Cationic metallocene polymerization catalysts based on tetrakis(pentafluorophenyl)borate and its derivatives. probing the limits of anion "noncoordination" via a synthetic, solution dynamic, structural, and catalytic olefin polymerization study

The synthesis and properties of two soluble, weakly coordinating derivatives of the tetrakis-(perfluoroaryl)borate anion B(4-C₆F₄TBS)₄^- and B(4-C₆F₄TIPS)₄^- (TBS = tert-butyldimethylsilyl and TIPS = triisopropylsilyl) are reported. Reaction of the trityl salts of the above anions with a variety of zirconium and thorium L₂MMe₂ complexes in benzene or toluene affords the cationic ion-paired methyl complexes L₂MMe⁺X^- or the corresponding hydrido complexes L₂MH⁺X^- (L₂ = bis(cyclopentadienyl)- or cyclopentadienylamido-type ligand) when the reaction is carried out under dihydrogen. The solid state structure of the complex (Me₅Cp)₂ThMe⁺B(C₆F₅) ₄^- has been characterized by X-ray diffraction. The B(C₆F₅)₄^--based zirconocenium methyl complexes L₂MMe⁺ are unstable at room temperature with respect to, among other factors, intramolecular C-H activation of the ligand framework. In general, the thermal stabilities of the B(C₆F₄TBS)₄^-- and B(C₆F₄TIPS)₄^--derived complexes are greater than those of the corresponding B(C₆F₅)₄^-- and MeB(C₆F₅)₃^--derived analogues. The relative coordinative tendencies of MeB(C₆F₅)₃^-, B(C₆F₅)₄^-, B(C₆F₄TBS)₄^-, and B(C₆F₄TIPS)₄^- are estimated from the solution spectroscopic information and the structural dynamics of the ion-pairs and follow the order MeB(C₆F₅)₃^- > B(C₆F₄TBS)₄^- ≈ B(C₆F₄TIPS)₄^- > B(C₆F₅)₄^-. The coordination of the neutral metallocene precursors to the cationic metallocenes is found to compete with counteranion coordination. Arene solvent coordination to the zirconium constrained geometry cation [(Me₄Cp)SiMe₂(N^tBu)]ZrMe⁺ is also observed when B(C₆F₅)₄^- is the counteranion. (1,2-Me₂Cp)₂ZrMe⁺B(C₆F ₄TBS)₄^- undergoes slow decomposition under an inert atmosphere to afford [(1,2-Me₂Cp)₂ZrF]₂(μ-F)⁺B(C ₆F₄TBS)₄^-, which has been characterized by X-ray diffraction. The olefin polymerization activity and thermal stability of the zirconocene catalysts reaches a maximum when B(C₆F₄TBS)₄^- and B(C₆F₄TIPS)₄^- are used as counteranions. The polymerization activity of the zirconium constrained geometry complex also reaches a maximum in aromatic solvents when B(C₆F₅)₄^- is used as the counteranion, apparently due to solvent coordination.