Dinitrogen complex formation

The dinitrogen complex formation is considered to take place via a series of steps including interaction of the electrophilic NO+ with the nucleophilic NH group of the free amine and elimination of water to form a co-ordinated diazonium ligand with subsequent ligand redox to the observed ruthenium product. 

The effect of (internal) coordination on the thermal stability of Cp TiR was not worked out in detail, but some qualitative results can be mentioned here. Coordination is more difficult than for Cp2TiR as was concluded from the insensitivity of Cp TiR spectra to solvent (pentane vs. THE) and also from the observation that dinitrogen complex formation, (CpWiR)2N2, does not take place in the temperature range (-120 - 20°C) studied. 

The reaction using C10 is not quantitative. When alkaline [Fe(CN)e] is used, however, four equivalents of oxidant are required. The ruthenium(ii) complexes may, however, be oxidized to Ru both by cyclic voltammetry and by titration with Ce . In the latter case, however, some over-oxidation occurs since five moles of oxidant are necessary for complete reaction. Dinitrogen complex formation has been postulated from the cleavage of a C—N bond in a co-ordinated ethylenediamine of ruthenium(iii). Treatment of [Ru(en)3] + with NO in alkaline media yields as one product [(en)aRu(N2)(OH2)] +. An initial rapid reaction may be associated with reduction of the metal to the + 2 oxidation state and the reaction may proceed via either (4) or (5) as intermediates. 

Solan E, Da Silva C, lacono B et al (2001) Photochemical activation of the NN bond in a dimolybdenum-dinitrogen complex formation of a molybdenum nitride. Angew Chem Int Ed 40 3907-3909... 

Monomeric, soluble Mo(III) precursors are extremely useful chemical intermediates, their uses ranging from transformation to other Mo(III) coordination compounds,1 reduction to dinitrogen complexes,2 conversion to metal-metal bonded compounds,3 and formation of new materials.4... 

Dinitrogen complexes of tungsten have been studied in great detail in relation to the function of nitrogenase. They show considerable versatility both with respect to reduction of the coordinated N2 and in reactions leading to carbon-nitrogen bond formation. 

Figure 20 The formation of hydrazine from the bridged dinitrogen complex, [ M(S2CNEt2)3 2(p-N2)] (M = Nb or Ta).

Two studies on the mechanism of formation of hydrazido(2-) complexes by the reaction of dinitrogen complexes with acids have been reported (186,187). The salient features of these two mechanisms (one found in methanol, the other in tetrahydrofuran), will be discussed separately, and then compared. 

The increased basicity of a ligand when coordinated to the heavier metals in a transition metal triad has been appreciated for some time (311). This is manifest in (1) the rate of alkylation of alkyldiazenido complexes (kw/kMo = 5.4) (93), and (2) the rate of formation of hydra-zido(2-) complexes by the reaction of dinitrogen complexes with acid, in methanol ( w// Mo = 9.2 x 102) (186), and in tetrahydrofuran (kw/ kMo = 29-85, dependent upon the acid and substrate employed) (187). Clearly the electron-releasing capability of the metal has conflicting influences on the rate, but as in Section VI,El the basicity influence dominates. 

The formation of ammonia and/or hydrazine by protonation of binuclear bridging dinitrogen complexes of Groups IV and V is well established and has been reviewed elsewhere (82, 148). In general, stoichiometries are fairly well defined [Eq. (51)], although as with mononuclear complexes, mechanisms are not clearly understood (282). 

Generally, when dinitrogen complexes are treated with acid the strongly reduced metal is protonated, leading to oxidation of the metal and formation of a hydrido complex. This is accompanied usually by loss of dinitrogen and often by subsequent protic attack on the hydride to evolve dihydrogen (7). Of the stable mononuclear dinitrogen com-... 

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