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

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Molecules, spectroscopy, kinetics, environment, & general theory, 2012-07, Vol.116 (26), p.7228-7237
• Main Authors: Camaioni, Donald M, Ginovska-Pangovska, Bojana, Schenter, Gregory K, Kathmann, Shawn M, Autrey, Tom
• Format: Article
• Language: English
• ISSN: 1089-5639; 1520-5215
• DOI: 10.1021/jp3039829

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 p N, σH–H, σ*H–H, and p 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 H2.