Electronic Origins of the Variable Efficiency of Room-Temperature Methane Activation by Homo- and Heteronuclear Cluster Oxide Cations [XYO2]+ (X, Y = Al, Si, Mg): Competition between Proton-Coupled Electron Transfer and Hydrogen-Atom Transfer

Activation and Transformation of Methane on Boron-Doped Cobalt Oxide Cluster Cations CoBO2. Inorg Chem 2024;63:1537-1542. [PMID: 38181068 DOI: 10.1021/acs.inorgchem.3c03112] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

The cleavage of inert C-H bonds in methane at room temperature and the subsequent conversion into value-added products are quite challenging. Herein, the reactivity of boron-doped cobalt oxide cluster cations CoBO2+ toward methane under thermal collision conditions was studied by mass spectrometry experiments and quantum-chemical calculations. In this reaction, one H atom and the CH3 unit of methane were transformed separately to generate the product metaboric acid (HBO2) and one CoCH3+ ion, respectively. Theoretical calculations strongly suggest that a catalytic cycle can be completed by the recovery of CoBO2+ through the reaction of CoCH3+ with sodium perborate (NaBO3), and this reaction generates sodium methoxide (CH3ONa) as the other value-added product. This study shows that boron-doped cobalt oxide species are highly reactive to facilitate thermal methane transformation and may open a way to develop more effective approaches for methane (CH4) activation and conversion under mild conditions.

Ion-molecule reactions of mass-selected ions

Gas-phase reactions of mass-selected ions with neutrals covers a very broad area of fundamental and applied mass spectrometry (MS). Oftentimes, ion-molecule reactions (IMR) can serve as a viable alternative to collision-induced dissociation and other ion dissociation techniques when using tandem MS. This review focuses on the literature pertaining applications of IMR since 2013. During the past decade considerable efforts have been made in analytical applications of IMR, including advances in one of the major techniques for characterization of unsaturated fatty acids and lipids, ozone-induced dissociation, and the development of a new technique for sequencing of large ions, hydrogen atom attachment/abstraction dissociation. Many advances have also been made in identifying gas-phase chemistry specific to a functional group in organic and biological compounds, which are useful in structure elucidation of analytes and differentiation of isomers/isobars. With "soft" ionization techniques like electrospray ionization having become mainstream for quite some time now, the efforts in the area of metal ion catalysis have firmly moved into exploring chemistry of ligated metal complexes in their "natural" oxidation states allowing to model individual steps of mechanisms in homogeneous catalysis, especially in combination with high-level DFT calculations. Finally, IMR continue to contribute to the body of knowledge in the area of chemistry of interstellar processes.

Computational Screening of Supported Metal Oxide Nanoclusters for Methane Activation: Insights into Homolytic versus Heterolytic C-H Bond Dissociation

Since its discovery in zeolites, the [CuOCu]²⁺ motif has played an important role in our understanding of selective methane activation over supported metal oxide nanoclusters. Although there are two known C-H bond dissociation mechanisms, namely, homolytic and heterolytic cleavage, most computational studies on optimizing metal oxide nanoclusters for improved methane activation reactivity have focused only on the homolytic mechanism. In this work, both mechanisms were examined for a set of 21 mixed metal oxide complexes of the form of [M₁OM₂]²⁺ (M₁ and M₂ = Mn, Fe, Co, Ni, Cu, and Zn). Except for pure copper, heterolytic cleavage was found to be the dominant C-H bond activation pathway for all systems. Furthermore, mixed systems including [CuOMn]²⁺, [CuONi]²⁺, and [CuOZn]²⁺ are predicted to possess methane activation activity similar to pure [CuOCu]²⁺. These results suggest that both homolytic and heterolytic mechanisms should be considered in computing methane activation energies on supported metal oxide nanoclusters.

Current Progress on Methods and Technologies for Catalytic Methane Activation at Low Temperatures

Methane (CH4 ) is an attractive energy source and important greenhouse gas. Therefore, from the economic and environmental point of view, scientists are working hard to activate and convert CH4 into various products or less harmful gas at low-temperature. Although the inert nature of CH bonds requires high dissociation energy at high temperatures, the efforts of researchers have demonstrated the feasibility of catalysts to activate CH4 at low temperatures. In this review, the efficient catalysts designed to reduce the CH4 oxidation temperature and improve conversion efficiencies are described. First, noble metals and transition metal-based catalysts are summarized for activating CH4 in temperatures ranging from 50 to 500 °C. After that, the partial oxidation of CH4 at relatively low temperatures, including thermocatalysis in the liquid phase, photocatalysis, electrocatalysis, and nonthermal plasma technologies, is briefly discussed. Finally, the challenges and perspectives are presented to provide a systematic guideline for designing and synthesizing the highly efficient catalysts in the complete/partial oxidation of CH4 at low temperatures.

