B(C₆F₅)₃- vs Al(C₆F ₅)₃-derived metallocenium ion pairs. Structural, thermochemical, and structural dynamic divergences

The thermodynamic and structural characteristics of Al(C₆F ₅)₃-derived vs B(C₆F₅) ₃-derived group 4 metallocenium ion pairs are quantified. Reaction of 1.0 equiv of B(C₆F₅)₃ or 1.0 or 2.0 equiv of Al(C₆F₅)₃ with rac-C₂H ₄(η⁵-Ind)₂Zr(CH₃)₂ (rac-(EBI)Zr(CH₃)₂) yields rac-(EBI)Zr(CH ₃)⁺H₃CB(C₆F₅) ₃^- (1a), rao(EBI)-Zr(CH₃)⁺H ₃CAl(C₆F₅)₃^- (1b), and rac-(EBI)Zr²⁺[H₃CAl(C₆F₅) ₃]^-₂ (1c), respectively. X-ray crystallographic analysis of 1b indicates the H₃CAl(C₆F₅) ₃^- anion coordinates to the metal center via a bridging methyl in a manner similar to B(C₆F₅)₃-derived metallocenium ion pairs. However, the Zr-(CH₃)_bridging and Al-(CH₃)_bridging bond lengths of 1b (2.505(4) Å and 2.026(4) Å, respectively) indicate the methyl group is less completely abstracted in 1b than in typical B(C₆F₅) ₃-derived ion pairs. Ion pair formation enthalpies (ΔH _ipf) determined by isoperibol solution calorimetry in toluene from the neutral precursors are -21.9(6) kcal mol^-1 (1a), -14.0(15) kcal mol^-1 (1b), and -2.1(1) kcal mol^-1 (1b→1c), indicating Al(C₆F₅)₃ to have significantly less methide affinity than B(C₆F₅)₃. Analogous experiments with Me₂Si(η⁵-Me₄C ₅)(t-BuN)Ti(CH₃)₂ indicate a similar trend. Furthermore, kinetic parameters for ion pair epimerization by cocatalyst exchange (ce) and anion exchange (ae), determined by line-broadening in VT NMR spectra over the range 25-75 °C, are ΔH^‡ _ce = 22(1) kcal mol^-1, ΔS^‡ _ce = 8.2(4) eu, ΔH^‡_ae = 14(2) kcal mol^-1, and ΔS^‡_ae = -15(2) eu for 1a. Line broadening for 1b is not detectable until just below the temperature where decomposition becomes significant (∼75-80 °C), but estimation of the activation parameters at 72 °C gives ΔH^‡ _ce ≈ 22 kcal mol^-1 and ΔH^‡ _ae ≈ 16 kcal mol^-1, consistent with the bridging methide being more strongly bound to the zirconocenium center than in 1a.