Electronic Origin of the Competitive Mechanisms in the Thermal Activation of Methane by the Heteronuclear Cluster Oxide [Al2 ZnO4 ]

Reactivity of Atomic Oxygen Radical Anions in Metal Oxide Clusters

Atomic oxygen radical anion (O⋅-) represents an important type of reactive centre that exists in both chemical and biological systems. Gas-phase atomic clusters can be studied under isolated and well controlled conditions. Studies of O⋅--containing clusters in the gas-phase provide a unique strategy to interpret the chemistry of O⋅- radicals at a strictly molecular level. This review summarizes the research progresses made since 2013 for the reactivity of O⋅- radicals in the atomically precise metal oxide clusters including negatively charged, nanosized, and neutral heteronuclear metal clusters benefitting from the development of advanced experimental techniques. New electronic and geometric factors to control the reactivity and product selectivity of O⋅- radicals under dark and photo-irradiation conditions have been revealed. The detailed mechanisms of O⋅- generation have been discussed for the reaction systems of nanosized and heteroatom-doped metal oxide clusters. The catalytic reactions mediated by the O⋅- radicals in metal clusters have also been successfully established and the microscopic mechanisms about the dynamic generation and depletion of O⋅- radicals have been clearly understood. The studies of O⋅- containing metal oxide clusters in the gas-phase provided new insights into the chemistry of reactive oxygen species in related condensed-phase systems.

A Lanthanide-Incorporated Phospho(III)tungstate Aggregate Constructed from [HPIIIW8O31]10- and [W11O39]12- Building Blocks and Its Nanocomposite with CdS for Ultrasensitive Photoelectrochemical Detection of Oxytetracycline

A novel tartronic acid decorated hexa-CeIII-incorporated phospho(III)tungstate aggregate (C4H12NO)6Na18H2[(HPW8O31)2[W11O39]2(H2TAD)4(H2O)4W4Ce6H2P2O14]·84H2O (1, H3TAD = tartronic acid) was synthesized by a one-step assembly strategy. Its main skeleton is constructed from two [W11O39]12- fragments, two [HPIIIW8O31]10- segments and one H2TAD--ornamented dodecanuclear heterometallic [W4Ce6H2PIII2O14(H2TAD)4(H2O)4]18+ cluster. In the structure, the [HPIIIO3]2- groups not only work as the heteroatom template to induce the formation of lacunary [HPIIIW8O31]10- segments but also function as the connector to bridge Ce3+ cations. With the help of a reaction strategy of combining ultrasonication treatment with the continuous ion layer adsorption method, the 1/CdS composite was constructed and exhibits prominent photoelectrochemical activity. The 1/CdS composite was used as a photoelectrochemical sensor for oxytetracycline detection at 0 V (vs Ag/AgCl), which displays excellent properties with quick response and low limit of detection (0.042 nM). This work can provide some helpful references in the construction of novel PIII-induced polyoxometalates consisting of different building blocks and can extend the applications of polyoxometalate-based nanocomposites into photoelectrochemical detection for antibiotics as well as biomolecules.

Thermal Reactions of NiAl3O6+ and Al4O6+ with Methane: Reactivity Enhancement by Doping

Investigation of the reactivity of heteronuclear metal oxide clusters is an important way to uncover the molecular-level mechanisms of the doping effect. Herein, we performed a comparative study on the reactions of CH4 with NiAl3O6+ and Al4O6+ cluster cations at room temperature to understand the role of Ni during the activation and transformation of methane. Mass spectrometric experiments identify that both NiAl3O6+ and Al4O6+ could bring about hydrogen atom abstraction reaction to generate CH3• radical; however, only NiAl3O6+ has the potential to stabilize [CH3] moiety and then transform [CH3] to CH2O. Density functional theory calculations demonstrate that the terminal oxygen radicals (Ot-•) bound to Al act as the reactive sites for the two clusters to activate the first C-H bond. Although the Ni atom cannot directly participate in methane activation, it can manipulate the electronic environment of the surrounding bridging oxygen atoms (Ob) and enable such Ob to function as an electron reservoir to help Ot-• oxidize CH4 to [H-O-CH3]. The facile reduction of Ni3+ to Ni+ also facilitates the subsequent step of activating the second C-H bond by the bridging "lattice oxygen" (Ob2-), finally enabling the oxidation of methane into formaldehyde. The important role of the dopant Ni played in improving the product selectivity of CH2O for methane conversion discovered in this study allows us to have a possible molecule-level understanding of the excellent performance of the catalysts doping with nickel.

