Synthesis and Structure of Imine–N-Heterocyclic Carbene Palladium Complexes and Their Catalytic Behavior in Norbornene Polymerization

On
the basis of the steric effects of ligand, a series of imine–N-heterocyclic
carbene (NHC) ligands and their corresponding five-membered palladium
complexes with bulky substituents on both the imine and the NHC moieties
were synthesized and characterized. Transpalladation of silver carbene
complexes with (COD)­PdCl2 and (COD)­PdMeCl afforded the
palladium dichloride and methylpalladium complexes, respectively.
Bulky cationic palladium complexes were further obtained by treatment
of the methylpalladium complexes with sodium tetrakis­(3,5-bis­(trifluoromethyl)­phenyl)­borate
(NaBAF) in CH3CN. Well-defined cationic palladium complexes
were confirmed by X-ray crystal diffraction to have trans forms. Palladium
dichloride complexes and methylpalladium complexes after activation
with MMAO show high activity for norbornene polymerization, whereas
cationic palladium complexes can polymerize norbornene alone without
any cocatalysts and exhibit a high thermostability. Norbornene polymerization
with the cationic palladium catalyst was proven to proceed through
a coordination–insertion mechanism by NMR studies. Analysis
of oligomers obtained by polymerizing the monomer in the presence
of H2 reveals the existence of a C7 linkage in the polynorbornene
(PNB) by σ-bond metathesis, which may be the reason for the
insolublity of polynorbornenes obtained by palladium catalysts.