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.