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

Water reduction using <i>p-</i>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 (<b>BPB</b>) and borylphosphine (<b>BP</b>) 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 <b>BPB</b> and <b>BP</b> while systematically varying the substituents on borane (R1) and phosphorus (R2) to modulate steric and electron-withdrawing/donating properties. Our results indicate that <b>BPB</b> exhibits lower barriers than <b>BP</b>. According to the distortion-interaction analysis, when the borane receptor B2 is intermolecular (<b>BP</b>) it undergoes greater geometric distortion upon hydride transfer than in <b>BPB</b>, where the borane receptor is intramolecular. Despite the high computed barriers for <b>BP</b>, these can be reduced (rendering <b>BP</b> competitive with <b>BPB</b>) by incorporating strongly electron-donating R2 substituents at the phosphorus.