Counterion Engineering of Cationic Molecular Cage Host Modulates the Microenvironment of Palladium Clusters for Selective Hydrogenation

Developing highly efficient catalysts for the selective hydrogenation of nitroarenes─an environmentally benign approach for producing industrially relevant aniline intermediates─remains a significant challenge. While Pd catalysts are known for their high activity, they often suffer from poor selectivity in this reaction. Here, we introduce a strategy to enhance the selectivity of Pd clusters encapsulated within quaternary ammonium cages (QA-Cage) by engineering the surrounding counterions (F^–, I^–, and BF₄^–) to modify the Pd microenvironment. Notably, Pd⊂QA-Cage-F achieves complete conversion and exceptional selectivity (≥99%) toward nitro group reduction across various nitroarenes while preserving other reducible functional groups (e.g., halides) under mild conditions. Experimental results and Density functional theory calculations reveal that electrostatic interactions between counterions and the cationic cage, which vary in electronegativity ability, modulate the electronic structure of the cage and, in turn, regulate the electronic state of the encapsulated Pd surface. The tailored Pd surface, combined with C–H (from cage skeleton)···anion interactions, enhances substrate adsorption and configuration stabilization, and promotes the preferential adsorption of nitro groups on the electron-deficient Pd, enabling selective hydrogenation of substituted nitroarenes to the desired anilines.