Coordinated N2 ligands

There are many dinitrogen complexes, but only a small group of Re, Mo and W complexes have coordinated N2 ligands which are sufficiently reactive (equations 139 and 140). 

Reactivity Patterns of Coordinated N2 Ligands 2.2.7 Reactivity Toward Electrophiles and Nucleophiles... 

Simple Lewis formulas can be used to describe the bonding in M(q -N2) compounds and also correcdy predict their reactivity, when treated with nucleophiles or electrophiles (Fig. 4). The coordinated N2 ligand is attacked by nucleophiles at the N atom adjacent to the metal atom (a-N position), which carries a formal positive charge. Although this reactivity is long known, only a few examples have been reported. 

Photoinduced cleavage of a coordinated N2 ligand to form metal nitridos was first reported for [ (Mes)3Mo 2(p-N2)] (162). Although no thermally activated N2 cleavage is observed for 162, the mixed-valent nitrido dimer [ (Mes)3Mo 2(p-N)] (165) is isolated when irradiated at A, = 365 nm (Scheme 44). Floriani and coworkers proposed a mechanism to account for this transformation that included the initial formation of [(Mes)3Mo=N] (163) firom 162, and in a subsequent step two molecules of 163 react with [ (Mes)3Mo 2(p-N2)] (162) to generate intermediate 164 before N2 is released to yield [ Mo(Mes)3 2(p-N2)J (165). Unfortunately, no followup chemistry or more detailed mechanistic studies were reported. Cummins later on demonstrated that the mixed-valent nitrido dimer [ (N(R) Ar)3Mo 2(p-N)] (166) can be prepared from [ (N(R)Ar)3Mo 2(p-N2)] (152) on irradiation at A = 546 nm. ... 

Chatt and coworkers demonstrated that the coordinated N2 ligand in the Re(I) dinitrogen complex [ReCl(r -N2)(PMe2Ph)4] (179) is not amenable to protonation. Nevertheless, functionalization can be achieved by the addition of organic acid chlorides to form the corresponding Re(III) diazenido compounds [ReCl2(r -N2C(0)R)(PMe2Ph)3] (R = Me, Ph) (180). ... 

Scheme 57 Functionalization of a coordinated N2 ligand in the coordination sphere of a metallalumatrane complex. ...

In order to mimic the sulfur-rich coordination sphere in the active site of nitrogenase, Holland and coworkers prepared a molecular model system, in which upon reduction an Fe-S bond is broken and N2 coordinates (Scheme 58). " Complex 229 represents the first synthetic Fe—N2 complex, in which iron is bonded both to sulfur as well as carbon atoms. The terminally coordinated N2 ligand is significantly activated with Vnn = 1880 cm which is the lowest value observed so far for Fe complexes with terminal Fe-N2 ligands.The thiolate ligands act as powerful electron donors enabling substantial backbonding from the Fe d-orbitals into the Jt -N2 orbitals. 

However, minor variations in the P-diketiminato ligand allowed the group of HoUand to accomplish the first complete reduction a coordinated N2 ligand to two nitrido (N ) ligands in the iron coordination sphere. The tetrairon bis (nitrido) complex 253 is formed after addition of 2 equiv. of... 

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