Reduction of dinitrogen

The systems involving Nb(III), Ta(III), Ti(II) and Cr(II) also reduced N2 in protic media. Mo(III) complexes catalyzed reduction of N2 by Ti(OH)3 and sodium amalgam (Volpin and Shilov, 1995). 

A study was conducted to elucidate the mechanism of FeMo-cofactor catalytic activity in non-enzymic conditions and to compare its catalytic behavior with that of the nitrogenase (Bazhenova et al., 2000). The kinetics were investigated of C2H2 redaction by multielectron donors, Zn and Eu amalgams, catalyzed by isolated FeMo-cofactor and 

a number of systems of the catalytic and noncatalytic reduction of dinitrogen to hydrazin and ammonia and the successful synthesis of model iron- and iron-molibdenum (vanadium) clusters have been reported. These investigations have formed a basis for subsequent progress in mimicking the nitrogenase reaction. 

---

Reduction of dinitrogen oxide to ammonia (which gives the ammonium ion with the acid) ... 

ELECTROCATALYTIC REDUCTION OF DINITROGEN AND NITROGEN OXIDES 9.10.6.1 Electrocatalytic Activation of Dinitrogen... 

The catalytic reduction of dinitrogen to ammonia in biological systems is thermodynamically favorable overall and takes place according to equation 6.2 ... 

In summary, reduction of dinitrogen by nitrogenase requires cooperativity among nitrogenase s subunits and involves three basic types of electron transfer steps ... 

A series of model studies initiated in the early 1960s by the groups of Chatt and Hidai demonstrated that dinitrogen could be bound and reduced to ammonia at a single metal centre by Mo and W complexes (Chatt et al., 1978 Hidai, 1999). However, although examples of virtually all the proposed intermediates in a Chatt cycle were isolated, no catalytic reduction of N2 to NH3 was ever achieved. Catalytic reduction of dinitrogen to ammonia... 

Structural basis of biological nitrogen fixation, Phil. Trans. R. Soc., 363, 971-984. Schrock, R.R. (2005) Catalytic reduction of dinitrogen to ammonia at a single molybdenum center,... 

In the earlier examples, the binuclear complexes with a M02S3 core clearly cannot be seen as modeling the metal sites in FeMo—co since it is known to contain a single molybdenum center [74]. However, what is suggested by these studies is that several steps (if not all) of the reduction of dinitrogen to ammonia by Mo-nitrogenase could take place at a... 

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

Scheme 5 Mechanism suggested for the reduction of dinitrogen by the suspension of freshly prepared V" and Mg11...

A spectrophotometric method was used to study144 the rate equation (3). The first term corresponds to the reduction of dinitrogen to ammonia and the second to the reduction of the protons of solvent to H2. 

The stoichiometry (equation 2) requires eight electrons transferred from vanadium(II) ions to dinitrogen atoms (six electrons) and hydrogen atoms (two electrons) and this kinetic equation (3) was interpreted as evidence for a polynuclear structure of the transition state during the reduction of dinitrogen. An alternative mechanism was suggested.145... 

In summary, a number of features of the nitrogenase-catalyzed reduction of dinitrogen may be noted. 

Figure 1. The Nitrogenase Reaction. The electron transfer proteins ferredoxin (Fd) and flavodoxin (Fid) serve to couple the nitrogenase reaction to metabolically generated reducing equivalents. Ammonia synthesis requires 8 electrons 6 for the reduction of dinitrogen and 2 for the coupled, obligatory synthesis of H2. These reactions are catalyzed by the terminal component in the complex, the MoFe-protein. The electrons are transferred to the MoFe-protein from the Fe-protein in a process coupled to the hydrolysis of 2ATP/electron (Howard and Rees, 1994,1996).

Scheme 1. Pathways for the stepwise reduction of dinitrogen to ammonia and hydrazine.

Schrauzer has shown that compounds derived from molyb-denum(V)-cysteine derivatives also catalyze the reduction of dinitrogen. Subsidiary experiments confirm that the active species contain molybdenumdV) rather than molybdenum Ill), the latter tending to lead to the production of dihydrogen (279). Molybdenum isonitrile derivatives can also lead to dinitrogen-reduction catalysis (252). These systems are very difficult to analyze, and they often work best at low molybdenum concentrations (<10 3 M). The intermediates cannot be isolated or detected directly. Their nature must be inferred on the basis of circumstantial evidence, and this is sometimes difficult to interpret. 

Scheme 18. Proposed mechanism for catalytic reduction of dinitrogen by Mo(red). ...

Scheme 19. Proposed mechanism for the reduction of dinitrogen to ammonia (via diazene) by V(OH)2-...

Dihydrogen is evolved prior to reduction of dinitrogen, as exemplified by transition metal complexes [Eqs. (86) and (87)]. 

In this section we attempt to summarize objectively what can clearly be stated concerning the reduction of dinitrogen by metal-based systems. As has been stated earlier, it is sometimes necessary to extrapolate information from analogous complexes containing alkyl or aryl substituents, because of the elusive nature of some of the intermediates. 

---