Structures of the dehydrogenation products of methane activation by 5d transition metal cations

Dynamics of carbene formation in the reaction of methane with the tantalum cation in the gas phase

The controlled activation of methane has drawn significant attention throughout various disciplines over the last few decades. In gas-phase experiments, the use of model systems with reduced complexity compared to condensed-phase catalytic systems allows us to investigate the intrinsic reactivity of elementary reactions down to the atomic level. Methane is rather inert in chemical reactions, as the weakening or cleavage of a C-H bond is required to make use of methane as C1-building block. The simplest model system for transition-metal-based catalysts is a mono-atomic metal ion. Only a few atomic transition-metal cations activate methane at room temperature. One of the most efficient elements is tantalum, which forms a carbene and releases molecular hydrogen in the reaction with methane: Ta+ + CH4 → TaCH2+ + H2. The reaction takes place at room temperature due to efficient intersystem crossing from the quintet to the triplet surface, i.e., from the electronic ground state of the tantalum cation to the triplet ground state of the tantalum carbene. This multi-state reactivity is often seen for reactions involving transition-metal centres, but leads to their theoretical treatment being a challenge even today. Chemical reactions, or to be precise reactive collisions, are dynamic processes making their description even more of a challenge to experiment and theory alike. Experimental energy- and angle-differential cross sections allow us to probe the rearrangement of atoms during a reactive collision. By interpreting the scattering signatures, we gain insight into the atomistic mechanisms and can move beyond stationary descriptions. Here, we present a study combining collision energy dependent experimentally measured differential cross sections with ab initio calculations of the minimum energy pathway. Product ion velocity distributions were recorded using our crossed-beam velocity map imaging experiment dedicated to studying transition-metal ion molecule reactions. TaCH2+ velocity distributions reveal a significant degree of indirect dynamics. However, the scattering distributions also show signatures of rebound dynamics. We compare the present results to the oxygen transfer reaction between Ta+ and carbon dioxide, which we recently studied.

IR spectroscopic characterization of [M,C,2H]+ (M = Ru and Rh) products formed by reacting 4d transition metal cations with oxirane: Spectroscopic evidence for multireference character in RhCH2. Phys Chem Chem Phys 2024;26:11445-11458. [PMID: 38572552 PMCID: PMC11022548 DOI: 10.1039/d4cp00012a]

A combination of infrared multiple-photon dissociation (IRMPD) action spectroscopy and quantum chemical calculations was employed to investigate the [M,C,2H]+ (M = Ru and Rh) species. These ions were formed by reacting laser ablated M+ ions with oxirane (ethylene oxide, c-C2H4O) in a room-temperature ion trap. IRMPD spectra for the Ru species exhibit one major band and two side bands, whereas spectra for the Rh species contain more distinct bands. Comparison with density functional theory (DFT), coupled-cluster (CCSD), and equation-of-motion spin-flip CCSD (EOM-SF-CCSD) calculations allows assignment of the [M,C,2H]+ structures. For the spectrum of [Ru,C,2H]+, a combination of HRuCH+ and RuCH2+ structures reproduces the observed spectrum at all levels of theory. The well-resolved spectrum of [Rh,C,2H]+ could not be assigned unambiguously to any calculated structure using DFT approaches. The EOM-SF-CCSD calculations showed that the ground-state surface has multireference electronic character, and symmetric carbenes in both the 1A1 and 3A2 states are needed to reproduce the observed spectrum.

IR spectroscopic characterization of 3d transition metal carbene cations, FeCH2+ and CoCH2+: periodic trends and a challenge for DFT approaches

A combination of IR multiple-photon dissociation (IRMPD) action spectroscopy and quantum chemical calculations was employed to investigate the [M,C,2H]+ (M = Fe and Co) species. These were formed by reacting laser ablated M+ ions with oxirane (ethylene oxide, c-C2H4O) in a room temperature ion trap. IRMPD spectra for the Fe and Co species are very similar and exhibit one major band. Comparison with density functional theory (DFT) and coupled cluster with single and double excitations (CCSD) calculations allows assignment of the spectra to MCH2+ carbene structures. For these 3d transition metal systems, experimental IRMPD spectra compare relatively poorly with DFT calculated IR spectra, but CCSD calculated spectra are a much better match primarily because the M-C stretch gains significant intensity. The origins of this behavior are explored in some detail. The present results are also compared to previous results for the 4d and 5d congeners and the periodic trends in these structures are evaluated.

