A Comparative Theoretical Study of Water Reduction by B/P-Based Frustrated Lewis Pairs

Water reduction using p-block compounds has recently gained attention over traditional methods involving transition-metal complexes. Building on prior work on boron-phosphorus-based frustrated Lewis pairs (FLPs), specifically the bisborylphosphine (BPB) and borylphosphine (BP) systems, we here use density functional theory (DFT) to model water splitting, in which the nucleophilic substitution step plays a key role. Understanding this step computationally is crucial for deriving design rules for water splitting. However, the complete mechanistic details and the influence of substituents on the computed nucleophilic-substitution energy barrier remain unclear. In this theoretical study, we calculate reaction profiles for BPB and BP while systematically varying the substituents on borane (R1) and phosphorus (R2) to modulate steric and electron-withdrawing/donating properties. Our results indicate that BPB exhibits lower barriers than BP. According to the distortion-interaction analysis, when the borane receptor B2 is intermolecular (BP) it undergoes greater geometric distortion upon hydride transfer than in BPB, where the borane receptor is intramolecular. Despite the high computed barriers for BP, these can be reduced (rendering BP competitive with BPB) by incorporating strongly electron-donating R2 substituents at the phosphorus.