The Catalytic Reduction of Nitrite. Metal Coordination and Reaction of Nitrosyl with Hydrazine: Two-Electron Oxidants Can Also Lead to Ammonia

Hydrazine Nitrosation of a Metal-Bound Nitric Oxide: Structural Evidence for the Formation of an Ammine Complex

Hydrazine nitrosation of [Ru(NO)(py(si)S4)]Br.THF (1) (py(si)S4(2-) = 2,6-bis(3-triphenylsilyl-2-sulfanylphenylthiomethyl)pyridine2-) in methanol/DMF led to the formation of mononuclear ammine complex [Ru(NH3)(py(si)S4)] (2) and N2O, whereas the reaction performed in THF/CH2Cl2/toluene afforded thioether-bridged dinuclear ammine complex [(NH3)Ru(mu-py(si)S4)Ru(py(si)S4)] (3). Compound 2 dimerizes in solution at room temperature to form 3 and is regenerated upon treatment of 3 with NH3. A plausible mechanism for the hydrazine nitrosation of 1 has been proposed. The reaction of 1 with NH3 or N3- does not lead to a nucleophilic attack at the NO+ ligand but to a deprotonation that yields neutral nitrosyl complex [Ru(NO){py(si)S4(H+)}] (4), which is supported by density functional theory calculations.

Further evidence supporting an inner sphere mechanism in the NO reduction of the copper(II) complex Cu(dmp)22+ (dmp=2,9-dimethyl-1,10-phenanthroline)

Described are further studies directed towards elucidating the mechanism of the nitric oxide reduction of the copper(II) model system, Cu(dmp)2(2+) (I, dmp=2,9-dimethyl-1,10-phenanthroline). The reaction of I with NO in methanol results in the formation of Cu(dmp)2+ (II) and methyl nitrite (CH3ONO), with a second order rate constant kNO=38.1 M-1 s-1 (298K). The activation parameters for this reaction in buffered aqueous medium were measured to be DeltaH(double dagger)=41.6 kJ/mol and DeltaS(double dagger)=-82.7 kJ/mol deg. The addition of azide ion (N3-) as a competing nucleophile results in a marked acceleration in the rate of the copper(II) reduction. Analysis of the kinetics for the NO reduction of the bulkier Cu(dpp)(2)2+ (IV, dpp=2,9-diphenyl-1,10-phenanthroline) and the stronger oxidant, Cu(NO2-dmp)2(2+) (V, NO2-dmp=5-nitro-2,9-dimethyl-1,10-phenanthroline), gave the second order rate constants kNO=21.2 and 29.3 M-1 s-1, respectively. These results argue against an outer sphere electron transfer pathway and support a mechanism where the first step involves the formation of a copper-nitrosyl (Cu(II)-NO or Cu(I)-NO+) adduct. This would be followed by the nucleophilic attack on the bound NO and the labilization of RONO to form the nitrite products and the cuprous complex.

Theoretical characterization of stable eta1-N2O-, eta2-N2O-, eta1-N2-, and eta2-N2-bound species: intermediates in the addition reactions of nitrogen hydrides with the pentacyanonitrosylferrate(II) ion

The addition of nitrogen hydrides (hydrazine, hydroxylamine, ammonia, azide) to the pentacyanonitrosylferrate(II) ion has been analyzed by means of density functional calculations, focusing on the identification of stable intermediates along the reaction paths. Initial reversible adduct formation and further decomposition lead to the eta(1)- and eta(2)-linkage isomers of N(2)O and N(2), depending on the nucleophile. The intermediates (adducts and gas-releasing precursors) have been characterized at the B3LYP/6-31G level of theory through the calculation of their structural and spectroscopic properties, modeling the solvent by means of a continuous approach. The eta(2)-N(2)O isomer is formed at an initial stage of adduct decompositions with the hydrazine and azide adducts. Further conversion to the eta(1)-N(2)O isomer is followed by Fe-N(2)O dissociation. Only the eta(1)-N(2)O isomer is predicted for the reaction with hydroxylamine, revealing a kinetically controlled N(2)O formation. eta(1)-N(2) and eta(2)-N(2) isomers are also predicted as stable species.

Kinetics and mechanism of the cobalt phthalocyanine catalyzed reduction of nitrite and nitrate by dithionite in aqueous solution

The reaction of sodium nitrite with sodium dithionite was studied in the presence of cobalt(II) tetrasulfophthalocyanine, COII(TSPc)4-, in aqueous alkaline solution. The overall mechanism comprises the reduction of CoII(TSPc)4- by dithionite, followed by the formation of an intermediate complex between COI(TSPc)5- and nitrite, which undergoes two parallel subsequent reactions with and without nitrite as a reagent. Kinetic parameters for the different reaction steps of the catalytic process were determined. The final product of the reaction was found to be ammonia. Contrary to those found for the catalytic reduction of nitrite, the products of the catalytic reduction of nitrate were found to be dinitrogen and nitrous oxide. The possible catalytic reduction of nitrous oxide was confirmed by independent experiments. The striking differences in the reduction products of nitrite and nitrate are explained in terms of different structures of the intermediate complex between CoI(TSPc)5- and substrate, in which nitrite and nitrate are suggested to coordinate via nitrogen and oxygen, respectively.

The electrophilic reactions of pentacyanonitrosylferrate(II) with hydrazine and substituted derivatives. Catalytic reduction of nitrite and theoretical prediction of eta(1)-, eta(2)-N(2)O bound intermediates

The electrophilic reactivity of the pentacyanonitrosylferrate(II) ion, [Fe(CN)(5)NO](2)(-), toward hydrazine (Hz) and substituted hydrazines (MeHz, 1,1-Me(2)Hz, and 1,2-Me(2)Hz) has been studied by means of stoichiometric and kinetic experiments (pH 6-10). The reaction of Hz led to N(2)O and NH(3), with similar paths for MeHz and 1,1-Me(2)Hz, which form the corresponding amines. A parallel path has been found for MeHz, leading to N(2)O, N(2), and MeOH. The reaction of 1,2-Me(2)Hz follows a different route, characterized by azomethane formation (MeNNMe), full reduction of nitrosyl to NH(3), and intermediate detection of [Fe(CN)(5)NO](3)(-). In the above reactions, [Fe(CN)(5)H(2)O](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 degrees 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 eta(1)- and eta(2)-linkage isomers of N(2)O.