Group 6 Transition Metal Dinitrogen Complexes
Résumé
The interaction of dinitrogen (N2) with a transition metal fragment can be understood in a simplified manner through the classical Dewar-Chatt-Duncanson model. The choice of the metal precursors as well as the reducing agent for the synthesis of the metal-N2 complexes is often vital. Several group 6 metal-dinitrogen complexes are efficient catalysts for the N2-to-N(SiMe3)3 process. The most efficient ones involve polydentate phosphine ligands, either two bidentate phosphines or a tridentate phosphine at Mo centers, or amacrocyclic tetradentate phosphine at Cr. In these best cases, the reaction is slow at room temperature (typically requiring days), which is linked to the kinetics of generation and reactivity of the Si radicals. One of the crucial features of the complexes is therefore to be stable for extended periods under strongly reducing media, as well as react fast with the silyl radical to limitate the competitive pathways: radical dimerization and radical attack of the solvent.