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

The steric and electronic effects exerted by the substituents R/R 0 on the heterolytic H 2 -splitting by phosphine-boranes R 3 B/PR 0 3 [R ¼ C 6 F 5 (1), Ph (2); R 0 ¼ C 6 H 2 Me 3 (a), tBu (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 0 3 is strongly dependent on the BAP distance. With short BAP 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 BAP distances (>4.0 Å ), the correlation and dispersion energy (DE 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 BÁÁÁH 2 ÁÁÁPR 0 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 0 3 is higher than 340.0 kcal/mol, the ion-pair [R 3 BH À ][HPR 0þ 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 0 3 ), and the binding energy DE b (BR 3 /PR 0 3 ) as predictor variables:(BR 3 /PR 0 3 ) þ 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 0 3 ), and DE b can be significantly influenced by the steric overcrowding.a Structural parameters are based on the optimal geometries at the B3LYP/6-31G(d,p) level; the basis sets used for calculations are cc-pVDZ. b Calculated at the RI-MP2/cc-pVDZ level. a 1HHa refers to the ion-pair complex obtained from Lewis pair 1a and so on. b DE zpe;cp b refers to binding energy after ZPE and 50% BSSE corrections.

Reactivity Models of Hydrogen Activation by Frustrated Lewis Pairs: Synergistic Electron Transfers or Polarization by Electric Field?

“…The starting point of our computational analysis of the six H 2 cleavage reactions is a comparison of the general features of the TSs, including the question whether the incoming H 2 has undergone any notable activation. The geometries of all TSs considered here have been published at various levels of theory; ,,,,,,,,− now, we present a comparison on the basis of ωB97X-D/6-311G(d,p) results (Figure ).…”

Experimental and Theoretical Study of Molecular Response of Amine Bases in Organic Solvents

“…However, experimental evidence of key intermediates and weakly interacting FLP encounter complexes in solution has been inconclusive, and recent investigations into reaction mechanisms have relied heavily on molecular simulation methods. Although the formation of a dative bond is “frustrated” in such systems, gas-phase calculations − find interaction energies between the Lewis acid and base in the range of 5–15 kcal/mol, whereas molecular dynamics , modeling of encounter complexes in solution have revealed interactions along the potential of mean force on the order of k B T .…”

Mechanistic Studies on the Metal-Free Activation of Dihydrogen by Antiaromatic Pentarylboroles

“…However, we were able to characterize an additional transition state in which H 2 adds to the external B–C bond involving the aromatic substituent bound to the boron center. Significantly, these transition states were found to be 72 and 46 kJ/mol higher in energy than the TS1 of boroles 1 and 2 , respectively . Furthermore, following the internal reaction coordinate toward products clearly showed that the characterized transition state leads to breakup of the B–C bond and subsequent formation of a borole with a B–H functionality and a molecule of benzene/pentafluorobenzene.…”