Manipulating Reactivity of Ir(CH2)0-2+ Cations toward Dinitrogen at Room Temperature: A Unique Dependence on the Organic Ligand Structures

Nitrogen (N2) activation at room temperature has long been a great challenge. Therefore, the rational design of reactive species to adsorb N2, which is a prerequisite for cleavage of the strong N≡N triple bond in industrial and biological processes, is highly desirable and meaningful. Herein, the N2 adsorption process is controlled by regulating the types and numbers of organic ligands, and the organic ligands are produced through the reactions of Ir+ with methane and ethane. CH4 molecules dissociate on the Ir+ cations to form Ir(CH2)1,2+. The reaction of Ir+ with C2H6 can generate HIrC2H3+, which is different from the structure of Ir(CH2)2+ obtained from Ir+/CH4. The reactivity order of N2 adsorption is Ir(CH2)2+ > HIrC2H3+ ≫ HIrCH+ ≈ Ir+ (almost inert under similar reaction conditions), indicating that different organic ligand structures affect reactivity dramatically. The main reason for this interesting reactivity difference is that the lowest unoccupied molecular orbital (LUMO) level of Ir(CH2)2+ is much closer to the highest occupied molecular orbital (HOMO) level of N2 than those of the other three systems. This study provides new insights into the adsorption of N2 on metal-organic ligand species, in which the organic ligand dominates the reactivity, and it discovers new clues in designing effective transition metal carbine species for N2 activation.

Quantitative Aspects of Gas-Phase Metal Ion Chemistry: Conservation of Spin, Participation of f Orbitals, and C-H Activation and C-C Coupling

In this Featured Article, I reflect on over 40 years of guided ion beam tandem mass spectrometry (GIBMS) studies involving atomic metal cations and their clusters throughout the periodic table. Studies that have considered the role of spin conservation (or lack thereof) are a primary focus with a quantitative assessment of the effects examined. A need for state-specific studies of heavier elements is noted, as is a more quantitative assessment of spin-orbit interactions in reactivity. Because GIBMS experiments explicitly evaluate the kinetic energy dependence of reactions over a wide range, several interesting and unusual observations are highlighted. More detailed studies of such unusual reaction events would be welcome. Activation of C-H bonds and ensuing C-C coupling events are reviewed, with future work encouraged. Finally, studies of lanthanides and actinides are examined with an eye on understanding the role of f orbitals in the chemistry, both as participants (or not) in the bonding and as sources/sinks of electron density. This area seems to be ripe for more quantitative experiments.

On the performance of the M-C (M = Fe, Ru, Os) unit toward methane activation

Gas-phase reactions of [MC]+ (M = Os and Ru) with methane at ambient temperature have been studied by using quadrupole-ion trap (Q-IT) mass spectrometry combined with quantum chemical calculations. Theoretical calculations reveal the influence of electronic signatures and that it is the energy gap of the associated frontier molecular orbitals that dominates the ability of the cluster in the initial H3C-H bond breaking. By extension, a theoretical consideration upon changing the ligand from carbide to carbyne and eventually to carbene reveals that the reactivities of the M-complex (M = Os, Ru and Fe) are determined by the energy gap of the involved orbitals. In addition, a few factors like the dipole moment, spin density and charge distributions influence the orbital energy gap to different extents. Thus, altering the local structure of the active center to modulate the orbital distribution may be a possible means of regulation of the activity.

Dynamics of the Oxygen Atom Transfer Reaction between Carbon Dioxide and the Tantalum Cation

The understanding of fundamental atomic-level processes often requires well-defined model systems. The oxygen atom transfer from CO₂ to a transition metal cation in the gas phase presents such a model system. We investigate the reaction of Ta⁺ + CO₂ for which the formation of TaO⁺ is highly efficient and attributed to multistate reactivity. Here, we study the atomistic dynamics of the oxygen atom transfer reaction by recording experimental energy and angle differential cross sections by crossed beam velocity map imaging supported by ab initio quantum chemical calculations. Product ion velocity distributions are dominated by signatures for indirect dynamics, despite the reaction being highly exothermic. Product kinetic energy distributions show little dependence on additional collision energy even with only four atoms involved, which hints at dynamical trapping behind a submerged barrier.

