Analysis of the Activation and Heterolytic Dissociation of H₂ by Frustrated Lewis Pairs: NH₃/BX₃ (X = H, F, and Cl)

We performed a computational study of H₂ activation and heterolytic dissociation promoted by prototype Lewis acid/base pairs NH₃/BX₃ (X = H, F, and Cl) to understand the mechanism in frustrated Lewis pairs (FLPs). Although the NH₃/BX₃ pairs form strong dative bonds, electronic structure theories make it possible to explore the potential energy surface away from the dative complex, in regions relevant to H₂ activation in FLPs. A weakly bound precursor complex, H₃N·H₂·BX₃, was found in which the H₂ molecule interacts side-on with B and end-on with N. The BX₃ group is pyramidal in the case of X = H, similar to the geometry of BH₅, but planar in the complexes with X = F and Cl. The latter complexes convert to ion pairs, [NH₄⁺]­[BHX₃^–] with enthalpy changes of 7.3 and −9.4 kcal/mol, respectively. The minimum energy paths between the FLP and the product ion pair of the chloro and fluoro complexes were calculated and analyzed in great detail. At the transition state (TS), the H₂ bond is weakened and the BX₃ moiety has undergone significant pyramidal distortion. As such, the FLP is prepared to accept the incipient proton and hydride ion on the product-side. The interaction energy of the H₂ with the acid/base pair and the different contributions for the precursor and TS complex from an energy decomposition analysis expose the dominant factors affecting the reactivity. We find that structural reorganization of the precursor complex plays a significant role in the activation and that charge-transfer interactions are the dominant stabilizing force in the activated complex. The electric field clearly has a role in polarizing H₂, but its contribution to the overall interaction energy is small compared to that from the overlap of the <i>p</i>_N, σ_H–H, σ*_H–H, and <i>p</i>_B orbitals at the TS. Our detailed analysis of the interaction of H₂ with the FLP provides insight into the important components that should be taken into account when designing related systems to activate H<sub>2