Highly electrophilic olefin polymerization catalysts. Quantitative reaction coordinates for fluoroarylborane/alumoxane methide abstraction and ion-pair reorganization in group 4 metallocene and 'constrained geometry' catalysts

Reaction enthalpies of group 4 metallocenes having the general formula L₂M(CH₃)₂ (L = Cp, 1,2-Me₂Cp, Me₅Cp; L₂ = Me₂Si(Me₄Cp)((t)BuN); M = Ti, Zr, and Hf) with the strong organo-Lewis acid B(C₆F₅)₃ were measured using batch titration calorimetry in toluene. Methide abstraction to form the corresponding L₂MCH₃⁺CH₃B(C₆F₅)₃^- contact ion pairs is highly exothermic in all cases. Exothermicity increases with increasing Cp methyl substitution: for M = Zr, ΔH = -23.1(3), -24.3(4), and -36.7(5) kcal mol^-1 for L = Cp, Me₂Cp, and Me₅Cp, respectively for M = Hf and L = 1,2-Me₂Cp, ΔH = -20.8(5) kcal mol^-1. 'Constrained geometry' complexes (L₂ = Me₂Si(Me₄Cp)((t)BuN)) exhibit similar exothermicities, with ΔH = -22.6(2), -23.9(4), and -19.3(6) kcal mol^-1 for M = Ti, Zr, and Hf, respectively. In contrast, analogous reactions with methylalumoxane (M:Al = 1:50) are less exothermic, with ΔH = -10.9(3) and -8.9(4) kcal mol^-1 for L = 1,2-Me₂Cp and M = Zr and Hf, respectively. Under identical conditions, (1,2-Me₂Cp)₂M-(CH₃)₂ (M = Zr, Hf) complexes also undergo methide abstraction with the less Lewis-acidic triarylboranes (C₆F₅)2BAr (AT = 3,5-C₆H₃F₂, Ph, and 3,5-C₆H₃Me₂); however, conversions to the corresponding (Me₂-Cp)₂MCH₃⁺ CH₃B(C₆F₅)₂Ar^- ion pairs, are incomplete. Variable-temperature NMR measurements yield thermodynamic parameters for partial methide abstraction by these less Lewis-acidic boranes. For Ar = 3,5-C₆H₃F₂, ΔH = -18.7(7) and -15.2(8) kcal mol^-1 with ΔS = -42(2) and -35(3) e.u:; for Ar = Ph, ΔH = -14.8(8) and -13.3(6) kcal mol^-1 with ΔS = -31(2) and -39(2) e.u.; for Ar = 3,5-C₆H₃Me₂, ΔH = -10.8(6) and -12.7(5) with ΔS = -19(2) and -36(4) e.u., in each case for M = Zr and Hf, respectively. Dynamic NMR analyses reveal that the activation barriers for methide abstraction from the neutral metallocene dialkyls are small and relatively insensitive to the borane identity (AH = 2-6 kcal mol^-1) while ion-pair separation/recombination processes are greatly facilitated by polar solvents. Ethylene polymerization activities for eight (Me₂Cp)₂MCH₃⁺CH₃B(C₆F₅)₂Ar^- complexes measured in toluene solution (25°C, 1 atm) follow a trend in metal (Zr > Hf)as well as a substantial trend in triarylborane (Ar = C₆F₅ > 3,5-C₆H₃F₂ > Ph ~3,5-C₆H₃-Me₂). Polymerization activities correlate roughly with MCH₃^{+ 13}C NMR chemical shifts and enthalpies of methide abstraction.