Linkage Isomers of Triangular Pd Metallacycles and Catalysis in Aqueous Suzuki Coupling Reaction.

The water-soluble trinuclear Pd metallacycles [Pd(tmeda)(4-Spy)] 3 (X) 3 (tmeda = tetramethylethylenediamine, X = OTf, 2 ; NO 3 , 3 ) were synthesized from the ambidentate ligand 4-pyridylthiolate (Spy - ) and [Pd(tmeda)X 2 ] in 80 and 70% yield, respectively. Two possible linkage isomers are found in solution (slow interconversion found in the NMR) and in the solid state. Density functional calculations showed that the energy of the isomer with a D 3 -symmetric arrangement of the SPy ligand and all Pd atoms having N∧NPdSN coordination is only 7 kcal/mol lower. When reacting [Pd(tmeda)(NO 3 ) 2 ] with 4,4'-biphenyldithiolate (S 2 bph 2- ), the tetranuclear [{Pd(tmeda)} 4 (μ-S 2 bph) 2 ](NO 3 ) 4 ( 1 ) was formed. A new type of undecanuclear Pd cluster was separated as a minor product from an acetone solution of 2 in air. The new complexes represent the first examples of water-soluble Pd metallacycles constructed from a pyridine-thiolate ligand. They show catalytic activity with turnover numbers ranging from 9 to 420 in aqueous Suzuki cross-coupling reactions using phenyl boronic acid and a number of aryl halides. An optimized system gave a TON of 6,900,000 and a TOF of 492,857 h -1 . The catalyst could be reused eight times, and the activity has been attributed to the formation of PdNPs.