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 H-2 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 H-2 activation in FLPs. A weakly bound precursor complex, H3N center dot H-2 center dot BX3, was found in which the H-2 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 fluor complexes were calculated and analyzed in great detail. At the transition state (TS), the H-2 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 H-2 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 H-2, but its contribution to the overall interaction energy is small compared to that from the overlap of the p(N), sigma(H-H), sigma(H-H)* and p(B) orbitals at the TS. Our detailed analysis of the interaction of H-2 with the FLP provides insight into the important components that should be taken into account when designing related systems to activate H-2.