Intrinsic Aptitude of Cationic Methyl- and Ethylpalladium To Associate Ethylene and To Further Undergo Subsequent Migratory Insertion. A Theoretical Study

Abstract

Density Functional Theory (DFT) calculations at the B3LYP level and PCI-80 calculations have been carried out for the following reaction sequence: (a) association of ethylene to cationic (σ-alkyl)palladium(II) complexes, forming a π-ethylene σ-alkyl intermediate, (b) subsequent migratory insertion, and (c) β-hydride elimination of the insertion product. Ethylene coordinates strongly to the “naked” complexes PdCH3+ and PdC2H5+ (43.9 kcal/mol and 36.0 kcal/mol respectively). Nitrogen ligands modify association exothermicity (or conversely dissociation endothermicity): Pd(NH3)2CH3+ (27.3 kcal/mol), Pd(NH3)2C2H5+ (14.9 kcal/mol) and Pd(CHNH)2CH3+ (29.8 kcal/mol), where (CHNH)2 is chelating diimine. Substantial agostic interaction between metal and β-hydrogens and differences in charge on palladium account for the differences. The migratory insertion step is endothermic for naked 12-electron (π-ethylene)(σ-alkyl)palladium complexes and exothermic for 16-electron complexes with two nitrogen ligands. Calculate...