Radical and migratory insertion reaction mechanisms in Schiff base zirconium alkyls

Four salicylaldimine derivatives H2L4–7 of 2,2′-diamino-6,6′-dimethylbiphenyl, where the Cdouble bond; length as m-dashN bond is sterically protected by substituents on the phenol ring, form alkyls of zirconium, cis-α-[ZrL4–7(CH2Ph)2]. Rather than decomposing via the established pathway of 1,2-migratory insertion of an alkyl group to imine, they undergo a radical mechanism. This is evidenced by the large number of products observed, kinetic and thermodynamic data (Rice-Herfeld, 3/2 order, positive ΔS‡), response to steric factors, and the fact that switching to a less stable radical leaving group inhibits the reaction. In contrast, the 1,2-migratory insertion is a clean, first-order intramolecular process with negative ΔS‡. The steric modification of the ligands nevertheless transforms an inactive precatalyst into a stable system for the polymerisation of ethene. Closely related unbridged salicylaldimine catalysts are known to be highly active catalysts, but in most cases they appear to suffer from high temperature instability. The first examples of zirconium alkyls of this class are isolated, and it is found that they are inherently much more resistant to decomposition by either pathway (migratory insertion or radical). Structural studies are used to interpret this variance in behaviour; the biaryl-bridged complexes are pre-organised for both reactions, while the unbridged systems would have to undergo significant ordering prior to activation. Correspondingly, the unbridged systems are not noticeably affected by the same steric modification of the ligand, and it is concluded that the more likely mechanism of catalyst death in the latter is ligand loss (i.e. transfer to aluminium from co-catalyst).