Benzimidazolium-pyrazole-palladium(II) complexes: New and efficient catalysts for Suzuki, Heck and Sonogashira reactions

Abstract

Three unsymmetrical benzimidazoliumpyrazole N-N ligands 2-(1-propylbenzimidazolylmethyl)-3,5-di-R-pyrazole (R=H, Me, t-Bu) have been conveniently prepared and structurally analyzed. The solid lattice packingof the R=Me compound at 223 K reveals one-dimensional "zig-zag" water chains stabilized by organic molecular channels through N⋯H-O and O-H⋯O bonding. These hybrid ligands add to palladium(II) to give high yields of air-stable complexes that are fully characterized by NMR, ESI, and X-ray single-crystal crystallography. They are active Suzuki catalysts at room temperature towards cross-couplingof unactivated aryl bromides and 5- or 6-membered heteroaryl bromides with arylboronic acids with turnover frequencies (TOF) reachingas high as 60,000 h-1. Their catalytic efficiency is significantly better than that of the C-N carbene-imidazole analogue. These catalysts are also active in Heck and Sonogashira cross-coupling reactions of aryl bromides giving the desired products in good yields. These results suggested that these Pd(II) complexes with N-based hybrid ligands are versatile and efficient catalysts for different types of cross-couplingreactio ns under aerobic conditions. © 2008 Wiley-VCH VerlagGmbH & Co. KGaA.