Photocatalytic Oxidative Dehydrogenation of Propane for Selective Propene Production with TiO2

Oxidative dehydrogenation of propane (ODHP) as an exothermic process is a promising method to produce propene (C₃H₆) with lower energy consumption in chemical industry. However, the selectivity of the C₃H₆ product is always poor because of overoxidation. Herein, the ODHP reaction into C₃H₆ on a model rutile(R)-TiO₂(110) surface at low temperature via photocatalysis has been realized successfully. The results illustrate that photocatalytic oxidative dehydrogenation of propane (C₃H₈) into C₃H₆ can occur efficiently on R-TiO₂(110) at 90 K via a stepwise manner, in which the initial C-H cleavage occurs via the hole coupled C-H bond cleavage pathway followed by a radical mediated C-H cleavage to the C₃H₆ product. An exceptional selectivity of ∼90% for C₃H₆ production is achieved at about 13% propane conversion. The mechanistic model constructed in this study not only advances our understanding of C-H bond activation but also provides a new pathway for highly selective ODHP into C₃H₆ under mild conditions.

CH4 activation by PtX+ (X = F, Cl, Br, I)

Reactions of PtX⁺ (X = F, Cl, Br, I) with methane have been investigated at the density functional theory (DFT) level. These reactions take place more easily along the low-spin potential energy surface. For HX (X = F, Cl, Br, I) elimination, the formal oxidation state of the metal ion appears to be conserved, and the importance of this reaction channel decreases in going as the sequence: X = F, Cl, Br, I. A reversed trend is observed in the loss of H₂ for X = F, Cl, Br, while it is not favorable for PtI⁺ in the loss of either HI or H₂. For HX eliminations, the transfer form of H is from proton to atom, last to hydride, and the mechanisms are from PCET to HAT, last to HT for the sequence of X = F, Cl, Br, I. One reason is mainly due to the electronegativity of halogens. Otherwise, the mechanisms of HX eliminations also can be explained by the analysis of Frontier Molecular Orbitals. While for the loss of H₂, the transfer of H is in the form of hydride for all the X ligands. Noncovalent interactions analysis also can be explained the reaction mechanisms.

Gas-Phase Mechanism of O.- /Ni2+ -Mediated Methane Conversion to Formaldehyde

The gas-phase reaction of NiAl2 O4 + with CH4 is studied by mass spectrometry in combination with vibrational action spectroscopy and density functional theory (DFT). Two product ions, NiAl2 O4 H+ and NiAl2 O3 H2 + , are identified in the mass spectra. The DFT calculations predict that the global minimum-energy isomer of NiAl2 O4 + contains Ni in the +II oxidation state and features a terminal Al-O.- oxygen radical site. They show that methane can react along two competing pathways leading to formation of either a methyl radical (CH3 ⋅) or formaldehyde (CH2 O). Both reactions are initiated by hydrogen atom transfer from methane to the terminal O.- site, followed by either CH3 ⋅ loss or CH3 ⋅ migration to an O2- site next to the Ni2+ center. The CH3 ⋅ attaches as CH3 + to O2- and its unpaired electron is transferred to the Ni-center reducing it to Ni+ . The proposed mechanism is experimentally confirmed by vibrational spectroscopy of the reactant and two different product ions.

Lanthanide Complexes Containing a Terminal Ln═O Oxo Bond: Revealing Higher Stability of Tetravalent Praseodymium versus Terbium

We report on the reactivity of gas-phase lanthanide-oxide nitrate complexes, [Ln(O)(NO₃)₃]^- (denoted LnO²⁺), produced via elimination of NO₂^• from trivalent [Ln^III(NO₃)₄]^- (Ln = Ce, Pr, Nd, Sm, Tb, Dy). These complexes feature a Ln^III-O^• oxyl, a Ln^IV═O oxo, or an intermediate Ln^III/IV oxyl/oxo bond, depending on the accessibility of the tetravalent Ln^IV state. Hydrogen atom abstraction reactivity of the LnO²⁺ complexes to form unambiguously trivalent [Ln^III(OH)(NO₃)₃]^- reveals the nature of the oxide bond. The result of slower reactivity of PrO²⁺ versus TbO²⁺ is considered to indicate higher stability of the tetravalent praseodymium-oxo, Pr^IV═O, versus Tb^IV═O. This is the first report of Pr^IV as more stable than Tb^IV, which is discussed with respect to ionization potentials, standard electrode potentials, atomic promotion energies, and oxo bond covalency via 4f- and/or 5d-orbital participation.

Hydrogen Abstraction by Alkoxyl Radicals: Computational Studies of Thermodynamic and Polarity Effects on Reactivities and Selectivities

Density functional theory calculations (ωB97X-D) are reported for the reactions of methoxy, tert-butoxy, trichloroethoxy, and trifluoroethoxy radicals with a series of 26 C-H bonds in different environments characteristic of a variety of hydrocarbons and substituted derivatives. The variations in activation barriers are analyzed with modified Evans-Polanyi treatments to account for polarity and unsaturation effects. The treatments by Roberts and Steel and by Mayer have inspired the development of a simple treatment involving the thermodynamics of reactions, the difference between the reactant radical and product radical electronegativities, and the absence or presence of α-unsaturation. The three-parameter equation (ΔH^⧧ = 0.52ΔH_rxn(1 - d) - 0.35Δχ_AB² + 10.0, where d = 0.44 when there is α-unsaturation to the reacting C-H bond), correlates well with quantum mechanically computed barriers and shows the quantitative importance of the thermodynamics of reactions (dictated by the reactant and the product bond dissociation energies) and polar effects.

