Conversion of Dinitrogen to Ammonia

1 Nitrogen Fixation in Nature The nitrogenase enzyme is a two-component protein that consists of an electron-transfer Fe protein and a catalytic protein [85]. Three different nitrogenase enzymes are known, which differ primarily in the nature of the putative active site within the catalytic protein. The most common form is the MoFe protein, in which the active site for nitrogen reduction, the so-called FeMo cofactor (FeMoco), is composed of seven irons, one molybdenum, and nine sulfides 

Notable progress in ammonia synthesis has also been made by Chirik and coworkers, who have shown that ammonia can be evolved from a zirconium metallocene complex. Using a tetramethylated-bis-Cp-dichloride complex, they were able to first add N2 to form a dimeric Zr complex and subsequently add excess H2, with heating, to evolve ammonia [99]. Their work, though not catalytic, also provides basic mechanistic insights into transition-metal-mediated N-H bond catalysis. 

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

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... 

The conversion of dinitrogen to ammonia is shown in Equations 33 and 34 to emphasize two approaches. 

The key steps involved in the conversion of dinitrogen to ammonia at the conserved bis(di-tertiary tiiosphine) metal - site are those outlined in Scheme M. Protonation of bis -dinitrogen complexes with acids HX give stable hydrazides of the type trans -... 

The conversion of dinitrogen to ammonia by nitrogenase enzyme in bacteria (sometimes in symbiosis with plants) is called nitrogen fixation. This process is essential to life, because it is the starting point of the synthesis of amino acids that are the components of proteins. Three kinds of nitrogenase enzymes are known, each of them containing two air-sensitive metalloproteins that can be easily separated ... 

The nitrogenase enzymes are the sole mediators of biological nitrogen fixation, the microbial reduction of dinitrogen to ammonia. The enzymatic conversion entails the intervention of a set of biometalloclusters with distinctive, often unique, properties. The enzyme system is complex and particularly resistant to biochemical and biophysical analysis at the molecular level, and the clusters themselves have very limited synthetic precedent in form and reactivity. As a result, a comprehensive chemical understanding of the nitrogenase clusters remains elusive despite decades of study. 

Nitrogenase is a multicomponent metalloenzyme that catalyzes the conversion of atmospheric dinitrogen to ammonia. For decades, it has been generally believed that the [8Fe-7S] P-cluster of nitrogenase component 1 is indispensable for nitrogenase activity. However, Hu et al identified two cataly-tically active P-cluster variants by ESR spectroscopic studies, and found that both P-cluster variants resemble [4Fe-4S]-like centers based on XAS experiments. 

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. 

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