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

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Abstract

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(NtBu)]ZrMe+ is also observed when B(C6F5)4- is the counteranion. (1,2-Me2Cp)2ZrMe+B(C6F4TBS)4- undergoes slow decomposition under an inert atmosphere to afford [(1,2-Me2Cp)2ZrF]2(μ-F)+B(C6F4TBS)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.

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