Visible Light-Driven Cross-Coupling Reactions at Lower Temperatures Using a Photocatalyst of Palladium and Gold Alloy Nanoparticles

Abstract

Palladium (Pd)-catalyzed cross-coupling reactions are among the most important methods in organic synthesis. We report the discovery of highly efficient and green photocatalytic processes by which cross-coupling reactions, including Sonogashira, Stille, Hiyama, Ullmann, and Buchwald−Hartwig reactions, can be driven with visible light at temperatures slightly above room temperature using alloy nanoparticles of gold and Pd on zirconium oxide, thus achieving high yields. The alloy nanoparticles absorb visible light, and their conduction electrons gain energy, which is available at the surface Pd sites. Results of the density functional theory calculations indicate that transfer of the light excited electrons from the nanoparticle surface to the reactant molecules adsorbed on the nanoparticle surface activates the reactants. When the light intensity was increased, a higher reaction rate was observed, because of the increased population of photoexcited electrons. The irradiation wavelength also has an important impact on the reaction rates. Ultraviolet irradiation can drive some reactions with the chlorobenzene substrate, while visible light irradiation failed to, and substantially improve the yields of the reactions with the bromobenzene substrate. The discovery reveals the possibility of using low-energy and -density sources such as sunlight to drive chemical transformations.

Improving the rates of Pd-catalyzed reactions by exciting the surface plasmons of AuPd bimetallic nanotriangles

“…16,17,26 Aer the excitation of surface plasmons, they can decay non-radiatively via generation of hot electrons in the metal nanoparticles. The energy of these hot electrons is either redistributed among the ground state electrons through electron-electron scattering or dissipated in the form of heat by electron-phonon scattering.…”

“…As a result of this transfer, the number of active catalytic sites on Pd increases, and this improves the overall rate of the reaction. 16 Such hot electron transfer from Au to Pd was described by recording a single particle photoluminescence (PL) spectra from an Au-Pd nanorod and an Au nanorod. 35 The authors observed a decrease in the PL intensity for the AuPd nanorod compared to the Au nanorod, which was attributed to hot electron transfer from Au and Pd.…”

“…Also, heterogeneous Pd catalysts can be easily recovered from the reaction mixture and reused for several reaction cycles. Recently, by using plasmonic nanoparticles, the rates of Pd-catalyzed reactions such as crosscoupling, 16,17 hydrogenation, [18][19][20][21] and oxidation reactions [22][23][24] have been improved, and it has been found that bimetallic nanoparticles show superior activity compared to the pure metallic nanoparticles. 25,26 The most studied bimetallic catalysts in plasmon-enhanced Pd-catalyzed reactions are AuPd bimetallic nanoparticles.…”

“…and spherical nanoparticles 16,20,26,37 have been used to improve the catalytic activity of palladium. Here, we employed plasmonic Au nanotriangles to harvest light in order to photophysically improve the rates of Pd-catalyzed reactions.…”

“…21,29,30,44,45 However, in addition to the plasmonic heating effect, many reports have observed that plasmonic hot electron transfer contributes to the rate of a cross-coupling reaction. 16,17 In many of these reports, a high power laser source was used to excite the Au nanoparticle surface plasmons, whereas, in the current study, we used a relatively low power ($500 mW) and unfocused LED light source. The generation of plasmonic hot electrons is highly dependent on the power of the excitation light source.…”