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"Bare" metal-carbene complexes, when generated in the gas phase and exposed to thermal reactions under (near) single-collision conditions, exhibit rather unique reactivities in addition to the well-known metathesis and cyclopropanation processes. For example, at room temperature the unligated [AuCH2](+) complex brings about efficient C-C coupling with methane to produce C2Hx (x = 4, 6), and the couple [TaCH2](+)/CO2 gives rise to the generation of the acetic acid equivalent CH2═C═O. Entirely unprecedented is the thermal extrusion of a carbon atom from halobenzenes (X = F, Cl, Br, I) by [MCH2](+) (M = La, Hf, Ta, W, Re, Os) and its coupling with the methylene ligand to deliver C2H2 and [M(X)(C5H5)](+). Among the many noteworthy C-N bond-forming processes, the formation of CH3NH2 from [RhCH2](+)/NH3, the generation of CH2═NH2(+) from [MCH2](+)/NH3 (M = Pt, Au), and the production of [PtCH═NH2](+) from [PtCH2](+)/NH3 are of particular interest. The latter species are likely to be involved as intermediates in the platinum-mediated large-scale production of HCN from CH4/NH3 (the DEGUSSA process). In this context, a few examples are presented that point to the operation of co-operative effects even at a molecular level. For instance, in the coupling of CH4 with NH3 by the heteronuclear clusters [MPt](+) (M = coinage metal), platinum is crucial for the activation of methane, while the coinage metal M controls the branching ratio between the C-N bond-forming step and unwanted soot formation. For most of the gas-phase reactions described in this Account, detailed mechanistic insight has been derived from extensive computational work in conjunction with time-honored labeling and advanced mass-spectrometry-based experiments, and often a coherent description of the experimental findings has been achieved. As for some transition metals, in particular those from the third row, the metal-carbene complexes can be formed directly from methane, coupling of the so-generated [MCH2] species with an inert molecule such as CH4, CO2, or NH3 constitutes a route to activate and functionalize methane under ambient conditions. Clearly, while these gas-phase studies cannot be translated directly to formally related processes in solution or those that occur at a surface, they nevertheless provide a conceptual mechanistic understanding and permit researchers to probe directly the remarkable intrinsic features of these elusive molecules and, in a broader context, help to identify the active site of a catalyst, the so-called "aristocratic atoms".

The thermal gas‐phase reactions of [Al2VO5]+ and [AlV2O6]+ with methane have been explored by using Fourier transform ion cyclotron resonance (FT‐ICR) mass spectrometry complemented by high‐level quantum chemical calculations. Both cluster ions chemisorbed methane as the major reaction channels at room temperature. [Al2VO5]+ could break only one C−H bond to liberate CH3, whereas [AlV2O6]+ exhibited higher oxidizing ability such that it brings about the selective generation of formaldehyde. Mechanistic aspects are revealed and the crucial roles of the metal centers are discussed. The reactivity of cluster oxides [AlnV3−nO7−n]+ (n=1, 2) towards methane was tuned by varying the composition of the metal centers. The thermal gas‐phase reactions of [Al2VO5]+ and [AlV2O6]+ with methane have been explored by using Fourier transform ion cyclotron resonance (FT‐ICR) mass spectrometry complemented by high‐level quantum chemical calculations. Mechanistic aspects are revealed and the crucial roles of the metal centers are discussed. [ABSTRACT FROM AUTHOR]

While pristine [TaO2]+ is rather inert in its thermal reactions with both methane and molecular hydrogen, the cluster oxide becomes reactive upon ligation with the closed‐shell ligand CO2. While the presence of CO2 increases the intrinsic barriers of the C−H and H−H bond activation steps, the efficiency of the overall reaction to generate the insertion product [Ta(R)(OH)O]+ (R=CH3, H) is greatly enhanced by the superb leaving‐group properties of CO2. The ligand effect of CO2 has been well mimicked by the electric field of a negative point charge, and mechanistic aspects are addressed by high‐level quantum chemical calculations Electric ligands: Upon the addition of a closed‐shell ligand CO2, the inertness of [TaO2]+ towards CH4 (or H2) is removed (see scheme, Ta purple). This remarkable ligand effect can be modeled by an electric field. [ABSTRACT FROM AUTHOR]

