INTRINSIC APTITUDE OF CATIONIC METHYLPALLADIUM AND ETHYLPALLADIUM TO ASSOCIATE ETHYLENE AND TO FURTHER UNDERGO SUBSEQUENT MIGRATORY INSERTION - A THEORETICAL-STUDY

Density Functional Theory (DFT) calculations at the B3LYP level and PC I-80 calculations have been carried out for the following reaction seq uence: (a) association of ethylene to cationic (sigma-alkyl)palladium( II) complexes, forming pi-ethylene sigma-alkyl intermediate, (b) subse quent migratory insertion, and (c) beta-hydride elimination of the ins ertion product. Ethylene coordinates strongly to the ''naked'' complex es PdCH3+ and PdC2H5+ (43.9 kcal/mol and 36.0 kcal/mol respectively). Nitrogen ligands modify association exothermicity (or conversely disso ciation endothermicity): Pd(NH3)(2)CH3+ (27.3 kcal/mol), Pd(NH3)(2)C2H 5+ (14.9 kcal/mol) and Pd(CHNH)(2)CH3+ (29.8 kcal/mol), where (CHNH)(2 ) is chelating diimine. Substantial agostic interaction between metal and beta-hydrogens and differences in charge on palladium account for the differences. The migratory insertion step is endothermic for naked 12-electron (pi-ethylene)(sigma-alkyl)palladium complexes and exother mic for 16-electron complexes with two nitrogen ligands. Calculated va lues for the barriers of migratory insertion agree remarkably with rep orted values experimentally found. Thus, the barrier for migratory ins ertion of PdCH3+(C2H4) is calculated to 18.3 kcal/mol and 18.0 kcal/mo l for the bisamine complex, whereas Brookhart recently found a value o f 18.5 kcal/mol for corresponding phenanthroline complex (see ref 1). The lowest value (16.4 kcal/mol) is calculated for Pd(CHNH)(2)CH3+(C2H 4) and the lowest found (17.2 kcal/mol) experimentally is for the diim ine complex Pd(HCN[2,6-C6H3(i-Pr)(2)])(2)CH3+(C2H4) (see ref 2). The p roduct of the insertion reaction readily undergoes beta-hydride elimin ation, which is thermoneutral for ''naked'' complexes and slightly end othermic (3.2-4.8 kcal/mol) for 16-electron complexes. The results sug gest that (a) DFT calculations at the B3LYP level seem to provide valu es close to experimentally found energetics for this type of organomet allic chemistry and (b) alkene coordination is sensitive to coordinate d ligands but insertion kinetics are less so.