Electronic Origins of the Variable Efficiency of Room-Temperature Methane Activation by Homo- and Heteronuclear Cluster Oxide Cations [XYO2]+ (X, Y = Al, Si, Mg): Competition between Proton- Coupled Electron Transfer and Hydrogen-Atom Transfer.

The reactivity of the homo- and heteronuclear
oxide clusters [XYO2]+ (X, Y = Al, Si, Mg) toward methane was
studied using Fourier transform ion cyclotron resonance mass
spectrometry, in conjunction with high-level quantum mechanical
calculations. The most reactive cluster by both experiment
and theory is [Al2O2]•+. In its favorable pathway, this cluster
abstracts a hydrogen atom by means of proton-coupled electron
transfer (PCET) instead of following the conventional hydrogen-
atom transfer (HAT) route. This mechanistic choice
originates in the strong Lewis acidity of the aluminum site of
[Al2O2]•+, which cleaves the C−H bond heterolytically to form
an Al−CH3 entity, while the proton is transferred to the
bridging oxygen atom of the cluster ion. In addition, a
comparison of the reactivity of heteronuclear and homonuclear oxide clusters [XYO2]+ (X, Y = Al, Si, Mg) reveals a striking
doping effect by aluminum. Thus, the vacant s−p hybrid orbital on Al acts as an acceptor of the electron pair from methyl anion
(CH3
−) and is therefore eminently important for bringing about thermal methane activation by PCET. For the Al-doped cluster
ions, the spin density at an oxygen atom, which is crucial for the HAT mechanism, acts here as a spectator during the course of
the PCET mediated C−H bond cleavage. A diagnostic plot of the deformation energy vis-à-vis the barrier shows the different
HAT/PCET reactivity map for the entire series. This is a strong connection to the recently discussed mechanism of oxidative
coupling of methane on magnesium oxide surfaces proceeding through Grignard-type intermediates.