Direct Coupling of Methane and Carbon Dioxide on Tantalum Cluster Cations

Understanding molecular-scale reaction mechanisms is crucial for the design of modern catalysts with industrial prospect. Through joint experimental and computational studies, we investigate the direct coupling reaction of CH₄ and CO₂ , two abundant greenhouse gases, mediated by Ta_1,4 ⁺ ions to form larger oxygenated hydrocarbons. Coherent with proposed elementary steps, we expose products of CH₄ dehydrogenation [Ta_1,4 CH₂ ]⁺ to CO₂ in a ring electrode ion trap. Product analysis and reaction kinetics indicate a predisposition of the tetramers for C-O coupling with a conversion to products of CH₂ O, whereas atomic cations enable C-C coupling yielding CH₂ CO. Selected experimental findings are supported by thermodynamic computations, connecting structure, electronic properties, and catalyst function. Moreover, the study of bare Ta_1,4 ⁺ compounds indicates that methane dehydrogenation is a significant initial step in the direct coupling reaction, enabling new, yet unknown reaction pathways.

Structural Determination of Lysine-Linked Cisplatin Complexes via IRMPD Action Spectroscopy: NNs and NO- Binding Modes of Lysine to Platinum Coexist

Despite its success as an anticancer drug, cisplatin suffers from resistance and produces side effects. To overcome these limitations, amino-acid-linked cisplatin analogues have been investigated. Lysine-linked cisplatin, Lysplatin, (Lys)PtCl₂, exhibited outstanding reactivity toward DNA and RNA that differs from that of cisplatin. To gain insight into its differing reactivity, the structure of Lysplatin is examined here using infrared multiple photon dissociation (IRMPD) action spectroscopy. To probe the influence of the local chemical environment on structure, the deprotonated and sodium-cationized Lysplatin complexes are examined. Electronic structure calculations are performed to explore possible modes of binding of Lys to Pt, their relative stabilities, and to predict their infrared spectra. Comparisons of the measured IRMPD and predicted IR spectra elucidate the structures contributing to the experimental spectra. Coexistence of two modes of binding of Lys to Pt is found where Lys binds via the backbone and side-chain amino nitrogen atoms, NN_s, or to the backbone amino and carboxylate oxygen atoms, NO^-. Glycine-linked cisplatin and arginine-linked cisplatin complexes have previously been found to bind only via the NO^- binding mode. Present results suggest that the NN_s binding conformers may be key to the outstanding reactivity of Lysplatin toward DNA and RNA.

IR Spectroscopic Characterization of Methane Adsorption on Copper Clusters Cun+ (n = 2-4)

The interaction of CH₄ with cationic copper clusters has been studied with infrared-multiple photon dissociation (IRMPD) spectroscopy. Cu_n⁺ (n = 2-4) formed by laser ablation were reacted with CH₄. The formed complexes were irradiated with the IR light of the free-electron laser for intracavity experiments (FELICE), and the fragments were mass-analyzed with a reflectron time-of-flight mass spectrometer. The structures of the Cu_n⁺-CH₄ complexes are assigned on the basis of comparison between the resulting IRMPD spectra to spectra of different isomers calculated with density functional theory (DFT). For all sizes n, the structure found is one with molecularly adsorbed CH₄. Only slight deformations of the CH₄ molecule have been identified upon adsorption on the clusters, which results in redshifts of the spectroscopic bands. This deformation can be explained by charge transfer from the cluster to the adsorbed methane molecule.

C-H Bond Activation and C-C Coupling of Methane on a Single Cationic Platinum Center: A Spectroscopic and Theoretical Study

We spectroscopically investigated the activation products resulting from reacting one and multiple methane molecules with Pt⁺ ions. Pt⁺ ions were formed by laser ablation of a metal target and were cooled to the electronic ground state in a supersonic expansion. The ions were then transferred to a room temperature ion trap, where they were reacted with methane at various partial pressures in an argon buffer gas. Product masses observed were [Pt,C,2H]⁺, [Pt,2C,4H]⁺, [Pt,4C,8H]⁺, and [Pt,2C,O,6H]⁺, which were mass-isolated and characterized using infrared multiple-photon dissociation (IRMPD) spectroscopy employing the free electron laser for intra-cavity experiments (FELICE). The spectra for [Pt,2C,4H]⁺ and [Pt,4C,8H]⁺ have several well-defined bands and, when compared to density functional theory-calculated spectra for several possible product structures, lead to unambiguous assignments to species with ethene ligands, proving Pt⁺-mediated C–C coupling involving up to four methane molecules. These findings contrast with earlier experiments where Pt⁺ ions were reacted in a flow-tube type reaction channel at significantly higher pressures of helium buffer gas, resulting in the formation of a Pt(CH₃)₂⁺ product. Our DFT calculations show a reaction barrier of +0.16 eV relative to the PtCH₂⁺ + CH₄ reactants that are required for C–C coupling. The different outcomes in the two experiments suggest that the higher pressure in the earlier work could kinetically trap the dimethyl product, whereas the lower pressure and longer residence times in the ion trap permit the reaction to proceed, resulting in ethene formation and dihydrogen elimination.

