Mechanistic interpretation of selective catalytic hydrogenation and isomerization of alkenes and dienes by ligand deactivated Pd nanoparticles

Abstract

Unsupported thiolate-capped palladium nanoparticle catalysts are found to be highly substrate-selective for alkene hydrogenation and isomerization. Steric and poisoning effects from thiolate ligands on the nanoparticle surface control reactivity and selectivity by influencing alkene adsorption and directing either di-σ or mono-σ bond formation. The presence of overlapping p orbitals and α protons in alkenes greatly influences the catalytic properties of deactivated palladium nanoparticles leading to easily predictable hydrogenation or isomerization products.

Effects of Noncovalent Interactions on the Catalytic Activity of Unsupported Colloidal Palladium Nanoparticles Stabilized with Thiolate Ligands

“…23–25 The formation of hydrogenation products would require the addition of two surface Pd–H to di- σ -Pd alkyl intermediate B . 42 The presence of thiolate ligands on PdNP surface would make this direct hydrogenation process kinetically slower than the process requiring the addition of only single Pd–H.…”

“…25,42 The previous work from our group has shown that the presence of activating groups such as phenyl group in the substrate increases the formation of di- σ -bonded Pd-alkyl species as the results of the overlapping p orbitals on the surface of octanethiolate-capped PdNP (C8 PdNP). 25 It appears that the formation of di- σ -bonded Pd-alkyl species on C6 PdNP also boosted the reaction of 1-phenyl-2-propen-1-ol, resulting in the increased formation of hydrogenation products. In contrast, the catalytic reaction of 1-phenyl-2-propen-1-ol with Ph PdNP resulted in ∼87% combined yields for the isomerization and hydrogenolysis products but only ∼13% yields for the hydrogenation product (entry 2).…”

“…25,29 On the basis of the improved selectivity of Ph PdNP toward the mono- σ -bonded Pd-alkyl intermediate over the di- σ -bonded Pd-alkyl intermediate (Table 2, entry 2), the catalytic conversion of aromatic allylic alcohols in polar protic solvent, methanol, was also attempted.…”

“…21–24 The same catalysts have also been efficient for the isomerization of terminal alkenes to internal alkenes, the selective hydrogenation of dienes to alkenes, and the tandem hydrogenation–isomerization of propargylic alcohols to saturated carbonyls. 25,26 The good catalytic activity and selectivity of these catalysts are due to the lower ligand surface coverage of PdNP synthesized using the thiosulfate protocol, which provides an adequate balance between the partial poisoning of excessively active Pd sites and the suitable accessibility of substrates to the remaining active Pd surface. 27 …”

Alkanethiolate-capped palladium nanoparticles for selective catalytic hydrogenation of dienes and trienes

“…25 Building upon this previous work, in this article we investigate the catalytic applications of C8 PdNP for the selective hydrogenation of various conjugated dienes and trienes. The mechanism for the high selectivity of C8 PdNP is proposed based on kinetic studies and catalysis results for various substrates.…”