Chemistry with methane: concepts rather than recipes

Phenyl-Group Exchange in Triphenylphosphine Mediated by Cationic Gold-Platinum Complexes-A Gas-Phase Mimetic Approach

The PPh₃ ligands in the heterodinuclear AuPt complex [(Ph₃P)AuPt(PPh₃)₃][BAr₄^F] (BAr₄^F = tetrakis[3,5-bis(trifluoromethyl)phenyl]borate) exhibit a high fluxionality on the AuPt core. Fast intramolecular and slow intermolecular processes for the reversible exchange of the PPh₃ ligands have been identified. When [(Ph₃P)AuPt(PPh₃)₃][BAr₄^F] is heated in solution, the formation of benzene is observed, and a trinuclear, cationic AuPt₂ complex is generated. This process is preceded by reversible phenyl-group exchange between the PPh₃ ligands present in the reaction mixture as elucidated by deuterium-labeling studies. Both the elimination of benzene and the preceding reversible phenyl-group exchange have originally been observed in mass-spectrometry-based CID experiments (CID = Collision-Induced Dissociation). While CID of mass-selected [Au,Pt,(PPh₃)₄]⁺ results exclusively in the loss of PPh₃, the resulting cation [Au,Pt,(PPh₃)₃]⁺ selectively eliminates C₆H₆. Thus, the dissociation of a PPh₃ ligand from [Au,Pt,(PPh₃)₃]⁺ is energetically not able to compete with processes which result in C-H- and C-P-bond cleavage. In both media, the heterobimetallic nature of the employed complexes is the key for the observed reactivity. Only the intimate interplay of the gas-phase investigations, studies in solution, and thorough DFT computations allowed for the elucidation of the mechanistic details of the reactivity of [(Ph₃P)AuPt(PPh₃)₃][BAr₄^F].

The Reaction of Sulfur Dioxide Radical Cation with Hydrogen and its Relevance in Solar Geoengineering Models

SO₂ has been proposed in solar geoengineering as a precursor of H₂ SO₄ aerosol, a cooling agent active in the stratosphere to contrast climate change. Atmospheric ionization sources can ionize SO₂ into excited states of S O 2 · + , which quickly reacts with trace gases in the stratosphere. In this work we explore the reaction of H 2 D 2 with S O 2 · + excited by tunable synchrotron radiation, leading to H S O 2 + + H ( D S O 2 + + D ), where H contributes to O₃ depletion and OH formation. Density Functional Theory and Variational Transition State Theory have been used to investigate the dynamics of the title barrierless and exothermic reaction. The present results suggest that solar geoengineering models should test the reactivity of S O 2 · + with major trace gases in the stratosphere, such as H₂ since this is a relevant channel for the OH formation during the nighttime when there is not OH production by sunlight. OH oxides SO₂ , triggering the chemical reactions leading to H₂ SO₄ aerosol.

2D Electrocatalysts for Converting Earth-Abundant Simple Molecules into Value-Added Commodity Chemicals: Recent Progress and Perspectives

The electrocatalytic conversion of earth-abundant simple molecules into value-added commodity chemicals can transform current chemical production regimes with enormous socioeconomic and environmental benefits. For these applications, 2D electrocatalysts have emerged as a new class of high-performance electrocatalyst with massive forward-looking potential. Recent advances in 2D electrocatalysts are reviewed for emerging applications that utilize naturally existing H₂ O, N₂ , O₂ , Cl^- (seawater) and CH₄ (natural gas) as reactants for nitrogen reduction (N₂ → NH₃ ), two-electron oxygen reduction (O₂ → H₂ O₂ ), chlorine evolution (Cl^- → Cl₂ ), and methane partial oxidation (CH₄ → CH₃ OH) reactions to generate NH₃ , H₂ O₂ , Cl₂ , and CH₃ OH. The unique 2D features and effective approaches that take advantage of such features to create high-performance 2D electrocatalysts are articulated with emphasis. To benefit the readers and expedite future progress, the challenges facing the future development of 2D electrocatalysts for each of the above reactions and the related perspectives are provided.

