Alkyldiazenido ligands

The third area is the synthesis and characterization of aryldiazenido complexes of transition metals. In 1964 King and Bisnette isolated the first metal complex with an aryldiazenido ligand. The interest of organometallic chemists was concentrated mainly on the isolation and characterization of stable aryldiazenido complexes and not on potential metastable intermediates involved in metal-catalyzed dediazonia-tions. The situation is different, however, for metal complexes with alkyl-diazenido ligands. Complexes with aryl- and alkyldiazenido ligands are the subject of Chapter 10 in the forthcoming second book (Zollinger, 1995). 

The first metal complex with an alkyldiazenido ligand was found three years later (Bagga et al., 1967), see Sect. 10.2. 

The nonspecialist reader of this section may come to the conclusion that the area of transition-metal complexes with alkyldiazenido ligands is not very large. This impression is reinforced by the fact that these complexes lose the diazo group relatively easily, as mentioned in the beginning of the section. It is wrong, however, if one considers the fact that diazoalkanes have been used, and still are, extensively in the preparation of carbenes via metastable diazenido intermediates of metal-complex catalysts in laboratory scale syntheses, in the Fischer-Tropsch reaction, and in olefin metathesis (see, e.g., Herrmann, 1978 Doyle 1986a, 1986b and Sects. 8.7 and 8.8 of this book). 

The reaction of alkyl halides with (rans-[M(N2)2(diphos)2] (diphos = chelating diphosphine ligand) can give rise to dialkylhydrazido(2-) complexes via the alkyldiazenido species (76, 93, 94) [Eqs. (37) and (38)1. 

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. 

Lappert and Poland (1969) were the first authors who reported on the discovery and characterization of mononuclear alkyldiazenido complexes by reaction of (tri-methylsilyl)diazomethane with tricarbonyl(cyclopentadienyl)molybdenum hydride and the analogous tungsten hydride. Scheme (10-18) demonstrates that these reactions are not simple substitutions of a carbonyl by a diazenido ligand, but that they are insertions of the diazoalkane into the M-H bond (see also Lappert and Lorberth, 1967). 

Some alkyldiazenido complexes of nickel, palladium, molybdenum, and ruthenium were obtained by ligand exchange or by ligand addition. They were later demonstrated not to have end-on structures, but a side-on (t ) coordination at both diazo N-atoms. Itsuka and coworkers demonstrated that in bis( r butylisocyanide)... 

Another synthesis of alkyldiazenido complexes is due to Chatt et al. (1977 b), and independently to the work of Hidai et al. (1976 b), namely a route by condensation of hydrazido(2 —) metal complexes with aldehydes or ketones. The original work of Hidai et al. (1976 a) concentrated on molybdenum complexes, that of Chatt et al. (1977 b) on tungsten, but both groups used the same type of ligands as their starting material complexes, namely [Mo(NNH2)(dppe)2] X (X = F, Cl, Br or I). The... 

Hussain, H., Leigh, G.J., Modh-Ali, H. and Pickett, C.J. Mechanism of alkylation of alkyldiazenido-complexes of molybdenum(II) and tungsten(II) influence of metal and co-ligands on the nucleophilicity of a diazenido-group J.Chem.Soc. Dalton Trans., (1986), 1473-1477... 

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