Heteronuclear Oxide Cluster Research Articles

Rare-Earth Elements Can Structurally and Energetically Replace the Calcium in a Synthetic Mn4CaO4-Cluster Mimicking the Oxygen-Evolving Center in Photosynthesis.

The oxygen-evolving center (OEC) in photosynthesis is a unique biological Mn4CaO5 cluster catalyzing the water-splitting reaction. A great current challenge is to achieve a robust and precise mimic of the OEC in the laboratory. Herein, we report synthetic Mn4XO4 clusters (X = calcium, yttrium, gadolinium) that closely resemble the OEC with regard to the main metal-oxide core and peripheral ligands, as well as the oxidation states of the four Mn ions and the redox potential of the cluster. We demonstrate that rare-earth elements can structurally replace the calcium in neutral Mn4XO4 clusters. All three Mn4XO4 clusters with different redox-inactive metal ions display essentially the same redox properties, challenging the conventional view that the Lewis acidity of the redox-inactive metal ions could modulate the redox potential of the heteronuclear-oxide clusters. The new synthetic rare-earth element-containing Mn4XO4 clusters reported here provide robust and structurally well-defined chemical models and shed new light on the design of new water-splitting catalysts in artificial photosynthesis.

Direct Identification of Acetaldehyde Formation and Characterization of the Active Site in the [VPO4].+/C2H4 Couple by Gas‐Phase Vibrational Spectroscopy

AbstractThe gas‐phase reaction of the heteronuclear oxide cluster [VPO4].+ with C2H4 is studied under multiple collision conditions at 150 K using cryogenic ion‐trap vibrational spectroscopy combined with electronic structure calculations. The exclusive formation of acetaldehyde is directly identified spectroscopically and discussed in the context of the underlying reaction mechanism. In line with computational predictions it is the terminal P=O and not the V=O unit that provides the oxygen atom in the barrier‐free thermal C2H4→CH3CHO conversion. Interestingly, in the course of the reaction, the emerging CH3CHO product undergoes a rather complex intramolecular migration, coordinating eventually to the vanadium center prior to its liberation. Moreover, the spectroscopic structural characterization of neutral C2H4O deserves special mentioning as in most, if not all, ion/molecule reactions, the neutral product is usually only indirectly identified.

Direct Identification of Acetaldehyde Formation and Characterization of the Active Site in the [VPO4 ].+ /C2 H4 Couple by Gas-Phase Vibrational Spectroscopy.

The gas‐phase reaction of the heteronuclear oxide cluster [VPO4].+ with C2H4 is studied under multiple collision conditions at 150 K using cryogenic ion‐trap vibrational spectroscopy combined with electronic structure calculations. The exclusive formation of acetaldehyde is directly identified spectroscopically and discussed in the context of the underlying reaction mechanism. In line with computational predictions it is the terminal P=O and not the V=O unit that provides the oxygen atom in the barrier‐free thermal C2H4→CH3CHO conversion. Interestingly, in the course of the reaction, the emerging CH3CHO product undergoes a rather complex intramolecular migration, coordinating eventually to the vanadium center prior to its liberation. Moreover, the spectroscopic structural characterization of neutral C2H4O deserves special mentioning as in most, if not all, ion/molecule reactions, the neutral product is usually only indirectly identified.

Thermal O-H Bond Activation of Water As Mediated by Heteronuclear [Al2Mg2O5]•+: Evidence for Oxygen-Atom Scrambling.

Mechanistic insight into the thermal O-H bond activation of water by the cubane-like, prototypical heteronuclear oxide cluster [Al2Mg2O5]•+ has been derived from a combined experimental/computational study. Experiments in the highly diluted gas phase using Fourier transform ion-cyclotron resonance mass spectrometry show that hydrogen-atom abstraction from water by the cluster cation [Al2Mg2O5]•+ occurs at ambient conditions accompanied by the liberation of an OH• radical. Because of a complete randomization of all oxygen atoms prior to fragmentation, about 83% of the oxygen atoms of the hydroxyl radical released originate from the oxide cluster itself. The experimental findings are supported by detailed high-level quantum chemical calculations. The theoretical analysis reveals that the transfer of a formal hydrogen atom from water to the metal-oxide cation can proceed mechanistically via proton- or hydrogen-atom transfer exploiting different active sites of the cluster oxide. In addition to the unprecedented oxygen-atom scrambling, one of the more general and quite unexpected findings concerns the role of spin density at the hydrogen-acceptor oxide atom. While this feature is crucial for [M-O]+/CH4 couples, it is much less important in the O-H bond activation of water.