Spin-orbit coupling and vibronic transitions of Ce(C3H4) and Ce(C3H6) formed by the Ce reaction with propene: Mass-analyzed threshold ionization and relativistic quantum computation

A Ce atom reaction with propene is carried out in a pulsed laser vaporization molecule beam source. Several Ce-hydrocarbon species formed by the C-H and C-C bond activation of propene are observed by time-of-flight mass spectrometry, and Ce(C₃H_n) (n = 4 and 6) are characterized by mass-analyzed threshold ionization (MATI) spectroscopy and density functional theory, multiconfiguration, and relativistic quantum chemical calculations. The MATI spectrum of each species consists of two vibronic band systems, each with several vibronic bands. Ce(C₃H₆) is identified as an inserted species with Ce inserting into an allylic C-H bond of propene and Ce(C₃H₄) as a metallocycle through 1,2-vinylic dehydrogenation. Both species have a C_s structure with the Ce 4f¹6s¹ ground valence electron configuration in the neutral molecule and the Ce 4f¹ configuration in the singly charged ion. The two vibronic band systems observed for each species are attributed to the ionization of two pairs of the lowest spin-orbit coupled states with each pair being nearly degenerate.

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.

The oxidation of cyclo-olefin by the S = 2 ground-state complex [FeIV(O)(TQA)(NCMe)]2

Density functional theory calculation is used to investigate the oxidation of cyclo-olefin (cyclobutene, cyclopentene, cyclohexene, cycloheptene, and cyclo-octene) by the complex [Fe^IV(O)(TQA)(NCMe)]²⁺, which has S = 2 ground state, and the effect of electronic factors and steric hindrance on reaction barriers. Our results suggest that the oxo-iron(IV) complex can oxidise C-H and C = C bonds via a single-state mechanism, and two different ways of electron transport exist. The energy barriers initially decrease with increasing substrate size, and the trend then reverses. Comparison of the energy barrier in different systems reveals that except for the reaction between [Fe^IV(O)(TQA)(NCMe)]²⁺ and cycloheptene, oxo-iron(IV) complexes prefer epoxidation to hydroxylation. However, the hydroxylated product is more stable than the corresponding epoxidated product. This result indicates that the products of epoxidation tend to decompose first. The energy barrier of hydroxylation and epoxidation originates from the balance of orbital interaction and Pauli repulsion from the equatorial ligand and protons on the approaching substrate. In this regard, we calculate the weak interaction between two fragments (oxo-iron complex and substrates) using the independent gradient model and drawn the corresponding 3D isosurface representations of reactants.

Operando Photoelectron Photoion Coincidence Spectroscopy Unravels Mechanistic Fingerprints of Propane Activation by Catalytic Oxyhalogenation

Herein, we demonstrate operando photoelectron photoion coincidence (PEPICO) spectroscopy as a pivotal technique for evidencing unprecedented mechanistic insights by isomer-selective radical detection within complex hydrocarbon-functionalization reaction networks, such as those of catalyzed propane oxychlorination and oxybromination. In particular, while the oxychlorination is surface-confined, we show that in oxybromination alkane activation follows a gas-phase reaction mechanism with evolved bromine and bromine radicals, favoring 2-propyl over 1-propyl radical formation, as evidenced by isomer-selective threshold photoelectron analysis. Furthermore, we provide new mechanistic insights into the cracking and coking pathways that are observed in oxybromination. The first entails propargyl radical formation from consecutive hydrogen abstraction of propyl radicals, ultimately yielding benzene. The second originates from C-C bond cleavage in propane to ethyl and methyl radicals, which produce CH₄ and C₂H₄, or undergo chain-growth reactions, forming C₄-C₆ species.

Bioinspired chemistry at MOF secondary building units

This perspective describes recent developments and future directions in bioinorganic chemistry and biomimetic catalysis centered at metal–organic framework secondary building units.

