Synthesis, structures, and norbornene polymerization behavior of C(sp3), N-chelated palladacycles bearing o-aryloxide-N-heterocyclic carbene ligands

Abstract

Treatment of the o-hydroxyaryl imidazolium pro-ligands (2-OH-3,5-tBu₂C₆H₂)(R)(C₃H₃N₂)⁺Br⁻ [R = Mes (1a), Ph (1b), iPr (1c), Me (1d)] with Ag₂O afforded the corresponding silver complexes 2a–d. Subsequent metal-exchange reactions of 2a–d with [Pd(OAc)(8-Me-quin-H)]₂ (3) yielded the desired C(sp³), N-chelated and o-aryloxide-NHC-ligated palladacycle complexes 4a–d in 60–80% yields. When the N-tert-butyl substituted o-hydroxyaryl imidazolium pro-ligand 1e reacted with 3 in the presence of K₂CO₃ in dioxane, the palladacycle complex 4e, in which the NHC adopted an abnormal binding (C4-bonding), was obtained in 20% yield. All these complexes were fully characterized using ¹H and ¹³C NMR spectra, high-resolution mass spectrometry (HRMS), and elemental analysis. Single-crystal X-ray diffraction analysis results further confirmed the molecular structures of 4a–c and the abnormal binding of NHC in 4e. With methylaluminoxane (MAO) as the cocatalyst these palladacycles showed excellent catalytic activities of up to 10⁷ g of PNB (mol of Pd)⁻¹ h⁻¹ in the addition polymerization of norbornene.