Mechanistic studies on the formation of linear polyethylene chain catalyzed by palladium phosphine-sulfonate complexes: Experiment and theoretical studies

Linear polyethylene propagation starting from Pd phosphine-sulfonate complexes, Pd(CH₃)-(L)(Ar₂PC₆H ₄SO₃) (L = 2,6-lutidine, Ar ) o-MeOC₆H ₄ (2a) and L = pyridine, Ar = Ph (2b)), was studied both experimentally and theoretically. Experimentally, highly linear polyethylene was obtained with Pd(CH₃)(L)(Ar₂PC₆H ₄SO₃) complexes 2a and 2b. Formation of a long alkyl-substituted palladium complex (3) was detected as a result of ethylene oligomerization on a palladium center starting from methylpalladium complex. Additionally, well-defined ethyl and propyl complexes (6_Et and 6 _Pr) were synthesized as stable n-alkyl palladium complexes. In spite of the existence of β-hydrogens, the β-hydride elimination to give 1-alkenes was very slow or negligible in all cases. On the other hand, isomerization of 1-hexene in the presence of a methylpalladium/phosphine- sulfonate complex 2a indicated that this catalyst system actually undergoes β-hydride elimination and reinsertion to release internal alkenes. On the theoretical side, the relative energies were calculated for intermediates and transition states for chain-growth, chain-walking, and chain-transfer on the basis of the starting model complex Pd(n-C₃H₇)(pyridine) (o-Me₂PC₆H₄SO₃) (8). First, cis/trans isomerization process via the Berry's pseudorotation was proposed for the Pd/phosphine-sulfonate system. The second oxygen atom of sulfonate group is involved in the isomerization process as the associative ligand, which is one of the most unique natures of the sulfonate group. Chain propagation was suggested to take place from the less stable alkylPd(ethylene) complex 10′ with the TS of 27.4/27.7 ((E+ZPC)/G) kcal/mol. Possible β-hydride elimination was suggested to occur under low concentration of ethylene: the highest-energy transition state to override for β-hydride elimination was either >37.4/25.3 kcal/mol (TS(9-12)) or 29.1/27.4 kcal/mol (TS(8′-9′) to reach 12′). The ethylene insertion to the iso-alkylpalladium species (14′) is allowed via a TS of 28.6/29.1 kcal/mol (TS(14′-15′)), slightly higher in energy than that for the normal-alkylpalladium species (TS(10′-11′)). Easy chain transfer was suggested to proceed from the more stable PdH(olefin) complex 12′ if β-hydride elimination to 12′ does take place. Thus, the production of linear polyethylene with high molecular weight under ethylene pressure suggests that the cis and trans PdH(alkene)(phosphine-sulfonate) complexes (12 and 12′) are merely accessible in the presence of excess amount of ethylene.