Porous nanographene formation on γ-alumina nanoparticles via transition-metal-free methane activation

γ-Al₂O₃ nanoparticles promote pyrolytic carbon deposition of CH₄ at temperatures higher than 800 °C to give single-walled nanoporous graphene (NPG) materials without the need for transition metals as reaction centers. To accelerate the development of efficient reactions for NPG synthesis, we have investigated early-stage CH₄ activation for NPG formation on γ-Al₂O₃ nanoparticles via reaction kinetics and surface analysis. The formation of NPG was promoted at oxygen vacancies on (100) surfaces of γ-Al₂O₃ nanoparticles following surface activation by CH₄. The kinetic analysis was well corroborated by a computational study using density functional theory. Surface defects generated as a result of surface activation by CH₄ make it kinetically feasible to obtain single-layered NPG, demonstrating the importance of precise control of oxygen vacancies for carbon growth.

Oxygen vacancies on the (100) surface of γ-Al₂O₃ nanoparticles catalyse CH₄-CVD for single-layered nanoporous graphenes with no transition metal reaction centre. The rate-limiting step is the proton transfer (PT) in the activation of CH₄ on them.

Origin and Regioselectivity of Direct Hydrogen Atom Transfer Mechanism of C(sp3)-H Arylation by [W10O32]4-/Ni Metallaphotoredox Catalysis

Polyoxometalates (POMs) have a broad array of applied platforms with well-characterized catalysis including photocatalysis to achieve aliphatic C(sp³)-H bond functionalization. However, the reaction mechanism of POMs in organic transformation remains unknown due to the complexity of POM structures. Here, a challenging [W₁₀O₃₂]^4-/Ni metallaphotoredox-catalyzed C(sp³)-H arylation of alkane has been investigated by density functional theory (DFT) calculations. The calculation revealed that the superficial active center located in bridged oxygen of *[W₁₀O₃₂]^4- is responsible for the abstraction of a foreign hydrogen atom and the activation of a C(sp³)-H bond. Furthermore, we discussed this activated process using the direct activation model of the C(sp³)-H σ-bond to deepen our mechanistic understanding of POM mediated C-H bond activation via the hydrogen atom transfer (HAT) pathway. Specifically, comparing three common mechanisms for nickel catalysis inducing by Ni⁰, Ni^I, and Ni^II to construct a C-C bond, the nickel catalytic cycle induced by the Ni^I active catalyst is profitable in kinetics and thermodynamics. Finally, a radical mechanism merging the ([W₁₀O₃₂]^4--*[W₁₀O₃₂]^4--[HW₁₀O₃₂]^4--[W₁₀O₃₂]^4-) decatungstate reductive quenching cycle, ([HW₁₀O₃₂]^4--[H₂W₁₀O₃₂]^4--[HW₁₀O₃₂]^4-) electron relay, and (Ni^I-Ni^II-Ni^I-Ni^III-Ni^I) nickel catalytic cycle is proposed to be favorable. We hope that this work would provide a better understanding of the unique catalytic activity of decatungstate anions for the direct functionalization of the C(sp³)-H bond.

C-H Bond Activation by the Excited Zinc Atom: Gas-Phase Formation of Methylzinc Hydride (HZnCH3) Based on Multireference Second-Order Perturbation Theory and Coupled Cluster Calculations

The pioneering spectroscopic observations of the methylzinc hydride [HZnCH₃(X¹A₁)] molecule were reported previously by the Ziurys group [J. Am. Chem. Soc. 2010, 132, 17186-17192], and the possible formation mechanisms were suggested therein, including those with the participation of excited zinc atoms in reaction with methane. Herein, the ground singlet state and the lowest excited triplet state potential energy surfaces of the Zn + CH₄ reaction have been explored using high-level electronic structure calculations with multireference second-order perturbation theory and coupled cluster singles and doubles with perturbative triples (CCSD(T)) methods in conjunction with all-electron basis sets (up to aug-cc-pV5Z) and scalar relativistic effects incorporated via the second-order Douglas-Kroll-Hess (DK) method. Based on the ab initio results, a plausible scenario for the formation of HZnCH₃(X¹A₁) is proposed involving the activation of the C-H bond of methane by the lowest excited ³P state atomic zinc. Calculations also highlight the importance of an agostic-like Zn···H-C interactions in the pre-activation complex and good agreement between the structure of the HZnCH₃(X¹A₁) molecule predicted at the DK-CCSD(T)/aug-cc-pVQZ-DK level of theory and that derived from rotational spectroscopy, as well as the discrepancies between the ab initio and density functional theory predictions.

