NHC-based Pd–PEPPSI complexes: Synthesis, characterization, DFT studies and catalytic activity in Suzuki–Miyaura cross coupling

Abstract

Three new Pd-PEPPSI complexes (2a-c) with the general formula [PdBr₂(NHC)(Py)] were prepared via unsymmetrical benzimidazolium salts containing a 1-3-dioxane group on one of the nitrogen atoms and cyanopropyl, cyclobutyl or 2-methylbenzyl on the other side. All the compounds were isolated in excellent yields (68–85 %) and fully characterized using ¹H and ¹³C NMR, FT-IR, and elemental analysis. Additionally, the DFT (Density functional Theory) calculations carried out on complexes 2a–c offers valuable insights on topics like physicochemical characteristics and reactivity tendencies, as well as further validations to the experimentally obtained data of examined Pd complexes. With regard to structural parameters, experimental and computational findings were revealed to be in alignment. A comparative analysis was conducted of the electronic properties, possible donor-acceptor interactions, and electrostatic surface properties of 2a-c. A close examination of the computational findings reveals that cyanopropyl substitution leads to a relative increase in structural stability. Also, Pd-PEPPSI-type complexes with NHC ligands were applied as catalyst precursors in Suzuki cross-coupling reactions forming a wide range of biaryls with high yields. Therefore, the catalytic activities of the synthesized complexes in the Suzuki reaction were tested. The reactions were carried out at very low catalyst loading (0.25 mol%) in the presence of air using a green water-based solvent (i-PrOH:H₂O/1:3). The results demonstrated that the catalysts exhibited high activity in this reaction.