Nitrogen fixation intermediates

Organohydrazine derivatives play an important role in the coordination chemistry of rhenium since they represent important intermediates in the preparation of nitrido, imido, or dinitrogen complexes as has been described in the previous sections. They are defined as complexes which possess metal-nitrogen-nitrogen bonds, which would (strictly applied) also include dinitrogen complexes, and indeed this class has attracted considerable attention in the exploration of biological nitrogen fixation. 

The electrochemistry of dinitrogen bridging two porphyrin ligated ruthenium centers has been studied as a possible route to fixed nitrogen [45 -47]. Diazene stabilized by bonding to two iron centers in a FeS system has been advanced as a structural model of a plausible intermediate in biological nitrogen fixation [48-50]. 

Production of hydrazine and its use as a chemical intermediate, reducing agent, rocket fuel and boiler-water treatment agent may result in its release to the environment through various waste streams. Hydrazine is also naturally produced by Azotobacter agile during nitrogen fixation. It has been detected at low levels in wastewater samples (United States National Library of Medicine, 1997). 

Ammonia is oxidized in nature to nitrate via several intermediates in the process of nitrification. Nitrate may be reduced to nitrite by either a dissimilatory or an assimilatory process. Nitrite may be assimilated into the cell via reduction to ammonia, or it may be reduced by microorganisms to N20 and N2 in denitrification. A major part of the total nitrogen in this pathway is lost to the atmosphere. However, in turn, atmospheric dinitrogen is converted to ammonia by various bacteria in nitrogen fixation. 

Figure 5.12 shows temperature dependencies of final and intermediate product accumulation or consumption for N2 hydroperoxide oxidation. Fixed nitrogen concentration in the reaction mixture reaches the maximum at 873 K and then decreases with a temperature increase to 923 K. Flence, hydrogen peroxide concentration decreases abruptly, which is associated with its consumption in the target reaction of nitrogen fixation HO and OH radical concentrations sharply increase with temperature. 

Hsu, H.F., W.C. Chu, C.H. Hung, and J.H. Liao. 2003. The first example of a seven-coordinate vanadium(III) thiolate complex containing the hydrazine molecule, an intermediate of nitrogen fixation. Inorg. Chem. 42 7369-7371. 

Fig. 3. Two diazene model compounds, 1(N2H2) and 2(N2H2), which are expected to emulate a key intermediate in the biological nitrogen fixation problem.

Dinitrogen is a weaker donor and acceptor than CO. However, N2 complexes are of great interest, especially as possible intermediates in reactions that may simulate natural processes of nitrogen fixation. 

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