"Soft" oxidative coupling of methane to ethylene: Mechanistic insights from combined experiment and theory

The oxidative coupling of methane to ethylene using gaseous disulfur (2CH₄ + S₂ → C₂H₄ + 2H₂S) as an oxidant (SOCM) proceeds with promising selectivity. Here, we report detailed experimental and theoretical studies that examine the mechanism for the conversion of CH₄ to C₂H₄ over an Fe₃O₄-derived FeS₂ catalyst achieving a promising ethylene selectivity of 33%. We compare and contrast these results with those for the highly exothermic oxidative coupling of methane (OCM) using O₂ (2CH₄ + O₂ → C₂H₄ + 2H₂O). SOCM kinetic/mechanistic analysis, along with density functional theory results, indicate that ethylene is produced as a primary product of methane activation, proceeding predominantly via CH₂ coupling over dimeric S-S moieties that bridge Fe surface sites, and to a lesser degree, on heavily sulfided mononuclear sites. In contrast to and unlike OCM, the overoxidized CS₂ by-product forms predominantly via CH₄ oxidation, rather than from C₂ products, through a series of C-H activation and S-addition steps at adsorbed sulfur sites on the FeS₂ surface. The experimental rates for methane conversion are first order in both CH₄ and S₂, consistent with the involvement of two S sites in the rate-determining methane C-H activation step, with a CD₄/CH₄ kinetic isotope effect of 1.78. The experimental apparent activation energy for methane conversion is 66 ± 8 kJ/mol, significantly lower than for CH₄ oxidative coupling with O₂ The computed methane activation barrier, rate orders, and kinetic isotope values are consistent with experiment. All evidence indicates that SOCM proceeds via a very different pathway than that of OCM.

Modulating the radical reactivity of phenyl radicals with the help of distonic charges: it is all about electrostatic catalysis

This manuscript reports the modulation of H-abstraction reactivity of phenyl radicals by (positive and negative) distonic ions. Specifically, we focus on the origins of this modulating effect: can the charged functional groups truly be described as "extreme forms of electron-withdrawing/donating substituents" - implying a through-bond mechanism - as argued in the literature, or is the modulation mainly caused by through-space effects? Our analysis indicates that the effect of the remote charges can be mimicked almost perfectly with the help of a purely electrostatic treatment, i.e. replacing the charged functional groups by a simple uniform electric field is sufficient to recover the quantitative reactivity trends. Hence, through-space effects dominate, whereas through-bond effects play a minor role at best. We elucidate our results through a careful Valence Bond (VB) analysis and demonstrate that such a qualitative analysis not only reveals through-space dominance, but also demonstrates a remarkable reversal in the reactivity trends of a given polarity upon a rational modification of the reaction partner. As such, our findings demonstrate that VB theory can lead to productive predictions about the behaviour of distonic radical ions.

Iron phosphate nanoparticle catalyst for direct oxidation of methane into formaldehyde: effect of surface redox and acid–base properties

The surface redox and the weakly basic properties of FePO4 nanoparticles would contribute to the selective CH4 oxidation to HCHO and the suppression of over-oxidation, respectively.

Catalytic cycle of the partial oxidation of methane to methanol over Cu-ZSM-5 revealed using DFT calculations

Density functional theory (DFT) calculations were performed to investigate the catalytic cycle of methane conversion to methanol over both [Cu2(O2)]2+ and [Cu2(μ-O)]2+ active sites in the Cu-ZSM-5 catalyst. The [Cu2(O2)]2+ site is found to be active for the partial oxidation of methane to methanol, and although it has a higher energy barrier in the methane activation step, it involves a very low energy barrier in the methanol formation step (36.3 kJ mol-1) as well as a lower methanol desorption energy (52.5 kJ mol-1). As the [Cu2(O2)]2+ active site is also thermodynamically stable, it may play an important role during methane conversion to methanol. Furthermore, the methane activation step follows the homolytic route and the heterolytic route for the [Cu2(O2)]2+ and [Cu2(μ-O)]2+ active sites, respectively, whereas the methanol formation step follows the direct radical rebound mechanism and the indirect rebound mechanism, respectively. Our calculations further indicate that the electronic properties of the reactive O atoms in the active site influence their reactivity toward methane oxidation. More specifically, the higher the spin density and the more negative the charge of the reactive O atom at the active site are, the lower the energy barrier for methane activation will be; and the more negative the charge of the hydroxyl group in the reaction intermediate during the partial oxidation of methane to methanol is, the higher energy barrier of the methanol formation step will be in the triplet state. Furthermore, we used a larger cluster model to predict the mechanism of the methane activation step and the effect of atomic charge of the O atom at the [Cu2(μ-O)]2+ and [Cu2(O2)]2+ active sites on the energy barriers of partial oxidation of methane to methanol, and the conclusions drawn employing the larger cluster model are consistent with those drawn using the smaller double-5T-ring cluster model. In addition, different from the traditional mechanism for methane activation at [Cu2(O2)]2+, which consists of two transition states, we find that the partial oxidation of methane at [Cu2(O2)]2+ can also occur via a single step by direct insertion of one of the O atoms at the active site into the C-H bond of methane.

