Ammonia, dinitrogen conversion

This section describes the properties of [M(L)(S )J complexes in which L represents various small molecules. The NO complexes, for example, are considered important intermediates in many reactions, ranging from waste water purifications to the bacterial nitrate-ammonia or nitrate-dinitrogen conversion or the mammalian bioregulation, in which NO acts as neurotransmitter (99-104). 

The conversion of molecular nitrogen to ammonia constitutes a potentially useful energy-storage reaction utilizing abundant raw material. The interaction of molecular dinitrogen with metal centers has been studied in considerable depth. It has been shown that N2 can be bound and reduced to NH3 at Mo, W, V, or Fe centers, particularly where these metals are in a low oxidation state and have a tertiary phosphine environment.312... 

Interest in these species stems largely from study of the mechanism of conversion of coordinated dinitrogen to hydrazine and ammonia. It seems abundantly clear that hydrazido(2—) species are involved in the conversion, in simple molecular systems at least, of N2 to NH3, and there is evidence to suggest their involvement enzymatically. 

Figure 18 Proposed conversion of dinitrogen to ammonia at a mononuclear site, based on structurally characterized complexes.

The synthesis of diazene complexes has been described (Sections V,B, C, and D). It is worth reiterating that no diazene complex has been prepared via reactions of coordinated dinitrogen and no stoichiometric conversion to hydrazine or ammonia has been demonstrated. 

The conversion of hydrazine (2 mol) to ammonia (2 mol), dihydrogen (1 mol), and dinitrogen (1 mol) by a heterogeneous catalyst in the presence of light has recently been discovered (263a ). The stoichiometry is clearly reminiscent of Eq. (84), but the reaction does not involve simple disproportionation of hydrazine and its relevance to nitrogenase is an open question. 

The conversion of dinitrogen to ammonia is one of the important processes of chemistry. Whereas the technical ammonia synthesis requires high temperature and pressure (1), this reaction proceeds at room temperature and ambient pressure in nature, mediated by the enzyme nitrogenase (2). There is evidence that N2 is bound and reduced at the iron-molybdenum cofactor (FeMoco), a unique Fe/Mo/S cluster present in the MoFe protein of nitrogenase. Although detailed structural information on nitrogenase has been available for some time (3), the mechanism of N2 reduction by this enzyme is still unclear at the molecular level. Nevertheless, it is possible to bind and reduce dinitrogen at simple mono- and binuclear transition-metal systems which allow to obtain mechanistic information on elemental steps involved... 

Hazari N. Homogeneous iron complexes for the conversion of dinitrogen into ammonia and hydrazine. Chem Soc Rev. 2010 39 4044-56. 

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