Catalytic CO Oxidation by O2 Mediated by Noble‐Metal‐Free Cluster Anions Cu2VO3–5−

AbstractCatalytic CO oxidation by molecular O2 is an important model reaction in both the condensed phase and gas‐phase studies. Available gas‐phase studies indicate that noble metal is indispensable in catalytic CO oxidation by O2 under thermal collision conditions. Herein, we identified the first example of noble‐metal‐free heteronuclear oxide cluster catalysts, the copper–vanadium bimetallic oxide clusters Cu2VO3–5− for CO oxidation by O2. The reactions were characterized by mass spectrometry, photoelectron spectroscopy, and density functional calculations. The dynamic nature of the Cu−Cu unit in terms of the electron storage and release is the driving force to promote CO oxidation and O2 activation during the catalysis.

Catalytic CO Oxidation by O2 Mediated by Noble-Metal-Free Cluster Anions Cu2 VO3-5.

Catalytic CO oxidation by molecular O2 is an important model reaction in both the condensed phase and gas-phase studies. Available gas-phase studies indicate that noble metal is indispensable in catalytic CO oxidation by O2 under thermal collision conditions. Herein, we identified the first example of noble-metal-free heteronuclear oxide cluster catalysts, the copper-vanadium bimetallic oxide clusters Cu2 VO3-5- for CO oxidation by O2 . The reactions were characterized by mass spectrometry, photoelectron spectroscopy, and density functional calculations. The dynamic nature of the Cu-Cu unit in terms of the electron storage and release is the driving force to promote CO oxidation and O2 activation during the catalysis.

Consecutive Oxidation of Three H2 Molecules by a Gold-Vanadium Oxide Cluster Cation AuVO4 +

Time-of-flight mass spectrometry experiments demonstrated that laser ablation generated and mass selected gold-vanadium heteronuclear oxide cluster AuVO4 + can oxidize three H2 molecules consecutively in an ion trap reactor. Quantum chemistry calculations were employed to reveal the elementary steps involved in the consecutive H2 oxidation. The positively charged gold in AuVO4 + functions as the active site to capture H2 and split the H–H bond in collaboration with the terminal lattice oxygen O2−, during the process of which the superoxide species O2 •− in AuVO4 + is activated and then dissociated to supply enough oxygen sites for the subsequent H2 oxidation. After the oxidation of three H2 molecules, the Au–O bond in AuVO4 + is converted to Au–V bond in product AuVO+ that is inert towards H2. In contast, cluster AuNbO4 + that is structurally related to AuVO4 + can oxdize only one H2 molecule. The origin of the different reactivities between two clusters was explored and the importance of different central metal in H2 oxidation was emphasized.

Methane Activation Mediated by Dual Gold Atoms Doped in Aluminium Oxide Cluster Cations Au2Al2O3+

AbstractThe study of chemical reactions between gold‐doped oxide clusters and methane can provide molecular level mechanisms to understand the activity of gold species supported on metal oxides. The role of single gold atom in methane activation has been identified, while the effect of two or more gold atoms is far from clear. By employing mass spectrometry and theoretical calculations, the reactivity of heteronuclear oxide clusters with dual gold atoms (Au2Al2O3+) toward methane was explored. Three major reaction products with the formation of CH3•, AuCH2, and Au were observed experimentally. Theoretical calculations indicate that Au2Al2O3+ contains two separate gold atoms and one oxygen−centred radical (O−•). Two types of mechanisms for methane activation are identified. The reaction initiated by the O−• site primarily generates the CH3• through hydrogen atom abstraction while the reaction starting from the gold site mainly contributes to the generation of AuCH2 and Au via the mechanism of Lewis acid‐base pairs. It was revealed that the formation of gold dimer species driven by the aurophilic interaction plays an important role in the second C−H bond activation.

