Synthesis, Structures, and Norbornene Polymerization Behavior of Palladium Complexes Bearing Tridentate o-Aryloxide-N-heterocyclic Carbene Ligands

Abstract

A series of new pincer-type tridentate o-aryloxide-N-heterocyclic carbene ligands 2a–d were synthesized. Treatment of the proligands with Ag2O and (COD)PdCl2 afforded the desired o-aryloxide-NHC tridentate palladium complexes 3a–d in high yields (NHC = N-heterocyclic carbene). In comparison with the above tridentate complexes, bidentate bis(aryloxide-NHC) palladium complex 3e was also synthesized. All of these complexes were fully characterized by 1H and 13C NMR spectroscopy, high-resolution mass spectrometry, and elemental analysis. The molecular structures of 3a,b,d,e were determined by single-crystal X-ray diffraction analysis. On activation with either methylaluminoxane (MAO) or diethylaluminum chloride (Et2AlCl), all palladium complexes exhibited excellent activities of up to 5.99 × 107 g of PNB (mol of Pd)−1 h–1 toward norbornene addition polymerization, and the monomer conversion is up to 99.9%. Notably, the tridentate palladium complexes show better activities than the corresponding bidentate bis(...