Tunable bis(pyridinium amidate) ligands efficiently promote palladium-catalyzed ethylene polymerization

A useful strategy for the co-polymerization of ethylene and functional olefins relies on palladium catalysts, as palladium typically shows in contrast to many other metals a high tolerance to a variety of functional groups. Here we have prepared a set of palladium complexes containing a <i>N</i>,<i>N</i>-bidentate coordinating bis(pyridinium amidate) (bisPYA) ligand. Ligand variation included either <i>para</i>- or an <i>ortho</i>-pyridinium amidate arrangement, with the pyridinium site either sterically flexible or locked through a dimethyl substitution <i>ortho</i> to the amidate. Activation of these complexes with NaBArF in the presence of ethylene indicated that sterically locked ligand structures promoted ethylene conversion and produced polymeric materials. In particular, complex 4d with an <i>ortho</i>-pyridinium amidate bisPYA ligand was active with a production of 10.8 kg polyethylene per mol palladium at room temperature and 1 bar ethylene. Synthesis of the complexes in the presence of K₂CO₃ or Ag₂CO₃ afforded adducts in which the K⁺ or Ag⁺ ion is bound by the two oxygens of the bisamidate core, thus leading to trimetallic Pd⋯K⋯Pd complexes. Such adduct formation indicates a dual role of NaBArF in halide abstraction and metal sequestration, thus rationalizing the need for 2.5 equivalent of NaBArF per palladium complex for effective polymerization.