Protonation of the Coordinated Dinitrogen

Protonation on a unidentate N2 results in the diazenido ligand N2H (24), which may be rearranged io 24a ox 24b io generate a basic site at either N or N, respectively. Upon the second protonation, a diazene HNNH (25) or a hydrazido(2-)NNH2 (26) ligand is produced. 

For example, the reactions of tra j-M(N2)2(dppe)2 (M = Mo, W) with an excess amount of hydrogen halide in THF at room temperature followed by treatment with a salt of noncoordinating anion result in the hydrazido(2-) complexes  

The X-ray structure of tra i-[WCl(NNH2)(dppe)2] [BPh4] indicates that the W—linkage is essentially linear with the N—distance (1.37(2) A) corresponding to a bond order of 1.5 and the W—distance (1.73(1) A) near the triple bond length.  

The diazenido complexes, which should be produced by the reaction of the N2 complex with 1 mol equivalent of HX, can not be isolated from the reaction mixtures. However, they are prepared cleanly by deprotonation of the hydrazido(2—) complexes in a medium in which the products are insoluble  

Treatment of the diazenido complexes with an equimolar amount of anhydrous acids regenerates the hydrazido(2-) compounds.  

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We also employed other ruthenium dihydrogen complexes for the protonation of the coordinated dinitrogen in complex (2b). Typical results are shown in Table 3 [21]. Ammonia is formed in higher yields (up to 79% based on tungsten) when dinitrogen complex (2b) is similarly treated with tra s-[RuCl(r -H2)(dppe)2]X... 

Yamamoto A, Miura Y, Ito T, et al. Preparation, x-ray molecular structure determination, and chemical properties of dinitrogen-coordinated cobalt complexes containing triphenylphosphine ligands and alkali metal or magnesium. Protonation of the coordinated dinitrogen to ammonia and hydrazine. Organometallics. 1983 2 1429-1436. 

Thus the 2 mol equivalents of protons which are necessary to dipro-tonate the coordinated dinitrogen are subsequently regenerated due to the relatively high acidity of coordinated methanol. Protons are consumed only in the neutralization of liberated phosphine. 

Structure 2.70 in Figure 2.6 shows a unique Mo complex that in the presence of AH and Bj catalyzes reaction 2.3.6.1 with low turnovers. The ligand is a very bulky trianionic tetradentate Ugand, and the oxidation state of molybdenum is 3+. The addition of six protons and electrons to the coordinated dinitrogen take place in a stepwise manner. This produces molybdenum complexes with N=NH, N-NH, N, and NH ligands. 

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... 

Also, it is worth mentioning (a) the process of N=N triple bond cleavage by Mo(III) complex [6], and (b) the reactivity of coordinated dinitrogen with electrophiles, including the protons and organic free radicals [7], and coordinated [8] and free [9] dihydrogen. 

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