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

The cationic Pd(II) ethyl complex [[(2,6-( i Pr) 2 C 6 H 3 )N C(An)C(An) N(2,6-( i Pr) 2 C 6 H 3 )]Pd(CH 2 CH 2 - μ -H)]BAr‘ 4 ( 8 ), which exhibits a β-agostic interaction, has been synthesized via protonation of the corresponding diethyl complex with H(OiPr 2 ) 2 BAr‘ 4 (Ar‘ = 3,5-(CF 3 ) 2 C 6 H 3 ). The fluxional behavior of this ethyl cation has been studied quantitatively using 1 H and 13 C 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 Δ G ⧧ = 7.1 kcal/mol at −108 °C. Eyring analysis of the 13 C NMR data indicates Δ H ⧧ = 6.1 ± 0.2 kcal/mol and Δ S ⧧ = −5.2 ± 0.9 eu. Rotation of the agostic methyl group occurs with Δ G ⧧ 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.