Mo NH3 and

Electrochemical reduction of Mo(NH3) is fully reversible in both TH F and PhF. Interestingly, at slow scan rates ( 10mVs ), waves corresponding to the MoN couple become discernible (near —1.8 V in THF). Therefore exchange of ammonia in Mo(NH3) vide infra) with dinitrogen takes place to an observable extent after the begirming of the Mo(NH3) reduction sweep (lOmVs ), a period of 30-40s. Since the potentials of CrCp 2 ° and Mo(NH3) in fluorobenzene are both -1.63 V, CrCp 2 is a viable reductant of Mo(NH3).  

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At temperatures above 45°C, however, (NRj MOs compounds decompose yielding gaseous ammonia, NH3, and oxygen as well as tantalum or niobium oxides ... 

Up to now, all of the BN hydrogenation data was in the absence of NH3 and Figure 57.24 shows the effects of adding NH3 to the reaction mixture for the Ni and Mo promoted Ni catalysts either with or without FT. The amounts of FT were 1.33 mmol H2CO per g of Ni and 2.0 mmol H2CO per g of Mo promoted Ni, because the BA yields for these catalysts were the highest at these FT levels in the absence of NH3. The amount of NH3 added to these reactions was 10 ml of a 32 wt.% aqueous NH3 solution and this is relatively much lower than what is normally added to... 

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... 

Model systems composed of Mo complexes and Fe-S clusters can give facile N2 reduction to NH3 under mild conditions. The Fe-S cluster is considered to act only as an electron transfer catalyst from reducing agents, such as BH4 and S2042, to the Mo complexes at which the reduction occurs [147-149]. Similarly the reduction of N2H4 to NH3 can be achieved by a catalyst of sodium molybdate, L-cysteine, and NaBH4 with a turnover number of 4.2 NH3 mol/(Mo-cysteine complex moljhr (148). 

Twenty-six moles of NH3 and 19 moles of H2 are produced per mole of cluster in 4 hr. The catalytic activity of [Mo-Fe]m is superior to that of [4-Fe]n with respect to NH3 formation, and in both cases, higher pH favors the fonnation of NH3. The maximum current efficiency using [Mo-Fe]m catalyst in MeOH/thf is 97%. 

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