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

Abstract

The synthesis and properties of two soluble, weakly coordinating derivatives of the tetrakis(perfluoroaryl)borate anion B(4-C 6 F 4 TBS) 4 - and B(4-C 6 F 4 TIPS) 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 L 2 MMe 2 complexes in benzene or toluene affords the cationic ion-paired methyl complexes L 2 MMe + X - or the corresponding hydrido complexes L 2 MH + X - (L 2 = bis(cyclopentadienyl)- or cyclopentadienylamido-type ligand) when the reaction is carried out under dihydrogen. The solid state structure of the complex (Me 5 Cp) 2 ThMe + B(C 6 F 5 ) 4 - has been characterized by X-ray diffraction. The B(C 6 F 5 ) 4 - -based zirconocenium methyl complexes L 2 MMe + 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(C 6 F 4 TBS) 4 - - and B(C 6 F 4 TIPS) 4 - -derived complexes are greater than those of the corresponding B(C 6 F 5 ) 4 - - and MeB(C 6 F 5 ) 3 - -derived analogues. The relative coordinative tendencies of MeB(C 6 F 5 ) 3 -, B(C 6 F 5 ) 4 -, B(C 6 F 4 TBS) 4 -, and B(C 6 F 4 TIPS) 4 - are estimated from the solution spectroscopic information and the structural dynamics of the ion-pairs and follow the order MeB(C 6 F 5 ) 3 - > B(C 6 F 4 TBS) 4 - ≈ B(C 6 F 4 TIPS) 4 - > B(C 6 F 5 ) 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 [(Me 4 Cp)SiMe 2 (N t Bu)]ZrMe + is also observed when B(C 6 F 5 ) 4 - is the counteranion. (1,2-Me 2 Cp) 2 ZrMe + B(C 6 F 4 TBS) 4 - undergoes slow decomposition under an inert atmosphere to afford [(1,2-Me 2 Cp) 2 ZrF] 2 (μ-F) + B(C 6 F 4 TBS) 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(C 6 F 4 TBS) 4 - and B(C 6 F 4 TIPS) 4 - are used as counteranions. The polymerization activity of the zirconium constrained geometry complex also reaches a maximum in aromatic solvents when B(C 6 F 5 ) 4 - is used as the counteranion, apparently due to solvent coordination.