The thermal reactions of the closed-shell, “naked” gold–carbene complex [Au(CH₂)]⁺ with methane have been explored by using FTICR mass spectrometry complemented by quantum chemical (QC) calculations at the CCSD(T)//BMK level of theory. Mechanistic aspects for this unprecedentedly efficient carbene insertion in the CH bond of methane have been addressed and the origin of the counterintuitive high reactivity of [Au(CH₂)]⁺ towards this most inert hydrocarbon is discussed.

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 CH₂O and CH₃OH occurs on the singlet-ground-state surface, while for the liberation of ³CH₂, a two-state reactivity scenario prevails.

The thermal reaction of [AuO]⁺ with methane has been explored using FT-ICR mass spectrometry complemented by high-level quantum chemical calculations. In contrast to the previously studied congener [CuO]⁺, and to [AgO]⁺, [AuO]⁺ reacts with CH₄ exclusively via oxygen-atom transfer to form CH₃OH, and a novel mechanistic scenario for this selective oxidation process has been revealed. Also, the origin of the inertness of the [AgO]⁺/CH₄ couple has been addressed computationally.

The ion/molecule reactions of molybdenum and tungsten dioxide cations with ethanol have been studied by Fourier transform ion-cyclotron resonance mass spectrometry (FT-ICR MS) and density functional theory (DFT) calculations. Dehydration of ethanol has been found as the dominant reaction channel, while generation of the ethyl cation corresponds to a minor product. Cleary, the reactions are mainly governed by the Lewis acidity of the metal center. Computational results, together with isotopic labeling experiments, show that the dehydration of ethanol can proceed either through a conventional concerted [1,2]-elimination mechanism or a step-wise process; the latter occurs via a hydroxyethoxy intermediate. Formation of C₂H₅⁺ takes place by transfer of OH⁻ from ethanol to the metal center of MO₂⁺. The molybdenum and tungsten dioxide cations exhibit comparable reactivities toward ethanol, and this is reflected in similar reaction rate constants and branching ratios.

Carbon-atom extrusion from the ipso-position of a halobenzene ring (C₆H₅X; X=F, Cl, Br, I) and its coupling with a methylene ligand to produce acetylene is not confined to [LaCH₂]⁺; also, the third-row transition-metal complexes [MCH₂]⁺, M=Hf, Ta, W, Re, and Os, bring about this unusual transformation. However, substrates with substituents X=CN, NO₂, OCH₃, and CF₃ are either not reactive at all or give rise to different products when reacted with [LaCH₂]⁺. In the thermal gas-phase processes of atomic Ln⁺ with C₇H₇Cl substrates, only those lanthanides with a promotion energy small enough to attain a 4fⁿ5d¹6s¹ configuration are reactive and form both [LnCl]⁺ and [LnC₅H₅Cl]⁺. Branching ratios and the reaction efficiencies of the various processes seem to correlate with molecular properties, like the bond-dissociation energies of the C−X or M⁺−X bonds or the promotion energies of lanthanides.

Die thermischen Reaktionen des geschlossenschaligen Metalloxidclusters [TaO₃]⁺ mit Methan wurden mittels FT-ICR-Massenspektrometrie sowie modernen quantenchemischen Methoden untersucht. Während die Bildung von Methanol und Formaldehyd zu erwarten war, ist die Fähigkeit von [TaO₃]⁺ zur Abstraktion von zwei Wasserstoffatomen unter Eliminierung von CH₂ sehr ungewöhnlich. Mechanistisch entstehen CH₂O und CH₃OH auf der Potentialfläche des Singulett-Grundzustandes, während die Eliminierung von ³CH₂ über eine Zweizustandsreaktivität verläuft.

Die thermischen Reaktionen von [AuO]⁺ mit Methan wurden mittels FT-ICR-Massenspektrometrie und quantenchemischen Rechnungen untersucht. Im Gegensatz zu den bereits untersuchten Oxiden der 11. Gruppe, [CuO]⁺ und [AgO]⁺, reagiert [AuO]⁺ mit CH₄ ausschließlich unter Bildung von CH₃OH durch Sauerstoffatomübertragung. Der neuartige Mechanismus dieser spezifischen Oxidation wird vorgestellt und die Ursachen der Trägheit des [AgO]⁺/CH₄-Reaktionspaares mittels theoretischer Untersuchungen aufgedeckt.

