Thermal Methane Activation by [Si2O5].+ and [Si2O5H2].+: Reactivity Enhancement by Hydrogenation

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

Electronic Origin of the Competitive Mechanisms in the Thermal Activation of Methane by the Heteronuclear Cluster Oxide [Al2ZnO4].+

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

On the Mechanisms of Hydrogen‐Atom Transfer from Water to the Heteronuclear Oxide Cluster [Ga2Mg2O5].+: Remarkable Electronic Structure Effects

Mechanistic insight into the homolytic cleavage of the O? H bond of water by the heteronuclear oxide cluster [Ga₂Mg₂O₅]^.+ has been derived from state‐of‐the‐art gas‐phase experiments in conjunction with quantum chemical calculations. Three pathways have been identified computationally. In addition to the conventional hydrogen‐atom transfer (HAT) to the radical center of a bridging oxygen atom, two mechanistically distinct proton‐coupled electron‐transfer (PCET) processes have been identified. The energetically most favored path involves initial coordination of the incoming water ligand to a magnesium atom followed by an intramolecular proton transfer to the lone‐pair of the bridging oxygen atom. This step, which is accomplished by an electronic reorganization, generates two structurally equivalent OH groups either of which can be liberated, in agreement with labeling experiments.     

The Oxidative Mechanism in Electrophilic C?H Activation: The Case of CH2F2 and CH2Cl2

The H^.‐atom transfer (HAT) reaction is investigated in the gas phase, starting from two different entrance channels, O₂^.+/CH₂X₂ and CH₂X₂^.+/O₂ (X=F, Cl), that correspond to a step of hydride transfer and to HAT, respectively. Analysis of the spin and charge along the reaction pathway shows that HAT occurs through the same reacting configuration, irrespective of whether the reactants are formed within the complex or are free isolated species.     

Thermal Methane Conversion to Formaldehyde Promoted by Single Platinum Atoms in PtAl2O4− Cluster Anions

Identification and mechanistic study of thermal methane conversion mediated by gas‐phase species is important for finding potentially useful routes for direct methane transformation under mild conditions. Negatively charged oxide species are usually inert with methane. This work reports an unexpected result that the bi‐metallic oxide cluster anions PtAl₂O₄^? can transform methane into a stable organic compound, formaldehyde, with high selectivity. The clusters are prepared by laser ablation and reacted with CH₄ in an ion trap reactor. The reaction is characterized by mass spectrometry and density functional theory calculations. It is found that platinum rather than oxygen activates CH₄ at the beginning of the reaction. The Al₂O₄^? moiety serves as the support of Pt atom and plays important roles in the late stage of the reaction. A new mechanism for selective methane conversion is provided and new insights into the surface chemistry of single Pt atoms may be obtained from this study.     

Gas‐Phase Reaction of CeV2O7+ with C2H4: Activation of CC and CH Bonds

The reactivity of metal oxide clusters toward hydrocarbon molecules can be changed, tuned, or controlled by doping. Cerium‐doped vanadium cluster cations CeV₂O₇⁺ are generated by laser ablation, mass‐selected by a quadrupole mass filter, and then reacted with C₂H₄ in a linear ion trap reactor. The reaction is characterized by a reflectron time‐of‐flight mass spectrometer. Three types of reaction channels are observed: 1) single oxygen‐atom transfer , 2) double oxygen‐atom transfer , and 3) C?C bond cleavage. This study provides the first bimetallic oxide cluster ion, CeV₂O₇⁺, which gives rise to C?C bond cleavage of ethene. Neither Ce_xO_y^± nor V_xO_y^± alone possess the necessary topological and electronic properties to bring about such a reaction.     

Gas‐Phase Reactions of Cationic Vanadium‐Phosphorus Oxide Clusters with C2Hx (x=4, 6): A DFT‐Based Analysis of Reactivity Patterns

The reactivities of the adamantane‐like heteronuclear vanadium‐phosphorus oxygen cluster ions [V_xP_4?xO₁₀]^.+ (x=0, 2–4) towards hydrocarbons strongly depend on the V/P ratio of the clusters. Possible mechanisms for the gas‐phase reactions of these heteronuclear cations with ethene and ethane have been elucidated by means of DFT‐based calculations; homolytic C? H bond activation constitutes the initial step, and for all systems the P? O^. unit of the clusters serves as the reactive site. More complex oxidation processes, such as oxygen‐atom transfer to, or oxidative dehydrogenation of the hydrocarbons require the presence of a vanadium atom to provide the electronic prerequisites which are necessary to bring about the 2e^? reduction of the cationic clusters.     

