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 splitting reaction of various Lewis pairs are in good agreement with the experimentally observed feasibility of H 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 splitting characterizes activation processes corresponding to high-energy transition states and, accordingly, is not operative for H-activating systems. One of the criteria of H-activation, actually, is the availability of a low-energy transition state that represents a different H 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 that facilities an efficient end-on acid-H interaction and to stabilize the charge separated "H-H" moiety in the transition state.