Die thermischen Reaktionen von Methan mit [HfO]^.+ und [XHfO]⁺ (X=F, Cl, Br, I) wurden mittels FT-ICR-Massenspektrometrie und modernen quantenchemischen Rechnungen untersucht. Während [HfO]^.+ gegenüber Methan inert ist, ermöglichen die geschlossenschaligen Ionen [XHfO]⁺ (X=F, Cl, Br) überraschenderweise die Aktivierung der H₃C-H-Bindung unter Bildung des Insertionsproduktes [Hf(X)(OH)(CH₃)]⁺. Mögliche Ursachen dieses außergewöhnlichen Ligandeneffekts werden diskutiert.

Die thermischen Reaktionen des geschlossenschaligen, “nackten” Gold-Carben-Komplexes [Au(CH₂)]⁺ mit Methan wurden mittels FT-ICR-Massenspektrometrie und quantenchemischen Rechnungen (CCSD(T)//BMK) untersucht. Die effiziente Insertion des Carbenliganden in die C-H-Bindung von Methan wird unter mechanistischen Gesichtspunkten beleuchtet, und die Ursache der unerwartet hohen Reaktivität von [Au(CH₂)]⁺ gegenüber dem höchst inerten Kohlenwasserstoff diskutiert.

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 H₃C−H bond to form the insertion products [Hf(X)(OH)(CH₃)]⁺. The possible origin of this remarkable ligand effect is discussed.

High-level electronic structure calculations, in combination with Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometric studies, permit the mechanism by which closed-shell, “naked” [TaO₂]⁺ brings about C−H bond activation of methane to be revealed. These studies also help to understand why the lighter congeners of [MO₂]⁺ (M=V, Nb) are unreactive under ambient conditions.

The thermal reaction of [Ho(CH₂S)]⁺ with toluene giving rise to [C₆H₅CHSHo]⁺ and CH₄ has been investigated using Fourier-transform ion cyclotron resonance (FT-ICR) mass spectrometry complemented by density functional theory (DFT) calculations. The high reactivity of [Ho(CH₂S)]⁺ which is in distinct contrast with the non-reactivity of “bare” Ho⁺ has its origin in the presence of a carbon-centered radical; the latter initiates hydrogen-atom abstraction from the methyl group of toluene as the first step of a sequence of hydrogen and sulfur transfer mediated by cationic holmium.

Gasphasenuntersuchungen sinnvoll dotierter Oxidcluster erlauben es, sich Herausforderungen wie der Tieftemperaturoxidation von CO oder der selektiven Umwandlung von Kohlenwasserstoffen zu stellen. Die gezielte Modifikation von Größe und Zusammensetzung freier Cluster ermöglicht es, lokale Ladungseffekte und den Spinzustand systematisch zu beeinflussen, und die Kombination von Rechnungen und Spektroskopie kann helfen, das aktive Zentrum eines Katalysators zu identifizieren und mechanistische Details von Reaktionen aufzudecken. Zudem ist es möglich, das Zusammenspiel von Trägermaterial und aktiver Komponente eines mehrkomponentigen, katalytisch aktiven Clusters zu untersuchen. Beispiele sollen zeigen, wie und warum die Gasphasenreaktivität heteronuklearer Cluster in Bezug auf kleine und üblicherweise recht inerte Moleküle gegenüber derjenigen ihrer homonuklearen Analoga erhöht oder vermindert sein kann.

Mechanistic aspects of an unusual reaction of [HoC₆H₄S]⁺ with CH₃X (X=Cl, Br, I) have been investigated using Fourier-transform ion cyclotron resonance mass spectrometry combined with density functional theory (DFT) calculations. In this thermal process, all four bonds of the methyl halides are cleaved.

The gas-phase reactions of chlorobenzene with all atomic lanthanide cations Ln⁺ (except Pm⁺) have been investigated by using Fourier transform ion cyclotron resonance mass spectrometry in conjunction with density functional theory calculations. According to the latter, a direct chlorine transfer to the lanthanide cation, which has been observed previously for fluorine abstraction from fluorobenzene, is not operative for the C₆H₅Cl/Ln⁺ couples; rather, chlorine transfer proceeds through an initial coordination of the lanthanide cation to the aromatic ring of the substrate. Both, the product distribution and the chlorine abstraction efficiencies are affected by the bond dissociation energy (BDE(Ln⁺Cl)) as well as the promotion energies of Ln⁺ to attain a 4fⁿ 5d¹ 6s¹ configuration. In addition, mechanistic aspects of some CH and CC bond activations are presented. Where appropriate, comparison with the previously studied C₆H₅F/Ln⁺ systems is made.

