期刊
ORGANOMETALLICS
卷 37, 期 20, 页码 3619-3630出版社
AMER CHEMICAL SOC
DOI: 10.1021/acs.organomet.8b00422
关键词
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资金
- Swiss National Science Foundation [200021_162868, 206021_128724, 206021_170755]
- European Research Council [CoG 615653]
- Italian MIUR [20154 X 9ATP_005]
- COST (CARISMA) [CM1205]
A series of cationic palladium complexes [Pd-((NN)-N-boolean AND')Me(NCMe)](+) was synthesized, comprising three different N<^>N'-bidentate coordinating pyridyl-pyridylidene amide (PYA) ligands with different electronic and structural properties depending on the PYA position (o-, m-, and p-PYA). Structural investigation in solution revealed cis/trans isomeric ratios that correlate with the donor properties of the PYA ligand, with the highest cis ratios for the complex having the most donating o-PYA ligand and lowest ratios for that with the weakest donor p-PYA system. The catalytic activity of the cationic complexes [Pd((NM)-M-boolean AND)Me(NCMe)](+) in alkene insertion and dimerization showed a strong correlation with the ligand setting. While complexes bearing more electron donating m- and o-PYA ligands produced butenes within 60 and 30 min, respectively, the p-PYA complex was much slower and only reached 50% conversion of ethylene within 2 h. Likewise, insertion of methyl acrylate as a polar monomer was more efficient with stronger donor PYA units, reaching a 32% ratio of methyl acrylate vs ethylene insertion. Mechanistic investigations about the ethylene insertion allowed detection, for the first time, by NMR spectroscopy both cis- and trans-Pd-ethyl intermediates and, furthermore, revealed a trans to cis isomerization of the Pd-ethyl resting state as the rate-limiting step for inducing ethylene conversion. These PYA palladium complexes induce rapid double-bond isomerization of terminal to internal alkenes through a chain-walking process, which prevents both polymerization and also the conversion of higher olefins, leading selectively to ethylene dimerization.
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