Bis(Imino)Acenaphthene (BIAN)-Supported Palladium(II) Carbene Complexes as Effective C-C Coupling Catalysts and Solvent Effects in Organic and Aqueous Media
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The synthesis and catalytic properties of two new 1,2-acenaphthenyl N-heterocyclic carbene-supported palladium(II) catalysts are presented. The acenaphthenyl carbene has been prepared with mesityl or 1,5-diisopropyl N-aryl substituents. Comprehensive catalytic studies for the Suzuki coupling of aryl halides with aryl boronic acids have been conducted. In general, the diisopropyl-functionalised catalyst showed superior selectivity and reactivity. A comparison of the catalytic performances in dichloromethane, toluene and water at low temperatures (30- 40 degrees C) is also presented. Both catalysts were proficient in the homogeneous Suzuki coupling of aryl iodides, bromides and chlorides with boronic acids in dichloromethane. Similar reactions in water led to the formation of insoluble colloidal catalytic species that still exhibited high activity in the Suzuki reaction with aryl chlorides. Reactions performed in toluene showed intermediate results; partial catalyst decomposition led to concomitant homogeneous and heterogeneous catalysis. The heterogeneous palladium precipitates could be easily recovered by filtration and reactivated for subsequent use. Activation energies determined for aryl bromide-based Suzuki reactions were found to be in the range of 159-171 kJ mol(-1) in organic solvents and 111-116 kJ mol(-1) in water. The corresponding activation energy for the aryl chloride was found to be 322 kJ mol(-1) in water.