B(C₆F₅)₃- vs Al(C₆F₅)₃-Derived Metallocenium Ion Pairs. Structural, Thermochemical, and Structural Dynamic Divergences

The thermodynamic and structural characteristics of Al(C6F5)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-C2H4(η5-Ind)2Zr(CH3)2 (rac-(EBI)Zr(CH3)2) yields rac-(EBI)Zr(CH3)+H3CB(C6F5)3- (1a), rac-(EBI)Zr(CH3)+H3CAl(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 (ΔHipf) 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-Me4C5)(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-1and ΔH⧧ae ≈ 16 kcal mol-1, consistent with the bridging methide being more strongly bound to the zirconocenium center than in 1a.