Revisiting the Intriguing Electronic Features of the BeOBeC Carbyne and Some Isomers: A Quantum-Chemical Assessment

Extensive high-level quantum-chemical calculations reveal that the rod-shaped molecule BeOBeC, which was recently generated in matrix experiments, exists in two nearly isoenergetic states, the 5 Σ quintet (5 6) and the 3 Σ triplet (3 6). Their IR features are hardly distinguishable at finite temperature. The major difference concerns the mode of spin coupling between the terminal beryllium and carbon atoms. Further, the ground-state potential-energy surface of the [2Be,C,O] system at 4 K is presented and differences between the photochemical and thermal behaviors are highlighted. Finally, a previously not considered, so far unknown C2v -symmetric rhombus-like four-membered ring 3 [Be(O)(C)Be] (3 5) is predicted to represent the global minimum on the potential-energy surface.

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].

On the Crucial Role of Isolated Electronic States in the Thermal Reaction of ReC+ with Dihydrogen

Presented here is that isolated, long‐lived electronic states of ReC⁺ serve as the root cause for distinctly different reactivities of this diatomic ion in the thermal activation of dihydrogen. Detailed high‐level quantum chemical calculations support the experimental findings obtained in the highly diluted gas phase using FT‐ICR mass spectrometry. The origin for the existence of these long‐lived excited electronic states and the resulting implications for the varying mechanisms of dihydrogen splitting are addressed.

Propene oxidation catalysis and electronic structure of M55 particles (M = Pd or Rh): differences and similarities between Pd55 and Rh55

Propene oxidation is one of the important reactions that occurs in the presence of a three-way catalyst but its reaction mechanism is unclear. The reaction mechanisms and differences in catalysis between Pd and Rh particles were investigated by DFT calculations employing Pd₅₅ and Rh₅₅ as the model catalysts. The O-attack mechanism, in which the O atom adsorbed on the Pd₅₅ and Rh₅₅ surfaces attacks the C[double bond, length as m-dash]C double bond of propene, needs to overcome a large activation barrier (E_a). On the other hand, C-H bond cleavage of the methyl group of propene easily occurs with moderate E_a; the mechanism initiated by this C-H activation is named H-transfer mechanism. In this mechanism, the next step is allyl alcohol formation, followed by the second C-H bond activation of the CH₂OH species of allyl alcohol, and the final step is proton transfer from OH-substituted π-allyl species to the OH group on the metal surface to yield acrolein and water molecules with the regeneration of M₅₅. The rate-determining step is the second C-H bond activation. Its E_a is 17.4 kcal mol^-1 for the reaction on Pd₅₅ and 34.4 kcal mol^-1 for the reaction on Rh₅₅. These results indicate that Pd particles are more active than Rh particles in propene oxidation, which agrees with the experimental findings. The larger E_a for Rh₅₅ than that for Pd₅₅ arises from the stronger Rh-OH bond than the Pd-OH bond. The higher energy d-valence band-top of Rh₅₅ than that of Pd₅₅ is the origin of the stronger Rh-OH bond than the Pd-OH bond. Thus, the d-valence band-top energy is an important property for understanding and designing catalysts for alkene oxidation.

Carboxylate breaks the arene C-H bond via a hydrogen-atom-transfer mechanism in electrochemical cobalt catalysis

Combined computational and experimental studies elucidated the distinctive mechanistic features of electrochemical cobalt-catalyzed C-H oxygenation. A sequential electrochemical-chemical (EC) process was identified for the formation of an amidylcobalt(iii) intermediate. The synthesis, characterization, cyclic voltammetry studies, and stoichiometric reactions of the related amidylcobalt(iii) intermediate suggested that a second on-cycle electro-oxidation occurs on the amidylcobalt(iii) species, which leads to a formal Co(iv) intermediate. This amidylcobalt(iv) intermediate is essentially a cobalt(iii) complex with one additional single electron distributed on the coordinating heteroatoms. The radical nature of the coordinating pivalate allows the formal Co(iv) intermediate to undergo a novel carboxylate-assisted HAT mechanism to cleave the arene C-H bond, and a CMD mechanism could be excluded for a Co(iii/i) catalytic scenario. The mechanistic understanding of electrochemical cobalt-catalyzed C-H bond activation highlights the multi-tasking electro-oxidation and the underexplored reaction channels in electrochemical transition metal catalysis.