The secondary building units (SBUs) in metal–organic frameworks (MOFs) support metal ions in well-defined and site-isolated coordination environments with ligand fields similar to those found in metalloenzymes. This burgeoning class of materials has accordingly been recognized as an attractive platform for metalloenzyme active site mimicry and biomimetic catalysis. Early progress in this area was slowed by challenges such as a limited range of hydrolytic stability and a relatively poor diversity of redox-active metals that could be incorporated into SBUs. However, recent progress with water-stable MOFs and the development of more sophisticated synthetic routes such as postsynthetic cation exchange have largely addressed these challenges. MOF SBUs are being leveraged to interrogate traditionally unstable intermediates and catalytic processes involving small gaseous molecules. This perspective describes recent advances in the use of metal centers within SBUs for biomimetic chemistry and discusses key future developments in this area.

Insight into the active site and reaction mechanism for selective oxidation of methane to methanol using H2O2 on a Rh1/ZrO2 catalyst

Five-coordinated Rh leads to the over-oxidation of CH₄, while four-coordinated Rh stabilizes CH₃ and facilitates methanol formation via the CH₃OOH intermediate.

Thermal activation of methane by MgO+: temperature dependent kinetics, reactive molecular dynamics simulations and statistical modeling

The kinetics methane activation (MgO⁺ + CH₄) was studied experimentally and computationally by running and analyzing reactive atomistic simulations.

Mechanism of selective and complete oxidation in La2O3-catalyzed oxidative coupling of methane

The catalytic mechanism and reaction network of oxidative coupling of methane over La₂O₃ are thoroughly investigated by density functional theory calculations.

Theoretical study of methane adsorption and C─H bond activation over Fe-embedded graphene: Effect of external electric field

The effect of an external electric field (EF) on the methane adsorption and its activation on iron-embedded graphene (Fe-GPs) are investigated by using the M06-L density functional method. The EF is applied in the perpendicular direction to the graphene in the range of -0.015 to +0.015 a.u. with the interval of 0.005 a.u. The effects of EF on the adsorption, transition state and product complexes of the methane activation reaction are revealed. The binding energies of methane on Fe site in Fe-GPs are increased from -12.9 to -15.3, -18.1 and -21.5 kcal/mol for the negative EF of -0.005, -0.010 and -0.015, respectively. By applying positive EF, the activation barriers for methane activation are reduced in range of 3-8 kcal/mol (around 12-31%) and the reaction energies are more exothermic. The positive EF kinetically favors the reaction compared to the system without EF. The adsorption and activation of methane on Fe-GPs can be easily tuned by adjusting the external electric field for various applications. © 2019 Wiley Periodicals, Inc.

Polymeric tungsten carbide nanoclusters: structural evolution, ligand modulation, and assembled nanomaterials

Seeking novel superatoms with tunable electronic and magnetic properties has attracted much interest due to their potential application in cluster assembly nanomaterials. By employing density functional theory (DFT) calculations, the recently observed superatomic WC cluster was adopted as the basic unit to construct larger polymeric clusters, namely (WC)_n (n = 2-7), and their structural evolution was explored to understand the growth pattern of these superatomic clusters into nanoscale materials. An unusual odd-even pattern in structural evolution was disclosed, in which the (WC)₂ unit is considered as the basic building block. Moreover, W₄C₄ is found to possess a cubic structure, based on which the CO and PH₃ ligands were attached to examine their ligation effects on W₄C₄. Theoretical results show that the electronic properties of W₄C₄ can be dramatically altered during the ligation process. Intriguingly, the continuous attachment of CO and PH₃ ligands strongly increases and decreases the electron affinities (EA) and ionization potentials (IP) of the ligated W₄C₄ clusters, respectively, leading to the formation of superhalogen and superalkali species with high magnetic moments. The observed ligand induced strategy highlighted here could serve as an effective way to tune the electronic and magnetic properties of clusters resulting in the formation of novel superatoms. Finally, studies on the geometrical and electronic structures of the W₄C₄ cluster solid unveil its special 3-D cubic honeycomb geometry and metallic properties with predominant contribution from the 5d of W, which may have potential applications in electro-catalysis.

