Abstract
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 H2 splitting reaction of various Lewis pairs are in good agreement with the experimentally observed feasibility of H2 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 H2 splitting characterizes activation processes corresponding to high-energy transition states and, accordingly, is not operative for H2-activating systems. One of the criteria of H2-activation, actually, is the availability of a low-energy transition state that represents a different H2 splitting mechanism, in which the electrostatic field generated in the cavity of Lewis pair plays a critical role: to induce a strong polarization of H2 that facilities an efficient end-on acid-H2 interaction and to stabilize the charge separated “H+–H−” moiety in the transition state.
Highlights
Since the articulation of the notion, a vast number of studies probing the chemistry of FLPs and their applications for various transformations have emerged and the developments made in less than a decade are already too diverse and numerous to list comprehensively
In this comprehensive computational study we systematically investigated the reaction of six Lewis pairs with molecular hydrogen, shown schematically in Fig. 3, including three classical Lewis acid-base pair (CLP) (Me3P–BF3, Me3P–B(C6F5)[3] and lut–B(C6F5)3), amongst which only lut–B(C6F5)[3] facilitates the splitting of H2, and three FLPs (carb · B(C6F5)[3], tBu3P · B(C6F5)[3], Mes3P · BPh3), amongst which only Mes3P · BPh3 does not promote heterolytic H2 splitting
The computed solution-state ΔGr and activation ΔG‡ parameters are in good agreement with the available experimental findings: heterolytic H2 splitting is not preferred for Me3P–BF3, Me3P–B(C6F5)[3] and Mes3P · BPh3 whereas it takes place through a thermally accessible activation barrier and is an exothermic process in the case of lut–B(C6F5)[3], carb · B(C6F5)[3] and tBu3P · B(C6F5)[3]
Summary
Since the articulation of the notion, a vast number of studies probing the chemistry of FLPs and their applications for various transformations have emerged and the developments made in less than a decade are already too diverse and numerous to list comprehensively. It was pointed out that the rather facile hydrogen-splitting reaction could hardly be explained in terms of a termolecular collision of the reactants, and a weak pre-association of the acid and base molecules was envisioned and identified on the PES as a key ingredient of the reaction. This preorganized donor-acceptor complex, was shown to be a highly flexible species, held together by weak, secondary, non-covalent interactions, including multiple C–H...F interactions and dispersion. A single, low-lying early transition state (TS)
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