Gas-phase investigations of judiciously doped oxide clusters permit to address fundamental challenges related to, for example, the low-temperature oxidation of CO or the selective conversion of hydrocarbons. Modifying the size and composition of a free cluster in a controlled way enables the modification of local charge effects and of spin states, and spectroscopic studies in combination with computational work help to identify the active site of a catalyst and to unravel mechanistic details. Also, the interplay of the support material with the reactive part of a composite catalyst cluster can be addressed. Examples will be presented demonstrating how and why the gas-phase reactivities of heteronuclear clusters, in comparison with their homonuclear counterparts, toward small, generally rather inert molecules can be increased, decreased, or not significantly affected.

Mechanistic insight into the homolytic cleavage of the OH bond of water by the heteronuclear oxide cluster [Ga₂Mg₂O₅]^.+ 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.

Der Mechanismus der durch den heteronuklearen Oxidcluster [Ga₂Mg₂O₅]^.+ vermittelten homolytischen Spaltung der O-H-Bindung in Wasser wurde mittels moderner Gasphasenexperimente und quantenchemischer Rechnungen eingehend untersucht. Basierend auf den theoretischen Untersuchungen lassen sich drei Reaktionswege unterscheiden: Neben der klassischen Übertragung eines Wasserstoffatoms (hydrogen-atom transfer, HAT) auf ein radikalisches Sauerstoffatom existieren zwei mechanistisch unterschiedliche, protonengekoppelte Elektronenübertragungen (proton-coupled electron transfer, PCET). Nach Koordination des Wassermoleküls an ein Magnesiumatom des Clusters zeichnet sich der energetisch günstigste Reaktionsweg durch einen intramolekularen Protonentransfer auf das freie Elektronenpaar eines verbrückenden Sauerstoffatoms sowie durch eine Reorganisation der Elektronenstruktur des Clusters aus. Dabei werden zwei strukturell identische OH-Gruppen generiert, von denen eine – in Übereinstimmung mit Isotopenmarkierungsexperimenten – im weiteren Verlauf der Reaktion abgespalten wird.

Der heteronukleare Oxidcluster [Ga₂Mg₂O₅]^.+, der ein ungepaartes Elektron an einem verbrückenden Sauerstoffatom (O_b^.−) besitzt, wurde mittels Fouriertransformations-Ionenzyklotronresonanz-Massenspektrometrie (FT-ICR-MS) auf seine Reaktivität gegenüber Methan und Ethan untersucht. In diesen Experimenten wird sowohl in der Reaktion mit Methan als auch mit Ethan die Übertragung eines Wasserstoffatoms (hydrogen-atom transfer, HAT) vom Substrat auf den Cluster beobachtet. Die Reaktionsmechanismen konnten durch quantenchemische Rechnungen aufgeklärt werden. Die durch die theoretischen Untersuchungen verdeutlichte Rolle von Spindichte und Ladungsverteilung für HAT-Prozesse vertieft das mechanistische Verständnis von C-H-Bindungsaktivierungen und stellt zudem eine wichtige Orientierung für das rationale Design von Katalysatoren unter besonderer Betonung von Dotierungseffekten dar.

Die thermischen Reaktionen des heteronuklearen Oxidclusters [Ga₂MgO₄]^.+ mit Methan und Wasser wurden mit modernsten Gasphasenexperimenten und quantenchemischen Rechnungen untersucht. Im Gegensatz zur H-Abstraktion aus Methan folgt die Aktivierung von Wasser einem protonengekoppelten Elektronentransfermechanismus (proton-coupled electron transfer, PCET); ihm ist die deutlich höhere Reaktivität gegenüber der starken O-H-Bindung geschuldet. Die Erkenntnisse dieser Studie vertiefen das mechanistische Verständnis, wie inerte R-H-Bindungen durch Metalloxide gespalten werden können.

