Mechanistic analysis of transformative Pd/NHC catalyst evolution in the 1,2-diphenylacetylene semihydrogenation using molecular hydrogen

Abstract

This study explored the dynamic transformations of the Pd/NHC catalytic system during the semihydrogenation of alkynes using molecular hydrogen under mild reaction conditions. Focusing on the appearance of a “cocktail”-type system, we harnessed advanced characterization methods, such as NMR, ESI-HRMS, and SEM/TEM. In the hydrogenation process of 1,2–diphenylacetylene, the Pd/NHC complex evolves to produce novel palladium-based compounds and a heterogeneous phase, which partially precipitates into catalytically active nanoparticles. A pivotal finding revealed that the modification of the Pd nanoparticle surface by organic ligands under reaction conditions significantly influenced the catalytic activity of the Pd particles. The split test results suggest an enhanced selectivity toward (Z)-stilbene when the precipitate is maintained in the reaction mixture, highlighting that the palladium nanoparticles act as active catalysts (or reservoirs of active species) synergistically with dissolved molecular complexes. This research reports important findings for understanding the dynamics of the Pd/NHC system, contributing to the development of more efficient catalytic systems.