Water-Soluble Palladium Click Chelating Complex: An Efficient and Reusable Precatalyst for Suzuki-Miyaura and Hiyama Reactions in Water

A water-soluble ionic palladium(II) nitrogen-containing chelating complex, [palladium(II) 1-(4-N,N,N-trimethylbutylammonium)-4-(2-pyridyl)-1H-1,2,3-triazole dichloride] chloride (3), was prepared through the click reaction of 1-chloro-4-bromobutane, sodium azide, and 2-ethynylpyridine, followed by the quarternization of Me3N and subsequent reaction with [Pd(cod)Cl2] (cod=1,5-cyclooctadiene). The catalytic performances of complex 3 were preliminarily evaluated through Suzuki-Miyaura and Hiyama cross-coupling reactions of aryl bromides; excellent catalytic activity in water was observed. TEM analysis revealed that small palladium nanoparticles (NPs) with a narrow size distribution were formed after the catalytic reaction. The NPs were stabilized by the synergetic effect of coordination and electrostatic interactions from the ionic, bidentate, nitrogen-containing ligand; no palladium black was detected after the aqueous solution of palladium NPs was stored in air for months. The use of 3 as a precursor in the formation of palladium NPs was further explored by using NaBH4 and hydrogen as reductive reagents. The resulting NPs displayed different sizes, surface properties, and catalytic performances in the Suzuki-Miyaura cross-coupling reaction in water.