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

Group 4 metal complexes bearing new phenoxy(benzimidazolyl)-imine, -amine and -amide ligands have been synthesized. A series of metal chloride derivatives has been prepared via treatment of MCl4(THF)2 (M = Ti, Zr, Hf) with the in situ generated sodium salt of the (benzimidazolyl)imine phenol 1. Reaction of the pro-ligand 2 with TiCl4(THF)2 afforded the corresponding complex 8 in which the amine proton remains bound to the nitrogen donor. Benzyl complexes of zirconium and hafnium were synthesized via treatment of pro-ligands 1 and 2 with M(CH2Ph)4 precursors. The complexes [NNO]M(CH2Ph)3 (6 M = Zr, 7 M = Hf) were found to undergo benzyl migration from the metal centre to the imine carbon of the ligand backbone giving complexes 11 and 12; the migration follows first order kinetics. The reaction of 1 with Ti(NMe2)4 led to the formation of an unusual C–C coupled product in which a new piperazine ring has formed. Complexes 11 and 12 undergo related transformations, leading to analogous C–C coupled products which were characterized by X-ray crystallography. Deuterium labelling experiments were carried out to determine the mechanistic pathway of the reactions. Chloride and benzyl complexes 3–12 were screened as pre-catalysts for olefin polymerization.

Graphical abstractA series of Group 4 metal complexes bearing new phenoxy(benzimidazolyl)-imine, -amine and -amide ligands has been synthesized. Derivatives containing benzyl ligands undergo benzyl migration and an unusual C–C coupling process to afford binuclear species in which the metals are linked by a central piperazine unit. Deuterium labelling experiments have been employed to determine the mechanistic pathways for these reactions.Figure optionsDownload full-size imageDownload as PowerPoint slide