The Electrophilic Reactions of Pentacyanonitrosylferrate(II) with Hydrazine and Substituted Derivatives. Catalytic Reduction of Nitrite and Theoretical Prediction of η1-, η2-N2O Bound Intermediates

The electrophilic reactivity of the pentacyanonitrosylferrate(II) ion, [Fe(CN)5NO]2-, toward hydrazine (Hz) and substituted hydrazines (MeHz, 1,1-Me2Hz, and 1,2-Me2Hz) has been studied by means of stoichiometric and kinetic experiments (pH 6−10). The reaction of Hz led to N2O and NH3, with similar paths for MeHz and 1,1-Me2Hz, which form the corresponding amines. A parallel path has been found for MeHz, leading to N2O, N2, and MeOH. The reaction of 1,2-Me2Hz follows a different route, characterized by azomethane formation (MeNNMe), full reduction of nitrosyl to NH3, and intermediate detection of [Fe(CN)5NO]3-. In the above reactions, [Fe(CN)5H2O]3- was always a product, allowing the system to proceed catalytically for nitrite reduction, an issue relevant in relation to the behavior of the nitrite and nitric oxide reductase enzymes. The mechanism comprises initial reversible adduct formation through the binding of the nucleophile to the N-atom of nitrosyl. The adducts decompose through OH- attack giving the final products, without intermediate detection. Rate constants for the adduct-formation steps (k = 0.43 M-1 s-1, 25 °C for Hz) decrease with methylation by about an order of magnitude. Among the different systems studied, one-, two-, and multielectron reductions of bound NO+ are analyzed comparatively, with consideration of the role of NO, HNO (nitroxyl), and hydroxylamine as bound intermediates. A DFT study (B3LYP) of the reaction profile allows one to characterize intermediates in the potential hypersurface. These are the initial adducts, as well as their decomposition products, the η1- and η2-linkage isomers of N2O.