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.

Jilai, Li and Shaodong, Zhou and Jun, Zhang and Maria, Schlangen and Thomas, W. and Usharani, D. (2016) 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. Journal of the American Chemical Society, 138. pp. 7973-7981.

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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.

Item Type:	Article
Uncontrolled Keywords:	methane activation cluster oxide cations
Subjects:	500 Natural Sciences and Mathematics > 04 Chemistry and Allied Sciences > 24 Organic Chemistry
Divisions:	Dept. of Lipid Science
Depositing User:	Food Sci. & Technol. Information Services
Date Deposited:	30 Jun 2017 09:18
Last Modified:	30 Jun 2017 09:18
URI:	http://ir.cftri.res.in/id/eprint/12725

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