Mechanistic Studies of the Palladium(II)-Catalyzed Copolymerization of Ethylene with Carbon Monoxide

The microscopic steps responsible for the perfectly alternating copolymerization of ethylene and CO catalyzed by 1,10-phenanthroline (phen) based palladium complexes have been studied. Palladium carbonyl alkyl, carbonyl acyl, ethylene alkyl, and ethylene acyl complexes [(phen)Pd(R)(L)+Ar‘4B- (Ar‘ = 3,5-(CF3)2C6H3; R, L = CH3, CO (2); CH3, C2H4 (3); CH2CH3, C2H4 (7); C(O)CH3, CO (8); C(O)CH3, C2H4 (13); CH2CH2C(O)CH3, C2H4 (15); CH2CH2C(O)CH3, CO (16); C(O)CH2CH2C(O)CH3, C2H4 (17); C(O)CH2CH2C(O)CH3, CO (18)); and the β- and γ-keto chelate complexes (phen)PdCH2CH2C(O)CH3 + (14) and (phen)PdC(O)CH2CH2C(O)CH3 + (19)] have been prepared. An X-ray structure of the carbonyl acyl complex (phen)Pd(C(O)CH3)(CO)+Ar‘4B-·CH2Cl2 (8·CH2Cl2) has been obtained. The migratory insertion reactions of 2, 3, 7, 13, 16, and 17 have been studied by low-temperature NMR techniques. The barriers for insertion increase in the following order: ΔG ⧧ R → CO ≈ 15 kcal/mol (−66 °C) < ΔG ⧧ Ac → C 2 H 4 ≈ 17 kcal/mol (ca. −45 °C) < ΔG ⧧ R → C 2 H 4 ≈ 19 kcal/mol (−25 °C). The relative binding affinities of ethylene and CO to Pd methyl, acyl, and chelate complexes have been determined by combining stepwise measurements of the binding affinities of ligands with intermediate strength to (phen)Pd(CH3)(L)+ (CO > MeSPh > CH3CN ≈ C2H4 > C6H5CN ≫ OEt2) with relative equilibrium constants for ethylene/CO binding between acyl and alkyl complexes. The copolymerization mechanism has been determined from the kinetic and thermodynamic data. The catalyst resting state is a carbonyl acyl complex which is in equilibrium (K 5(25 °C) = (7.1 ± 3.5) × 10-4) with a less stable ethylene acyl intermediate which undergoes β-acyl migratory insertion to generate a Pd alkyl species followed by rapid reaction with 2 equiv of CO to reform the resting state. This model is tested by comparing calculated and experimental turnover frequencies.