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-center dot). Whenever the radical is delocalized, e.g., in [MgO](n)(center dot+) the HAT barrier increases due to the penalty of radical localization. Adding a dopant (Ga2O3) to [MgO](2)(center dot+) localizes the radical and HAT transpires. Whenever the radical is located on the metal centers as in [Al2O2](n) 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 [MgO](2)(center dot+) by Al2O3 enables HAT and PCET to compete. Similarly, [ZnO](center dot+) activates methane by PCET generating many products. Adding a CH3CN ligand to form [(CH3CN)-ZnO](center dot+) 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, Cut, 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+.