Thermal Ethane Activation by Bare [V2O5]+ and [Nb2O5]+ Cluster Cations: on the Origin of Their Different Reactivities

The gas‐phase reactivity of [V₂O₅]⁺ and [Nb₂O₅]⁺ towards ethane has been investigated by means of mass spectrometry and density functional theory (DFT) calculations. The two metal oxides give rise to the formation of quite different reaction products; for example, the direct room‐temperature conversions C₂H₆→C₂H₅OH or C₂H₆→CH₃CHO are brought about solely by [V₂O₅]⁺. In distinct contrast, for the couple [Nb₂O₅]⁺/C₂H₆, one observes only single and double hydrogen‐atom abstraction from the hydrocarbon. DFT calculations reveal that different modes of attack in the initial phase of C?H bond activation together with quite different bond‐dissociation energies of the M?O bonds cause the rather varying reactivities of [V₂O₅]⁺ and [Nb₂O₅]⁺ towards ethane. The gas‐phase generation of acetaldehyde from ethane by bare [V₂O₅]⁺ may provide mechanistic insight in the related vanadium‐catalyzed large‐scale process.     

A Reaction-Induced Localization of Spin Density Enables Thermal C−H Bond Activation of Methane by Pristine FeC4+

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, FeC₄H⁺ 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.     

Activation of Methane Promoted by Adsorption of CO on Mo2C2− Cluster Anions

Atomic clusters are being actively studied for activation of methane, the most stable alkane molecule. While many cluster cations are very reactive with methane, the cluster anions are usually not very reactive, particularly for noble metal free anions. This study reports that the reactivity of molybdenum carbide cluster anions with methane can be much enhanced by adsorption of CO. The Mo₂C₂^? is inert with CH₄ while the CO addition product Mo₂C₃O^? brings about dehydrogenation of CH₄ under thermal collision conditions. The cluster structures and reactions are characterized by mass spectrometry, photoelectron spectroscopy, and quantum chemistry calculations, which demonstrate that the Mo₂C₃O^? isomer with dissociated CO is reactive but the one with non‐dissociated CO is unreactive. The enhancement of cluster reactivity promoted by CO adsorption in this study is compared with those of reported systems of a few carbonyl complexes.     

{Bis­[2‐(2‐oxido‐1‐naphthylmethylidene­amino)­phenyl] disulfide}chloroiron(III)

The crystal structure of the title compound, {bis­[2‐(2‐oxido‐2‐naphthyl­idene­amino)­phenyl] di­sulfide‐κ⁵O,N,S,N′,O′}chloroiron(III), [FeCl(C₃₄H₂₂N₂O₂S₂)], has been determined. The structure consists of monomeric iron(III) complexes with distorted octahedral coordination. The di­sulfide functions as a pentadentate ligand and the Fe^III atom is coordinated through two N, two O and one S atom, and one chloride ion. The distance between the second S atom and the Fe^III atom is a non‐bonding 3.8473 (14) Å.     

First‐ and second‐ sphere coordination of the uranyl ion by bis­[2‐(2‐hydroxy­phenoxy)­ethoxy]­ethane

Uranyl nitrate hexahydrate reacts with bis­[2‐(2‐hydroxy­phenoxy)­ethoxy]­ethane (C₁₈H₂₂O₆), denoted LH₂ hereafter, in the presence of triethylamine to give ­triethylammonium aqua[2,2′‐(3,6‐dioxaoctane‐1,8‐diyldioxy)diphenolato‐κ²O,O′](nitrato‐κ²O,O′)dioxouranium(VI), (Et₃NH)[UO₂(H₂O)L(NO₃)], which possesses a symmetry plane. The uranyl ion is coordinated to the two phenoxide O atoms, a nitrate ion and a water mol­ecule (first sphere); the water mol­ecule is itself held in the crown ether chain by hydrogen‐bonding interactions, thus ensuring second‐sphere coordination by the ligand L.     

Experimental and Computational Analysis of the Solvent‐Dependent O2/Li+‐O2− Redox Couple: Standard Potentials,Coupling Strength,and Implications for Lithium–Oxygen Batteries

Understanding and controlling the kinetics of O₂ reduction in the presence of Li⁺‐containing aprotic solvents, to either Li⁺‐O₂⁻ by one‐electron reduction or Li₂O₂ by two‐electron reduction, is instrumental to enhance the discharge voltage and capacity of aprotic Li‐O₂ batteries. Standard potentials of O₂/Li⁺‐O₂⁻ and O₂/O₂⁻ were experimentally measured and computed using a mixed cluster‐continuum model of ion solvation. Increasing combined solvation of Li⁺ and O₂⁻ was found to lower the coupling of Li⁺‐O₂⁻ and the difference between O₂/Li⁺‐O₂⁻ and O₂/O₂⁻ potentials. The solvation energy of Li⁺ trended with donor number (DN), and varied greater than that of O₂⁻ ions, which correlated with acceptor number (AN), explaining a previously reported correlation between Li⁺‐O₂⁻ solubility and DN. These results highlight the importance of the interplay between ion–solvent and ion–ion interactions for manipulating the energetics of intermediate species produced in aprotic metal–oxygen batteries.     

