Why Only the 2,6-Bis(o-Carborano)Pyridine-Stabilized Phosphenium Cation Has Succeeded in Splitting H₂?: Key Design Insights for Next-Gen Phosphenium Pincer Catalysts

Phosphenium pincer complexes have emerged as promising alternatives to transition metal catalysts for small-molecule activation. Among them, only the 2,6-bis(o-carborano)pyridine-stabilized phosphenium cation (1+) has been shown to activate molecular hydrogen (H2). This study investigates the origin of this unique reactivity by comparing 1+ with previously reported phosphenium cations by using density functional theory (DFT) calculations. Orbital analysis reveals that earlier phosphenium cations fail to exhibit metallomimetic H2 activation due to the inaccessibility of suitable molecular orbitals, stemming from the structural features of the ligands. In contrast, an in-depth examination of the H2 activation pathway by 1+ suggests that inducing ligand flexibility facilitates access to a reactive state through rehybridization at the phosphorus center. This hypothesis was tested by introducing flexible prototypical phosphenium cations, resulting in an ∼8 kcal/mol decrease in the activation energy for H2 splitting. Additionally, the interaction between the phosphorus atom and the nitrogen atom of the pyridine ring in the pincer ligand plays a critical role in stabilizing the cationic product of the reaction. These findings underscore the significant influence of ligand architecture on the reactivity of 1+ toward H2 activation. These structural features offer valuable design principles for developing next-generation phosphenium pincer complexes for small-molecule activation.