Substituted NHC–PEPPSI–Pd(ii) complexes: synthesis, characterization, DFT calculations and catalytic application in thiazole C–H arylation

Abstract

In this study, a series of functionalized benzimidazolium salts and their PEPPSI-type Pd(II)NHC complexes 3a–g were synthesized and characterized by FT-IR, ¹H NMR and ¹³C NMR spectroscopy, elemental analysis, and mass spectrometry. Quantum chemistry computations at the B3LYP/6-311G(d,p)/LANL2DZ level were used to examine the molecular structure, electronic characteristics, and chemical reactivity of the ligand and its Pd complex. The new salt and PEPPSI-type Pd-NHC complex were fully characterized by spectroscopic and analytical methods. Under the optimized reaction conditions, the catalytic activity of the NHC–PEPPSI–Pd(II) complexes was tested by direct arylation of 2-n-propylthiazole and 4,5-dimethylthiazole with various aryl halides at 120 °C for 1 h and 3 h, respectively. It was observed that the palladium complexes gave high yields and were selective at the C₅ and C₂-positions of heteroaryl derivatives. Complex 3f, with an NHC substituted by a 3,4,5-trimethoxybenzyl group, showed the highest conversion probably due to the flexibility, electronic and mostly steric effects of this group.