Olefin Dimerization and Isomerization Catalyzed by Pyridylidene Amide Palladium Complexes

A series of cationic palladium complexes [Pd(N^N')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 (ortho-, meta-, and para-PYA). Structural investigation in solution revealed cis/trans isomeric ratios that correlate with the donor properties of the PYA ligand, with highest cis ratios for the complex having the most donating ortho-PYA ligand and lowest ratios for that with the weakest donor para-PYA system. The catalytic activity of the cationic complexes [Pd(N^N')Me(NCMe)]+ in alkene insertion and dimerization showed a strong correlation with the ligand setting. While complexes bearing more electron-donating meta- and ortho-PYA ligands produced butenes within 60 and 30 min respectively, the para-PYA complex was much slower and only reached 50% conversion of ethylene within 2 h. Likewise, insertion of methyl acrylate as 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 to detect, 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.