Radical and migratory insertion reaction mechanisms in Schiff base zirconium alkyls

Group 4 metal complexes bearing new tridentate (NNO) ligands: Benzyl migration and formation of unusual C–C coupled products

“…Eyring plots revealed negative DS # values (À79 and À139 J K À1 mol À1 , respectively for the conversion of 6 into 11 and 7 into 12; Fig. 6) indicative of non-dissociative intramolecular processes [46].…”

“…Scott and coworkers reported that titanium and zirconium benzyl complexes bearing bridged phenoxyimine ligands can undergo migrations of one of the benzyls to the imino carbon of the ligand backbone [19,[44][45][46]. Other systems based on pyrrolylimines and a-diimine ligands show similar reactivities [47][48][49][50].…”

Highly active self-assembled group-IV-metal multinuclear catalysts for ethylene polymerization

“…However, experimental results demonstrated that the tetradentate ligands III and XII (see Scheme 3) did not afford olefin polymerization catalysts, principally because of a destructive 1,2-migratory insertion of a metal-bound alkyl/polymeryl chain into the imine C]N unit [18e20]. Subsequently, Knight et al [19,20] found that introducing an alkyl group at the position R 4 (see ligand XI in Scheme 3) of a zirconium salicylaldiminato complex leads to a long-lived catalyst (1 h test) for ethylene polymerization because of steric blocking of an intramolecular 1,2-migratory insertion. However, this steric blocking promotes a new radical catalyst decomposition mechanism in certain instances, thus resulting in far lower activities compared to the corresponding catalyst based on the FI ligand.…”

“…However, this steric blocking promotes a new radical catalyst decomposition mechanism in certain instances, thus resulting in far lower activities compared to the corresponding catalyst based on the FI ligand. In addition, all the zirconium complexes of ligands IV~X have no activity probably due to the lack of steric bulk in the phenolate 2-position [20]. In their further studies, Clarkson et al [21] investigated more tetradentate ligands (see XIII-XVII in Scheme 3).…”

“…The complexes [(XVI) ZrCl 2 ] and [(XVII)ZrCl 2 ] demonstrated only low activities. Similarly, introducing a methyl group at the phenolate 5-position of the simple phenoxy-imine ligand may block the intramolecular 1,2-migratory insertion, however this steric blocking failed to improve the catalytic lifetime [20]. Therefore, after many investigations of various tetradentate ligands, the development of long-lived and highly efficient phenoxy-imine-based Group 4 metal catalysts is still a challenge.…”