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; NO₃, 3) were synthesized from the ambidentate ligand 4-pyridylthiolate (Spy^-) and [Pd(tmeda)X₂] 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 D₃-symmetric arrangement of the SPy ligand and all Pd atoms having N∧NPdSN coordination is only 7 kcal/mol lower. When reacting [Pd(tmeda)(NO₃)₂] with 4,4'-biphenyldithiolate (S₂bph^2-), the tetranuclear [{Pd(tmeda)}₄(μ-S₂bph)₂](NO₃)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.