Insights into the alternating vs. non-alternating copolymerization of ethylene and CO catalyzed by P,O-coordinated nickel and palladium diphosphazane monoxide complexes

Abstract

Copolymerization of ethylene with carbon monoxide (CO), catalyzed by late transition metal complexes, is a promising approach for producing functionalized polymers. P,O-coordinated nickel and palladium diphosphazane monoxide complexes (Ni-PNPO and Pd-PNPO) have recently shown distinct differences in selectivity and activity: Ni-PNPO yields polyketones without extra ethylene incorporation, while Pd-PNPO exhibits lower activity and incorporates additional ethylene. Herein, we performed density functional theory (DFT) calculations to investigate the chain-growth step of this copolymerization. Two mechanistic pathways were examined: (i) the non-alternating path involving consecutive ethylene (C<inf>2</inf>H<inf>4</inf>) insertions and (ii) the alternating path involving sequential CO and ethylene insertions. Our results reveal that CO coordination to the β-carbonyl chelate starting complex is significantly more favorable than ethylene coordination, primarily due to the stronger σ-donation of CO, rather than the commonly assumed π-back-donation. In the alternating path, CO insertion into the metal–C(alkyl) bond of the β-carbonyl chelate complex to form a γ-carbonyl chelate intermediate is facile, while the subsequent ethylene insertion into the metal–C(acyl) bond is rate-determining. For Ni-PNPO, shorter metal–ligand distances and a larger bite angle essentially destabilize the six-membered γ-carbonyl chelate, lowering the free energy required for ethylene insertion. In contrast, Pd-PNPO stabilizes this γ-carbonyl chelate, increasing the ethylene insertion barrier, in correspondence to its lower catalytic activity. Although the alternating path is favored for both metals, the smaller free energy difference between the ethylene insertion into the metal–C(acyl) bond and the decarbonylation of Pd enables competing non-alternating insertions, resulting in extra ethylene incorporation. These insights into metal-dependent activity and selectivity will assist with the rational design of efficient late-transition metal catalysts.