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J Am Chem Soc. 2017 Dec 6;139(48):17201-17212. doi: 10.1021/jacs.7b10139. Epub 2017 Nov 17.

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

Author information

1
Institut für Chemie, Technische Universität Berlin , Straße des 17. Juni 135, 10623 Berlin, Germany.
2
Institute of Chemistry, The Hebrew University of Jerusalem , Jerusalem 91904, Israel.
3
Institute of Theoretical Chemistry, Jilin University , Changchun 130023, P.R. China.

Abstract

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 (Ga2O3) to [MgO]2•+ 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 [MgO]2•+ 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+.

PMID:
29112810
DOI:
10.1021/jacs.7b10139

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