Heterolytic Splitting of Molecular Hydrogen by Frustrated and Classical Lewis Pairs: A Unified Reactivity Concept

Using a set of state-of-the-art quantum chemical techniques we scrutinized the characteristically different reactivity of frustrated and classical Lewis pairs towards molecular hydrogen. The mechanisms and reaction profiles computed for the H-2 splitting reaction of various Lewis pairs are in good agreement with the experimentally observed feasibility of H-2 activation. More importantly, the analysis of activation parameters unambiguously revealed the existence of two reaction pathways through a low-energy and a high-energy transition state. An exhaustive scrutiny of these transition states, including their stability, geometry and electronic structure, reflects that the electronic rearrangement in low-energy transition states is fundamentally different from that of high-energy transition states. Our findings reveal that the widespread consensus mechanism of H-2 splitting characterizes activation processes corresponding to high-energy transition states and, accordingly, is not operative for H-2-activating systems. One of the criteria of H-2-activation, actually, is the availability of a low-energy transition state that represents a different H-2 splitting mechanism, in which the electrostatic field generated in the cavity of Lewis pair plays a critical role: to induce a strong polarization of H-2 that facilities an efficient end-on acid-H-2 interaction and to stabilize the charge separated H+-H- moiety in the transition state.