Abstract: The thermal gas‐phase reaction of [Al2CuO5]+ with methane has been explored by using FT‐ICR mass spectrometry complemented by high‐level quantum chemical calculations. The generation of atomic [Cu]+ from the [Al2CuO5]+/CH4 couple corresponds to the only reaction channel. Labeling experiments and computational studies strongly suggest that methane activation is indeed involved in the production of [Cu]+, and generation of CH2O prevails. Mechanistic aspects and the associated doping effects are discussed. [ABSTRACT FROM AUTHOR]

Abstract: In thermal reactions of methane with diatomic metal oxides [MO].+ of the zinc triad (M=Zn, Cd, Hg), protonated formaldehyde [CH2OH]+ is generated as the major product only for the [HgO].+/CH4 couple. Mechanistic insight is provided by high‐level quantum‐chemical calculations, and relativistic effects are suggested to be the root cause for the unexpected thermal production of [CH2OH]+ from [HgO].+/CH4. [ABSTRACT FROM AUTHOR]

The thermal gas-phase reactions of [ReO4]+ with methane have been explored by using Fourier Transform Ion Cyclotron Resonance (FT-ICR) mass spectrometry complemented by high-level quantum chemical calculations. Upon reacting with methane, this cluster oxide, having an even-number of valence electrons, brings about both hydrogen-atom abstraction (HAT) to generate [ReO4H].+ and the formation of formaldehyde. Mechanistically, HAT occurs on the ground-state triplet surface, while for the generation of formaldehyde a two-state reactivity scenario prevails. The branching ratio of these competing processes is affected by the rather inefficient spin-orbit coupling to bring about the required triplet-singlet intersystem crossing. [ABSTRACT FROM AUTHOR]

The thermal gas-phase reactions of [Al2ZnO4].+ with methane have been explored by using FT-ICR mass spectrometry complemented by high-level quantum chemical calculations. Two competitive mechanisms, that is, hydrogen-atom transfer (HAT) and proton-coupled electron transfer (PCET) are operative. Interestingly, while the HAT process is influenced by the polarity of the transition structure, both the ionic nature of the metal-oxygen bond and the structural rigidity of the cluster oxide affect the PCET pathway. As compared to the previously reported homonuclear [Al2O3].+ and [ZnO].+, the heteronuclear oxide [Al2ZnO4].+ exhibits a much higher chemoselectivity towards methane. The electronic origins of the doping effect have been explored. [ABSTRACT FROM AUTHOR]

The thermal gas-phase reactions of [Al2ZnO4].+ with methane have been explored by using FT-ICR mass spectrometry complemented by high-level quantum chemical calculations. Two competitive mechanisms, that is, hydrogen-atom transfer (HAT) and proton-coupled electron transfer (PCET) are operative. Interestingly, while the HAT process is influenced by the polarity of the transition structure, both the ionic nature of the metal-oxygen bond and the structural rigidity of the cluster oxide affect the PCET pathway. As compared to the previously reported homonuclear [Al2O3].+ and [ZnO].+, the heteronuclear oxide [Al2ZnO4].+ exhibits a much higher chemoselectivity towards methane. The electronic origins of the doping effect have been explored. [ABSTRACT FROM AUTHOR]

The gas-phase reactions of [NiL]+ (L=C6H5, C5H4N, CN) with methane have been explored by using electrospray-ionization mass spectrometry (ESI-MS) complemented by quantum chemical calculations. Though the phenyl Ni complex [Ni(C6H5)]+ exclusively abstracts one hydrogen atom from methane at ambient conditions, the cyano Ni complex [Ni(CN)]+ brings about both H-atom abstraction and ligand exchange to generate [Ni(CH3)]+. In contrast, the complex 2-pyridinyl Ni [Ni(C5H4N)]+ is inert towards this substrate. The presence of the empty 4s(Ni) orbital dominates the proton-coupled electron transfer (PCET) processes for the investigated systems. [ABSTRACT FROM AUTHOR]

