Ammonia, coordinated dinitrogen

Molybdenum(0) also forms a variety of dinitrogen complexes (41), especially when there are phosphine ligands in the molybdenum coordination sphere (see Fig. 7c). This type of complex has been extensively studied because the coordinated dinitrogen is reduced to ammonia upon acidification. 

There are two types of interesting and important reactions involving the coordinated dinitrogen, its reduction to hydrazine and ammonia, and the reactions leading to carbon-nitrogen bond formation. 

Interest in these species stems largely from study of the mechanism of conversion of coordinated dinitrogen to hydrazine and ammonia. It seems abundantly clear that hydrazido(2—) species are involved in the conversion, in simple molecular systems at least, of N2 to NH3, and there is evidence to suggest their involvement enzymatically. 

The simplest member of this class is the NNH2 ligand, which, as mentioned above, is a crucial and persistent intermediate in the protonation of coordinated dinitrogen. Indeed it remains the only coordinated protonated nitrogen ligand identifiable between dinitrogen and ammonia.455,456 Spectroscopic studies have provided a detailed mechanistic picture of the protonation of [Mo(N2)2(depe)2] (M = Mo, W depe = Et2PCH2CH2PEt2).472... 

The synthesis of diazene complexes has been described (Sections V,B, C, and D). It is worth reiterating that no diazene complex has been prepared via reactions of coordinated dinitrogen and no stoichiometric conversion to hydrazine or ammonia has been demonstrated. 

Zero-valent iron complexes can activate small molecules. For example, reduction of FeCl2(diphos)2 under nitrogen atmosphere resulted in formation of Fe(N2)(diphos)2 [diphos = dmpe or depe] (Scheme 10.13). Interestingly protonolysis of Fe(N2)(dmpe)2 was reported to give ammonia under ambient conditions [75]. In addition, the coordinated dinitrogen in Fe(N2)(depe)2 [76-77] can be readily replaced with CO or CO2. 

Thus we had reactivity of coordinated dinitrogen to make both N-H and N-C bonds, opening up the prospect of routes to ammonia, hydrazine and organo-nitrogen componds using dinitrogen complexes as catalysts or catalyst precursors. For convenience, I will outline the advances made in the two areas separate-... 

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... 

Recently, the first mechanistic study of the conversion of a coordinated dinitrogen to a hydrazido complex was reported/ Treatment of cis-[M(N2)2(PMe2Ph)4] with excess HCl, HBr, or H2SO4 in methanol yields [M(NNH2)(OMe)2(PMe2Ph)3] in a reaction that would ultimately give ammonia. The stoichiometry of the reaction is shown in Equation (27) ... 

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