Methane Activation by [AlFeO3 ]+ : the Hidden Spin Selectivity

The performance of heteronuclear cluster [AlFeO3 ]+ in activating methane has been explored by a combination of high-level quantum chemical calculations with gas-phase experiments. At room temperature, [AlFeO3 ]+ is a mixture of 7 [AlFeO3 ]+ and 5 [AlFeO3 ]+ , in which two states lead to different reactivity and chemoselectivity for methane activation. While hydrogen extracted from methane is the only product channel for the 7 [AlFeO3 ]+ /CH4 couple, 5 [AlFeO3 ]+ is able to convert this substrate to formaldehyde. In addition, the introduction of an external electric field may regulate the reactivity and product selectivity. The interesting doping effect of Fe and the associated electronic origins are discussed, which may guide one on the design of Fe-involved catalyst for methane conversion.

Size Effects in Gas-phase C-H Activation

The gas-phase clusters reaction permits addressing fundamental aspects of the challenges related to C-H activation. The size effect plays a key role in the activation processes as it may substantially affect both the reactivity and selectivity. In this paper, we reviewed the size effect related to the hydrocarbon oxidation by early transition metal oxides and main group metal oxides, methane activation mediated by late transition metals. Based on mass-spectrometry experiments in conjunction with quantum chemical calculations, mechanistic discussions were reviewed to present how and why the size greatly regulates the reactivity and product distribution.

Thermal Methane Conversion to Formaldehyde Mediated by NiAlO3+ in the Gas Phase

Understanding the active sites and reaction mechanisms of Ni-based catalysts, such as Ni/Al₂O₃, toward methane is a prerequisite for improving their rational design. Here, the gas-phase reactivity of NiAlO₃⁺ cations toward CH₄ is studied using mass spectrometry combined with density functional theory. Similar to our previous study on NiAl₂O₄⁺, we find evidence for the formation of both the methyl radical (CH₃^•) and formaldehyde (CH₂O). The first step for methane activation is hydrogen atom abstraction by the terminal oxygen radical Ni(O)₂AlO^• from methane forming a [Ni(O)₂AlOH⁺, ^•CH₃] complex and leaving the Ni-oxidation state unchanged. The second C-H bond is subsequently activated by the association of a bridged Ni-O^2--Al. The oxidation state of the Ni atom is reduced from +3 to +1 during the formation of formaldehyde. Compared to Al₂O₃⁺/CH₄ and YAlO₃⁺/CH₄ systems, the Ni-atom substitution increases the overall reaction rate by roughly an order of magnitude and yields a CH₃^•/CH₂O branching ratio of 0.62/0.38. The present study provides molecular-level insights into the highly efficient gas-phase reaction mechanism contributing to an improved understanding of methane conversion by Ni/Al₂O₃ catalysts.

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.

On the origin of reactivity variation upon sequential ligation: the [Re(Cl)x]+/CH4 (x = 1-3) couples

The potential of [ReCl_x]⁺ (x = 1-3) in activating methane has been explored by using a combination of gas-phase experiments and high-level quantum calculations. When the number of Cl ligands increases, the reactivity towards methane activation varies accordingly. While [ReCl_x]⁺ (x = 1-2) are able to dehydrogenate methane by a three-state reactivity scenario, [ReCl₃]⁺ shows inertness towards methane at ambient conditions. Furthermore, the product ion [ClRe(H)CH]⁺ of the [ReCl]⁺/CH₄ couple could continue to activate methane and liberate molecular dihydrogen but another product ion [Cl₂ReCH₂]⁺ is unreactive with methane. Obviously, the nature and the number of ligands make a difference to the reactivity towards methane activation. The associated reaction mechanism and the electron origins for the rather different reactivities are discussed in detail. Finally and more importantly, instructive information concerning the rational design of Re-catalysts for methane conversion is obtained.

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].

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.

Selective C-O Coupling Hidden in the Thermal Reaction of [Al2 CuO5 ]+ with Methane

The thermal gas-phase reaction of [Al₂ CuO₅ ]⁺ with methane has been explored by using FT-ICR mass spectrometry complemented by high-level quantum chemical calculations. The generation of atomic [Cu]⁺ from the [Al₂ CuO₅ ]⁺ /CH₄ couple corresponds to the only reaction channel. Labeling experiments and computational studies strongly suggest that methane activation is indeed involved in the production of [Cu]⁺ , and generation of CH₂ O prevails. Mechanistic aspects and the associated doping effects are discussed.