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 MgOn•+ the HAT barrier increases due to the penalty of radical localization. Adding a dopant (Ga2O3) to MgO2•+ localizes the radical and HAT transpires. Whenever the radical is located on the metal centers as in Al2O2•+ 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 CH3 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 MgO2•+ by Al2O3 enables HAT and PCET to compete. Similarly, ZnO•+ activates methane by PCET generating many products. Adding a CH3CN ligand to form (CH3CN)ZnO•+ leads to a single HAT product. The CH3CN 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+.