Mechanistic Variants in Gas-Phase Metal-Oxide Mediated Activation of Methane at Ambient Conditions

The C−H bond activation of methane
mediated by a prototypical heteronuclear metal-oxide cluster,
[Al2Mg2O5]•+, was investigated by using Fourier transform ion
cyclotron resonance mass spectrometry (FT-ICR-MS) in
conjunction with high-level quantum mechanical calculations.
Experimentally, hydrogen-atom abstraction from methane by
the cluster ion [Al2Mg2O5]•+ takes place at ambient
conditions. As to the mechanism, according to our computational
findings, both the proton-coupled electron transfer
(PCET) and the conventional hydrogen-atom transfer (HAT)
are feasible and compete with each other. This is in distinct
contrast to the [XYO2]+ (X, Y = Mg, Al, Si) cluster oxide ions
which activate methane exclusively via the PCET route (Li, J.;
Zhou, S.; Zhang, J.; Schlangen, M.; Weiske, T.; Usharani, D.; Shaik, S.; Schwarz, H. J. Am. Chem. Soc. 2016, 138, 7973−7981).
The electronic origins of the mechanistically rather complex reactivity scenarios of the [Al2Mg2O5]•+/CH4 couple were
elucidated. For the PCET mechanism, in which the Lewis acid−base pair [Al+−O−] of the cluster acts as the active site, a clear
correlation has been established between the nature of the transition state, the corresponding barrier height, the Lewis aciditybasicity
of the [M+−O−] unit, as well as the bond order of the M+−O− bond. Also addressed is the role of the spin and charge
distributions of a terminal oxygen radical site in the direct HAT route. The knowledge of the factors that control the reactivity of
PCET and HAT pathways not only deepens our mechanistic understanding of metal-oxide mediated C−H bond activation but
may also provide guidance for the rational design of catalysts.