Synthesis, Structures, and Norbornene Polymerization Behavior of Bis(aryloxide-N-heterocyclic carbene) Palladium Complexes

A series of o-hydroxyaryl imidazolinium pro-ligands 3a−3e were synthesized by the reactions of 4-bromo-2,4,6-tri-tert-butyl-2,5-cyclohexadien-1-one (1) with different N-substituted imidazoles. Treatment of the pro-ligands with Pd(OAc)2 afforded the corresponding bis(aryloxide-NHC) palladium complexes (NHC = N-heterocyclic carbene). The N-substituents at the NHCs were found to have significant effects on the structures of the bis(aryloxide-NHC) palladium complexes. Trans isomers were obtained when R = Me and iPr, while cis isomers were obtained for R = Ph and Mes (Mes = mesityl). However, for R = tBu, a cis isomer with a normal and an abnormal NHC ligand was obtained. All these complexes have been characterized by 1H and 13C NMR and HRMS spectroscopy. The molecular structures of the pro-ligand 3b and complexes 4−8 were determined by single-crystal X-ray diffraction analysis. With methylaluminoxane (MAO) as cocatalyst these bis(aryloxide-NHC) palladium complexes showed excellent catalytic activities up to 107 g of PNB (mol of Pd)−1 h−1 in the addition polymerization of norbornene.