FeMoco models

The possibility of N2 coordination to up to four (six) iron atoms has been proposed by Dance on the basis of restricted frozen-core Kohn-Sham calculations on a FeMoco model (32,33). It was found that a binding mode intermediate between p Vn and p4,r 2 coordination to be most stable. However, these propositions are not necessarily the final answer since open-shell states are most likely to become important and exact exchange was not present in the density functional chosen to cure the singlet preference of the pure density functional (cf. discussion in the Appendix). 

Exhaustive periodic DFT studies have been undertaken by Rod, Norskov, and collaborators (31,37,38), who investigated FeMoco models from the point of view of heterogeneous catalysis. Two of their most important results are that FeMoco does not dissociate N2 (N-N bond breaking occurs in the final reduction step) and the reduction takes place at the Fe atoms. It is interesting to note that end-on binding to a single Fe center was found rather than bonding to a Fe4 face of the cluster. 

Sellmann, D. Sutter, J. Biological N2 fixation Molecular mechanism of the nitrogen-ase catalyzed N2 dependent HD-formation, the N2 fixation inhibition and the open-side FeMoco model, Perspectives in Coordination Chemistry , Vol. 5 Eds. Trzeciak, A. M. Sobota, D. Ziolkowski, J. University of Wroclaw Poland, 2000. 

In 2004 and 2005 the photochemical activation of dinitrogen with transition metal model complexes of the Sellmann type nitrogenase was studied using CPMD [193, 194], A dinuclear complex — designed to emulate the open-side FeMoco model -was simulated. Several side reactions were observed which have to be suppressed in order to arrive at the reduced species [194]. Chelate effects and their partial dissociation as well as low temperatures led to successful events. An optimized design of the complexes to inhibit side reactions was suggested [194]. 

Fig. 3. The tetrameric structure of the MoFe protein (Kpl) from Klebsiella pneumoniae (7). The two FeMoco clusters and the P clusters are depicted by space-filling models and the polypeptides by ribbons diagrams (MOLSCRIPT (196)). The FeMoco clusters are bound only to the a subunits, whereas the P clusters span the interface of the a and j8 subunits.

It is probable that the negative charge induced by these three electrons on FeMoco is compensated by protonation to form metal hydrides. In model hydride complexes two hydride ions can readily form an 17-bonded H2 molecule that becomes labilized on addition of the third proton and can then dissociate, leaving a site at which N2 can bind (104). This biomimetic chemistry satisfyingly rationalizes the observed obligatory evolution of one H2 molecule for every N2 molecule reduced by the enzyme, and also the observation that H2 is a competitive inhibitor of N2 reduction by the enzyme. The bound N2 molecule could then be further reduced by a further series of electron and proton additions as shown in Fig. 9. The chemistry of such transformations has been extensively studied with model complexes (15, 105). 

Many researchers have considered models for possible intermediates in the nitrogenase reaction. Two possible dinitrogen attachments to the FeMoco factor of MoFe-protein have been put forward. Symmetric, edge- or side-on modes discussed by Dance48 would lead to a reaction sequence such as is shown in reaction 6.11. In contrast, the asymmetric end-on terminal mode discussed in the work of Nicolai Lehnert50 may be favored thermodynamically and by molecular orbital calculations. Reaction sequence 6.13 below illustrates one scenario for the asymmetric model. 

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. 

A large number of synthetic metal-sulfur cluster analogs have been prepared and some proposed as suitable models for FeMoco (see Section II). Some of the analogs exhibit spectroscopic properties very similar to those of the cofactors, but none shows nitrogen fixing ability. 

Figure 25 Coordination modes for N2 bound to FeMoco, as predicted by various theoretical models.

The FeMoco can be extracted intact (20) from the acid-denatured MoFe protein, which has the important consequence that modeling of its function may start with studies on the structure of the isolated cluster. The protein environment (and also solvent molecules) may be neglected in the first modeling step though they are essential for the reduction mechanism. This simplification appears to be mandatory since the electronic structure of the isolated cluster is extremely complicated. 

Pioneering studies with semi-empirical methods were carried out shortly after the discovery of the FeMoco structure via X-ray diffraction experiments (21,22). These extended-Hiickel calculations have been carried out for structurally fixed models of FeMoco (23,24) and of FeMoco, FeVco, and FeFeco (25), in which the three amino acid ligands have been replaced by hydride ligands. The main results of these studies are deduced based on the assumption that the character of the frontier orbitals, i.e., their composition in terms of atomic orbitals,... 

In view of the many approximations in these semi-empirical calculations their results can only be regarded as first hints for more elaborate methods. Such are DFT calculations on FeMoco-type clusters and model complexes. As it is discussed, for instance, in Ref. (31), N2 usually binds very weakly to transition metal complexes. It is thus not certain whether end-on coordination to one or two iron centers can lead to sufficiently strong coordination of dinitrogen. 

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