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.

The reactivity of the homo- and heteronuclear oxide clusters [XYO2](+) (X, Y = Al, Si, Mg) toward methane was studied using Fourier transform ion cyclotron resonance mass spectrometry, in conjunction with high-level quantum mechanical calculations. The most reactive cluster by both experiment and theory is [Al2O2](•+). In its favorable pathway, this cluster abstracts a hydrogen atom by means of proton-coupled electron transfer (PCET) instead of following the conventional hydrogen-atom transfer (HAT) route. This mechanistic choice originates in the strong Lewis acidity of the aluminum site of [Al2O2](•+), which cleaves the C-H bond heterolytically to form an Al-CH3 entity, while the proton is transferred to the bridging oxygen atom of the cluster ion. In addition, a comparison of the reactivity of heteronuclear and homonuclear oxide clusters [XYO2](+) (X, Y = Al, Si, Mg) reveals a striking doping effect by aluminum. Thus, the vacant s-p hybrid orbital on Al acts as an acceptor of the electron pair from methyl anion (CH3(-)) and is therefore eminently important for bringing about thermal methane activation by PCET. For the Al-doped cluster ions, the spin density at an oxygen atom, which is crucial for the HAT mechanism, acts here as a spectator during the course of the PCET mediated C-H bond cleavage. A diagnostic plot of the deformation energy vis-à-vis the barrier shows the different HAT/PCET reactivity map for the entire series. This is a strong connection to the recently discussed mechanism of oxidative coupling of methane on magnesium oxide surfaces proceeding through Grignard-type intermediates.

Dehydrogenation of propylene mediated by CeVO4+: An interesting example for the chemistry of binary Ce–V transition-metal oxide cluster cations

Laser-ablation-generated cerium-vanadium oxide cluster cations CeVO4+ were mass-selected by applying a quadrupole mass filter, and reacted with propylene in a linear ion trap reactor at ambient conditions. Time-of-flight mass spectrometry and density functional theory calculations were used to characterize the cluster reactions. The CeVO4+ cluster has a closed-shell electronic structure, and is reactive toward propylene leading to the formation of CeVO3C3H4+/H2O and CeVO4C3H6+. The computed results indicate that this reaction involves the occurrence of spin conversions (singlet→triplet), and a new mechanism of dehydrogenation of alkenes, which is different from the reported ones, was proposed. Compared with another closed-shell cluster V3O7+, possessing the similar mass with CeVO4+, the heteronuclear oxide cluster cations show higher reactivity toward propylene. The computational results are in good agreement with the experimental observations. This work provides insights into nature of active sites and reaction mechanisms in related condensed-phase systems at a molecular level.

Activation and Transformation of Ethane by Au2 VO3(+) Clusters with Closed-Shell Electronic Structures.

The study of chemical reactions between gold-containing heteronuclear oxide clusters and small molecules can provide molecular level mechanisms to understand the excellent activity of gold supported by metal oxides. While the promotion role of gold in alkane transformation was identified in the clusters with atomic oxygen radicals (O(-.)), the role of gold in the systems without O(-.) is not clear. By employing mass spectrometry and quantum chemistry calculations, the reactivity of Au2 VO3(+) clusters with closed-shell electronic structures toward ethane was explored. Both the dehydrogenation and ethene elimination channels were identified. It is gold rather than oxygen species initiating the C-H activation. The Au-Au dimer formed during the reactions plays important roles in ethane transformation. The reactivity comparison between Au2 VO3(+) and bare Au2(+) demonstrates that Au2 VO3(+) not only retains the property of bare Au2(+) that transforming ethane to dihydrogen, but also exhibits new functions in converting ethane to ethene, which reveals the importance of the composite system. This study provides a further understanding of the reactivity of metal oxide supported gold in alkane activation and transformation.

