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.