Reductive Dinitrogen Binding

The historical move to early transition metals in rather high oxidation states and the use of a- and 7r-donor ligands, contrary to what was observed in the late and low-oxidation-state transition metals, has been the preferred approach to achieve a real electron-transfer from the metal to the N2 ligand. In this context, two major strategies have been particularly successful, namely the use of the ancillary ligands mentioned above and the macrocyclic effect. 

---

Hypotheses for dinitrogen binding and reduction at nitrogenase s M center include ... 

It seems likely that the active site for dinitrogen binding involves the molybdenum atom. It has been established by EXAFS that the coordination sphere consists of several sulfur atoms at distances of about 235 pm. An Mo=0 double bond, so common in complexes of Mo(IV) and Mo(Vl), is not present. There are other heavy atoms, perhaps iron, nearby (—270 pm). The ultimate source of reductive capacity is pyruvate, and the electrons are transferred via ferredoxin (see page 911) to nitro-... 

Figure 17 Dihydrogen production and dinitrogen binding in the reduction of [MoH2(T 2-02CMe)(dppe)2]+.

Elegant chemistry by Yoshida s group has shown that dinitrogen binds to molybdenum in a macrocyclic tetrathioether complex and that some of the reactions of the coordinated dinitrogen parallel those of the Mo bis(diphosphine) systems [38]. Comparison of the reduction potentials for... 

Synthetic structural models for clusters within the nitrogenase proteins or of the molybdenum or vanadium iron - sulphur cofactors isolated from them, are at an eaily stage of development and there are as yet no well - defined systems which model electron - transfer chemistry involving the binding and reduction of molecular nitrogen or other substrates dinitrogen binding to synthetic iron - sulphur clusters or heterometallic iron - sulphur clusters of any type has not been established structurally nor spectroscopically, [44- 46]. 

Dinitrogen-Reducing Systems. The binding of N2 to a metal center is the first step in activating molecular nitrogen toward reduction. 

---