NO disproportionation reactivity of Fe tropocoronand complexes

The synthesis and characterization of divalent [Fe(TC-5,5)] (1) and trivalent [Fe(OTf)(TC-5,5)] (2) tropocoronand complexes are described. Compound 1 reacts with 1 equiv of NO to form the {FeNO}⁷ complex 3. A single-crystal X-ray structure determination of 3 reveals a trigonal bipyramidal geometry with a linearly coordinated nitrosyl (Fe-N-O = 174.3(4)°) having a short Fe-N distance of 1.670(4) Å. EPR and Mossbauer spectroscopy, SQUID susceptometry, and normal coordinate analysis indicate 3 to be a low-spin {Fe(III)(NO^-)}²⁺ species. In the presence of excess NO, 3 converts to a metastable nitrosyl-nitrito complex that decomposes by losing NO2, which subsequently nitrates the aromatic tropolone rings of the ligand. The final products of the NO disproportionation reaction are N2O and [Fe(NO)(TC-5,5-NO2)] (4). The v(NO) stretching band of 4 is increased to 1716 cm^-1 from its value of 1692 cm^-1 in 3, owing to the electron- withdrawing nitro groups on the ligand, and the compound no longer promotes the disproportionation of NO. Mechanistic aspects of the reaction are discussed.

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Author Katherine J. Franz Chemistry