Solvated Pd Atoms as Precursors of NHC-stabilized Pd NPs: the Role of the Ligand in Limonene Hydrogenation

Transition metal-nanoparticles (MNPs) have received a great deal of attention in fields such as organometallic drugs, materials, and catalysis. The choice of an appropriate stabilizer (e.g. polymers, surfactants or ligands) is a crucial aspect to consider for the synthesis of MNPs in order to finely control their size, shape and surface state. Due to their strong coordination properties, N-heterocyclic carbenes (NHC) are excellent stabilizers for MNPs, able to tune their surface properties and consequently their catalytic behavior. NHC-stabilized MNPs have been obtained by using organometallic complexes as precursors [1] or by a ligand-exchange procedure. We report herein a new route to the synthesis of a long chained NHC-stabilized Pd NPs (NHC-Pd NPs) by exploiting the metal vapor synthesis (MVS) technique (Figure 1). This synthetic approach provides access to various NHC-M NPs with tailored size and composition (heterometallic M NPs). The NHC-Pd NPs were characterized by TEM (dm = 2.5 nm), NMR and TGA and further immobilized onto a carbonaceous support (Vulcan XC-72). The catalytic efficiency of carbon-supported NHC-Pd NPs was evaluated in the hydrogenation of R-(+)-limonene under mild conditions (P(H2) = 5 Bar, T = 30°C). The NHC surface modification of Pd NPs resulted in significant differences in reaction selectivity compared to analogous carbon-supported Pd NPs synthesized without the ligand, affording high yield toward (+)-p-1-menthene product.