Benzimidazolium-Pyrazole-Palladium(II) Complexes: New and Efficient Catalysts for Suzuki, Heck and Sonogashira Reactions

Three unsymmetrical benzimidazolium-pyrazole 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 packing of the R=Me compound at 223 K reveals one-dimensional zig-zag water chains stabilized by organic molecular channels through N center dot center dot center dot H-O and O-H center dot center dot center dot 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-coupling of unactivated aryl bromides and 5- or 6-membered heteroaryl bromides with arylboronic acids with turnover frequencies (TOF) reaching as 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-coupling reactions under aerobic conditions.