FeMo nitrogenase

As shown schematically in Figure 30, the FeMo-nitrogenase is composed of two proteins ... 

Despite the availability of the molecular structures of the different active sites of the FeMo-nitrogenases, the mechanism of nitrogen fixation remains obscure. The interest of our discussion, however, is centred on the various modes proposed to describe how molecular nitrogen might coordinate the FeMoco. Some of the reported schemes are inspired by the type of coordination found in model compounds. 

It is recalled that in Chapter 9, Section 2, the electrochemical behaviour of the FeMo cofactor from FeMo-nitrogenase, was reported. It possesses a heteronuclear iron-molybdenum-sulfur (MoFe7S9) cluster, which has similarities with the above discussed iron-sulfur proteins. 

In 1960, Burris and co-workers (153) observed that the obligatory H2 evolved by FeMo nitrogenase contained HD when D2 had been added to the gas phase (Eq. 59). 

Studying iron models for the FeMo cofactor of FeMo nitrogenases, Sellmann et al. isolated iron complexes with sulfur ligands [58, 59]. Generally, complexes such as 34-36 are only soluble in organic solvents like CH2C12, THF, DMSO, or DMF. 

Sulfur plays a central role in enzymatic catalysis and metal sulfido enzymes can activate various small molecules in biological conditions [20]. Much work has been devoted to the isolation and understanding of the intimate mechanisms of activation of biomimetic sulfur-containing complexes [21]. However, these systems do not present significant solubility in water under the studied conditions. We can nevertheless refer to the Sellman group s work in which the authors isolated sulfur-containing representative [FeMo] complexes of the cofactor of [FeMo] nitrogenases [22, 23], and were able to produce water-soluble complexes by introduction of... 

Since nature depends on FeMo-nitrogenase, complexes containing these metals are of particular interest in terms of investigating N2 to NH3 conversion. Complexes of type 27.6 have been a starting point for a number of studies involving intermediates such as 27.7 and 27.8. However, such interconversions produce only moderate 3rields of NH3 when 27.8 is protonated. 

X-ray Structure Analysis of FeMo Nitrogenase—Is the Problem of N2 Fixation Solved D. Sellmann, Angew. Chem., Int. Ed. Engl. (1993) 52, 64. 

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