σ-H-H, σ-C-H, and σ-Si-H Bond Activation Catalyzed by Metal Nanoparticles

Activation of H-H, Si-H, and C-H bonds through σ-bond coordination has grown in the past 30 years from a scientific curiosity to an important tool in the functionalization of hydrocarbons. Several mechanisms were discovered via which the initially σ-bonded substrate could be converted: oxidative addition, heterolytic cleavage, σ-bond metathesis, electrophilic attack, etc. The use of metal nanoparticles (NPs) in this area is a more recent development, but obviously nanoparticles offer a much richer basis than classical homogeneous and heterogeneous catalysts for tuning reactivity for such a demanding process as C-H functionalization. Here, we will review the surface chemistry of nanoparticles and catalytic reactions occurring in the liquid phase, catalyzed by either colloidal or supported metal NPs. We consider nanoparticles prepared in solution, which are stabilized and tuned by polymers, ligands, and supports. The question we have addressed concerns the differences and similarities between molecular complexes and metal NPs in their reactivity toward σ-bond activation and functionalization.

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.

Gas-Phase Synthesis and Reactivity of Ligated Group 10 Ions in the Formal +1 Oxidation State

Electrospray ionization of the group 10 complexes [(phen)M(O₂CCH₃)₂] (phen=1,10-phenanthroline, M = Ni, Pd, Pt) generates the cations [(phen)M(O₂CCH₃)]⁺, whose gas-phase chemistry was studied using multistage mass spectrometry experiments in an ion trap mass spectrometer with the combination of collision-induced dissociation (CID) and ion-molecule reactions (IMR). Decarboxylation of [(phen)M(O₂CCH₃)]⁺ under CID conditions generates the organometallic cations [(phen)M(CH₃)]⁺, which undergo bond homolysis upon a further stage of CID to generate the cations [(phen)M]^+· in which the metal center is formally in the +1 oxidation state. In the case of [(phen)Pt(CH₃)]⁺, the major product ion [(phen)H]⁺ was formed via loss of the metal carbene Pt=CH₂. DFT calculated energetics for the competition between bond homolysis and M=CH₂ loss are consistent with their experimentally observed branching ratios of 2% and 98% respectively. The IMR of [(phen)M]^+· with O₂, N₂, H₂O, acetone, and allyl iodide were examined. Adduct formation occurs for O₂, N₂, H₂O, and acetone. Upon CID, all adducts fragment to regenerate [(phen)M]^+·, except for [(phen)Pt(OC(CH₃)₂)]^+·, which loses a methyl radical to form [(phen)Pt(OCCH₃)]⁺ which upon a further stage of CID regenerates [(phen)Pt(CH₃)]⁺ via CO loss. This closes a formal catalytic cycle for the decomposition of acetone into CO and two methyl radicals with [(phen)Pt]^+· as catalyst. In the IMR of [(phen)M]^+· with allyl iodide, formation of [(phen)M(CH₂CHCH₂)]⁺ was observed for all three metals, whereas for M = Pt also [(phen)Pt(I)]⁺ and [(phen)Pt(I)₂(CH₂CHCH₂)]⁺ were observed. Finally, DFT calculated reaction energetics for all IMR reaction channels are consistent with the experimental observations.

Structural Properties of Gas-Phase Molybdenum Oxide Clusters [Mo4O13]2-, [HMo4O13]-, and [CH3Mo4O13]- Studied by Collision-Induced Dissociation

Molybdenum oxide-based catalysts are widely used for the ammoxidation of toluene, methanation of CO, or hydrodeoxygenation. As a first step towards a gas-phase model system, we investigate here structural properties of mass-selected [Mo₄O₁₃]^2-, [HMo₄O₁₃]^-, and [CH₃Mo₄O₁₃]^- by a combination of collision-induced dissociation (CID) experiments and quantum chemical calculations. According to calculations, the common structural motif is an eight-membered ring composed of four MoO₂ units and four O atoms. The 13th O atom is located above the center of the ring and connects two to four Mo centers. For [Mo₄O₁₃]^2- and [HMo₄O₁₃]^-, dissociation requires opening or rearrangement of the ring structure, which is quite facile for the doubly charged [Mo₄O₁₃]^2-, but energetically more demanding for [HMo₄O₁₃]^-. In the latter case, the hydrogen atom is found to stay preferentially with the negatively charged fragments [HMo₂O₇]^- or [HMoO₄]^-. The doubly charged species [Mo₄O₁₃]^2- loses one MoO₃ unit at low energies while Coulomb explosion into the complementary fragments [Mo₂O₆]^- and [Mo₂O₇]^- dominates at elevated collision energies. [CH₃Mo₄O₁₃]^- affords rearrangements of the methyl group with low barriers, preferentially eliminating formaldehyde, while the ring structure remains intact. [CH₃Mo₄O₁₃]^- also reacts efficiently with water, leading to methanol or formaldehyde elimination.