Near‐Infrared Light‐Driven Hydrogen Evolution from Water Using a Polypyridyl Triruthenium Photosensitizer

In order to realize artificial photosynthetic devices for splitting water to H₂ and O₂ (2 H₂O+hν→2 H₂+O₂), it is desirable to use a wider wavelength range of light that extends to a lower energy region of the solar spectrum. Here we report a triruthenium photosensitizer [Ru₃(dmbpy)₆(μ‐HAT)]⁶⁺ (dmbpy=4,4′‐dimethyl‐2,2′‐bipyridine, HAT=1,4,5,8,9,12‐hexaazatriphenylene), which absorbs near‐infrared light up to 800 nm based on its metal‐to‐ligand charge transfer (¹MLCT) transition. Importantly, [Ru₃(dmbpy)₆(μ‐HAT)]⁶⁺ is found to be the first example of a photosensitizer which can drive H₂ evolution under the illumination of near‐infrared light above 700 nm. The electrochemical and photochemical studies reveal that the reductive quenching within the ion‐pair adducts of [Ru₃(dmbpy)₆(μ‐HAT)]⁶⁺ and ascorbate anions affords a singly reduced form of [Ru₃(dmbpy)₆(μ‐HAT)]⁶⁺, which is used as a reducing equivalent in the subsequent water reduction process.     

Experimental and Computational Analysis of the Solvent‐Dependent O2/Li+‐O2− Redox Couple: Standard Potentials,Coupling Strength,and Implications for Lithium–Oxygen Batteries

Understanding and controlling the kinetics of O₂ reduction in the presence of Li⁺‐containing aprotic solvents, to either Li⁺‐O₂^? by one‐electron reduction or Li₂O₂ by two‐electron reduction, is instrumental to enhance the discharge voltage and capacity of aprotic Li‐O₂ batteries. Standard potentials of O₂/Li⁺‐O₂^? and O₂/O₂^? were experimentally measured and computed using a mixed cluster‐continuum model of ion solvation. Increasing combined solvation of Li⁺ and O₂^? was found to lower the coupling of Li⁺‐O₂^? and the difference between O₂/Li⁺‐O₂^? and O₂/O₂^? potentials. The solvation energy of Li⁺ trended with donor number (DN), and varied greater than that of O₂^? ions, which correlated with acceptor number (AN), explaining a previously reported correlation between Li⁺‐O₂^? solubility and DN. These results highlight the importance of the interplay between ion–solvent and ion–ion interactions for manipulating the energetics of intermediate species produced in aprotic metal–oxygen batteries.     

A Novel Expansion Mode of Polyoxovanadate Clusters: Synthesis,Crystal Structure and Properties of {[Cu(H2O)(C5H14N2)2]2[V16O38(Cl)]}·4(C5H16N2)

The new polyoxovanadate (POV) compound {[Cu(H₂O)(C₅H₁₄N₂)₂]₂[V₁₆O₃₈(Cl)]} · 4(C₅H₁₆N₂) was synthesized under solvothermal conditions and crystallizes in the tetragonal space group I4₁/amd with a = 13.8679(6), c = 45.558(2) Å, V = 8761.7(7) ų. The central structural motif is a {V₁₆O₃₈(Cl)} cluster constructed by condensation of 16 square‐pyramidal VO₅ polyhedra. The cluster hosts a central Cl^– anion. According to valence bond sum calculations, chemical analysis and magnetic properties the cluster anion may be formulated as [V₁₅^IVV^VO₃₈(Cl)]^12–, i.e., only one vanadium atom is not reduced. To the best of our knowledge this is the first reported {V₁₆O₃₈(X)} cluster in this V^IV:V^V ratio. The presence of the two different vanadium oxidation states is clearly seen in the IR spectrum. An unusual and hitherto never observed structural feature is the binding mode between the [Cu(H₂O)(C₅H₁₄N₂)₂]²⁺ complexes and the [V₁₅^IVV^VO₃₈(Cl)]^12– anion. The Cu²⁺ ion binds to a μ₂‐O atom of the cluster anion whereas in all other transition metal complex‐augmented POVs bonding between the transition metal cation and the anion occurs through terminal oxygen atoms of the POV. The magnetic properties are dominated by strong antiferromagnetic exchange interactions between the V⁴⁺ d¹ centers, whereas the Cu²⁺ d⁹ cations are magnetically decoupled from the cluster anion. Upon heating, the title compound decomposes in a complex fashion.     

