Disproportionation of nitric oxide promoted by a Mn tropocoronand

The synthesis and characterization of the divalent tropocoronand, [Mn(THF)(TC-5,5)] (1), are described. Compound 1 reacts with NO to afford a discrete mononitrosyl [Mn(NO)(TC-5,5)] (2), which was isolated and shown formally to contain the {(MnNO-)-N-III}(2+) fragment by infrared spectroscopy, normal coordinate analysis, X-ray crystallography, and SQUID susceptometry. The Mn center in 2 has idealized trigonal bipyramidal geometry with the NO ligand occupying a coordination site in the equatorial plane. The Mn-N-O bond angle is nearly linear, 174.1(3)degrees. Compounds 1 and 2 react with excess NO to afford N2O and [Mn(NO2)(TC-5,5)] (3). The X-ray structure of 3 reveals coordination of the nitrito ligand through one oxygen atom, the Mn-O bond length being 2.09(4) Angstrom. The other oxygen atom interacts weakly with the manganese center at a distance of 2.61(5) Angstrom. Quantitation by gas chromatographic analysis of released N2O confirms that 1 equiv forms for each molecule of 2 which converts to 3. Infrared spectroscopic experiments with (NO)-N-14 and (NO)-N-15 suggest that a dinitrosyl intermediate is present during the reaction, mechanistic aspects of which are discussed.