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 Mössbauer 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 NO 2, which subsequently nitrates the aromatic tropolone rings of the ligand. The final products of the NO disproportionation reaction are N 2 O and [Fe(NO)(TC-5,5-NO 2 )] (4) . The ν 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.