Double‐Oxygen‐Atom Transfer in Reactions of CemO2m+ (m=2–6) with C2H2

Cerium oxide cluster cations (Ce_mO_n⁺, m=2–16; n=2m, 2m±1 and 2m±2) are prepared by laser ablation and reacted with acetylene (C₂H₂) in a fast‐flow reactor. A time‐of‐flight mass spectrometer is used to detect the cluster distribution before and after the reactions. Reactions of stoichiometric Ce_mO_2m⁺ (m=2–6) with C₂H₂ produce Ce_mO_2m?2⁺ clusters, which indicates a “double‐oxygen‐atom transfer” reaction Ce_mO_2m⁺+C₂H₂→Ce_mO_2m?2⁺+(CHO)₂ (ethanedial). A single‐oxygen‐atom transfer reaction channel is also identified as Ce_mO_2m⁺+C₂H₂→Ce_mO_2m?1⁺+C₂H₂O (at least for m=2 and 3). Density functional theory calculations are performed to study reaction mechanisms of Ce₂O₄⁺+C₂H₂, and the calculated results confirm that both the single‐ and double‐oxygen‐atom transfer channels are thermodynamically and kinetically favourable.     

Tuning the Reactivities of the Heteronuclear [AlnV3−nO7−n]+ (n=1, 2) Cluster Oxides towards Methane by Varying the Composition of the Metal Centers

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

C−−H activation on aluminum-vanadium bimetallic oxide cluster anions

Aluminum–vanadium bimetallic oxide cluster anions (BMOCAs) have been prepared by laser ablation and reacted with ethane and n‐butane in a fast‐flow reactor. A time‐of‐flight mass spectrometer was used to detect the cluster distribution before and after the reactions. The observation of hydrogen‐containing products AlVO₅H^? and Al_xV_4?xO_11?xH^? (x=1–3) strongly suggests that AlVO₅^? and Al_xV_4?xO_11?x^? (x=1–3) can react with ethane and n‐butane by means of an oxidative dehydrogenation process at room temperature. Density functional theory studies have been carried out to investigate the structural, bonding, electronic, and reactive properties of these BMOCAs. Terminal‐oxygen‐centered radicals (O_t^.) were found in all of the reactive clusters, and the O_t^. atoms, which prefer to be bonded with Al rather than V atoms, are the active sites of these clusters. All the hydrogen‐abstraction reactions are favorable both thermodynamically and kinetically. To the best of our knowledge, this is the first example of hydrogen‐atom abstraction by BMOCAs and may shed light on understanding the mechanisms of C? H activation on the surface of alumina‐supported vanadia catalysts.     

Neue molekulare Indium‐Zinn‐Sauerstoff‐ und Indium‐Zinn‐Natrium‐Sauerstoff‐Chlor‐Cluster

Abstract. The five‐membered heteroelement cluster THF · Cl₂In(O^tBu)₃Sn reacts with the sodium stannate [Na(O^tBu)₃Sn]₂ to produce either the new oxo‐centered alkoxo cluster ClInO[Sn(O^tBu)₂]₃ ( 1 ) (in low yield) or the heteroleptic alkoxo cluster Sn(O^tBu)₃InCl₃Na[Sn(O^tBu)₂]₂ ( 2 ). X‐ray diffraction analyses reveal that in compound 1 the polycyclic entity is made of three tin atoms which together with a central oxygen atom form a trigonal, almost planar triangle, perpendicular to which a further indium atom is connected through the oxygen atom. The metal atoms thus are arranged in a Sn₃In pyramid, the edges of which are all saturated by bridging tert‐butoxy groups. The indium atom has a further chloride ligand. Compound 2 has two trigonal bipyramids as building blocks which are fused together at a six coordinate indium atom. One of the bipyramids is of the type SnO₃In with tert‐butyl groups on the oxygen atoms, while the other has the composition InCl₃Na with chlorine atoms connecting the two metals. The sodium atom in 2 has further contacts to two plus one alkoxide groups which are part of a[Sn(O^tBu)₂]₂ dimer disposing of a Sn₂O₂ central cycle. The hetero element cluster in 2 thus combines three closed entities and its skeleton SnO₃InCl₃NaO₂Sn₂O₂ consists of three different metallic and two different non‐metallic elements.