Frontispiece: Distinct Mechanistic Differences in the Hydrogen‐Atom Transfer from Methane and Water by the Heteronuclear Oxide Cluster [Ga2MgO4].+

Frontispiece: Distinct Mechanistic Differences in the Hydrogen‐Atom Transfer from Methane and Water by the Heteronuclear Oxide Cluster [Ga₂MgO₄]^.+

On the Mechanisms of Hydrogen-Atom Transfer from Water to the Heteronuclear Oxide Cluster [Ga2 Mg2 O5 ](.+) : Remarkable Electronic Structure Effects.

Mechanistic insight into the homolytic cleavage of the O-H bond of water by the heteronuclear oxide cluster [Ga2 Mg2 O5 ](.+) 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.

Distinct Mechanistic Differences in the Hydrogen‐Atom Transfer from Methane and Water by the Heteronuclear Oxide Cluster [Ga2MgO4].+

The thermal reactions of the heteronuclear oxide cluster [Ga2 MgO4 ](.+) with methane and water have been studied using state-of-the-art gas-phase experiments in conjunction with quantum-chemical calculations. The significant reactivity differences, favoring activation of the strong OH bond, can be ascribed to a proton-coupled electron transfer (PCET) mechanism operative in the activation of water. This study deepens our mechanistic understanding on how inert RH bonds are cleaved by metal oxides.

Thermal conversion of methane to formaldehyde promoted by gold in AuNbO3(+) cluster cations.

In addition to generation of a methyl radical, formation of a formaldehyde molecule was observed in the thermal reaction of methane with AuNbO3(+) heteronuclear oxide cluster cations. The clusters were prepared by laser ablation and mass-selected to react with CH4 in an ion-trap reactor under thermal collision conditions. The reaction was studied by mass spectrometry and DFT calculations. The latter indicated that the gold atom promotes formaldehyde formation through transformation of an Au-O bond into an Au-Nb bond during the reaction.

On the role of the electronic structure of the heteronuclear oxide cluster [Ga2Mg2O5 ](.+) in the thermal activation of methane and ethane: an unusual doping effect.

The reactivity of the heteronuclear oxide cluster [Ga2 Mg2 O5 ](.+) , bearing an unpaired electron at a bridging oxygen atom (Ob (.-) ), towards methane and ethane has been studied using Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS). Hydrogen-atom transfer (HAT) from both methane and ethane to the cluster ion is identified experimentally. The reaction mechanisms of these reactions are elucidated by state-of-the-art quantum chemical calculations. The roles of spin density and charge distributions in HAT processes, as revealed by theory, not only deepen our mechanistic understanding of CH bond activation but also provide important guidance for the rational design of catalysts by pointing to the particular role of doping effects.

CO Oxidation Promoted by Gold Atoms Supported on Titanium Oxide Cluster Anions

Laser ablation generated Au(x)(TiO2)(y)O(z)(-) (x = 0, 1; y = 2, 3; z = 1, 2) oxide cluster anions have been mass-selected using a quadrupole mass filter and reacted with CO in a hexapole collision cell. The reactions have been characterized by time-of-flight mass spectrometry and density functional theory calculations. Gold-titanium bimetallic oxide clusters Au(TiO2)(y)O(z)(-) are more reactive in CO oxidation than pure titanium oxide clusters (TiO2)(y)O(z)(-). The computational studies identify the dual roles that the gold atom plays in CO oxidation: functioning as a CO trapper and electron acceptor. Both factors are important for the high reactivity of Au(TiO2)(y)O(z)(-) clusters. To the best of our knowledge, this is the first example of CO oxidation by gold-containing heteronuclear oxide clusters, which provides molecular-level insights into the roles of gold in CO oxidation over oxide supports.

