Reversible Heterolytic Methane Activation of Metal-Free Closed-Shell Molecules: A Computational Proof-of-Principle Study

Utilization of the acid/base effects simultaneously is one of the basic principles used by transition-metal (TM) complexes to activate H-H/C-H sigma bonds. In principle, the high reactivity of metal-free FLPs (frustrated Lewis pairs) towards H-2 can also be attributed to these effects. On the basis of our proposed integrated FLPs, we pushed the effects to a higher (if not a limit) level at which the extremely unreactive methane C-H sigma bond can be activated by our designed metal-free closed-shell molecules. Three molecules (M3c, M4b, and M4c) among the reported have activation free energies (22.4, 20.0, and 20.2 kcal mol(-1), respectively) comparable with (or less than) the 22.3 kcal mol(-1) of a TM model complex that features a Ti=N double bond. The derivative of the TM model has been experimentally shown to be capable of activating methane. Moreover, some of the activation reactions are (or nearly) thermoneutral. For example, the methane activations of M3c, M4b, and M4c are exothermic by -1.9, -4.5, and -5.2 kcal mol(-1) of free energies, respectively. The kinetics and thermodynamics imply that the molecules could be further developed to realize catalytic methane activation. The electronic structure analyses reveal that, although our metal-free molecules and the TM model complex share the same principle in activating the C-H bond, there are differences as to how they go about maintaining the effective active sites. The reported molecules could be the targets for experimental realizations. The strategy could be applied to the design of similar molecules to realize more general C-H bond activation of alkanes.