Rationalizing the Reactivity of Frustrated Lewis Pairs: Thermodynamics of H2 Activation and the Role of Acid-Base Properties

The acid-base strengths of recently reported frustrated Lewis pairs and their relation with the thermodynamic feasibility of heterolytic hydrogen splitting reactions are analyzed in terms of quantum chemical calculations. Reaction free energies of hydrogenation processes are computed, and an energy partitioning scheme is introduced, which involves quantitative measures of the acidity and basicity of the reacting Lewis centers. Additional terms are also included that account for possible dative bond formation between the active sites and for stabilizing electrostatic interactions occurring in the product species. For intermolecular combinations of donor-acceptor components, the calculated reaction free energies are found to correlate well with the cumulative acid-base strengths. Product stabilization for these systems represents a notable contribution to the overall energetics; however, it generally shows only a slight variation for the investigated series. The reactivity of linked donor-acceptor pairs is primarily governed by acid-base properties as well, but the magnitude of stabilizing effects arising from acid-base cooperativity of active sites is also of significant importance in determining the thermodynamic feasibility of the reactions.