Solution Catalytic Cycle of Incompatible Steps for Ambient Air Oxidation of Methane to Methanol

Direct chemical synthesis from methane and air under ambient conditions is attractive yet challenging. Low-valent organometallic compounds are known to activate methane, but their electron-rich nature seems incompatible with O₂ and prevents catalytic air oxidation. We report selective oxidation of methane to methanol with an O₂-sensitive metalloradical as the catalyst and air as the oxidant at room temperature and ambient pressure. The incompatibility between C-H activation and O₂ oxidation is reconciled by electrochemistry and nanomaterials, with which a concentration gradient of O₂ within the nanowire array spatially segregated incompatible steps in the catalytic cycle. An unexpected 220 000-fold increase of the apparent reaction rate constants within the nanowire array leads to a turnover number up to 52 000 within 24 h. The synergy between nanomaterials and organometallic chemistry warrants a new catalytic route for CH₄ functionalization.

M-O Bonding Beyond the Oxo Wall: Spectroscopy and Reactivity of Cobalt(III)-Oxyl and Cobalt(III)-Oxo Complexes

Terminal oxo complexes of late transition metals are frequently proposed reactive intermediates. However, they are scarcely known beyond Group 8. Using mass spectrometry, we prepared and characterized two such complexes: [(N4Py)CoIII (O)]+ (1) and [(N4Py)CoIV (O)]2+ (2). Infrared photodissociation spectroscopy revealed that the Co-O bond in 1 is rather strong, in accordance with its lack of chemical reactivity. On the contrary, 2 has a very weak Co-O bond characterized by a stretching frequency of ≤659 cm-1 . Accordingly, 2 can abstract hydrogen atoms from non-activated secondary alkanes. Previously, this reactivity has only been observed in the gas phase for small, coordinatively unsaturated metal complexes. Multireference ab-initio calculations suggest that 2, formally a cobalt(IV)-oxo complex, is best described as cobalt(III)-oxyl. Our results provide important data on changes to metal-oxo bonding behind the oxo wall and show that cobalt-oxo complexes are promising targets for developing highly active C-H oxidation catalysts.

Efficient and Selective Methane Borylation Through Pore Size Tuning of Hybrid Porous Organic-Polymer-Based Iridium Catalysts

As a new energy source that could replace petroleum, the global reserves of methane hydrate (combustible ice) are estimated to be approximately 20 000 trillion cubic meters. A large amount of methane hydrate has been found under the seabed, but the transportation and storage of methane gas far from coastlines are technically unfeasible and expensive. The direct conversion of methane into value-added chemicals and liquid fuels is highly desirable but remains challenging. Herein, we prepare a series of iridium complexes based on porous polycarbazoles with high specific areas and good thermochemical stabilities. Through structure tuning we optimized their catalytic activities for the selective monoborylation of methane. One of these catalysts (CAL-3-Ir) can produce methyl boronic acid pinacol ester (CH₃ Bpin) in 29 % yield in 9 h with a turnover frequency (TOF) of approximately 14 h^-1 . Because its pore sizes favor monoborylated products, it has a high chemoselectivity for monoborylation (CH₃ Bpin:CH₂ (Bpin)₂ =16:1).

Selective Activation of the C-H Bond in Methane by Single Platinum Atomic Anions

Mass spectrometric analysis of the anionic products of interaction between platinum atomic anions, Pt^- , and methane, CH₄ and CD₄ , in a collision cell shows the preferred generation of [PtCH₄ ]^- and [PtCD₄ ]^- complexes and a low tendency toward dehydrogenation. [PtCH₄ ]^- is shown to be H-Pt-CH₃ ^- by a synergy between anion photoelectron spectroscopy and quantum chemical calculations, implying the rupture of a single C-H bond. The calculated reaction pathway accounts for the observed selective activation of methane by Pt^- . This study presents the first example of methane activation by a single atomic anion.

