Dinitrogen reduction

For many decades chemists faced the problem of nitrogen fixation under mild conditions. The energy of electron detachment from the binding orbitals of dinitrogen or molecule excitation to the excited states is very high (369 and 143 kcal/mole, 

In 1970 Likhtenshtein and Shilov advanced the supposition that the enzyme nitrogenase by-passed the above mentioned energy difficulties by realizing a reaction mechanism that provides the rupture of two bonds in N2 with simultaneous compensation due to the formation of four new bonds with catalytic transition atoms. This supposition was based on the following thermodynamic grounds and kinetics considerations. 

In order to estimate the thermodynamics of a reaction that occurs in the coordination sphere of a metal (M), it is expedient to conditionally divide the reaction 

According to the principle of dynamic adaptation (Likhtenshtein, 1976a), the multi-orbital interaction between a substrate and metal atoms in a bi- or polynuclear center and the consequent chemical conversion require a certain optimum flexibility of metal atoms involved in the catalytic process. Such flexibility would allow the space provision for each step of the consecutive chemical reaction, i.e. complexation, product formation and release. 

Studies on model polynuclear catalytic systems have confirmed that for the catalytic reduction of dinitrogen under mild conditions, it is necessary to use a polynuclear transition metal complex capable of donating four electrons to form the hydrazine derivative. 

In recent work, Pickett and Talarmin have shown that protolysis of tram-[W(N2)2(dppe)2] can be made to proceed beyond the hydrazido stage by coupling protonation with electronation using controlled-potential electrolysis at a Hg-pool cathode  

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Dilworth MJ, RR Eady (1991) Hydrazine is a product of dinitrogen reduction by the vanadium-nitrogenase from Azotobacter chroococcum. Biochem J 277 465 68. 

The alternative mechanistic scenario for the protonation and reduction of end-on terminally coordinated N2 through the Schrock cycle is represented by the Chatt cycle which has been developed many years earlier (5). This system is based on Mo(0) and W(0) dinitrogen complexes with phosphine coligands (Fig. 3). As expected, the intermediates of the dinitrogen reduction scheme are very similar to those of the Schrock cycle. Moreover, a cyclic generation of NH3 from N2 has been demonstrated on the basis of this system, however, with very small yields (3,4a). In order to obtain general insight into the mechanism of the Chatt cycle we have studied most of the intermediates of Fig. 3 with... 

The first step in dinitrogen reduction is highly endothermic because the dinitrogen triple bond is stable both kinetically and thermodynamically. For example, if the... 

Lehnert and Tuczek further studied end-on terminal coordination by density functional theory (DFT) calculations on the compounds [Mo(N2)2(dppe)2], [MoF(NNH)(dppe)2], and [MoF(NNH2)(dppe)2]+, where dppe= 1,2-bis(diphenyl-phosphino)ethane.50 They proposed a reaction scheme, shown in reaction 6.13, for asymmetric dinitrogen reduction and protonation. The end-on model favored by Lehnert in reference 50, as shown in reaction 6.13, appears to be a less thermodynamically unfavorable pathway, at least to reach the M-NNH3 intermediate. Step 1 produces a metal-attached diazenido ion (NNH-), step 2 produces a hydrazido ion (NNH2 ), and step 3 produces a hydrazidium ion (NNHj). 

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. 

The most efficient system of this type is obtained by the reduction of bovine serum albumin in the presence of molybdate. Apparently disulfide links in the peptide are broken and form thiolate groups which then bind molybdenum. In a borate buffer, this system will reduce dinitrogen and acetylene, although not using dithionite as an electron source. The turnover is similar to that of the iron-molybdenum cofactor (see Section XII), and dinitrogen reduction is inhibited by carbon monoxide and stimulated by ATP. The yield of ammonia is linearly dependent upon PN2, and the yield is also depressed in the presence of fumarate and, more surprisingly, succinate. It is calculated that the... 

The V(OH)2/Mg(OH)2 gel is believed to be a solid solution with a lattice similar to Cdl2, and the yield of hydrazine reaches a maximum with the magnesium to vanadium ratio in the range 1 5-10 (133). Shilov finds (1) no 14N isotope effect corresponding to the Schrauzer mechanism, (2) that there is no evidence for vanadium IV), (3) that the reduction of allyl alcohol is independent of, and competitive with, dinitrogen reduction, and (4) a different dependence on P 2 from the square dependence claimed. In short, no evidence for diazene, but much more for a direct reduction to hydrazine (136). 

In the absence of substrate (other than the proton) the final product of electron transfer is dihydrogen. In the presence of dinitrogen, electrons are diverted into the substrate, but dihydrogen is always a product. The limiting stoichiometry for dinitrogen reduction appears to be as shown in Eq. (84). 

Fig. 8. Energetics of dinitrogen reduction with the mononuclear catalyst 2 (solid lines). The energy D0 of N2 bound to 2 and three unbound H2 molecules has been chosen as the zero-energy reference point AD0 = AScp, B3LYP + AZPE. Note that the non-reacting H2 molecules (e.g., two H2 molecules at the N2 + H2 step) are not mentioned explicitly. The energetics of dinitrogen reduction without the catalyst are depicted for comparison (dotted lines). Here, the D0 energy of N2 and three H2 has been chosen as the energy reference point.

Only dinitrogen reduction is inhibited by dihydrogen or dideuterium. 

As shown in Figure 3, the first step in dinitrogen reduction could involve binding of dinitrogen to the enzyme, with the mode of binding... 

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