Metal Oxo Research Articles

Mono- and Diatomic Reactive Oxygen Species Produced upon O2 Interaction with the (111) Facet of Cobalt Spinel at Various Conditions—Molecular DFT and Atomistic Thermodynamic Investigations

Periodic spin unrestricted DFT + U calculations joined with atomistic thermodynamics were used to study the location, structure, and stability along with the electronic and magnetic properties of various surface oxygen species and oxygen vacancies, produced under different thermodynamic conditions on the (111) surface of the cobalt spinel nano-octahedra. The density of state alignment diagrams between dioxygen and cobalt centers were used to rationalize speciation of the surface oxygen varieties into diatomic superoxo (μ-CoO3c–O2–CoT3c, η2-O2–CoO3c) or intrafacial peroxo ([O–Osurf.]2–) and monoatomic metal–oxo (CoT3c–O, CoO3c–O) entities. It was shown that the surface cobalt cations work in tandem constituting dual CoO3c–CoT3c sites for O2 adsorption, where the CoO3c dxz(β) and dyz(β) states act as spin-polarized electron donor centers, producing the most stable bridging μ-CoO3c–O2–CoT3c adducts (ΔEa = −1.86 eV). The single site mono- and bidentate binding modes η1-O2–CoO3c (ΔEa = −1.66 eV) and η2-O2–CoO3...

Oxygen-Oxygen Bond Cleavage and Formation in Co(II)-Mediated Stoichiometric O2 Reduction via the Potential Intermediacy of a Co(IV) Oxyl Radical.

In reactions of significance to alternative energy schemes, metal catalysts are needed to overcome kinetically and thermodynamically difficult processes. Often, high-oxidation-state, high-energy metal oxo intermediates are proposed as mediators in elementary steps involving O-O bond cleavage and formation, but the mechanisms of these steps are difficult to study because of the fleeting nature of these species. Here we utilized a novel dianionic pentadentate ligand system that enabled a detailed mechanistic investigation of the protonation of a cobalt(III)-cobalt(III) peroxo dimer, a known intermediate in oxygen reduction catalysis to hydrogen peroxide. It was shown that double protonation occurs rapidly and leads to a low-energy O-O bond cleavage step that generates a Co(III) aquo complex and a highly reactive Co(IV) oxyl cation. The latter was probed computationally and experimentally implicated through chemical interception and isotope labeling experiments. In the absence of competing chemical reagents, it dimerizes and eliminates dioxygen in a step highly relevant to O-O bond formation in the oxygen evolution step in water oxidation. Thus, the study demonstrates both facile O-O bond cleavage and formation in the stoichiometric reduction of O2 to H2O with 2 equiv of Co(II) and suggests a new pathway for selective reduction of O2 to water via Co(III)-O-O-Co(III) peroxo intermediates.

Facile fabrication of hierarchical micro-meso-macro porous metal oxide with high photochemical and electrochemical performances

A feasible route with a dual-templating method including both ice and SiO2 hard templates to fabricate hierarchical porous TiO2 with micro-, meso-, and macro-pores from inorganic/organic metal oxo cluster precursors is proposed. As-prepared TiO2 shows high photocatalytic activity and excellent electrochemical performances. The current strategy can be applied to fabrication of a broad range of metal oxide materials for catalysis and energy related applications.

Property–Activity Relations for Methane Activation by Dual‐Metal Cu–Oxo Trimers in ZSM‐5 Zeolite

AbstractThe identification of correlations between the experimentally or computationally measurable parameters of a catalytic system and its reactivity is one of the key steps toward the realization of a catalysis by design strategy. Here, periodic density functional theory calculations to establish such correlations for perspective catalysts for selective methane oxidation based on dual‐metal cation‐exchanged zeolites are employed. A representative trimeric metal–oxo active site is considered as a model reactive center. Computations reveal that the activation barrier for the homolytic CH bond cleavage in methane correlates well with the thermodynamic stability of the resulting CH3·⋯HO intermediate. The stability of the HO part approximated by the hydrogen affinity of the active site correlates with the activity trend, but deviations are observed due to inability of this descriptor to account for the stabilization of the CH3∙ moiety. Such fundamental characteristics as the atomic spin density and the basicity of reactive oxygen sites cannot be directly correlated with the catalyst reactivity, implying the complexity in property–reactivity relationship for methane activation. Calculations suggest that the initial screening of the potent zeolite‐based catalyst for methane activation can be established based on the analysis of both the thermodynamics and perturbation of base molecular adsorption probes such as pyrrole.