A Reaction-Induced Localization of Spin Density Enables Thermal C-H Bond Activation of Methane by Pristine FeC4. Chemistry 2019;25:12940-12945. [PMID: 31268193 PMCID: PMC6852486 DOI: 10.1002/chem.201902572]

The reactivity of the cationic metal-carbon cluster FeC₄ ⁺ towards methane has been studied experimentally using Fourier-transform ion cyclotron resonance mass spectrometry and computationally by high-level quantum chemical calculations. At room temperature, FeC₄ H⁺ is formed as the main ionic product, and the experimental findings are substantiated by labeling experiments. According to extensive quantum chemical calculations, the C-H bond activation step proceeds through a radical-based hydrogen-atom transfer (HAT) mechanism. This finding is quite unexpected because the initial spin density at the terminal carbon atom of FeC₄ ⁺ , which serves as the hydrogen acceptor site, is low. However, in the course of forming an encounter complex, an electron from the doubly occupied sp-orbital of the terminal carbon atom of FeC₄ ⁺ migrates to the singly occupied π*-orbital; the latter is delocalized over the entire carbon chain. Thus, a highly localized spin density is generated in situ at the terminal carbon atom. Consequently, homolytic C-H bond activation occurs without the obligation to pay a considerable energy penalty that is usually required for HAT involving closed-shell acceptor sites. The mechanistic insights provided by this combined experimental/computational study extend the understanding of methane activation by transition-metal carbides and add a new facet to the dizzying mechanistic landscape of hydrogen-atom transfer.

Reassessment of the Mechanisms of Thermal C-H Bond Activation of Methane by Cationic Magnesium Oxides: A Critical Evaluation of the Suitability of Different Density Functionals

The mechanisms of the thermal reactions of the two iconic magnesium oxide cations MgO^.+ and Mg₂ O₂ ^.+ with methane have been re-evaluated at the CCSD(T)/CBS//CCSD/def2-TZVP level of theory. For the reaction of MgO^.+ with CH₄ , only the classical hydrogen-atom transfer (HAT) was found; in contrast, for the Mg₂ O₂ ^.+ /CH₄ couple, both HAT and proton-coupled electron-transfer (PCET) exist as mechanistic variants. In order to evaluate the suitability of density functional theory (DFT) methods, the reactions were computed by using 27 density functionals. The results obtained demonstrate that the various DFT methods often deliver rather different results for both geometric and energetic features. As to the prediction of the apparent barriers, pure functionals give the largest mean absolute errors. BMK, ωB97XD, and the double-hybrid functional mPW2PLYP were confirmed to come closest to the results provided by CCSD(T)/CBS. Thus, mechanistic conclusions based on a single DFT method should be viewed with great caution. In summary, this study may assist in the selection of a suitable quantum chemical method to unravel the mechanistic details of C-H bond activation by charged metal oxides.

Release of Formic Acid from Copper Formate: Hydride, Proton-Coupled Electron and Hydrogen Atom Transfer All Play their Role

Although the mechanism for the transformation of carbon dioxide to formate with copper hydride is well understood, it is not clear how formic acid is ultimately released. Herein, we show how formic acid is formed in the decomposition of the copper formate clusters Cu(II)(HCOO)3- and Cu(II)2 (HCOO)5- . Infrared irradiation resonant with the antisymmetric C-O stretching mode activates the cluster, resulting in the release of formic acid and carbon dioxide. For the binary cluster, electronic structure calculations indicate that CO2 is eliminated first, through hydride transfer from formate to copper. Formic acid is released via proton-coupled electron transfer (PCET) to a second formate ligand, evidenced by close to zero partial charge and spin density at the hydrogen atom in the transition state. Concomitantly, the two copper centers are reduced from Cu(II) to Cu(I). Depending on the detailed situation, either PCET or hydrogen atom transfer (HAT) takes place.

Room-Temperature Activation of the C-H Bond in Methane over Terminal ZnII-Oxyl Species in an MFI Zeolite: A Combined Spectroscopic and Computational Study of the Reactive Frontier Molecular Orbitals and Their Origins

Oxygenase reactivity toward selective partial oxidation of CH₄ to CH₃OH requires an atomic oxygen-radical bound to metal (M-O^•: oxyl intermediate) that is capable of abstracting an H atom from the significantly strong C-H bond in CH₄. Because such a reaction is frequently observed in metal-doped zeolites, it has been recognized that the zeolite provides an environment that stabilizes the M-O^• intermediate. However, no experimental data of M-O^• have so far been discovered in the zeolite; thus, little is known about the correlation among the state of M-O^•, its reactivity for CH₄, and the nature of the zeolite environment. Here, we report a combined spectroscopic and computational study of the room-temperature activation of CH₄ over Zn^II-O^• in the MFI zeolite. One Zn^II-O^• species does perform H-abstraction from CH₄ at room temperature. The resultant CH₃^• species reacts with the other Zn^II-O^• site to form the Zn^II-OCH₃ species. The H₂O-assisted extraction of surface methoxide yields 29 μmol g^-1 of CH₃OH with a 94% selectivity. The quantum mechanics (QM)/molecular mechanics (MM) calculation determined the central step as the oxyl-mediated hydrogen atom transfer which requires an activation energy of only 10 kJ mol^-1. On the basis of the findings in gas-phase experiments regarding the CH₄ activation by the free [M-O^•]⁺ species, the remarkable H-abstraction reactivity of the Zn^II-O^• species in zeolites was totally rationalized. Additionally, the experimentally validated QM/MM calculation revealed that the zeolite lattice has potential as the ligand to enhance the polarization of the M-O^• bond and thereby enables to create effectively the highly reactive M-O^• bond required for low-temperature activation of CH₄. The present study proposes that tuning of the polarization effect of the anchoring site over heterogeneous catalysts is the valuable way to create the oxyl-based functionality on the heterogeneous catalyst.

