Measurement of the barrier to β-hydride elimination in a β-agostic palladium-ethyl complex: A model for the energetics of chain-walking in (α-diimine)PdR+ olefin polymerization catalysts

Article abstract of DOI:10.1021/om010197j

The cationic Pd(II) ethyl complex [[(2,6-(ⁱPr)₂C₆H₃)N=C(An)C(An)=N(2,6 -(ⁱPr)₂C₆H₃)]Pd (CH₂CH₂-μ-H)]BAr′₄ (8), which exhibits a β-agostic interaction, has been synthesized via protonation of the corresponding diethyl complex with H(OiPr₂)₂BAr′₄ (Ar′ = 3,5-(CF₃)₂C₆H₃). The fluxional behavior of this ethyl cation has been studied quantitatively using ¹H and ¹³C NMR spectroscopy. Two dynamic processes have been observed: (1) β-H elimination and reinsertion with opposite regiochemistry and (2) rotation about the C-C bond of the ethyl group. β-H elimination is the lower energy process, with ΔG? = 7.1 kcal/mol at -108°C. Eyring analysis of the ¹³C NMR data indicates ΔH? = 6.1 ± 0.2 kcal/mol and ΔS? = -5.2 ± 0.9 eu. Rotation of the agostic methyl group occurs with ΔG? ca. 8.4 kcal/mol at -108°C. These data provide insight into the energetics of chain walking, which is responsible for the unusual polyolefin microstructures produced using (α-diimine)PdR⁺ catalysts.