Dinitrogen bond formation

The third chapter presented by E. Schulz deals with the use of dinitrogen-containing ligands in three important asymmetric methods of C - C bond formation asymmetric cyclopropanation, the Dields-Alder reaction and al-lylic substitution. 

The principal mechanistic events include N-N bond formation stage, where the coordinated NO reactant is transformed into the NzO semi-product via dinitrosyl ( M-(NO)2]Z) or dinitrogen dioxide ( M-N202 Z) intermediates, depending on the nature of TMI (vide infra). Simultaneously, the primary (M Z active sites are converted into the secondary [M-0]Z active sites involved in the dioxygen formation cycle [5], The mononitrosyl complexes are usually postulated to be the key intermediate species of this step [2,5,41], whereas the mechanistic role of dinitrosyls and dinitrogen dioxide is more indistinct as yet. 

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. 

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. 

C-N Bond Formation at Coordinated Dinitrogen via Hydrazido(2-) Species... 

Significant progress in the application of this concept has also been made in catalytic C—C bond formation reactions. Chirik and co-workers recently reported an interesting case involving [2jc +2jc] cycloaddition of dienes and enynes using the bis-dinitrogen... 

If the radical is too unstable e.g. BrCH2CH2 ) it will decompose before N-C bond formation. Alternatively, a radical which is too stable will not attack N2. It appears that alkylation and acylation of coordinated dinitrogen have similar mechanisms. ... 

In the reaction of coordinated dinitrogen with acidic ruthenium dihydrogen complexes shown in the previous section, only one of the two hydrogen atoms of the coordinated dihydrogen is used for the N-H bond formation, while the other is not used for the product formation and remains at the Ru center. However, several examples are now known where both of the hydrogen atoms of coordinated dihydrogen are catalytically transferred to products via the heterolytic splitting. 

Quite recently, Fryzuk and co-workers observed the N-H bond formation when a dinuclear zirconium complex with a side-on bridging dinitrogen ligand was treated with molecular dihydrogen. However, no ammonia was formed (a) M. D. Fryzuk, J. B. Love, S. J. Rettig, V. G. Young, Science 275 (1997) 1445. (b) H. Basch, D. G. Musaev, K. Morokuma, M. D. Fryzuk, J. B. Love, W. W. Seidel, A. Albinati, T. F. Koetzle, W. T. Klooster, S. A. Mason, J. Eckert, J. Am. Chem. Soc. 121 (1999) 523. 

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. 

Fixation of carbon dioxide and formaldehyde in their intact form on a metal centre is a primary goal in metal-promoted transformations of a C molecule, provided it forms metal-carbon bonds. Formation of formaldehyde and carbon dioxide complexes is, however, a quite rare reaction, in spite of the various strategies applied so far. On the contrary, coordination of dinitrogen has been found in a number of complexes all the mononuclear compounds so far identified prefer the end-on bonding mode. Activation of dinitrogen, however, can be much more pronounced in ca.se N2 binds the metal in a side-on fashion. 

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... 

Only a limited number of group 7 metal complexes carrying dinitrogen (N2) ligands are known, but some of them exhibit interesting reactivity including N—C bond formation. 

PhBP 3 = [PhB(CH2CH2P Pr2)3] ) with Mg under an N2 atmosphere yields [ PhBP 3 Fe(p-T r -N2)MgCl(thf)2] (199) with a strongly activated N2 ligand (Vnn=1830 cm ). Addition of MeOTs forms the diazenido compound [ PhBP 3 Fe—N=NMe] (200), which represented at the time the first Fe-mediated N—C bond formation on dinitrogen (Scheme 52). Flow-ever, addition of acids produced no detectable quantities of N2H4 or NFls. 

Dinitrogen Bond Scission and Ammonia Formation with the B-Diketimate Ligand System... 

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