Dinitrogen to ammonia

Multinuclear M centre Nitrogenases (Mo, V, Fe) Mo-nitrogenase Dinitrogen to ammonia... 

The catalytic reduction of dinitrogen to ammonia in biological systems is thermodynamically favorable overall and takes place according to equation 6.2 ... 

A series of model studies initiated in the early 1960s by the groups of Chatt and Hidai demonstrated that dinitrogen could be bound and reduced to ammonia at a single metal centre by Mo and W complexes (Chatt et al., 1978 Hidai, 1999). However, although examples of virtually all the proposed intermediates in a Chatt cycle were isolated, no catalytic reduction of N2 to NH3 was ever achieved. Catalytic reduction of dinitrogen to ammonia... 

Structural basis of biological nitrogen fixation, Phil. Trans. R. Soc., 363, 971-984. Schrock, R.R. (2005) Catalytic reduction of dinitrogen to ammonia at a single molybdenum center,... 

Photoenhanced adsorption of SO2 occurs on goethite (Inoue et al., 1988) and on hematite (Toledano et al., 1998). Ultrafine particles of goethite promote photoreduction of adsorbed dinitrogen to ammonia (Tennakone et al., 1991). 

In the earlier examples, the binuclear complexes with a M02S3 core clearly cannot be seen as modeling the metal sites in FeMo—co since it is known to contain a single molybdenum center [74]. However, what is suggested by these studies is that several steps (if not all) of the reduction of dinitrogen to ammonia by Mo-nitrogenase could take place at a... 

A spectrophotometric method was used to study144 the rate equation (3). The first term corresponds to the reduction of dinitrogen to ammonia and the second to the reduction of the protons of solvent to H2. 

Nitrogenases (EC 1.18.6.1) are the enzymes responsible for the biological fixation of atmospheric dinitrogen to ammonia. These enzymes are found only in bacteria, although some of the bacteria form symbioses with plants. 

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

Scheme 1. Pathways for the stepwise reduction of dinitrogen to ammonia and hydrazine.

G. N. Schrauzer has developed a series of molybdenum-based systems, which, in the presence of a reductant (generally Na[BH4]) and a buffer (normally borate), convert dinitrogen to ammonia and acetylene to ethylene (and sometimes further to ethane). The general mechanism postulated some years ago is that shown in Scheme 18. 

Scheme 19. Proposed mechanism for the reduction of dinitrogen to ammonia (via diazene) by V(OH)2-...

Vanadium(II) homogeneous systems reduce dinitrogen to ammonia very cleanly and efficiently, and there is no dispute about them. The original discovery was that vanadium(II) plus polyphenols in homogeneous alkaline solution in a very restricted pH range reduces dinitrogen to ammonia according to Eq. (79) (198). 

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... 

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