Evaluation of Thermal and Radiation Induced Chemistries of Metal Oxo–Hydroxo Clusters for Next-Generation Nanoscale Inorganic Resists

Thin films formed by the condensation of metal oxo–hydroxo clusters offer a promising approach to ultrahigh-resolution patterning including next-generation photolithography using extreme ultraviolet (EUV) radiation and electron-beam lithography. In this work, we elucidate the thermal and radiative mechanisms that drive the chemical transformations in these materials and therefore control the patterning performance. Beginning from aqueous hafnium clusters, peroxide and sulfate additions serve to modify the clusters and, upon spin coating to form a thin film, provide the chemical contrast necessary to create resist. The coordination and functionality of peroxide and sulfate in hafnium-based metal oxo–hydroxo clusters were monitored at various stages of the patterning process which provided insight into the chemical and structural evolution of the material. Peroxide serves as the radiation sensitive species while sulfate enhances solubility and controls the concentration of hydroxide in the films. Peroxide a...

Isolation of a Terminal Co(III)-Oxo Complex.

Late transition metal oxo complexes with high d-electron counts have been implicated as intermediates in a wide variety of important catalytic reactions; however, their reactive nature has often significantly limited their study. While some examples of these species have been isolated and characterized, complexes with d-electron counts >4 are exceedingly rare. Here we report that use of a strongly donating tris(imidazol-2-ylidene)borate scaffold enables the isolation of two highly unusual CoIII-oxo complexes which have been thoroughly characterized by a suite of physical techniques including single crystal X-ray diffraction. These complexes display O atom and H atom transfer reactivity and demonstrate that terminal metal oxo complexes with six d-electrons can display strong metal-oxygen bonding and sufficient stability to enable their characterization. The unambiguous assignment of these complexes supports the viability of related species that are frequently invoked, but rarely observed, in the types of catalytic reactions mentioned above. The studies described here change our understanding of the reactivity and bonding in late transition metal oxo complexes and open the door to further study of the properties of this class of elusive and important intermediates.

Enhancement of Electrocatalytic Reduction of Carbon Dioxide at Electrodes Modified with Highly-Acidic Mixed-Metal-Oxide Films

Electroreduction of carbon dioxide to simple organic fuels and chemicals is a topic of growing scientific and technological interest. The reaction provides means for both reducing emissions of CO2 into atmosphere and storing renewable energy. The presentation will address low-temperature CO2-conversion processes based on electrocatalytic and photoelectrochemical approaches. Among important issues are choice of the catalytic or semiconducting materials, their morphology and operating conditions including temperature, solvent, electrolyte, pH etc. There is a need to improve the reaction dynamics and selectivity toward specific products. In practical electrolysis cells, the CO2-reduction (at cathode) is accompanied by water oxidation (at anode or photoanode). Given the fact that the CO2 molecule is very stable, its electroreduction processes are characterized by large overpotentials. It is often postulated that, during electroreduction, the rate limiting step is the protonation of the adsorbed CO product to form the CHO adsorbate. In this respect, the proton availability and its mobility at the photo(electro)chemical interface has to be addressed. On the other hand, competition between such parallel processes as hydrogen evolution and carbon dioxide reduction has also to be considered. Recently, mixed oxide systems stabilized through Zr-O-W bonds have been demonstrated to be very attractive acid catalysts exhibiting high catalytic activities and good stabilities in many demanding industrial reactions. The mixed WO3/ZrO2 systems are characterized by fast charge (electron, proton) propagation during the system’s redox transitions. By dispersing metallic Cu electrocatalytic nanoparticles over, under or within, such active WO3/ZrO2 matrices, the electrocatalytic activities of the respective systems toward the reduction of carbon dioxide have been significantly enhanced even at decreased loadings in acid media. The enhancement effects should be attributed to features of the mixed metal oxide support such as porosity and high population of hydroxyl groups (due to presence of ZrO2), high Broensted acidity of sites formed on mixed WO3/ZrO2, fast electron transfers coupled to unimpeded proton displacements (e.g. in HxWO3), as well as strong metal-support interactions between nanosized metals (Cu) and the metal (W,Zr) oxo species. Application of the hybrid system composed of both above-mentioned oxides resulted in high current densities originating from electroreduction of carbon dioxide mostly to methanol (CH3OH) as demonstrated upon identification of final products.

The Role of Bi3+ in Promoting and Stabilizing Iron Oxo Clusters in Strong Acid.

