water molecules and decomposition of the constitutional acetate ligands for Mo₇₂Fe₃₀. Furthermore, the performed measurements of the temperature dependency of the electron paramagnetic resonance spectra for the pure nanocluster and that treated with NO₂ allowed us to draw up a conclusion about the delocalization of weak-bonded NO₂ molecules in the pores of the Mo₇₂Fe₃₀ crystal at 25 °C. The opposite situation was observed under cryogenic temperatures. The localization of NO₂ molecules occurs resulting in the distortion of FeO₆ octahedra towards tetrahedral symmetry accompanied with the appearance of the signal at g-factor 4.3. The produced complex compound [POM–(NO₂)_x]·(NO₂)_y possesses sufficient NO₂ capacity, water solubility and pH-dependant decomposition; these are important properties of a potential NO_x donor, which can be hypothetically applied in biomedicine.