Shaodong Zhou

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.

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.

On the Role of the Electronic Structure of the Heteronuclear Oxide Cluster [Ga2Mg2O5].+ in the Thermal Activation of Methane and Ethane: An Unusual Doping Effect

The reactivity of the heteronuclear oxide cluster [Ga2 Mg2 O5 ](.+) , bearing an unpaired electron at a bridging oxygen atom (Ob (.-) ), towards methane and ethane has been studied using Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS). Hydrogen-atom transfer (HAT) from both methane and ethane to the cluster ion is identified experimentally. The reaction mechanisms of these reactions are elucidated by state-of-the-art quantum chemical calculations. The roles of spin density and charge distributions in HAT processes, as revealed by theory, not only deepen our mechanistic understanding of CH bond activation but also provide important guidance for the rational design of catalysts by pointing to the particular role of doping effects.

Spin‐Selective Thermal Activation of Methane by Closed‐Shell [TaO3]+

Thermal reactions of the closed-shell metal-oxide cluster [TaO3 ](+) with methane were investigated by using FTICR mass spectrometry complemented by high-level quantum chemical calculations. While the generation of methanol and formaldehyde is somewhat expected, [TaO3 ](+) remarkably also has the ability to abstract two hydrogen atoms from methane with the elimination of CH2 . Mechanistically, the generation of CH2 O and CH3 OH occurs on the singlet-ground-state surface, while for the liberation of (3) CH2 , a two-state reactivity scenario prevails.

Distinct Mechanistic Differences in the Hydrogen‐Atom Transfer from Methane and Water by the Heteronuclear Oxide Cluster [Ga2MgO4].+

The thermal reactions of the heteronuclear oxide cluster [Ga2 MgO4 ](.+) with methane and water have been studied using state-of-the-art gas-phase experiments in conjunction with quantum-chemical calculations. The significant reactivity differences, favoring activation of the strong OH bond, can be ascribed to a proton-coupled electron transfer (PCET) mechanism operative in the activation of water. This study deepens our mechanistic understanding on how inert RH bonds are cleaved by metal oxides.

Thermal Activation of Methane by [HfO].+ and [XHfO]+ (X=F, Cl, Br, I) and the Origin of a Remarkable Ligand Effect

The thermal reactions of methane with [HfO](.+) and [XHfO](+) (X=F, Cl, Br, I) were investigated by using FT-ICR mass spectrometry complemented by high-level quantum chemical calculations. Surprisingly, in contrast to the inertness of [HfO](.+) towards methane, the closed-shell oxide ions [XHfO](+) (X=F, Cl, Br) activate the H3 C-H bond to form the insertion products [Hf(X)(OH)(CH3 )](+) . The possible origin of this remarkable ligand effect is discussed.

Hidden Hydride Transfer as a Decisive Mechanistic Step in the Reactions of the Unligated Gold Carbide [AuC]+ with Methane under Ambient Conditions

The reactivity of the cationic gold carbide [AuC]+ (bearing an electrophilic carbon atom) towards methane has been studied using Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS). The product pairs generated, that is, Au+ /C2 H4 , [Au(C2 H2 )]+ /H2 , and [C2 H3 ]+ /AuH, point to the breaking and making of C-H, C-C, and H-H bonds under single-collision conditions. The mechanisms of these rather efficient reactions have been elucidated by high-level quantum-chemical calculations. As a major result, based on molecular orbital and NBO-based charge analysis, an unprecedented hydride transfer from methane to the carbon atom of [AuC]+ has been identified as a key step. Also, the origin of this novel mechanistic scenario has been addressed. The mechanistic insights derived from this study may provide guidance for the rational design of carbon-based catalysts.

Control of Product Distribution and Mechanism by Ligation and Electric Field in the Thermal Activation of Methane

An unexpected mechanistic switch as well as a change of the product distribution in the thermal gas-phase activation of methane have been identified when diatomic [ZnO].+ is ligated with acetonitrile. Theoretical studies suggest that a strong metal-carbon attraction in the pristine [ZnO].+ species plays an important role in the rebound of the incipient CH3. radical to the metal center, thus permitting the competitive generation of CH3. , OH. , and CH3 OH. This interaction is drastically weakened by a single CH3 CN ligand. As a result, upon ligation the proton-coupled single electron transfer that prevails for [ZnO].+ /CH4 switches to the classical hydrogen-atom-transfer process, thus giving rise to the exclusive expulsion of CH3. . This ligand effect can be modeled quite well by an oriented external electric field of a negative point charge.

Efficient Room-Temperature Methane Activation by the Closed-Shell, Metal-Free Cluster [OSiOH]+: A Novel Mechanistic Variant

The closed-shell cluster ion [OSiOH](+) is generated in the gas phase and its reactivity towards the thermal activation of CH4 has been examined using Fourier transform-ion cyclotron resonance (FT-ICR) mass spectrometry in conjunction with state-of-the-art quantum chemical calculations. Quite unexpectedly at room temperature, [OSiOH](+) efficiently mediates C-H bond activation, giving rise to [SiOH](+) and [SiOCH3 ](+) with the concomitant formation of methanol and water, respectively. Mechanistic aspects for this unprecedented reactivity pattern are presented, and the properties of the [OSiOH](+) /CH4 couple are compared with those of the closed-shell systems [OCOH](+) /CH4 and [MgOH](+) /CH4 ; the last two couples exhibit an entirely different reactivity scenario.

On the Mechanisms of Hydrogen‐Atom Transfer from Water to the Heteronuclear Oxide Cluster [Ga2Mg2O5].+: Remarkable Electronic Structure Effects

Mechanistic insight into the homolytic cleavage of the O-H bond of water by the heteronuclear oxide cluster [Ga2 Mg2 O5 ](.+) has been derived from state-of-the-art gas-phase experiments in conjunction with quantum chemical calculations. Three pathways have been identified computationally. In addition to the conventional hydrogen-atom transfer (HAT) to the radical center of a bridging oxygen atom, two mechanistically distinct proton-coupled electron-transfer (PCET) processes have been identified. The energetically most favored path involves initial coordination of the incoming water ligand to a magnesium atom followed by an intramolecular proton transfer to the lone-pair of the bridging oxygen atom. This step, which is accomplished by an electronic reorganization, generates two structurally equivalent OH groups either of which can be liberated, in agreement with labeling experiments.