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Kinetics, Mechanism and Theoretical Studies of Norbornene-Ethylene Alternating Copolymerization Catalyzed by Organopalladium(II) Complexes Bearing Hemilabile ?-Amino-pyridine.

ABSTRACT: Cationic methylpalladium complexes bearing hemilabile bidentate ?-amino-pyridines can serve as effective precursors for catalytic alternating copolymerization of norbornene (N) and ethylene (E), under mild conditions. The norbornyl palladium complexes in the formula of {[RHNCH?(o-C?H?N)]Pd(C?H10Me)(NCMe)}(BF?) (R = iPr (2a), tBu (2b), Ph (2c), 2,6-Me?C?H? (2d), 2,6-iPr?C?H? (2e)) were synthesized via single insertion of norbornene into the corresponding methylpalladium complexes 1a-1e, respectively. Both square planar methyl and norbornyl palladium complexes exhibit facile equilibria of geometrical isomerization, via sterically-controlled amino decoordination-recoordination of amino-pyridine. Kinetic studies of E-insertion, N-insertion of complexes 1 and 2, and the geometric isomerization reactions have been examined by means of VT-NMR, and found in excellent agreement with the results estimated by DFT calculations. The more facile N-insertion in the cis-isomers, and ready geometric isomerization, cooperatively lead to a new mechanism that accounts for the novel catalytic formation of alternating COC.

SUBMITTER: Yu KH

PROVIDER: S-EPMC6152412 | biostudies-literature | 2017 Jun

REPOSITORIES: biostudies-literature

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Kinetics, Mechanism and Theoretical Studies of Norbornene-Ethylene Alternating Copolymerization Catalyzed by Organopalladium(II) Complexes Bearing Hemilabile α-Amino-pyridine.

Yu Kuo-Hsuan KH Huang Shou-Ling SL Liu Yi-Hung YH Wang Yu Y Liu Shiuh-Tzung ST Cheng Yuan-Chung YC Lin Ya-Fan YF Chen Jwu-Ting JT

Molecules (Basel, Switzerland) 20170630 7

Cationic methylpalladium complexes bearing hemilabile bidentate α-amino-pyridines can serve as effective precursors for catalytic alternating copolymerization of norbornene (N) and ethylene (E), under mild conditions. The norbornyl palladium complexes in the formula of {[RHNCH₂(o-C₆H₄N)]Pd(C₇H10Me)(NCMe)}(BF₄) (R = iPr (2a), tBu (2b), Ph (2c), 2,6-Me₂C₆H₃ (2d), 2,6-iPr₂C₆H₃ (2e)) were synth ...[more]

PMID: 28665348

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Project description:Since the discovery of α-diimine catalysts in 1995, an extensive series of Brookhart-type complexes have shown their excellence in catalyzing ethylene polymerizations with remarkable activity and a high molecular weight. However, although this class of palladium complexes has proven proficiency in catalyzing ethylene copolymerization with various polar monomers, the α-diimine nickel catalysts have generally exhibited a much worse performance in these copolymerizations compared to their palladium counterparts. Recently, Brookhart et al. reported a notable exception, demonstrating that α-diimine nickel catalysts could catalyze the ethylene copolymerization with some vinylalkoxysilanes effectively, producing functionalized polyethylene incorporating trialkoxysilane (-Si(OR)3) groups. This breakthrough is significant since Pd-catalyzed copolymerizations are commercially less usable due to the high cost of palladium. Thus, the utilization of Ni, given its abundance in raw materials and cost-effectiveness, is a landmark in ethylene/polar vinyl monomer copolymerization. Inspired by these findings, we used density functional theory (DFT) calculations to investigate the mechanistic study of ethylene copolymerization with vinyltrimethoxysilane (VTMoS) catalyzed by Brookhart-type nickel catalysts, aiming to elucidate the molecular-level understanding of this unique reaction. Initially, the nickel complexes and cationic active species were optimized through DFT calculations. Subsequently, we explored the mechanisms including the chain initiation, chain propagation, and chain termination of ethylene homopolymerization and copolymerization catalyzed by Brookhart-type complexes. Finally, we conducted an energetic analysis of both the in-chain and chain-end of silane enchainment. It was found that chain initiation is the dominant step in the ethylene homopolymerization catalyzed by the α-diimine Ni complex. The 1,2- and 2,1-insertion of vinylalkoxysilane exhibit similar barriers, explaining the fact that both five-membered and four-membered chelates were identified experimentally. After the VTMoS insertion, the barriers of ethylene reinsertion become higher, indicating that this step is the rate-determining step, which could be attributed to the steric hindrance between the incoming ethylene and the bulky silane substrate. We have also reported the energetic analysis of the distribution of polar substrates. The dominant pathway of chain-end -Si(OR)3 incorporation is suggested as chain-walking → ring-opening → ethylene insertion, and the preference of chain-end -Si(OR)3 incorporation is primarily attributed to the steric repulsion between the pre-inserted silane group and the incoming ethylene molecule, reducing the likelihood of in-chain incorporation.

Dataset Information

Kinetics, Mechanism and Theoretical Studies of Norbornene-Ethylene Alternating Copolymerization Catalyzed by Organopalladium(II) Complexes Bearing Hemilabile ?-Amino-pyridine.

Publications

Kinetics, Mechanism and Theoretical Studies of Norbornene-Ethylene Alternating Copolymerization Catalyzed by Organopalladium(II) Complexes Bearing Hemilabile α-Amino-pyridine.

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