Metal oxo clusters and metal oxides assemble and precipitate from water in processes that depend on pH, temperature, and concentration. Other parameters that influence the structure, composition, and nuclearity of "molecular" and bulk metal oxides are poorly understood, and have thus not been exploited. Herein, we show that Bi3+ drives the formation of aqueous Fe3+ clusters, usurping the role of pH. We isolated and structurally characterized a Bi/Fe cluster, Fe3 BiO2 (CCl3 COO)8 (THF)(H2 O)2 , and demonstrated its conversion into an iron Keggin ion capped by six Bi3+ irons (Bi6 Fe13 ). The reaction pathway was documented by X-ray scattering and mass spectrometry. Opposing the expected trend, increased cluster nuclearity required a pH decrease instead of a pH increase. We attribute this anomalous behavior of Bi/Fe(aq) solutions to Bi3+ , which drives hydrolysis and condensation. Likewise, Bi3+ stabilizes metal oxo clusters and metal oxides in strongly acidic conditions, which is important in applications such as water oxidation for energy storage.

The Role of Bi3+ in Promoting and Stabilizing Iron Oxo Clusters in Strong Acid

AbstractMetal oxo clusters and metal oxides assemble and precipitate from water in processes that depend on pH, temperature, and concentration. Other parameters that influence the structure, composition, and nuclearity of “molecular” and bulk metal oxides are poorly understood, and have thus not been exploited. Herein, we show that Bi3+ drives the formation of aqueous Fe3+ clusters, usurping the role of pH. We isolated and structurally characterized a Bi/Fe cluster, Fe3BiO2(CCl3COO)8(THF)(H2O)2, and demonstrated its conversion into an iron Keggin ion capped by six Bi3+ irons (Bi6Fe13). The reaction pathway was documented by X‐ray scattering and mass spectrometry. Opposing the expected trend, increased cluster nuclearity required a pH decrease instead of a pH increase. We attribute this anomalous behavior of Bi/Fe(aq) solutions to Bi3+, which drives hydrolysis and condensation. Likewise, Bi3+ stabilizes metal oxo clusters and metal oxides in strongly acidic conditions, which is important in applications such as water oxidation for energy storage.

Methane adsorption and hydrogen atom abstraction at diatomic radical cation metal oxo clusters: first-principles calculations

A thermochemical assessment of the C–H bond activation of methane over a series of monoxide MO•+ radical cations (M = Be, Mg, Ca, Sr, Ti, Cr, Fe, Ni, Zn, Pd and Pt) is presented in this paper within a hybrid density functional theory framework. Although all oxo clusters could implement chemisorption, only three (MgO•+, CaO•+ and SrO•+) could spontaneously perform H transfer. The stabilisation of methane at the adsorption stage, which was accompanied by partial electron transfers (0.026–0.210 e) from methane to the radical cation, was found to be a key player in the exergonic CH4/MO•+ reaction systems. The thermodynamic favourability followed the order of CrO•+ < TiO•+ < FeO•+ < PtO•+ < PdO•+ < NiO•+ < ZnO•+ < MgO•+ < SrO•+ < CaO•+ < BeO•+ as understood from the free energy changes. The activation barriers ranged from 6.86 kcal/mol (SrO•+) to 37.29 kcal/mol (TiO•+). The density of states implied that a promising radical cation system was able to maintain its occupied molecular orbital on the O atom while reserving the metal atom for the unoccupied one.

Highly-Acidic Mixed-Metal-Oxides (WO3-ZrO2) As Active Supports for Dispersed Metal Centers: Enhancement of Electrocatalytic Reduction of Oxygen and Carbon Dioxide

