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→C2H4
≈ 17 kcal/mol (ca. −45 °C) <
ΔG⧧R→C2H4
≈ 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 (K5(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.