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 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 (similar to 2.1 angstrom), 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 angstrom), the correlation and dispersion energy (Delta 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 R3B center dot center dot center dot H-2 center dot center dot center dot 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'(+)(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(BR3), PA(PR'(3)), and the binding energy Delta E-b(BR3/PR'(3)) as predictor variables: Delta E-R([R3BH-][HPR'(+)(3)]) = -0.779HA(BR3) - 0.695PA(PR'(3)) - 1.331 Delta E-b(zpe,cp) (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 H-2-splitting reactions. The electronic effects are reflected by HA(BR3) and PA(PR'(3)), and Delta E-b can be significantly influenced by the steric overcrowding. (C) 2010 Wiley Periodicals, Inc. Int J Quantum Chem 111: 3761-3775, 2011