Methane is reacted with gas-phase atomic platinum cations in a room temperature ion trap, and the resulting product ions are characterized using IR light of the Free Electron Laser for Intra-Cavity Experiments (FELICE). We observed the formation of PtCH₂⁺, Pt(ethene)⁺, and Pt(ethene)₂⁺. These findings demonstrate the C−H bond activation and C−C coupling of methane molecules.

Ion spectroscopy in methane activation

This review is devoted to ion spectroscopy studies of complexes relevant for the understanding of methane activation with metal ions and clusters. Methane activation starts with the formation of a complex with a metal ion. The degree of the interaction between an intact methane molecule and the ion can be monitored by the perturbations of C-H stretch vibrations in the methane molecule. Binding mediated by the electrostatic interaction results in a η³ type coordination of methane. In contrast, binding governed by orbital interactions results in a η² type coordination of methane. We further review the spectroscopic characterization of activation products of metal-methane reactions, such as the metal-carbene and carbyne products resulting from the interaction of selected 5d metals with methane. The focus of recent research in the field has shifted towards the investigation of interactions between methane and metal clusters. We show examples highlighting that metal clusters can be more reactive in methane activation reactions.

Room-Temperature Methane Activation Mediated by Free Tantalum Cluster Cations: Size-by-Size Reactivity

The energetics of small cationic tantalum clusters and their gas-phase adsorption and dehydrogenation reaction pathways with methane are investigated with ion-trap experiments and spin-density-functional-theory calculations. Ta_n⁺ clusters are exposed to methane under multicollision conditions in a cryogenic ring electrode ion-trap. The cluster size affects the reaction efficiency and the number of consecutively dehydrogenated methane molecules. Small clusters (n = 1-4) dehydrogenate CH₄ and concurrently eliminate H₂, while larger clusters (n > 4) demonstrate only molecular adsorption of methane. Unique behavior is found for the Ta⁺ cation, which dehydrogenates consecutively up to four CH₄ molecules and is predicted theoretically to promote formation of a [Ta(CH₂-CH₂-CH₂)(CH₂)]⁺ product, exhibiting C-C coupled groups. Underlying mechanisms, including reaction-enhancing couplings between potential energy surfaces of different spin-multiplicities, are uncovered.

Methane to Methanol Conversion Facilitated by Anionic Transition Metal Centers: The Case of Fe, Ni, Pd, and Pt

Density functional theory and high-level ab initio electronic structure calculations are performed to study the mechanism of the partial oxidation of methane to methanol facilitated by the titled anionic transition metal atoms. The energy landscape for the overall reaction M^- + N₂O + CH₄ → M^- + N₂ + CH₃OH (M = Fe, Ni, Pd, Pt) is constructed for different reaction pathways for all four metals. The comparison with earlier experimental and theoretical results for cationic centers demonstrates the better performance of the metal anions. The main advantage is that anionic centers interact weakly with the produced methanol. This fact facilitates the fast removal of methanol from the catalytic center and prevents the overoxidation of methane. Moreover, a moderate or high energy barrier for the M^- + CH₄ → HMCH₃^- reaction step is observed, which protects the metal center from deactivation. Future work should focus on the identification of proper ligands, which stabilize the negative charge on the metal (electronic factors) and prevent the formation of the global CH₃MOH^- minimum (steric factors). Finally, a composite electronic structure method (combining size extensive coupled clusters approaches and accurate multireference configuration interaction) is proposed for computationally demanding systems and is applied to Fe^-.

IR photofragmentation of the phenyl cation: spectroscopy and fragmentation pathways

We present the gas-phase infrared spectra of the phenyl cation, phenylium, in its perprotio (C₆H₅⁺) and perdeutero (C₆D₅⁺) forms, in the 260–1925 cm⁻¹ (5.2–38 μm) spectral range, and investigate the observed photofragmentation.

