N2-reducing systems

A photochemical N2-reducing system, based on Ti02 in the rutile form containing approximately 0.4% water and impregnated with 0.2% Fe203, is known (99). This material (0.2 g) gives up to 6 jumol of NH3, plus some N2H4, at 40°C under 1 atm N2 in 3 hr on irradiation with a 360-watt mercury arc lamp. 

The following is a brief survey on these chemical N2-reducing systems giving ammonia (and sometimes hydrazine). At first reactions of N2 promoted by the mixtures of transition-metal species with reducing agents and proton sources are illustrated, and then transformations of the N2 ligand in well-defined complexes to form ammonia are summarized. 

Dinitrogen has a dissociation energy of 941 kj/mol (225 kcal/mol) and an ionisation potential of 15.6 eV. Both values indicate that it is difficult to either cleave or oxidize N2. For reduction, electrons must be added to the lowest unoccupied molecular orbital of N2 at —7 eV. This occurs only in the presence of highly electropositive metals such as lithium. However, lithium also reacts with water. Thus, such highly energetic interactions ate unlikely to occur in the aqueous environment of the natural enzymic system. Even so, highly reducing systems have achieved some success in N2 reduction even in aqueous solvents. 

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

A related homogeaous aqueous—alcohoHc system, composed of V(II) complexes of catechol and its derivatives, reduces N2 to ammonia and H2. Only catecholates are active ia this system, which is seasitive to pH. This system has beea likened to nitrogeaase by suggestiag that both use a sequeace of two four-electroa reductioas to evolve oae H2 for every N2 reduced (201). 

In such matters some progress can be achieved by combinations of the decomposition method and the method of separation of variables. For example, this can be done using the method of separation of variables for the reduced system (6) upon eliminating the unknown vectors with odd subscripts j. This trick allows one to solve problem (2) here the expenditures of time are Q 2nin2 og N2 arithmetic operation, half as much than required before in the method of separation of variables. 

In their 1960 paper, Carnahan et al. reported that ATP was inhibitory to nitrogenase activity in their cell-free preparations. Hence, when McNary and Burris [24] reported that ATP was needed to support fixation, the report was met with a good deal of skepticism. But experiments in a number of other laboratories verified the absolute need for ATP. Not only is ATP needed, it is needed in substantial amounts. Under ideal conditions 16 ATP are required per N2 reduced to 2 NH3. Under normal conditions in nature the requirement probably is in the 20 to 30 ATP per N2 range. N2 reduction is energy demanding whether it is accomplished chemically in the Haber process or enzymatically by the nitrogenase system. 

It has been found useful to represent the interaction potential for a dimer of homonuclear diatomic molecules [4,5,46,58] as a spherical harmonic expansion, separating radial and angular dependencies. The radial coefficients include different types of contributions to the interaction potential (electrostatic, dispersion, repulsion due to overlap, induction, spin-spin coupling). For the three dimers of atmospheric relevance, we provided compact expansions, where the angular dependence is represented by spherical harmonics and truncating the series to a small number of physically motivated terms. The number of terms in the series are six for the N2-O2 systems, corresponding to the number of configurations of the dimer (for N2-N2 and O2-O2 this number of terms is reduced to five and four, respectively). 

The transition metals of groups IV, V and VI, particularly Ti, V, Cr, Mo, and W, have the strongest N2-reducing capacity. Ti compounds are particularly active. In the first row of transition metals, the ammonia yields decrease generally from left to right, in line with the decreasing stability of the metal nitrides. Co and Ni compounds are usually of low or no activity. Palladium, copper and platinum complexes have no activity in any system tested. 

In contrast, two of the best known aqueous systems that reduce N2 are based on vanadium (55, 56) but are difficult to characterize and mechanistic conclusions are often controversial. Of these, the V(II) catechol system, which functions at alkaline pH, provides a good analog for the reactions catalyzed by nitrogenase. In the absence of N2 this system reduces protons to H2. As with nitrogenase, this reaction, which is inhibited by N2 and the limiting stoichiometry that occurs at room temperature and pressure, is... 

Only on rare occasions is it possible to synthesize and purify a whole series of N2 complexes with different ligands the Mo, W, and Re systems shown above are perhaps the most versatile in this respect. N2 can often displace ti -H2, as shown in Eq. 16.25 if this were the last step in the catalytic cycle, it would explain why N2 always produces at least one mole of H2 per mole of N2 reduced. 

The systems included are only a few examples of the many oxidation-reduction potentials recorded. The range of values encompasses almost all biological systems, but many more powerful oxidizing and reducing systems are known, with potentials extending from -1-3.09 volts (J N2 - - H+ - - e —> HNa) to —3.06 volts (Fj - - 2H+ - -2e- 2HF). 

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