The thermal gas-phase reactions of methane with [OMoH]+ and [MoH]+ were investigated by using electrospray-ionization mass spectrometry (ESI-MS) complemented by quantum-chemical calculations. In contrast to the inertness of [MoH]+ towards methane, [OMoH]+ activates the C−H bond to form the ionic product [OMo(CH3)]+ concomitantly with the liberation of H2. The origin of the varying reactivities is traced back to a different influence of the oxo ligand on the Mo−C and Mo−H bonds. While the presence of this ligand weakens both the Ti−H and the Ti−CH3 bonds, both the Mo−H and Mo−CH3 bonds are strengthened. The more pronounced strengthening of the Mo−CH3 bond compared to the Mo−H bond favors the exothermicity of the reaction of [OMoH]+ with CH4. [ABSTRACT FROM AUTHOR]

The thermal gas-phase reactions of rhenium carbonyl complexes [Re(CO) x]+ ( x=0-3) with methane have been explored by using FT-ICR mass spectrometry complemented by high-level quantum chemical calculation. While it had been concluded in previous studies that addition of closed-shell ligands in general decreases the reactivity of metal ions, the current work provides an exception: the previously demonstrated inertness of atomic Re+ towards methane is completely changed upon ligation with CO. Both [Re(CO)]+ and [Re(CO)2]+ bring about efficient dehydrogenation of the hydrocarbon under ambient conditions. However, addition of a third ligand to form [Re(CO)3]+ completely quenches the reactivity. [ABSTRACT FROM AUTHOR]

The thermal reaction of methane with the metal-free cluster cation [Si2O4].+ has been examined by using Fourier transform-ion cyclotron resonance (FT-ICR) mass spectrometry. In addition to generating a methyl radical via hydrogen-atom abstraction, [Si2O4].+ can selectively oxidize methane to formaldehyde. The mechanisms of these rather efficient reactions have been elucidated by high-level quantum-chemical calculations. [ABSTRACT FROM AUTHOR]

The thermal reaction of methane with the metal-free cluster cation [Si2O4].+ has been examined by using Fourier transform-ion cyclotron resonance (FT-ICR) mass spectrometry. In addition to generating a methyl radical via hydrogen-atom abstraction, [Si2O4].+ can selectively oxidize methane to formaldehyde. The mechanisms of these rather efficient reactions have been elucidated by high-level quantum-chemical calculations. [ABSTRACT FROM AUTHOR]

The thermal reactivity of the heteronuclear metal-oxide cluster cations [ XAlO4].+ ( X=V, Nb, and Ta) towards methane has been studied by using mass spectrometry in conjunction with quantum mechanical calculations. Experimentally, a hydrogen-atom transfer (HAT) from methane is mediated by all the three oxide clusters at ambient conditions. However, [VAlO4].+ is unique in that this cluster directly transforms methane into formaldehyde. The absence of this reaction for the Nb and Ta analogues demonstrates a striking doping effect on the chemoselectivity in the conversion of methane. Mechanistic aspects of the two reactions have been elucidated by quantum-chemical calculations. The HAT reactivity can be attributed to the significant spin density localized at the terminal oxygen atom (Ot.−) of the cluster ions, while the ionic/covalent character of the Lewis acid-base unit [ X−Ob] plays a crucial role for the generation of formaldehyde. The mechanistic insight derived from this combined experimental/computational investigation may provide guidance for a more rational design of catalysts. [ABSTRACT FROM AUTHOR]

The thermal gas-phase reactions of the closed-shell heteronuclear metal-oxide clusters [AlCeOx]+ ( x=2-4) with methane have been explored by FT-ICR mass spectrometry and high-level quantum-chemical calculation. Whereas [AlCeO2]+ and [AlCeO4]+ are inert towards methane under ambient conditions, [AlCeO3]+ spontaneously abstracts one hydrogen atom from methane. Mechanistic aspects have been addressed to reveal the reasons for the rather distinct reactivities of the [AlCeOx]+/CH4 couples, and the electronic origins of the unprecedented single hydrogen-atom abstraction from methane by closed-shell [AlCeO3]+ are discussed. [ABSTRACT FROM AUTHOR]

The thermal reaction of the diatomic rhenium nitride cation [ReN]+ with methane has been explored using FT-ICR mass spectrometry complemented by high-level quantum chemical calculations. Mechanistic aspects of the reaction have been addressed to uncover scenarios for the competitive generation of a methylene and a hydrogen cyanide ligand attached to the rhenium center. The role the nitrogen atom plays in the reactive [ReN]+/CH4 couple is elucidated, and the cause of the rather different reactivities of [ReN]+ and its lighter congener [MnN]+ is revealed. [ABSTRACT FROM AUTHOR]