Consecutive methane activation mediated by single metal boride cluster anions NbB4. Phys Chem Chem Phys 2021;23:12592-12599. [PMID: 34047332 DOI: 10.1039/d1cp01418h]

Cleavage of all C-H bonds in two methane molecules by gas-phase cluster ions at room temperature is a challenging task. Herein, mass spectrometry and quantum chemical calculations have been used to identify one single metal boride cluster anions NbB4- that can activate eight C-H bonds in two methane molecules and release one H2 molecule each time under thermal collision conditions. In these consecutive reactions, the loaded Nb atoms and the support B4 units play different roles but act synergistically to activate CH4, which is responsible for the interesting reactivity of NbB4-. Moreover, there are some mechanistic differences in these two reactions, including the mechanisms for the first C-H bond activation steps, dihydrogen desorption sites, and major electron donors. This study shows that non-noble metal boride species are reactive enough to facilitate thermal C-H bond cleavages, and boron-based materials may be one kind of potential support material facilitating surface hydrogen transport.

Carbide Dihydrides: Carbonaceous Species Identified in Ta4 + -Mediated Methane Dehydrogenation

The products of methane dehydrogenation by gas‐phase Ta₄⁺ clusters are structurally characterized using infrared multiple photon dissociation (IRMPD) spectroscopy in conjunction with quantum chemical calculations. The obtained spectra of [4Ta,C,2H]⁺ reveal a dominance of vibrational bands of a H₂Ta₄C⁺ carbide dihydride structure over those indicative for a HTa₄CH⁺ carbyne hydride one, as is unambiguously verified by studies employing various methane isotopologues. Because methane dehydrogenation by metal cations M⁺ typically leads to the formation of either MCH₂⁺ carbene or HMCH⁺ carbyne hydride structures, the observation of a H₂MC⁺ carbide dihydride structure implies that it is imperative to consider this often‐neglected class of carbonaceous intermediates in the reaction of metals with hydrocarbons.

C/C Exchange in Activation/Coupling Reaction of Acetylene and Methane Mediated by Os+: A Comparison with Ir+, Pt+, and Au. J Phys Chem Lett 2020;11:8346-8351. [PMID: 32885973 DOI: 10.1021/acs.jpclett.0c02068]

The activation and coupling reactions of methane and acetylene mediated by M⁺ (M = Os, Ir, Pt, and Au) have been comparatively studied at room temperature by the techniques of mass spectrometry in conjunction with theoretical calculations. Studies have shown that Os⁺ and Ir⁺ can mediate the activation/coupling reaction of CH₄ and C₂H₂, while Pt⁺ and Au⁺ cannot, which could be explained by the number of empty valence orbitals in the metal atom. In addition, there are different competition channels for the reaction mediated by Os⁺ and Ir⁺: an expected dehydrogenation and an unexpected C/C exchange. We find that if the rare C/C exchange reaction takes place, there are symmetric carbon atoms in the reaction intermediate and the C/C exchange reaction is favored kinetically. The C/C exchange reaction must be considered, which will affect the yield of the products in the primary reaction. This study shows the molecular-level mechanisms which include the C/C exchange reaction in the activation and coupling reaction of organic compounds mediated by different metals.

Single-Atom Pt+ Derived from the Laser Dissociation of a Platinum Cluster: Insights into Nonoxidative Alkane Conversion

In this study, we construct a 193 nm ultraviolet laser dissociation high-resolution mass spectrometry (HRMS) platform to produce Pt⁺ cations with high efficiency, which is in situ applied for monitoring the "Pt⁺ + alkanes" reactions (where alkanes include methane, ethane, and propane). The conversion intermediates and products could be accurately determined by an orbitrap detector with high resolution (up to 150 000). Importantly, methane conversion by Pt⁺ cations yields [Pt + ethane]⁺ and [Pt + ethylene]⁺ as the sole products formed via the cross-coupling reaction of the Pt-CH₂ intermediate with gaseous methane. However, the Pt⁺ cations promote only the nonoxidative dehydrogenation of ethane and propane to give the corresponding [Pt + alkenes]⁺ and [Pt + alkynes]⁺. The details of the reaction mechanism are corroborated by density functional theory (DFT) calculations. These results highlight the power of HRMS with the laser dissociation of metal clusters in the generation and reaction characterization of metal ions.

