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

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, Bu-i) has been studied. It was shown that outer-sphere ion pairs of the type [(FI)(TiMe2)-Me-IV](+)[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 Ti-III and Ti-II species. In the systems (FI)TiCl3/MAO and (FI)TiCl3/AlMe3/[Ph3C](+)[B(C6F5)(4)](-), complexes with the proposed structures (FI)(TiMe2)-Me-III, (FI)(TiCl)-Cl-II, and [(FI)Ti-II(S)](+)[A](-) ([A](-) = [MeMAO](-), [B(C6F5)(4))](-), S = solvent, vacancy) were observed (concentrations of Ti-III species was lower than those of the Ti-II congeners). In contrast, in the system (FI)TiCl3/MMAO, the concentrations of Ti-II species (ion pairs of the type [(FI)Ti-III(mu-H)(mu-Cl)(AlBu2)-Bu-i](+)[MeMMAO](-)) were higher than those of the Ti-II counterparts (ion pairs [(FI)Ti-II(S)(+)[MeMMAO](-)). The system (FI)TiCl3/MMAO displays lower activity and selectivity in 1-hexene formation, in comparison to (FI)TiCl3/MAO, due to undesirable PE generation. Probably, Ti-II and Ti-IV ion pairs are those participating in ethylene trimerization.