Hydrogen‐Atom Abstraction from Methane by Stoichiometric Vanadium–Silicon Heteronuclear Oxide Cluster Cations

Vanadium-silicon heteronuclear oxide cluster cations were prepared by laser ablation of a V/Si mixed sample in an O(2) background. Reactions of the heteronuclear oxide cations with methane in a fast-flow reactor were studied with a time-of-flight (TOF) mass spectrometer to detect the cluster distribution before and after the reactions. Hydrogen abstraction reactions were identified over stoichiometric cluster cations [(V(2)O(5))(n)(SiO(2))(m)](+) (n=1, m=1-4; n=2, m=1), and the estimated first-order rate constants for the reactions were close to that of the homonuclear oxide cluster V(4)O(10) (+) with methane. Density functional calculations were performed to study the structural, bonding, electronic, and reactivity properties of these stoichiometric oxide clusters. Terminal-oxygen-centered radicals (O(t)*) were found in all of the stable isomers. These O(t)* radicals are active sites of the clusters in reaction with CH(4). The O(t)* radicals in [V(2)O(5)(SiO(2))(1-4)](+) clusters are bonded with Si rather than V atoms. All the hydrogen abstraction reactions are favorable both thermodynamically and kinetically. This work reveals the unique properties of metal/nonmetal heteronuclear oxide clusters, and may provide new insights into CH(4) activation on silica-supported vanadium oxide catalysts.

Experimental and Theoretical Study of the Reactions between Vanadium−Silicon Heteronuclear Oxide Cluster Anions withn-Butane

Vanadium and silicon heteronuclear oxide cluster anions VxSiyOz− (x + y ≥ 2, z ≥ 4) are prepared by laser ablation and reacted with n-butane (C4H10) in a fast flow reactor. A time-of-flight mass spectrometer is used to detect the cluster distribution before and after the reactions. The observation of hydrogen-containing products (V2O5)m(SiO2)nOH− (m = 1, n = 1−4; m = 2, n = 1) strongly suggests the following reactions: (V2O5)m(SiO2)nO− + C4H10 → (V2O5)m(SiO2)nOH− + C4H9. Although V2O6− is produced in the cluster source, no V2O6H− product is produced under the same experimental condition. It indicates that specific heteronuclear oxide clusters V2O5(SiO2)1−4O− and (V2O5)2SiO2O− are more reactive than the homonuclear oxide cluster V2O6− (or V2O5O−). Density functional theory (DFT) calculations are performed to study reaction mechanisms of V2O5SiO2O− (or V2SiO8−) + C4H10. The calculated results are in good agreement with the experimental observations. The structural and bonding properties of (V2O5)m(SiO2)nO− ...

Methane activation by V3PO10˙+ and V4O10˙+ clusters: A comparative study

A series of vanadium and phosphorus heteronuclear oxide cluster cations (V(x)P(y)O(z)(+)) are prepared by laser ablation and the reactions of V(3)PO(10)˙(+) and V(4)O(10)˙(+) with methane in a fast flow reactor under the same conditions are studied. A time of flight mass spectrometer is used to detect the cluster distribution before and after reactions. In addition to previously identified reaction of V(4)O(10)˙(+) + CH(4)→ V(4)O(10)H(+) + CH(3)˙, the observation of hydrogen atom pickup cluster V(3)PO(10)H(+) suggests the reaction: V(3)PO(10)˙(+) + CH(4)→ V(3)PO(10)H(+) + CH(3)˙. The rate of the reaction of V(4)O(10)˙(+) with CH(4) is approximately 2.5 times faster than that of V(3)PO(10)˙(+) with CH(4). Density functional theory (DFT) calculations predict that structure of V(3)PO(10)˙(+) is topologically similar to that of V(4)O(10)˙(+), as well as that of P(4)O(10)˙(+), which is very similar to V(4)O(10)˙(+) in terms of methane activation in previous studies. The facile methane activation by the homo- and hetero-nuclear oxide clusters can all be attributed to the presence of an oxygen-centered radical (O˙) in these clusters. Further theoretical study indicates that the O˙ radical (or spin density of the cluster) can transfer within the high symmetry V(4)O(10)˙(+) and P(4)O(10)˙(+) clusters quite easily, and CH(4) molecule further enhances the rate of intra-cluster spin density transfer. In contrast, the intra-cluster spin density transfer within low symmetry V(3)PO(10)˙(+) is thermodynamically forbidden. The experimentally observed reactivity difference is consistent with the theoretical consideration of the intra-cluster spin density transfer.