CO₂ activation mediated by [LTiH]⁺ (L=Cp₂, O) is observed in the gas phase at room temperature using electrospray-ionization mass spectrometry, and reaction details are derived from traveling wave ion-mobility mass spectrometry. Wheresas oxygen-atom transfer prevails in the reaction of the oxide complex [OTiH]⁺ with CO₂, generating [OTi(OH)]⁺ under the elimination of CO, insertion of CO₂ into the metal–hydrogen bond of the cyclopentadienyl complex, [Cp₂TiH]⁺, gives rise to the formate complex [Cp₂Ti(O₂CH)]⁺. DFT-based methods were employed to understand how the ligand controls the observed variation in reactivity toward CO₂. Insertion of CO₂ into the TiH bond constitutes the initial step for the reaction of both [Cp₂TiH]⁺ and [OTiH]⁺, thus generating formate complexes as intermediates. In contrast to [Cp₂Ti(O₂CH)]⁺ which is kinetically stable, facile decarbonylation of [OTi(O₂CH)]⁺ results in the hydroxo complex [OTi(OH)]⁺. The longer lifetime of [Cp₂Ti(O₂CH)]⁺ allows for secondary reactions with background water, as a result of which, [Cp₂Ti(OH)]⁺ is formed. Further, computational studies reveal a good linear correlation between the hydride affinity of [LTi]²⁺ and the barrier for CO₂ insertion into various [LTiH]⁺ complexes. Understanding the intrinsic ligand effects may provide insight into the selective activation of CO₂.

Mechanistic aspects of an unusual gas-phase reaction of [LaCH₂]⁺ with halobenzenes have been investigated using Fourier-transform ion cyclotron resonance (FTICR) mass spectrometry combined with density functional theory (DFT) calculations. In this thermal process a carbon-atom from the benzene ring, most likely the ipso-position, and the carbene ligand are coupled to form C₂H₂.

The reactivity of the heteronuclear oxide cluster [Ga₂Mg₂O₅]^.+, bearing an unpaired electron at a bridging oxygen atom (O_b^.−), 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 CH bond activation but also provide important guidance for the rational design of catalysts by pointing to the particular role of doping effects.

The thermal reactions of the heteronuclear oxide cluster [Ga₂MgO₄]^.+ 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 OH 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 RH bonds are cleaved by metal oxides.

Recent progress in the gas-phase activation of methane is discussed. We demonstrate that cluster size, charge state, and ligands crucially affect both the reactivity and selectivity of metal-mediated bond activation processes. We outline the important role that relativistic effects and spin densities play and discuss the paradigm of two-state reactivity in thermal reactions. State-of-the-art mass-spectrometry based experiments, in conjunction with electronic structure calculations, permit identification of the elementary steps at a strictly molecular level and thus allow to uncover mechanistic features for four types of reactions: (i) metal-mediated dehydrogenation of methane, (ii) ligand-switch processes of the type ML + CH₄ M(CH₃) + HL, (iii) hydrogen-atom abstraction as the crucial step in the oxidative coupling of methane, and (iv) the mechanism of the challenging CH₄CH₃OH conversion.

With the exception of [Cu(OH)]⁺, the thermal reactions of the first-row transition-metal hydroxide cations, [Sc(OH)]⁺–[Zn(OH)]⁺, with ammonia have been studied by means of gas-phase experiments and by computational methods for the whole series. The primary reaction channels involve NH bond activation, forming [M(NH₂)]⁺ concomitantly with the elimination of water, and adduct formation, leading to [M(OH)(NH₃)]⁺. Furthermore, [Ti(OH)]⁺ and [V(OH)]⁺ react with ammonia under dehydrogenation conditions, leading to [M,O,N,H₂]⁺ (M=Ti, V), and for [Ni(OH)]⁺ ligand exchange is observed. Computations of the main reaction channels have been performed for the [M(OH)]⁺/NH₃ couples (M=Sc–Zn) to uncover the underlying reaction mechanism and periodic trends across the first row. For NH bond activation, σ-bond metathesis was found to be the underlying mechanism.

Invited for this month’s cover is the group of Prof. Helmut Schwarz from Technische Universität Berlin, Germany. For this special issue dedicated to the memory of Detlef Schröder, the cover picture shows the then young Detlef—a mastermind dedicated to the sciences. Read the full text of the article on page 952.