title: 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.
abstract: 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.
authors.affiliation: Institut für Chemie, Technische Universität Berlin , Straße des 17. Juni 135, 10623 Berlin, Germany.|Institute of Theoretical Chemistry, Jilin University , Changchun 130023, People's Republic of China.
authors.forename: Jilai
authors.initials: J
authors.lastname: Li
authors.affiliation: Institut für Chemie, Technische Universität Berlin , Straße des 17. Juni 135, 10623 Berlin, Germany.
authors.forename: Shaodong
authors.initials: S
authors.lastname: Zhou
authors.affiliation: Institute of Theoretical Chemistry, University of Cologne , Greinstraße 4, 50939 Cologne, Germany.
authors.forename: Jun
authors.initials: J
authors.lastname: Zhang
authors.affiliation: Institut für Chemie, Technische Universität Berlin , Straße des 17. Juni 135, 10623 Berlin, Germany.
authors.forename: Maria
authors.initials: M
authors.lastname: Schlangen
authors.affiliation: Institut für Chemie, Technische Universität Berlin , Straße des 17. Juni 135, 10623 Berlin, Germany.
authors.forename: Thomas
authors.initials: T
authors.lastname: Weiske
authors.affiliation: Department of Lipid Science, CSIR-Central Food Technological Research Institute , Mysore 570 020, India.
authors.forename: Dandamudi
authors.initials: D
authors.lastname: Usharani
authors.affiliation: Institute of Chemistry and the Lise-Meitner-Minerva Center for Computational Quantum Chemistry, The Hebrew University of Jerusalem , 91904 Jerusalem, Israel.
authors.forename: Sason
authors.initials: S
authors.lastname: Shaik
authors.affiliation: Institut für Chemie, Technische Universität Berlin , Straße des 17. Juni 135, 10623 Berlin, Germany.
authors.forename: Helmut
authors.initials: H
authors.lastname: Schwarz