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)1 (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)degrees) having a short Fe-N distance of 1.670(4) Angstrom. 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 YNO 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.