Steric and electronic effects on the heterolytic H₂‐splitting by phosphine‐boranes R₃B/PR′₃(R = C₆F₅, Ph; R′ = C₆H₂Me₃,tBu, Ph, C₆F₅, Me, H): A computational study

The steric and electronic effects exerted by the substituents R/R′ on the heterolytic H 2 ‐splitting by phosphine‐boranes R 3 B/PR′ 3 [R = C 6 F 5 ( 1 ), Ph ( 2 ); R′ = C 6 H 2 Me 3 ( a ), t Bu ( b ), Ph ( c ), C 6 F 5 ( d ), Me ( e ), H ( f )] have been studied by performing quantum mechanical density functional theory and RI‐MP2 calculations. Energy decomposition analyses based on the block‐localized wavefunction method show that the nature of the interaction between R 3 B and PR′ 3 is strongly dependent on the BP distance. With short BP distances (∼2.1 Å), the strength of Lewis pairs results from the balance among various energy terms, and both strong and weak dative bonds can be found in this group. However, at long BP distances (>4.0 Å), the correlation and dispersion energy (Δ E corr ) dominates. In other words, the van der Waals (vdW) interaction rules these weakly bound complexes. No ion‐pair structures of 1f and 2c – 2f can be located as they instantly converge to vdW complexes R 3 B···H 2 ···PR′ 3 . We thus propose a model, which predicts that when the sum ( E hp ) of the hydride affinity (HA) of BR 3 and the proton affinity (PA) of PR′ 3 is higher than 340.0 kcal/mol, the ion‐pair [R 3 BH − ][HPR′ ] can be observed, whereas with E hp below this value, the ion pair would instantly undergo the combination of proton and hydride with the release of H 2 . The overall reaction energies ( 1a – 1e and 2a – 2b ) can be best described by a fitting equation with HA(BR 3 ), PA(PR′ 3 ), and the binding energy Δ E b (BR 3 /PR′ 3 ) as predictor variables: Δ E R ([R 3 BH − ][HPR′ ]) = −0.779HA(BR 3 ) − 0.695PA(PR′ 3 ) − 1.331 Δ E (BR 3 /PR′ 3 ) + 245.3 kcal/mol. The fitting equation provides quantitative insights into the steric and electronic effects on the thermodynamic aspects of the heterolytic H 2 ‐splitting reactions. The electronic effects are reflected by HA(BR 3 ) and PA(PR′ 3 ), and Δ E b can be significantly influenced by the steric overcrowding.