Application of metal oxides as active matrices in electrocatalysis is particular importance. The hydrous behavior, which favors proton mobility and affects overall reactivity, reflects not only the oxide’s chemical properties but its texture and morphology as well. What is of importance to electrochemical science and technology, certain nonstoichiometric mixed-valence oxides could exhibit pseudo-metallic conductivity and possess appreciable catalytic activity. Recently, mixed oxide systems stabilized through Zr-O-W bonds have been demonstrated to be very attractive acid catalysts exhibiting high catalytic activities and good stabilities in many demanding industrial reactions. For electrocatalytic applications, mixed-valent tungsten(VI,V) oxide and zirconium(IV) oxide have been sequentially deposited and integrated through voltammetric potential cycling to form sub-microstructured films on glassy carbon electrode. The mixed WO3/ZrO2 systems are characterized by fast charge (electron, proton) propagation during the system’s redox transitions. By dispersing metallic Pt or Cu electrocatalytic nanoparticles over such active WO3/ZrO2 supports, the electrocatalytic activities of the respective systems toward the reduction of oxygen or carbon dioxide have been enhanced even at decreased loadings in acid media. The enhancement effects should be attributed to features of the mixed metal oxide support such as porosity and high population of hydroxyl groups (due to presence of ZrO2), high Broensted acidity of sites formed on mixed WO3/ZrO2, fast electron transfers coupled to unimpeded proton displacements (e.g. in HxWO3), as well as strong metal-support interactions between nanosized metals (Pt or Cu) and the metal (W, Zr) oxo species. The fact that WO3/ZrO2 nanostructures are in immediate contact with the metallic catalytic sites leads to the specific interactions (via the surface hydroxyl groups) with the reaction intermediates (H2O2 or CO adsorbates). Further mechanistic studies involving rotating ring disk voltammetry and gas-diffusion electrode measurements are planned in addition to spectroscopic and XPS measurements. The mixed-metal systems will also be considered together with nanostructured carbon carriers such as reduced graphene oxide, carbon nanotubes or carbon black.

Kallol Ray

Angewandte Chemie International EditionVolume 57, Issue 28 p. 8366-8366 Author ProfileFree Access Kallol Ray First published: 16 February 2018 https://doi.org/10.1002/anie.201801205AboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat Graphical Abstract When I was eighteen I wanted to be a fighter pilot in the Indian air force. The natural talent I would like to be gifted with is the ability to see the future ...” This and more about Kallol Ray can be found on page 8366. Kallol Ray The author presented on this page has recently published his 10th article in Angewandte Chemie in the last 10 years: “A Highly Reactive Oxoiron(IV) Complex Supported by a Bioinspired N3O Macrocyclic Ligand”: I. Monte Pérez, X Engelmann, Y.-M. Lee, M. Yoo, E. Kumaran, E. R. Farquhar, E. Bill, J. England, W. Nam, M. Swart, K. Ray, Angew. Chem. Int. Ed. 2017, 56, 14384; Angew. Chem. 2017, 129, 14576. The work of K. Ray has been featured on the inside cover of Angewandte Chemie: “Oxidation Reactions with Bioinspired Mononuclear Non-Heme Metal–Oxo Complexes”: X. Engelmann, I. Monte-Pérez, K. Ray, Angew. Chem. Int. Ed. 2016, 55, 7632; Angew. Chem. 2016, 128, 7760. Date of birth: December 13, 1978 Position: Professor of Inorganic Chemistry, Humboldt-Universität zu Berlin E-mail: kallol.ray@chemie.hu-berlin.de Homepage: https://www.chemie.hu-berlin.de/en/forschung-en/ray/ray-en/kallolray ORCID: 0000-0003-2074-8844 Education: 2001 MSc, Indian Institute of Technology, Kanpur 2005 PhD supervised by Prof. Dr. Karl Wieghardt, Ruhr-Universität Bochum and Max Planck Institute (MPI) for Bioinorganic Chemistry, Mülheim an der Ruhr 2005–2006 Postdoctoral position with Prof. Dr. Karl Wieghardt, MPI for Bioinorganic Chemistry 2006–2009 Postdoctoral position with Prof. Dr. Lawrence Que, Jr., University of Minnesota Awards: 2012 Ernst Haage Prize, MPI for Chemical Energy Conversion; 2013 Wöhler–EurJIC Young Investigator Prize; 2015 Carl Duisberg Memorial Award, GDCh; 2016 Heisenberg Professorship, DFG. Current research interests: Bioinspired catalysis; organometallic chemistry; water oxidation reactions; activation of C−H bonds; fuel cells; recyclable energy sources Hobbies: Cricket, soccer, fitness, wine tasting When I was eighteen I wanted to be a fighter pilot in the Indian air force. The natural talent I would like to be gifted with is the ability to see the future. The secret of being a successful scientist is to learn how to deal with criticism in life. My favorite molecule is hopefully yet to be discovered from an unexpected reaction in my laboratory. If I had one year of paid leave I would use the time to teach my kids my mother tongue Bengali. The principal aspect of my personality is impatience. What I appreciate most about my friends is that they accept me with all my flaws. Chemistry is fun because it is sometimes too difficult even for chemists to understand. My favorite drink is a Zombie. In a spare hour, I … wish I had that luxury. My favorite quote is “Don't criticize what you can't understand” (Bob Dylan). My biggest inspiration is my parents. I advise my students to be even (over)confident rather than being insecure in their lives. My 5 top papers: 1“Temperature Dependence of the Catalytic Two- versus Four-Electron Reduction of Dioxygen by a Hexanuclear Cobalt Complex”: I. Monte-Pérez et al., J. Am. Chem. Soc. 2017, 139, 15033. (A deep understanding of the factors that control 2e−/2H+ vs. 4e−/4H+ reductions of dioxygen.) 2“Redox Non–innocence of a N–Heterocyclic Nitrenium Cation Bound to a Nickel–Cyclam Core”: F. Heims, F. F. Pfaff, S.-L. Abram, E. R. Farquhar, M. Bruschi, C. Greco, K. Ray, J. Am. Chem. Soc. 2014, 136, 582. (The coordination chemistry of an N-heterocyclic nitrenium ion to a first-row transition metal and its implication in catalysis.) 3“A High-Valent Heterobimetallic [CuIII(μ-O)2NiIII]2+ Core involving Nucleophilic Oxo Groups”: S. Kundu et al., Angew. Chem. Int. Ed. 2013, 52, 5622; Angew. Chem. 2013, 125, 5732 (Novel reactivity patterns of heterodinuclear bis(μ-oxo) complexes compared to their homodinuclear analogues.) 4“Lewis Acid Trapping of an Elusive Copper–Tosylnitrene Intermediate Using Scandium Triflate”: S. Kundu, E. Miceli, E. Farquhar, F. F. Pfaff, U. Kuhlmann, P. Hildebrandt, B. Braun, C. Greco, K. Ray, J. Am. Chem. Soc. 2012, 134, 14710. (The long-sought reactive intermediate that is believed to be involved in the copper-mediated amination and aziridination reactions was trapped.) 5“An Oxocobalt(IV) Complex Stabilized by Lewis Acid Interactions with Scandium(III) Ions”: F. F. Pfaff et al., Angew. Chem. Int. Ed. 2011, 50, 1711; Angew. Chem. 2011, 123, 1749. (We trapped the elusive intermediate that is postulated to occur in cobalt-mediated oxidation reactions.) Volume57, Issue28July 9, 2018Pages 8366-8366 ReferencesRelatedInformation

