Synthesis, Structures, and Norbornene Polymerization Behavior of N-Heterocyclic Carbene-Sulfonate-Ligated Palladacycles

Abstract

A series of new N-heterocyclic carbene-sulfonate (NHC-sulfonate) ligands 5a–e were synthesized. Treatment of the NHC-sulfonate ligands with Ag2O and palladacycles {[Pd(OAc)(8-Me-quin-H)]2 or [Pd(dmba)(μ-Cl)]2 (dmba = Me2NCH2C6H5)} yielded the desired C(sp3),N-chelated and C(sp2),N-chelated NHC-sulfonate palladacycles 6a–e and 7a–e in high yields. All these complexes were fully characterized by 1H and 13C NMR, high-resolution mass spectrometry, and elemental analysis. The molecular structures of compounds 5a, 6d, 6e, and 7e were determined by single-crystal X-ray diffraction analysis. In the presence of MAO, the C(sp3),N-chelated NHC-sulfonate palladacycles 6a–e showed excellent catalytic activities [107 g of polynorbornene (PNB) (mol of Pd)−1 h–1], while the C(sp2),N-chelated palladacycles 7a–e showed moderate catalytic activities [106 g of PNB (mol of Pd)−1 h–1] in the vinyl polymerization of norbornene. The C(sp2),N-chelated palladacycles 7a–e showed high thermal stability and reached the highest activities at high temperature (100 °C).