Structural Properties of Gas Phase Molybdenum Sulfide Clusters [Mo3S13]2-, [HMo3S13]-, and [H3Mo3S13]+ as Model Systems of a Promising Hydrogen Evolution Catalyst

Amorphous molybdenum sulfide (MoS _x ) is a potent catalyst for the hydrogen evolution reaction (HER). Since mechanistic investigations on amorphous solids are particularly difficult, we use a bottom-up approach and study the [Mo₃S₁₃]^2- nanocluster and its protonated forms. The mass selected pure [Mo₃S₁₃]^2- as well as singly and triply protonated [HMo₃S₁₃]^- and [H₃Mo₃S₁₃]⁺ ions, respectively, were investigated by a combination of collision induced dissociation (CID) experiments and quantum chemical calculations. A rich variety of H _x S _y elimination channels was observed, giving insight into the structural flexibility of the clusters. In particular, it was calculated that the observed clusters tend to keep the Mo₃ ring structure found in the bulk and that protons adsorb primarily on terminal disulfide units of the cluster. Mo-H bonds are formed only for quasi-linear species with Mo centers featuring empty coordination sites. Protonation leads to increased cluster stability against CID. The rich variety of CID dissociation products for the triply protonated [H₃Mo₃S₁₃]⁺ ion, however, suggests that it has a large degree of structural flexibility, with roaming H/SH moieties, which could be a key feature of MoS _x to facilitate HER catalysis via a Volmer-Heyrovsky mechanism.

On the Role of Hydrogen Atom Transfer (HAT) in Thermal Activation of Methane by MnO+: Entropy vs. Energy

The temperature dependent kinetics and product branching fractions of first-row transition metal oxide cation MnO+ with CH4 and CD4 at temperatures between 200 and 600 K are measured using a selected-ion flow tube apparatus. Likely reaction mechanisms are determined by comparison of temperature dependent kinetics to statistical modeling along calculated reaction coordinates. The data is well-modeled with the reaction proceeding over a rate limiting four-centered transition state leading to an insertion intermediate, similar to reactions of NiO+ and FeO+, and showing characteristics of proton-coupled electron transfer (PCET). However, a more direct pathway traversing a transition state of hydrogen atom transfer (HAT) character to a hydroxyl intermediate is found to possibly be competitive, especially with increasing temperature. While uncertainties in calculated energetics limit quantitative assessment of the role of HAT at thermal energies, it is clear that this mechanism becomes increasingly prevalent in higher energy regimes.

Synthesis and Structures of Stable PtII and PtIV Alkylidenes: Evidence for π-Bonding and Relativistic Stabilization

Isolable cationic Pt^II and Pt^IV alkylidenes, proposed intermediates in catalytic organic transformations, are reported. The bonding in these species was probed by experimental, structural, spectroscopic, electrochemical and computational methods, providing direct evidence for π-bonding, the often-theorized relativistic stabilization of these species, and the influence of oxidation state.

Selective Generation of Free Hydrogen Atoms in the Reaction of Methane with Diatomic Gold Boride Cations

The thermal reaction of diatomic gold boride cation AuB+ with methane has been studied by using state-of-the-art mass spectrometry in conjunction with density functional theory calculations. The AuB+ ion can activate a methane molecule to produce exclusively the free hydrogen atom, an important intermediate in hydrocarbon transformation. This result is different from the reactivity of AuC+ and CuB+ counterparts with methane in previous studies. The AuC+ cation mainly transforms methane into ethylene. The CuB+ reaction system principally generates the free hydrogen atoms, but it also gives rise a portion of ethylene-like product H2B−CH2. The B atom of AuB+ is the active site to activate methane. The strong relativistic effect on gold plays an important role for the product selectivity. The mechanistic insights obtained from this study provide guidance for rational design of active sites with high product selectivity toward methane activation.