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 moiet...