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 acidity-basicity 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.