Synthesis, Characterization, and Comparative Study on Norbornene Polymerization of CNN and PCN Pincer Palladium...

Several CNN pincer Pd(II) complexes including chiral complexes 1a-e with 2-phenyl-6-(oxazolinyl)pyridines and achiral ones 2a-c with N-substituted-2-aminomethyl-6-phenylpyridines were prepared. In addition, the preparation of the achiral PCN pincer Pd(II) complexes 3a-e with aryl-based phosphinite-imine ligands and chiral 4a-c with aryl-based phosphinite-imidazoline ligands was also performed. Among them, the PCN Pd(II) pincers 3a-e were new complexes and were readily synthesized from commercially available materials in only two steps. The new complexes were characterized through elemental analyses, namely ¹H NMR, ¹³C{¹H} NMR, and ³¹P{¹H} NMR spectroscopies. Furthermore, the molecular structure of complex 3a was determined via X-ray single-crystal diffraction analysis. In the presence of EtAlCl₂, Et₂AlCl, or methylaluminoxane (MAO), the CNN pincer Pd(II) complexes and PCN pincer Pd(II) complexes exhibited excellent activities and monomer conversion rates in norbornene addition polymerization. Surprisingly, the CNN pincer Pd(II) complexes exhibited a higher conversion rate (99.5%) with Et₂AlCl as the cocatalyst, while the PCN pincer Pd(II) complexes showed a higher conversion rate (98.8%) with MAO.