Evidence for lactone formation during infrared multiple photon dissociation spectroscopy of bromoalkanoate doped salt clusters

Reaction mechanisms of organic molecules in a salt environment are of fundamental interest and are potentially relevant for atmospheric chemistry, in particular sea-salt aerosols. Here, we found evidence for lactone formation upon infrared multiple photon dissociation (IRMPD) of non-covalent bromoalkanoate complexes as well as bromoalkanoate embedded in sodium iodide clusters. The mechanism of lactone formation from bromoalkanoates of different chain lengths is studied in the gas phase with and without salt environment by a combination of IRMPD and quantum chemical calculations. IRMPD spectra are recorded in the 833-3846 cmT¹ range by irradiating the clusters with tunable laser systems while they are stored in the cell of a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer. The measurements of the binary complex Br(CH2)_mCOOH·Br(CH2)_mCOO^- for m = 4 indicate valerolactone formation without salt environment while lactone formation is hindered for longer chain lengths. When embedded in sodium iodide clusters, butyrolactone formation from 4-bromobutyrate seems to take place already during formation of the doped clusters in the electrospray process, evidenced by the infrared (IR) signature of the lactone. In contrast, IRMPD spectra of sodium iodide clusters containing 5-bromovalerate contain signatures for both valerate as well as valerolactone. In both cases, however, a neutral fragment corresponding to the mass of valerolactone is eliminated, indicating that ring formation can be activated by IR light in the salt cluster. Quantum chemical calculations show that already complexation with one sodium ion significantly increases the barrier for lactone formation for all chain lengths. IRMPD of sodium iodide clusters doped with neutral bromoalkanoic acid molecules proceeds by elimination of HI or desorption of the intact acid molecule from the cluster.

Methane activation by heteronuclear diatomic AuRh+ cation: comparison with homonuclear Au2+ and Rh2. Phys Chem Chem Phys 2020;22:6231-6238. [PMID: 32129335 DOI: 10.1039/c9cp05699h]

The ability to activate methane differs appreciably for different transition metals, and it is attractive to find the most suitable metal for the direct conversion of methane to value-added chemicals. Herein, we performed a comparative study on the reactions of CH₄ with Au₂⁺, AuRh⁺ and Rh₂⁺ cations by mass-spectrometry based experiments and DFT-based theoretical analysis. Different reactivity has been found for these cations: Au₂⁺ has the lowest reactivity, and it can activate methane but only produce H-Au₂-CH₃⁺ without H₂ release; Rh₂⁺ has the highest reactivity, and it can produce both carbene-type Rh₂-CH₂⁺ and carbyne-type H-Rh₂-CH⁺ with H₂ release; AuRh⁺ also has high reactivity to produce only AuRh-CH₂⁺ with H₂, avoiding the excessive dehydrogenation of CH₄. Our theoretical results demonstrate that Rh is responsible for the high reactivity, while Au leads to selectivity, which may be caused by the unique intrinsic bonding properties of the metals.

Catalytic Non-Oxidative Coupling of Methane on Ta8O2

Mass-selected Ta₈O₂⁺ cluster ions catalyze the transformation of methane in a gas-phase ion trap experiment via nonoxidative coupling into ethane and H₂, which is a prospective reaction for the generation of valuable chemicals on an industrial scale. Systematic variation of the reaction conditions and the isotopic labeling of methane by deuterium allow for an unambiguous identification of a catalytic cycle. Comparison with the proposed catalytic cycle for tantalum-doped silica catalysts reveals surprising similarities as the mechanism of the C-C coupling step, but also peculiar differences like the mechanism of the eventual formation of molecular hydrogen and ethane. Therefore, this work not only supplies insights into the mechanisms of methane coupling reactions but also illustrates how the study of trapped ionic catalysts can contribute to the understanding of reactions, which are otherwise difficult to study.

Infrared Spectroscopy of Gold Carbene Cation (AuCH2+): Covalent or Dative Bonding?

