Nitrogen fixation Dinitrogen complexes

Keywords Dinitrogen complexes, Organo-nitrogen compounds, Nitrogen fixation, Molybdenum complexes, Tungsten complexes... 

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 ... 

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. 

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

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... 

The first site of protonation in a dinitrogen complex, based on mechanistic studies of complexes, must be the dinitrogen itself, yielding the diazenido species (N2H). If protonation occurs at the metal then reaction proceeds no further, or results in the loss of coordinated dinitro-gen. The formal insertion of dinitrogen into a metal-hydride bond (a popular proposal in the early chemical nitrogen-fixation literature) is unknown. 

In addition to the classical Haber process, recent efforts have been directed to finding an alternative economic process for fixing N2 to meet-increasing demands for nitrogeneous fertilizers and others industrial requirements. Thus, numerous results of the recent advances in the Chemistry of Nitrogen Fixation via transition metal complexes have reported (Chatt et al. 1978 A treatise on dinitrogen fixation , Hardy ed. 1979 New trends in the chemistry of nitrogen fixation , Chatt ed.1980). 

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