Insight into the relative reactivity of “Frustrated Lewis pairs” and stable carbenes in activating H2 and CH4: A comparative computational study

Abstract

Computational study has been conducted to gain insight into the relative reactivity of stable carbenes (1 and 2) and typical frustrated Lewis pairs (FLPs, 3–6) in activating H₂ and CH₄. For the FLP H₂ activations, despite the quite different basicities of the Lewis base components, they have comparable reactivities. The unexpected relative reactivity can be attributed to the following two factors: (i) the vacant carbene C: p_π orbital, which is important when carbene works alone but does not participate in the FLP activation; and (ii) the electrostatic interaction between the Lewis base center and the approaching H atom which plays an important role and can either favor or disfavor a reaction. These explanations are also applicable to methane activations. The study brings two messages to the experimentalists for constructing FLPs: (i) it is recommended to use P- and N-centered Lewis bases to construct FLPs for H₂ activation because using more reactive components does not benefit the activation; and (ii) the FLPs are less reactive in activating CH₄ than H₂. In addition, using more reactive carbenes as Lewis bases in FLPs does not necessarily benefit the methane activation.