Dinitrogen nitrogen fixation

A remarkable process was reported by Mori that forms aniline from dinitrogen (Equation (26)).106 Titanium nitrogen fixation complexes were generated from reactions of titanium tetrachloride or tetraisopropoxide, lithium metal, TMS chloride, and dinitrogen. These complexes generated a mixture of aryl and diarylamines in yields as high as 80% when treated with aryl halide and a palladium catalyst containing DPPF ... 

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,... 

Interest in metal complexes able to bind molecular dinitrogen is mainly due to a desire to understand and reproduce in the laboratory the so-called nitrogen fixation reaction. This process, which in nature is catalysed by the metalloprotein nitrogenases, reduces the inert dinitrogen molecule to the metabolically useful NH3 ... 

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]. 

Fig. 3. Model complexes for nitrogen fixation (top) postulated mechanism for the reduction of dinitrogen with Sellmann-type complexes (bottom).

Soil-borne bacteria of the family Rhizobiaceae and leguminous plants form a symbiotic relationship during which a new organ, the root nodule, is developed. Within these root nodules the bacteria fix atmospheric dinitrogen and the product of nitrogen fixation, ammonia, is exported to the plant [69,70]. Root nodules develop from primordia which are established at specific sites in the root cortex shortly after Rhizobium infection. The peptide enod40 is believed to play a critical role in inducing the de-differentiation and the mitotic division of root cortical cells, i.e. the initial steps in nodule development. This however, is not entirely undisputed [3,4,69-72]. 

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. 

In order to provide a basis for the discussion, we shall use Scheme 1. This is based upon the chemistry of dinitrogen complexes as developed in the Unit of Nitrogen Fixation, but it is not intended to exclude consideration of schemes originating elsewhere. Indeed we hope to evaluate all the schemes in the literature. Our basic assumptions are... 

A low oxidation state of the metal would also tend to produce a coordination site at which ammonia would bind weakly and be displaced by the more strongly binding dinitrogen, thus facilitating the next stage of a nitrogen-fixation cycle. This substitution has indeed been observed on the [Mo(N2)(dppe)2] site (95). 

At least four vanadium(II) species have been identified in methano-lic aqueous solutions of VC12 and catechol (231). One is identified as Na2[V(C6H402)2] 2H20, and three contain three vanadium(II) ions on the basis of EPR evidence. One of the compounds is cyclic, and another, which binds dinitrogen, is open chain. This species reaches a maximum concentration when nitrogen fixation is fastest. These results modify the earlier picture to the sequence of reactions in Eq. (81). 

---