Steric and electronic effects on the heterolytic H_2-splitting by phosphine-boranes R_3B/PR′_3 (R = C_6F _5, Ph; R′ = C_6H_2Me_3, tBu, Ph, C_6F_5, Me, H): A computational study

摘要

The steric and electronic effects exerted by the substituents R/R′ on the heterolytic H2-splitting by phosphine-boranes R _3B/PR′_3 [R = C_6F_5 (1), Ph (2); R′ = C_6H_2Me_3 (a), tBu (b), Ph (c), C_6F_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_3B and PR′_3 is strongly dependent on the B-P distance. With short B-P distances (～2.1 ?), 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 ?), 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 R_3B?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_3BH-][HPR′ 3+] 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 ΔE_b(BR_3/PR′_3) as predictor variables: ΔE_R([R_3BH~-][HPR′ 3+]) = -0.779HA(BR_3) - 0.695PA(PR′_3) - 1.331 ΔE bzpe, cp (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 ΔE_b can be significantly influenced by the steric overcrowding.