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

Several CNN pincer Pd(II) complexes including chiral complexes <b>1a</b>-<b>e</b> with 2-phenyl-6-(oxazolinyl)pyridines and achiral ones <b>2a</b>-<b>c</b> with <i>N</i>-substituted-2-aminomethyl-6-phenylpyridines were prepared. In addition, the preparation of the achiral PCN pincer Pd(II) complexes <b>3a</b>-<b>e</b> with aryl-based phosphinite-imine ligands and chiral <b>4a</b>-<b>c</b> with aryl-based phosphinite-imidazoline ligands was also performed. Among them, the PCN Pd(II) pincers <b>3a</b>-<b>e</b> 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 <b>3a</b> 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.