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 H₂‐splitting by phosphine‐boranes R₃B/PR′₃ [R = C₆F₅ ( 1 ), Ph ( 2 ); R′ = C₆H₂Me₃ ( a ), tBu ( b ), Ph ( c ), C₆F₅ ( 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 R₃B 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 (ΔE_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 R₃B···H₂···PR′₃. We thus propose a model, which predicts that when the sum (E_hp) of the hydride affinity (HA) of BR₃ and the proton affinity (PA) of PR′₃ is higher than 340.0 kcal/mol, the ion‐pair [R₃BH^?][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 H₂. The overall reaction energies ( 1a – 1e and 2a – 2b ) can be best described by a fitting equation with HA(BR₃), PA(PR′₃), and the binding energy ΔE_b(BR₃/PR′₃) as predictor variables: ΔE_R([R₃BH^?][HPR′]) = ?0.779HA(BR₃) ? 0.695PA(PR′₃) ? 1.331 ΔE (BR₃/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 H₂‐splitting reactions. The electronic effects are reflected by HA(BR₃) and PA(PR′₃), and ΔE_b can be significantly influenced by the steric overcrowding. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011