Can silaborane serve as an effective bridging unit for Lewis pairs? A comparative discourse with carborane

Frustrated Lewis pairs (FLPs) have emerged as versatile metal-free catalysts for small-molecule activation, and strategies to control frustration through geometric immobilization and electronic tuning remain a topic of significant interest. Carborane cages, with their unique coordination-dependent electronic effects, have recently been shown to modulate FLP frustration. In this work, we introduce <i>o</i>-silaborane cages, whose coordination-based electronic effects had not previously been explored for modulating FLP reactivity. The -BH₂ and -PH₂ substituents were strategically placed at distinct coordinating sites of the cages to systematically compare the coordination dichotomy of <i>o</i>-carborane and <i>o</i>-silaborane frameworks in influencing the frustration of Lewis pairs. CO₂ activation was employed as the model reaction to assess and compare these effects. Electronic structure analysis reveals that silaborane exerts stronger electron-withdrawing and electron-donating influences than carborane, leading to pronounced differences in acidity-basicity balance, strain distribution, and the stability of CO₂ complexes. Furthermore, transition-state energetics demonstrate that site-dependent positioning of Lewis acid-base centres critically governs activation barriers. In the <i>o</i>-carborane framework, the acid and base groups positioned at the 1,4-sites exhibit the highest reactivity, whereas in the <i>o</i>-silaborane system, the 4,9-substituted arrangement is identified as the most reactive. These findings establish silaborane as a promising bridging unit, opening new design pathways for tuneable FLP-based CO₂ utilization and broader small-molecule activation.