Nitrogenase model compounds

Leigh32 has discussed chemical systems for dinitrogen activation and reduction 

Using protons from water (and an energy input) for reaction with metal-coordinated dinitrogen (the metalloenzyme nitrogenase) 

Reacting dinitrogen with dihydrogen on a catalytic iron surface (the Haber-Bosch process) 

Splitting of dinitrogen to form nitrido (N1 2 3- 4 5) complexes on a three-coordinate molybdenum(III) complex (process described below)33 

Development of systems using vanadium(II) that fix nitrogen in an aqueous environment34 

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NIS measurements have been performed on the rubredoxin (FeSa) type mutant Rm 2-A from Pyrococcus abyssi [103], on Pyrococcus furiosus rubredoxin [104], on Fe2S2 - and Fe4S4 - proteins and model compounds [105, 106], and on the P-cluster and FeMo-cofactor of nitrogenase [105, 107]. 

On the other hand, such approaches to the metalloenzymes described above in Figures 1 and 2 are still under way. Thus, the model clusters reproducing precisely their complex metal-sulfur assemblies in the native form have not yet been isolated. In this section, the studies aiming at the syntheses of the model compounds of two clusters in nitrogenase, FeMo cofactor and P-cluster, will be surveyed. The choice of these clusters as the representatives of the metal-sulfur clusters in metalloenzymes arises from the fact that these are the largest and most complicated metal-sulfur clusters known at present among those observed in natural enzymes. 

Table 3.4 lists values for A Eq and for some important oxidation and spin states found in bioinorganic molecules. Data are taken from reference 24 and from Table 1 of reference 25 for hemoglobin, myoglobin, and the picket-fence porphyrin model compound, FeTpivPP(l-Melm).25 The myoglobin and hemoglobin model compounds are discussed in Section 4.8.2. Reference 26 provides the Table 3.4 data on iron sulfur clusters found in many bioinorganic species.26 The unusual iron-sulfur and iron-molybdenum-sulfur clusters found in the enzyme nitrogenase are discussed more fully below and in Chapter 6. 

For updating the information presented in this chapter, a literature search on the keyword nitrogenase modified with structure, X ray, Mossbauer, iron sulfur cluster, or model compound will generate citations referring to the newest research results. A search of the Protein Data Bank (PDB) at the website address http //www.rcsb.org/pdb/ will yield the latest updates on X ray, NMR, and other submitted structural data. 

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. 

Of particular interest are those complexes which serve as model compounds for biological systems such as the active center of the MoFe protein in nitrogenase.107 These complexes will be discussed in detail in the relevant chapters. It should be noted here, however, that WS2- is a non-innocent ligand, and thus complexes such as [Co(WS4)2]" (n = 2,3)108>109 and [Fe(WS4)2]" (n = 2, 3) can easily be prepared.110,111,112 In addition, some unusual complexes have been synthesized with WS2- as ligands. One such example is [Fe3W3S12]4 which contains the Fe3(/r3-S)2 center as depicted in Figure 11. 

Lu, J. Evidence for the string-bag structure as a basic structural unit for the active center of nitrogenase and an attempted synthesis of string-bag model compounds, in Ref. 23, p. 50... 

When reacting with nitrogenase, and its other substrates (nitriles, isocyanides, ethyne, etc.) get bound to Fe while model compounds reducing at ambient conditions may... 

Table 4.9 Selected data from XAS investigations of vanadium nitrogenases from Azofobacter. The number of scatterers (next neighbours to vanadium) n and the distances d are provided. For the model compound, data from single-crystal structure results (XRD) are added for comparison. Data from refs. 102 and 106a. See also text.

In the context of alternative substrates for nitrogenases, in particular alkynes, [Equation (4.27)], model compounds are of interest in which the alkyne is coordinated side-on to vanadium. The side-on or rr coordination implies a weakening of the triple bond by TT-back-donation from vanadium-d into the tt orbitals of the ligand, and hence an activation. The activated siloxyacetylene in the complex [ClVdmpe)2 i7 -(Me3SiOC= COSiMe3)] [dmpe = bis(dimethylphosphino)ethane] is reduced by hydrogen to the respective ethene, [Equation (4.39)]. The precursor acetylene complex is formed by... 

The model compounds do not bind or reduce N2 or other nitrogenase However, some iron-vanadium-sulfur clusters will reduce hydrazine to ammonia. In a systematic evaluation of the metal M in double cubane clusters [M2Fe6Sg(SEt)9] (M = V, Nb, Mo, W, Re), it was found that the reduction potential is lower, and the Fe Mossbauer isomer shift higher, for Mo relative to However, the Fe isomer shifts are similar between MoFe and VFe proteins, suggesting that protein interactions (especially hydrogen bonding) are likely to play an important role in determining the electron density at the iron atoms in the FeMoco and... 

Fig. 5. (A) Fourier transform of Mo K-edge EXAFS spectrum of the Mo-Fe protein from nitrogenase [Adapted from S. P. Cramer, in X-Ray Absorption Principles, Applications, Techniques of EXAFS, SEXAFS and XANES (D. C. Koningsberger and R. Prins, eds.), p. 257. Wiley, New York, 1988]. The Fourier transform shows two peaks indicating that at least two major components, Mo-S (and Mo-O/N) and Mo-Fe, are required to explain the Mo EXAFS spectrum from nitrogenase. (B) Model for the FeMo-co proposed based on the Mo EXAFS data and comparison with data from model compounds [T. E. Wolff, J. M. Berg, C. Warrick, K. O. Hodgson, R. H. Holm, and R. B. Frankel, J. Am. Chem. Soc. 100, 4630 (1978)]. (C) Structure of the FeMo-co based on a more recent X-ray crystal structure [J Kim and D. C. Rees, Science 257, 1677 (1992)]. The similarities between the structures in (B) and (C) are remarkable.

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