The thermal gas-phase reactions of the closed-shell metal-oxide cluster [Al2TaO5]+ with methane have been explored by using FT-ICR mass spectrometry complemented by high-level quantum chemical calculations. Mechanistic aspects have been addressed to reveal the origins of the efficient addition process which results in activating the C−H bond of methane. The [Al2TaO5]+/CH4 couple has been compared with several other systems reported previously, and the electronic origins of their rather distinct performances are discussed. [ABSTRACT FROM AUTHOR]

The thermal reaction of the diatomic rhenium nitride cation [ReN]+ with methane has been explored using FT-ICR mass spectrometry complemented by high-level quantum chemical calculations. Mechanistic aspects of the reaction have been addressed to uncover scenarios for the competitive generation of a methylene and a hydrogen cyanide ligand attached to the rhenium center. The role the nitrogen atom plays in the reactive [ReN]+/CH4 couple is elucidated, and the cause of the rather different reactivities of [ReN]+ and its lighter congener [MnN]+ is revealed. [ABSTRACT FROM AUTHOR]

The thermal gas-phase reactions of the closed-shell metal-oxide cluster [Al2TaO5]+ with methane have been explored by using FT-ICR mass spectrometry complemented by high-level quantum chemical calculations. Mechanistic aspects have been addressed to reveal the origins of the efficient addition process which results in activating the C−H bond of methane. The [Al2TaO5]+/CH4 couple has been compared with several other systems reported previously, and the electronic origins of their rather distinct performances are discussed. [ABSTRACT FROM AUTHOR]

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+/C2H4, [Au(C2H2)]+/H2, and [C2H3]+/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. [ABSTRACT FROM AUTHOR]

The thermal reactions of methane with the oxygen-rich cluster cations [Si2O5]⋅+ and [Si2O5H2]⋅+ have been examined using Fourier transform-ion cyclotron resonance (FT-ICR) mass spectrometry in conjunction with state-of-the-art quantum chemical calculations. In contrast to the inertness of [Si2O5].+ towards methane, the hydrogenated cluster [Si2O5H2].+ brings about hydrogen-atom transfer (HAT) from methane with an efficiency of 28 % relative to the collision rate. The mechanisms of this process have been investigated in detail and the reasons for the striking reactivity difference of the two cluster ions have been revealed. [ABSTRACT FROM AUTHOR]

The thermal reaction of diatomic tantalum nitride cation [TaN]+ with methane has been explored using FT-ICR mass spectrometry complemented by high-level quantum chemical calculation; based on this combined experimental/computational approach, mechanistic aspects of this novel, highly efficient C−N coupling process have been uncovered. In distinct contrast to [TaN]+, its lighter congeners [VN]+ and [NbN]+ are inert towards methane under ambient conditions, and the origins of the remarkably variable efficiencies of the three metal nitrides are uncovered by CCSD(T) calculations. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The reactivity of the heteronuclear oxide cluster [Ga2Mg2O5].+, 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. [ABSTRACT FROM AUTHOR]

Methane conversion under mild conditions by Ru complexes. [Display omitted] • CH 4 can be converted to CF 3 COOCH 3 at 60 °C and 1 MPa as mediated by Ru catalysts. • Theory reveal a relatively low-lying, empty n *(Ru) orbital explain catalyst's effect. • The promising role Ru may play in liquid-phase methane conversion is indicated. Liquid-phase methane activation by ruthenium complexes has been studied using a combination of experiments and quantum-chemical calculation. Surprisingly, methane can be converted to methyl trifluoroacetate efficiently under mild conditions, and the bis(NHC) ligand outperforms the other two employed. The associated electronic origins of the excellent performance of the ruthenium complex – in particular the ligand effects – have been discussed, and the dative C → Ru interaction as well as the structural distortion of the bis(NHC) plane are found responsible. This work reveals the promising role ruthenium may play in the construction of organometallic catalysts for efficient methane conversion. [ABSTRACT FROM AUTHOR]