Palladium(II) beta-agostic alkyl cations and alkyl ethylene complexes: Investigation of polymer chain isomerization mechanisms

A series of stable dialkyl complexes of Pd, (a-diimine)PdR2 (alpha -diimine = aryl-substituted diimine, R = n-Pr, n-Bu, i-Bu), have been prepared via Grignard alkylation of the corresponding (alpha -diimine)PdCl2 complexes. P rotonation of these dialkyl species at low temperature results in loss of a lkane and formation of cationic Pd beta -agostic alkyl complexes, which hav e been observed as intermediates in the polymerization of ethylene and prop ylene by these Pd catalysts. Studies of the structure and dynamic behavior of these alkyl complexes are presented. along with the results of trapping reactions of these species with ligands such as NCMe, CO, and C2H4. Trappin g with ethylene results in formation of cationic alkyl ethylene complexes w hich model the catalyst resting state in these systems. These complexes hav e been used to obtain mechanistic details and kinetic parameters of several processes, including isomerization of the alkyl ethylene complexes, associ ative and dissociative exchange with free ethylene, and migratory insertion rates of both primary and secondary alkyl ethylene species. These studies indicate that the overall branching observed in polyethylenes produced by t hese Pd catalysts is governed both by the kinetics of migratory insertion a nd by the equilibria involving the alkyl ethylene complexes.