“Hidden” Nanoscale Catalysis in Alkyne Hydrogenation with Well-Defined Molecular Pd/NHC Complexes

Pd/NHC complexes are widely used as catalysts in hydrogenation reactions. Usually, the operating mode of these systems is referred to as homogeneous. In this work, we demonstrated that mixed homogeneous–heterogeneous catalysis can be realized in the hydrogenation reaction when Pd/NHC complexes were used as precatalysts. Palladium NPs are formed in situ and act as “hidden” nanoscale catalysts. Based on the quantum chemical calculations and experimental XPS results, the presence of surface NHC ligands on metal nanoparticles can be proposed. Herein, we propose a method for the determination of dynamic transformations of Pd/NHC complexes in transfer hydrogenation reactions via 13C labeling and NMR spectroscopy. This approach is based on the introduction of a 13C label in the C2 position of the imidazolium fragment of Pd/NHC, which is unique to the M–NHC bond. It was found using NMR, ESI-MS, and TEM monitoring of the transfer semihydrogenation of diphenylacetylene that Pd/NHC complexes disappear from the reaction mixtures at the early stage of reaction. Palladium atoms pass into a heterogeneous phase, forming NPs with sizes ranging from 1 to 9 nm. The experimental study and calculations performed in the present study revealed the role of the ligands on the surface of metal nanoparticles. Comparative modeling of hydrogenation reactions on ligand-free and NHC-modified Pd clusters showed that modification of the metal surface increased the catalytic activity by reducing the potential barriers of the alkyne syn-addition and reductive elimination stages. Since the presence of an NHC ligand in the catalytic system leads to a change in the rate-limiting stage of the reaction, we proposed a combined reaction mechanism, according to which oxidative addition proceeds on a bare metal surface, and the remaining two stages occur in the modified zone of NPs.