Intrinsic Reactivity of Diatomic 3d Transition-Metal Carbides in the Thermal Activation of Methane: Striking Electronic Structure Effects

Mechanistic aspects of the C-H bond activation of methane by metal-carbide cations MC⁺ of the 3d transition-metals Sc-Zn were elucidated by NEVPT2//CASSCF quantum-chemical calculations and verified experimentally for M = Ti, V, Fe, and Cu by using Fourier transform ion-cyclotron resonance mass spectrometry. While MC⁺ species with M = Sc, Ti, V, Cr, Cu, and Zn activate CH₄ at ambient temperature, this is prevented with carbide cations of M = Mn, Fe, and Co by high apparent barriers; NiC⁺ has a small apparent barrier. Hydrogen-atom transfers from methane to metal-carbide cations were found to proceed via a proton-coupled electron transfer mechanism for M = Sc-Co; wherein the doubly occupied π_xz/yz-orbitals between metal and carbon at the carbon site serve as electron donors and the corresponding metal-centered vacant π*_xz/yz-orbitals as electron acceptors. Classical hydrogen-atom transfer transpires only in the case of NiC⁺, while ZnC⁺ follows a mechanistic scenario, in which a formally hydridic hydrogen is transferred. CuC⁺ reacts by a synchronous activation of two C-H bonds. While spin density is often so crucial for the reactions of numerous MO⁺/CH₄ couples, it is much less important for the C-H bond activation by carbide cations of the 3d transition-metals, in which one notes large changes in bond dissociation energies, spin states, number of d-electrons, and charge distributions. All these factors jointly affect both the reactivity of the metal carbides and their mechanisms of C-H bond activation.

Methane Activation by Gas Phase Atomic Clusters

The increasing supply of natural gas has created a strong demand for developing efficient catalytic processes to upgrade methane, the most stable alkane molecule, into value-added chemicals. Currently, methane conversion in laboratory and industry is mostly performed under high-temperature conditions. A lot of effort has been devoted to exploring chemical entities that are able to activate the C-H bond of methane at lower temperatures, preferably room temperature. Gas phase atomic clusters with limited numbers of atoms are ideal models of active sites on heterogeneous catalysts. The cluster systems are being actively studied to activate methane under room-temperature conditions. State-of-the-art mass spectrometry, photoelectron imaging spectroscopy, and quantum chemistry calculations have been combined in our laboratory to reveal the molecular-level mechanisms of methane activation by atomic clusters. In this Account, we summarize our recent progress on thermal methane activation by metal oxide clusters doped with noble-metal atoms (Au, Pt, and Rh) as well as by oxygen-free species including carbides and borides of base metals (V, Ta, Mo, and Fe). In contrast to the generations of CH₃^• free radicals in many of the previously reported cluster reactions with methane, the generations of stable products such as formaldehyde, acetylene, and syngas as well as closed-shell species AuCH₃ and B₃CH₃ have been identified for the cluster reaction systems herein. Besides the well recognized mechanisms of methane activation by the O^-• radicals through hydrogen atom abstraction and by metal atoms through oxidative addition, the new mechanisms of synergistic methane activation by Lewis acid-base pairs (such as Au^δ+-O^δ- and B^δ+-B^δ-) and by dinuclear metal centers (such as Ta-Ta) have been recently revealed. In the reactions between methane and oxide clusters doped with noble-metal atoms, the oxide cluster "supports" can accept the H atoms and the CH _x species delivered through the noble-metal atoms and then transform methane into stable oxygenated compounds. The product selectivity (such as formaldehyde versus syngas) can be controlled by different noble-metal atoms (such as Pt versus Rh). The electronic structures of base metal centers can be engineered through carburization so that the low-spin states can be accessible to reduce the C-H bond of methane. Such active base metal centers in low-spin states resemble related noble-metal atoms in methane activation. The boron clusters (such as B₃ in VB₃⁺) can be polarized by the metal cations to form the Lewis acid-base pair B^δ+-B^δ- to cleave the C-H bond of methane very easily. These molecular-level mechanisms may well be operative in related heterogeneous catalysis and can be a fundamental basis to design efficient catalysts for activation and conversion of methane under mild conditions.

