Presence Of Palladium Nanoparticles Research Articles

Suzuki cross-coupling reaction catalyzed by sulfur-containing palladacycles: Formation of palladium active species

Air-stable cyclopalladated compounds derived from the ortho-metalation of benzylic tert-butyl thioethers efficiently promote C–C coupling reactions and were studied mechanistically in the Suzuki reaction. The catalytic reaction was monitored by 19F NMR and GC–MS and the decomposition of the S-palladacycle was studied under stoichiometric conditions, showing that these palladacycles serve as a reservoir of zerovalent palladium species. The formation of these species is initiated by attack of the arylboronic acid on the palladacycle resulting in the formation of an arylated palladacycle. Thereafter, this species undergoes a reductive elimination to form the active Pd(0) species. As a minor pathway, the Pd(0) species could also be generated in a pathway involving a reduction process that leads to the decomposition of the palladacycle to Pd(0) affording the ortho hydrogenated thioether. Poisoning studies and TEM analysis indicated the presence of palladium nanoparticles. Competitive experiments showed that electron-withdrawing substituents on the aryl halide and electron-donating substituents on the arylboronic acid facilitated the reaction. They also provide evidence that the soluble species formed from the oxidative addition, that are active for Heck and Suzuki reactions, are the same species.

FTIR Study of the Mode of Binding of the Reactants on the Pd Nanoparticle Surface during the Catalysis of the Suzuki Reaction

In the Suzuki reaction between phenylboronic acid and iodobenzene catalyzed by palladium nanoparticles, our previous studies suggested that the phenylboronic acid adsorbs on the nanoparticle surface and then interacts with the iodobenzene that is present in solution. In the present study, FTIR is used to examine the change in the vibrational frequencies of phenylboronic acid in films with and without the addition of palladium nanoparticles. The large change in the B-O stretching frequency of phenylboronic acid from 1348 to 1376 cm(-1) in the presence of sodium acetate and palladium nanoparticles strongly suggests that the mode of binding of phenylboronic acid to the Pd nanoparticle surface involves a B-O-Pd type of bonding. Shifts in the B-C stretching mode and the out-of-plane phenyl C-C ring deformation bands associated with phenylboronic acid provide additional confirmations of the binding process. It is also shown that the phenylboronic acid needs to be in the deprotonated form in the presence of sodium acetate (phenylboronate anion) to bind to the palladium nanoparticle surface. No changes in the characteristic bands of iodobenzene were observed in films made in the presence of the palladium nanoparticles. The FTIR studies provide proof of the mode of binding that occurs in the nanoparticle surface for the first time and also confirms the mechanism of the Suzuki reaction that we proposed previously.