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.