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Steric and electronic effects on the heterolytic H 2 ‐splitting by phosphine‐boranes R 3 B/PR′ 3 (R = C 6 F 5 , Ph; R′ = C 6 H 2 Me 3 , t Bu, Ph, C 6 F 5 , Me, H): A computational study
Author(s) -
Gao Shulin,
Wu Wei,
Mo Yirong
Publication year - 2010
Publication title -
international journal of quantum chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.484
H-Index - 105
eISSN - 1097-461X
pISSN - 0020-7608
DOI - 10.1002/qua.22901
Subject(s) - heterolysis , steric effects , chemistry , boranes , borane , van der waals force , phosphine , proton affinity , crystallography , density functional theory , frustrated lewis pair , stereochemistry , hydride , binding energy , ion , computational chemistry , lewis acids and bases , atomic physics , molecule , physics , hydrogen , protonation , biochemistry , organic chemistry , catalysis , boron
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 BP distance. With short BP 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 BP 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′ 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 Δ E b (BR 3 /PR′ 3 ) as predictor variables: Δ E R ([R 3 BH − ][HPR′ 3 + ]) = −0.779HA(BR 3 ) − 0.695PA(PR′ 3 ) − 1.331 Δ E b zpe, 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 Δ E b can be significantly influenced by the steric overcrowding. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011