Selective Ethylene Trimerization by Titanium Complexes Bearing Phenoxy-Imine Ligands: NMR and EPR Spectroscopic Studies of the Reaction Intermediates

Abstract

The catalyst systems (FI)TiCl3/MAO (FI = phenoxyimine ligand with an additional aryl–O–CH3 donor) display exceptionally high activity in selective ethylene trimerization. By means of NMR and EPR spectroscopy, the nature of the Ti species formed in the catalyst systems (FI)TiCl3/MAO, (FI)TiCl3/MMAO, and (FI)TiCl3/AlR3/[Ph3C]+[B(C6F5)4]− (R = Me, Et, iBu) has been studied. It was shown that outer-sphere ion pairs of the type [(FI)TiIVMe2]+[A]− ([A]− = [MeMAO]−, [MeMMAO]−, [B(C6F5)4]−) are formed at the initial stage of the reaction of (FI)TiCl3 with MAO, MMAO, and AlMe3/[Ph3C]+[B(C6F5)4]−. These ion pairs further partially convert into TiIII and TiII species. In the systems (FI)TiCl3/MAO and (FI)TiCl3/AlMe3/[Ph3C]+[B(C6F5)4]−, complexes with the proposed structures (FI)TiIIIMe2, (FI)TiIICl, and [(FI)TiII(S)]+[A]− ([A]− = [MeMAO]−, [B(C6F5)4)]−, S = solvent, vacancy) were observed (concentrations of TiIII species was lower than those of the TiII congeners). In contrast, in the system (FI)TiCl3/MMAO, the conc...