Electronic Effects on Room-Temperature, Gas-Phase C-H Bond Activations by Cluster Oxides and Metal Carbides: The Methane Challenge.

This Perspective discusses a story of one molecule (methane), a few metal-oxide cationic clusters (MOCCs), dopants, metal-carbide cations, oriented-electric fields (OEFs), and a dizzying mechanistic landscape of methane activation! One mechanism is hydrogen atom transfer (HAT), which occurs whenever the MOCC possesses a localized oxyl radical (M-O). Whenever the radical is delocalized, e.g., in [MgO] the HAT barrier increases due to the penalty of radical localization. Adding a dopant (GaO) to [MgO] localizes the radical and HAT transpires. Whenever the radical is located on the metal centers as in [AlO] the mechanism crosses over to proton-coupled electron transfer (PCET), wherein the positive Al center acts as a Lewis acid that coordinates the methane molecule, while one of the bridging oxygen atoms abstracts a proton, and the negatively charged CH moiety relocates to the metal fragment. We provide a diagnostic plot of barriers vs reactants' distortion energies, which allows the chemist to distinguish HAT from PCET. Thus, doping of [MgO] by AlO enables HAT and PCET to compete. Similarly, [ZnO] activates methane by PCET generating many products. Adding a CHCN ligand to form [(CHCN)ZnO] leads to a single HAT product. The CHCN dipole acts as an OEF that switches off PCET. [MC] cations (M = Au, Cu) act by different mechanisms, dictated by the M-C bond covalence. For example, Cu, which bonds the carbon atom mostly electrostatically, performs coupling of C to methane to yield ethylene, in a single almost barrier-free step, with an unprecedented atomic choreography catalyzed by the OEF of Cu.