The present work explores the structure of the gold carbene cation, AuCH₂⁺, using infrared multiple photon dissociation action spectroscopy and density functional theory (DFT). Unlike several other 5d transition-metal cations (M⁺ = Ta⁺, W⁺, Os⁺, Ir⁺, and Pt⁺) that react with methane by dehydrogenation to form MCH₂⁺ species, gold cations are unreactive with methane at thermal energies. Instead, the metal carbene is formed by reacting atomic gold cations formed in a laser ablation source with ethylene oxide (cC₂H₄O) pulsed into a reaction channel downstream. The resulting [Au,C,2H]⁺ product photofragmented by loss of H₂ as induced by radiation provided by the free-electron laser for intracavity experiments in the 300-1800 cm^-1 range. Comparison of the experimental spectrum, obtained by monitoring the appearance of AuC⁺, and DFT calculated spectra leads to the identification of the ground-state carbene, AuCH₂⁺ (¹A₁), as the species formed, as previously postulated theoretically. Unlike the covalent double bonds formed by the lighter, open-shell 5d transition metals, the closed-shell Au⁺ (¹S, 5d¹⁰) atom binds to methylene by donation of a pair of electrons from CH₂(¹A₁) into the empty 6s orbital of gold coupled with π back-bonding, i.e., dative bonding, as explored computationally. Contributions to the AuC⁺ appearance spectrum from larger complexes are also considered, and H₂CAu⁺(c-C₂H₄O) seems likely to contribute one band observed.

Thermal C–O coupling reactions of Ta methylene clusters [TanCH2]+ (n = 1, 4) with O2

Cationic tantalum carbenes [TaCH₂]⁺ and [Ta₄CH₂]⁺, products of methane dehydrogenation, are reacted with dioxygen in an ion trap. Detected products suggest a formation of value-added molecules originating from C–O coupling reactions.

Spectroscopic Identification of the Carbyne Hydride Structure of the Dehydrogenation Product of Methane Activation by Osmium Cations

The present work explores the structures of species formed by dehydrogenation of methane (CH₄) and perdeuterated methane (CD₄) by the 5d transition metal cation osmium (Os⁺). Using infrared multiple photon dissociation (IRMPD) action spectroscopy and density functional theory (DFT), the structures of the [Os,C,2H]⁺ and [Os,C,2D]⁺ products are explored. This study complements previous work on the related species formed by dehydrogenation of methane by four other 5d transition metal cations (M⁺ = Ta⁺, W⁺, Ir⁺, and Pt⁺). Osmium cations are formed in a laser ablation source, react with methane pulsed into a reaction channel downstream, and the resulting products spectroscopically characterized through photofragmentation using the Free-Electron Laser for IntraCavity Experiments (FELICE) in the 300-1800 cm^-1 range. Photofragmentation was monitored by the loss of H₂/D₂. Comparison of the experimental spectra and DFT calculated spectra leads to identification of the ground state carbyne hydride, HOsCH⁺ (²A') as the species formed, as previously postulated theoretically. Further, a full description of the systematic spectroscopic shifts observed for deuterium labeling of these complexes, some of the smallest systems to be studied using IRMPD action spectroscopy, is achieved. A full rotational contour analysis explains the observed linewidths as well as the observation of doublet structures in several bands, consistent with previous observations for HIrCH⁺ (²A'). Graphical Abstract ᅟ.

Structures of the dehydrogenation products of methane activation by 5d transition metal cations revisited: Deuterium labeling and rotational contours

A previous infrared multiple photon dissociation (IRMPD) action spectroscopy and density functional theory (DFT) study explored the structures of the [M,C,2H]⁺ products formed by dehydrogenation of methane by four, gas-phase 5d transition metal cations (M⁺ = Ta⁺, W⁺, Ir⁺, and Pt⁺). Complicating the analysis of these spectra for Ir and Pt was observation of an extra band in both spectra, not readily identified as a fundamental vibration. In an attempt to validate the assignment of these additional peaks, the present work examines the gas phase [M,C,2D]⁺ products of the same four metal ions formed by reaction with perdeuterated methane (CD₄). As before, metal cations are formed in a laser ablation source and react with methane pulsed into a reaction channel downstream, and the resulting products are spectroscopically characterized through photofragmentation using the free-electron laser for intracavity experiments in the 350-1800 cm^-1 range. Photofragmentation was monitored by the loss of D for [Ta,C,2D]⁺ and [W,C,2D]⁺ and of D₂ in the case of [Pt,C,2D]⁺ and [Ir,C,2D]⁺. Comparison of the experimental spectra and DFT calculated spectra leads to structural assignments for all [M,C,2H/2D]⁺ systems that are consistent with previous identifications and allows a full description of the systematic spectroscopic shifts observed for deuterium labeling of these complexes, some of the smallest systems to be studied using IRMPD action spectroscopy. Further, full rotational contours are simulated for each vibrational band and explain several observations in the present spectra, such as doublet structures in several bands as well as the observed linewidths. The prominent extra bands in the [Pt,C,2D/2H]⁺ spectra appear to be most consistent with an overtone of the out-of-plane bending vibration of the metal carbene cation structure.

