Dinuclear N2 complex

VII. Study of the First Dinuclear N2 Complex with Biologically Compatible Ligand Sphere... 

Holland and Peters earlier reported examples of three- and four-coordinate N2 complexes with iron in the formal +1 oxidation state, but in neither case were terminally bonded N2 species identified or isolated dinuclear end-on Fe-NN-Fe species were inevitably obtained (99,100). Therefore, Peters and coworkers recently began to turn their attention to new derivatives of the classic Sacconi-type tripodal ligands to achieve access to a terminally bound Fe-N2 complex. Indeed, by employing the mono-anionic ligand L8 — [f2-Ph2PC6H4)3Si] (Fig. 10) they were able to generate the desired complex [Fe(L8)(N2)] (10) (101). 

Regarding the structure and function of nitrogenases in producing ammonia from N2, Sellmann has studied several model systems wherein heterolytic activation of H2 occurs on sulfur ligands (87). A core geometry based on a hybrid of the FeMoco active site structure with a dinuclear diazene complex, [Fe( NHS4 )]2(p-N2H2), is a proposed model (Scheme 4). 

Detailed studies with several Ru(H2) complexes showed that the yield of NH3 critically depended upon the pKa value of the Ru(H2) complexes (89). When the W-N2 complex was treated with 10 equiv of [RuCl(H2)(dppe)2]+ (dppe = l,2-bis(diphenylphosphino)ethane) with pi a = 6.0 under 1 atm of H2, NH3 was formed in up to 79% total yield (free NH3 plus NH3 released on base distillation). If the pKa of the Ru(H2) complex was increased to 10, the yield of ammonia decreased remarkably. Heterolytic cleavage of H2 was proposed to occur at the Ru center via nucleophilic attack of the coordinated N2 on the coordinated H2 where the coordinated N2 is protonated and a hydride remains at the Ru atom. Only a very limited number of reactions of bound N2 with H2 are known, e.g., Eq. (23) which slowly occurs in toluene over 1-2 weeks for a dinuclear Zr complex capped by macro-cyclic ligands with N and P donor atoms (90). 

Other less bulky trisamido An complexes, for example, U[N(R)Ar]3 (R = Bu and Ar = 3-5-Me2C6H3), have been shown to elicit some interesting chemistry. When the U(NRAr)3 complex is produced in situ by reduction of UI(NRAr)3 in the presence of Mo[NPh(t-Bu)]3 and N2, an end-on N2-bridged complex of the form [(NRAr)3U(/r-N2)Mo"]. A similar reaction has also been found with cyanoimide where a dinuclear uranium complex was isolated, (/x j] , -NCN) U[N(R)Ar]3 2 (26). ... 

In a bid to prove that the ligand is the source of the hydrogen atoms, the reaction was repeated with the analogous Tp -complexes. We reasoned that hydrogen atom abstraction from the isopropyl groups would generated isopropyl radicals, which should disproportionate into isopropyl- and isopropenyl-substituted ligands. Reaction of Tp "Pf Co-02 with half an equivalent of [(Tp -Pf MeCo)2( 4-N2)l produced a mixture of products, one of which was the dinuclear hydroxide complex [(Tp Co)2(ti-OH)2]. Hydrolysis of the whole product mixture, and examination of the recovered pyrazoles indeed showed that 10 % of the pyrazole fraction was 3-isopropenyl-5-methylpyrazole. 

Heterolysis of H2 was proposed to occur at Ru via nucleophilic attack of coordinated N2 on bound H2 where the bound N2 is protonated and a hydride remains at Ru. Only a limited number of reactions of bound N2 or nitride with H2 are fof example. Equation (19) which slowly occurs for a dinuclear Zr complex... 

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