Mechanism of Ethylene Oligomerization by a Cationic Palladium(II) Alkyl Complex that Contains a (3,5-Me2-pyrazolyl)2CHSi(p-tolyl)3) Ligand

The reactivity of the ethylene oligomerization catalyst (N^N)Pd(Me)(ethylene) + ( 5 ), which contains a silyl-capped bis-pyrazolyl methane ligand (N^N = (3,5-Me 2 -pyrazolyl) 2 CHSi( p -tolyl) 3 ( 1 )) was investigated. The reaction of (N^N)PdMeCl ( 3 ) with Li[B(C 6 F 5 ) 4 ] and ethylene in CH 2 Cl 2 generates 5 . Complex 5 undergoes ethylene insertion at −10 °C with a rate constant of k insert,Me = 3.3(3) × 10 −3 s −1 . Complex 5 catalytically oligomerizes ethylene to branched C 6 –C 20 internal olefins (−10 to 20 °C; 2.7–30 atm ethylene). NMR studies show that an equilibrium mixture of base-free β-agostic secondary alkyl (N^N)PdR + species ( 8 ) and ethylene adducts (N^N)Pd(R)(ethylene) + ( 9 ) is present under oligomerization conditions. Complex 9 decomposes to Pd 0 at 20 °C, resulting in catalyst decomposition. (N^N)Pd(R)(L) + species containing R groups that can function as internal donors (either η 2 -acyl or agostic β-H) undergo a dynamic process that exchanges the two pz* rings and are thermally unstable. It is proposed that catalyst decomposition and pz* exchange both involve initial coordination of the internal donor to generate a configurationally labile five-coordinate intermediate, which isomerizes, resulting in pz* site exchange or displacement of one arm of the N^N ligand.