“Frustration” of Orbital Interactions in Lewis Base/Lewis Acid Adducts: A Computational Study of H₂ Uptake by Phosphanylboranes R₂P=BR′₂

Abstract

The reaction of molecular hydrogen with phosphanylboranes R 2 P=BRЈ 2 (1) is systematically studied by means of density functional theory calculations and second order Møller-Plesset perturbation theory. The potential energy barriers and the exothermicity of H 2 uptake are reported for a series of phosphanylboranes with different electron-donating and -withdrawing groups bound to phosphorus and boron. Systematic molecular orbital analysis reveals that the "frustration" between boron and phosphorus can be increased by modifying

Expanding the Scope of Metal‐Free Catalytic Hydrogenation through Frustrated Lewis Pair Design

“…It was reassuring to find that weakly bound complexes with preorganized active centers can be identified for both FLPs, and the interaction energies of the most stable forms (−8.4 and −7.6 kcal mol −1 for the 2 / 13 and 2 / 8 pairs, respectively) are similar to previously studied systems 2. 3f,i, 6a Nevertheless, the size difference between the bases has remarkable effects, as apparent from the potential energy curves with respect to the B⋅⋅⋅N distance (Figure 2).…”

A Prediction of Proton‐Catalyzed Hydrogenation of Ketones in Lewis Basic Solvent through Facile Splitting of Hydrogen Molecules

“…It is shown here that the H 2 splitting barrier height is highly tunable by conventionally used electron‐withdrawing groups; it is possible to reduce the potential energy barrier height to less than 10 kcal mol −1 . For comparison with our calculated barrier heights (Table ), we would like to mention that (i) for H 2 activation by phosphinoboranes R 2 PB(C 6 F 5 ) 2 , the calculated barrier is circa 22 kcal mol −1 ; (ii) splitting of hydrogen (H 2 ) by nucleophilic activation with singlet carbenes is considered facile with the calculated barriers of approximately 22 kcal mol −1 and more; (iii) facile FLP splitting of H 2 by LB/LA pairs generally corresponds to calculated barrier heights of less than 10 kcal mol −1 . Finally, for proton solvated in dioxane with a carbonyl compound as an additive, it is energetically more favorable to share a proton between dioxane and a carbonyl compound rather than between two dioxane molecules; for example, the proton‐sharing pair [dioxane−H (+) ⋅⋅⋅acetone] is more stable than [dioxane−H (+) ⋅⋅⋅dioxane] by approximately 5 kcal mol −1 and the potential energy barrier for the interconversion of these proton‐sharing pairs is around 4 kcal mol −1 .…”

Carbonyl Activation by Borane Lewis Acid Complexation: Transition States of H2 Splitting at the Activated Carbonyl Carbon Atom in a Lewis Basic Solvent and the Proton‐Transfer Dynamics of the Boroalkoxide Intermediate

“…Therefore, the Lewis acid complexation does not really activate the whole O=C bond for the polarization and the direct addition of H 2 . For comparison, the experimentally proven heterolytic H 2 cleavage by phosphine‐boranes R 2 PB(C 6 F 5 ) 2 corresponds to the calculated barrier of approximately 22 kcal mol −1 . In the interest of qualitative insights, let us mention that the addition of H 2 to the completely free acetone (without BCF) has a very high barrier of about 53 kcal mol −1 and in the TS structure the O⋅⋅⋅H and C⋅⋅⋅H distances are 1.53 and 1.56 Å, respectively (Figure S1 in the Supporting Information).…”

“…Therefore, the Lewis acid complexation does not really activate the whole O=Cb ond for the polarization and the direct addition of H 2 .F or comparison, the experimentally provenh eterolytic H 2 cleavage by phosphine-boranes R 2 PB(C 6 F 5 ) 2 corresponds to the calculated barrier of approximately 22 kcal mol À1 . [48,49] In the interest of qualitative insights, let us mention that the addition of H 2 to the completely free acetone( withoutB CF) has av ery high barrier of about 53 kcal mol À1 and in the TS structure the O···H and C···H distances are 1.53 and 1.56 ,r espectively ( Figure S1 in the Supporting Information).Acomparison with the O···H (+) and C···H (À) distances in TS direct ,F igure 2a,c learly indicates that the BCF-ketone adduct has al ess potent LB character at the carbonyl oxygen atom but am ore potent LA character at the carbonyl carbon atom in comparison to the free acetone.…”