A thiosemicarbazone–palladium(II)–imidazole complex as an efficient pre-catalyst for Suzuki–Miyaura cross-coupling reactions at room temperature in aqueous media

Abstract

A Pd(II) complex of two sterically crowded ligands, specifically an N,S-donor thiosemicarbazone and an N-donor imidazole, has been synthesized and characterized by physicochemical and spectroscopic methods. X-ray single-crystal analysis revealed that the coordination geometry around the palladium center is distorted square planar, and the chloride ligand is involved in intermolecular bifurcated X–H···Y-type (where X = C, N and Y = Cl) hydrogen bonding. This complex proved to be a highly active and retrievable pre-catalyst for additive-free Suzuki–Miyaura cross-coupling reactions of arylboronic acids with aryl bromides or chlorides at room temperature and 60 °C, respectively. The reactions require a low catalyst loading and the complex is converted to ~1.5–2.0 nm-sized Pd nanoparticles (probably the real catalyst). The catalyst can be reused up to seven times without significant loss in activity. Since the reaction proceeds under mild conditions in aqueous medium and the catalyst is recoverable, it provides an environmentally benign alternative to the existing protocols for Suzuki–Miyaura reactions. Biaryls can be synthesized in high yield under greener reaction conditions in the presence of efficient pre-catalyst, thiosemicarbazone–palladium(II)–imidazole.