Metal-catalyzed anaerobic disproportionation of hydroxylamine.: Role of diazene and nitroxyl intermediates in the formation of N2, N2O, NO+, and NH3

The catalytic disproportionation of NH2OH has been studied in anaerobic aqueous solution, pH 6-9.3, at 25.0 degreesC, with Na-3[Fe(CN)(5)NH3].3H(2)O as a precursor of the catalyst, [Fe-II(CN)(5)H2O](3-). The oxidation products are N-2, N2O, and NO+ (bound in the nitroprusside ion, NP), and NH3 is the reduction product. The yields of N-2/N2O increase with pH and with the concentration of NH2OH. Fast regime conditions involve a chain process initiated by the NH2 radical, generated upon coordination of NH2OH to [Fe-II(CN)(5)H2O](3-). NH3 and nitroxyl, HNO, are formed in this fast process, and HNO leads to the production of N-2, N2O, and NP. An intermediate absorbing at 440 nm is always observed, whose formation and decay depend on the medium conditions. It was identified by UV-vis, RR, and (NMR)-N-15 spectroscopies as the diazene-bound [Fe-II(CN)(5)N2H2](3-) ion and is formed in a competitive process with the radical path, still under the fast regime. At high pH's or NH2OH concentrations, an inhibited regime is reached, with slow production of only N-2 and NH3. The stable red diazene-bridged [(NC)SFeHN=NHFe(CN)(5)](6-) ion is formed at an advanced degree of NH2OH consumption.