Spectroscopy and formation of lanthanum-hydrocarbon radicals formed by association and carbon-carbon bond cleavage of isoprene

La atom reaction with isoprene is carried out in a laser-vaporization molecular beam source. The reaction yields an adduct as the major product and C-C cleaved and dehydrogenated species as the minor ones. La(C₅H₈), La(C₂H₂), and La(C₃H₄) are characterized with mass-analyzed threshold ionization (MATI) spectroscopy and quantum chemical computations. The MATI spectra of all three species exhibit a strong origin band and several weak vibronic bands corresponding to La-ligand stretch and ligand-based bend excitations. La(C₅H₈) is a five-membered metallacycle, whereas La(C₂H₂) and La(C₃H₄) are three-membered rings. All three metallacycles prefer a doublet ground state with a La 6s¹-based valence electron configuration and a singlet ion. The five-membered metallacycle is formed through La addition and isoprene isomerization, whereas the two three-membered rings are produced by La addition and insertion, hydrogen migration, and carbon-carbon bond cleavage.

Selective C-H Bond Cleavage in Methane by Small Gold Clusters

Methane represents the major constituent of natural gas. It is primarily used only as a source of energy by means of combustion, but could also serve as an abundant hydrocarbon feedstock for high quality chemicals. One of the major challenges in catalysis research nowadays is therefore the development of materials that selectively cleave one of the four C-H bonds of methane and thus make it amenable for further chemical conversion into valuable compounds. By employing infrared spectroscopy and first-principles calculations it is uncovered herein that the interaction of methane with small gold cluster cations leads to selective C-H bond dissociation and the formation of hydrido methyl complexes, H-Au_x⁺ -CH₃ . The distinctive selectivity offered by these gold clusters originates from a fine interplay between the closed-shell nature of the d states and relativistic effects in gold. Such fine balance in fundamental interactions could prove to be a tunable feature in the rational design of a catalyst.

On the Origin of Reactivity Enhancement/Suppression upon Sequential Ligation: [Re(CO)x ]+ /CH4 (x=0-3) Couples

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.

Activation of C-H Bonds in Pt(+) + x CH4 Reactions, where x = 1-4: Identification of the Platinum Dimethyl Cation

Activation of C-H bonds in the sequential reactions of Pt(+) + x(CH4/CD4), where x = 1-4, have been investigated using infrared multiple photon dissociation (IRMPD) spectroscopy and theoretical calculations. Pt(+) cations are formed by laser ablation and exposed to controlled amounts of CH4/CD4 leading to [Pt,xC,(4x-2)H/D](+) dehydrogenation products. Irradiation of these products in the 400-2100 cm(-1) range leads to CH4/CD4 loss from the x = 3 and 4 products, whereas PtCH2(+)/PtCD2(+) products do not decompose at all, and x = 2 products dissociate only when formed from a higher order product. The structures of these complexes were explored theoretically at several levels of theory with three different basis sets. Comparison of the experimental and theoretical results indicate that the species formed have a Pt(CH3)2(+)(CH4)x-2/Pt(CD3)2(+)(CD4)x-2 binding motif for x = 2-4. Thus, reaction of Pt(+) with methane occurs by C-H bond activation to form PtCH2(+), which reacts with an additional methane molecule by C-H bond activation to form the platinum dimethyl cation. This proposed reaction mechanism is consistent with theoretical explorations of the potential energy surface for reactions of Pt(+) with one and two methane molecules.

Methane Activation by 5 d Transition Metals: Energetics, Mechanisms, and Periodic Trends

Although it has been known for almost three decades that several 5d transition-metal cations will activate methane at room temperature, a more detailed examination of these reactions across the periodic table has only recently been completed. In this Minireview, we compare and contrast studies of the kinetic energy dependence of these reactions as studied using guided-ion-beam tandem mass spectrometry. Thermochemistry for the various products observed (MH⁺ , MH₂⁺ , MC⁺ , MCH⁺ , MCH₂⁺ , and MCH₃⁺ ) are collected and periodic trends evaluated and discussed. The mechanisms for the reactions as elucidated by synergistic quantum chemical calculations are also reviewed. Recent spectroscopic evidence for the structures of the MCH₂⁺ dehydrogenation products are discussed as well.