Measurement of the barrier to β-hydride elimination in 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-(Pr-i)(2)C6H3)N=C(An)C(An)=N(2,6-(Pr-i)(2)C6H3)]Pd(CH2-CH2-mu -H)]BAr'(4) (8), which exhibits beta -agostic interaction, has been synthesized via protonation of the corresponding diethyl complex with H(OiPr(2))(2)BAr'(4) (Ar' = 3,5-(CF3)(2)C6H3). The fluxional behavior of this ethyl cation has been studied quantitatively using H-1 and C-13 NMR spectroscopy. Two dynamic processes have been observed: (1) beta -H elimination and reinsertion with opposite regiochemistry and (2) rotation about the C-C bond of the ethyl group. beta -H elimination is the lower energy process, with DeltaG(double dagger) = 7.1 kcal/mol at -108 degreesC. Eyring analysis of the C-13 NMR data indicates DeltaH(double dagger) = 6.1 +/- 0.2 kcal/mol and DeltaS(double dagger) = -5.2 +/- 0.9 eu. Rotation of the agostic methyl group occurs with DeltaG(double dagger) ca. 8.4 kcal/mol at -108 degreesC. These data provide insight into the energetics of chain walking, which is responsible for the unusual polyolefin microstructures produced using (alpha -diimine)PdR+ catalysts.