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)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 boolean AND NPdSN coordination is only 7 kcal/mol lower. When reacting [Pd(tmeda)(NO3)(2)] with 4,4 '-biphenyldithiolate (S(2)bph(2-)), the tetranuclear [{Pd(tmeda)}(4)(mu-S(2)bph)(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.