Trapping Iron(III)-Oxo Species at the Boundary of the "Oxo Wall": Insights into the Nature of the Fe(III)-O Bond

Terminal non-heme iron(IV)-oxo compounds are among the most powerful and best studied oxidants of strong C-H bonds. In contrast to the increasing number of such complexes (>80 thus far), corresponding one-electron-reduced derivatives are much rarer and presumably less stable, and only two iron(III)-oxo complexes have been characterized to date, both of which are stabilized by hydrogen-bonding interactions. Herein we have employed gas-phase techniques to generate and identify a series of terminal iron(III)-oxo complexes, all without built-in hydrogen bonding. Some of these complexes exhibit ∼70 cm^-1 decrease in ν(Fe-O) frequencies expected for a half-order decrease in bond order upon one-electron reduction to an S = 5/2 center, while others have ν(Fe-O) frequencies essentially unchanged from those of their parent iron(IV)-oxo complexes. The latter result suggests that the added electron does not occupy a d orbital with Fe═O antibonding character, requiring an S = 3/2 spin assignment for the nascent Fe^III-O^- species. In the latter cases, water is found to hydrogen bond to the Fe^III-O^- unit, resulting in a change from quartet to sextet spin state. Reactivity studies also demonstrate the extraordinary basicity of these iron(III)-oxo complexes. Our observations show that metal-oxo species at the boundary of the "Oxo Wall" are accessible, and the data provide a lead to detect iron(III)-oxo intermediates in biological and biomimetic reactions.

Highlights from the 53rd EUCHEM conference on stereochemistry, Bürgenstock, Switzerland, May 2018

When we first heard of the Bürgenstock conference it was described as a guarded meeting in a remote location, undisturbed by modern diversions, with mysterious customs and a secret handshake. All of these rumors turned out to be completely true. We arrived at an undisclosed location and three young men, who knew our names upon sight, greeted us and gave us an agenda in which we discovered the identity of the speakers, moderators and other participants. We then had a few moments to bask in their glory before being treated to a delicious dose of chemistry. During the banquet, before the first lecture, this year's president, Prof. Ilan Marek, gave the opening address from a balcony reserved only for these meetings, rumors hold that it stands vacant all year in anticipation. He mentioned several of the traditions of the meeting, which we are not allowed to share of course, and described the joy of putting together this year's exciting program with the help of an exceptional organizing committee that included Prof. Cristina Nevado, Prof. Christian Bochet, Dr Fabrice Gallou and Dr Alain De Mesmaeker. He also introduced this year's guest of honor, Prof. Yitzhak Apeloig, and wished the best luck to the current Vice President and future President, Prof. Véronique Gouverneur, not only for preparing the 54th Bürgenstock Conference, but also with her duty this year, maintaining the good weather for the whole week. Although the official title of the conference is the EUCHEM conference on stereochemistry, the topics covered span a broad range of cutting edge chemical transformations and insights, which can appeal to anyone working in chemistry and its interfaces with other disciplines. We will briefly describe each of the talks following one of our favorite citable quotes from these inspiring speakers.

Rationally designing mixed Cu–(μ-O)–M (M = Cu, Ag, Zn, Au) centers over zeolite materials with high catalytic activity towards methane activation

The direct conversion of methane to methanol on [Cu(μ-O)M]²⁺ (M = Cu, Ag, Zn, Au) bimetal centers in ZSM-5 zeolite is investigated using periodic DFT for the first time.

Oriented external electric fields as mimics for probing the role of metal ions and ligands in the thermal gas-phase activation of methane

Unusual, if not unprecedented, effects of transition-metal ions and ligands are discovered when simple metal oxides or carbides activate methane in the gas phase in manners reminiscent of oriented external electric fields.

Origins of the enantioselectivity of a palladium catalyst with BINOL–phosphoric acid ligands

The enantioselectivity of the studied C–H activation is related to the Brønsted acidity and isopropyl groups of the effective catalysts.

Thermally Induced Oxidation of [FeII(tacn)2](OTf)2 (tacn = 1,4,7-triazacyclononane)

We previously reported the spin-crossover (SC) properties of [FeII(tacn)2](OTf)2 (1) (tacn = 1,4,7-triazacyclononane) [Eur. J. Inorg. Chem. 2013, 2115]. Upon heating under dynamic vacuum, 1 undergoes oxidation to generate a low spin iron(III) complex. The oxidation of the iron center was found to be facilitated by initial oxidation of the ligand via loss of an H atom. The resulting complex was hypothesized to have the formulation [FeIII(tacn)(tacn-H)](OTf)2 (2) where tacn-H is N-deprotonated tacn. The formulation was confirmed by ESI-MS. The powder EPR spectrum of the oxidized product at 77 K reveals the formation of a low-spin iron(III) species with rhombic spectrum (g = 1.98, 2.10, 2.19). We have indirectly detected H2 formation during the heating of 1 by reacting the headspace with HgO. Formation of water (1HNMR in anhydrous d6-DMSO) and elemental mercury were observed. To further support this claim, we independently synthesized [FeIII(tacn)2](OTf)3 (3) and treated it with one equiv base yielding 2. The structures of 3 was characterized by X-ray crystallography. Compound 2 also exhibits a low spin iron(III) rhombic signal (g = 1.97, 2.11, 2.23) in DMF at 77 K. Variable temperature magnetic susceptibility measurements indicate that 3 undergoes gradual spin increase from 2 to 400 K. DFT studies indicate that the deprotonated nitrogen in 2 forms a bond to iron(III) exhibiting double bond character (Fe-N, 1.807 Å).