Homogeneous versus supported ONN pincer-type gold and palladium complexes: catalytic activity.
The ONN-tridentate unsymmetrical pincer-type (S)-1-((6-(2-hydroxyphenyl)pyridin-2-yl)methyl)-N-methyl-N-(3-(triethoxysilyl)pro-pyl)pyrrolidine-2-carboxamide ligand has been synthesized by an easy method, in high purity and good yield. Its respective palladium(II) and gold(III) complexes have been prepared as air-stable solids. After deprotonation of the -OH group the coordination of the metal ion is completely stereospecific and gives rise to only one diastereoisomer. The complexes, immobilized onto ordered mesoporous silica (MCM-41), are shown to be very active catalysts in the hydrogenation of prochiral olefins (98% ee was achieved with the heterogenized chiral Au(MCM-41) complex), hydrosilylation, and Suzuki and Heck C-C coupling reactions, under mild conditions. The reactions were studied with the soluble catalysts as well as their heterogenized counterparts. The high accessibility introduced by the structure of the supports allows the preparation of highly efficient immobilized catalysts. The repeated use of the immobilized catalyst in four cycles demonstrates "homogeneous" catalysis with "heterogeneous" catalysts, thus reducing solvent waste and loss of precious metal and/or ligand.