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]n •+ the HAT barrier increases due to the penalty of radical localization. Adding a dopant (Ga2 O3 ) to [MgO]2 •+ localizes the radical and HAT transpires. Whenever the radical is located on the metal centers as in [Al2 O2 ]•+ 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 •+ by Al2 O3 enables HAT and PCET to compete. Similarly, [ZnO]•+ activates methane by PCET generating many products. Adding a CH3 CN ligand to form [(CH3 CN)ZnO]•+ leads to a single HAT product. The CH3 CN 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+ .