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

Abstract

AbstractThe 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′3is 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 (ΔEcorr) dominates. In other words, the van der Waals (vdW) interaction rules these weakly bound complexes. No ion‐pair structures of1fand2c–2fcan be located as they instantly converge to vdW complexes R3B···H2···PR′3. We thus propose a model, which predicts that when the sum (Ehp) of the hydride affinity (HA) of BR3and the proton affinity (PA) of PR′3is higher than 340.0 kcal/mol, the ion‐pair [R3BH−][HPR′] can be observed, whereas withEhpbelow this value, the ion pair would instantly undergo the combination of proton and hydride with the release of H2. The overall reaction energies (1a–1eand2a–2b) can be best described by a fitting equation with HA(BR3), PA(PR′3), and the binding energy ΔEb(BR3/PR′3) as predictor variables: ΔER([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 ΔEbcan be significantly influenced by the steric overcrowding. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011