Electrochemical Water Oxidation and Stereoselective Oxygen Atom Transfer Mediated by a Copper Complex.

Water oxidation by copper-based complexes to form dioxygen has attracted attention in recent years, with the aim of developing efficient and cheap catalysts for chemical energy storage. In addition, high-valent metal-oxo species produced by the oxidation of metal complexes in the presence of water can be used to achieve substrate oxygenation with the use of H2 O as an oxygen source. To date, this strategy has not been reported for copper complexes. Herein, a copper(II) complex, [(RPY2)Cu(OTf)2 ] (RPY2=N-substituted bis[2-pyridyl(ethylamine)] ligands; R=indane; OTf=triflate), is used. This complex, which contains an oxidizable substrate moiety (indane), is used as a tool to monitor an intramolecular oxygen atom transfer reaction. Electrochemical properties were investigated and, upon electrolysis at 1.30 V versus a normal hydrogen electrode (NHE), both dioxygen production and oxygenation of the indane moiety were observed. The ligand was oxidized in a highly diastereoselective manner, which indicated that the observed reactivity was mediated by metal-centered reactive species. The pH dependence of the reactivity was monitored and correlated with speciation deduced from different techniques, ranging from potentiometric titrations to spectroscopic studies and DFT calculations. Water oxidation for dioxygen production occurs at neutral pH and is probably mediated by the oxidation of a mononuclear copper(II) precursor. It is achieved with a rather low overpotential (280 mV at pH 7), although with limited efficiency. On the other hand, oxygenation is maximum at pH 8-8.5 and is probably mediated by the electrochemical oxidation of an antiferromagnetically coupled dinuclear bis(μ-hydroxo) copper(II) precursor. This constitutes the first example of copper-centered oxidative water activation for a selective oxygenation reaction.

Oxygen Activation and Radical Transformations in Heme Proteins and Metalloporphyrins.

As a result of the adaptation of life to an aerobic environment, nature has evolved a panoply of metalloproteins for oxidative metabolism and protection against reactive oxygen species. Despite the diverse structures and functions of these proteins, they share common mechanistic grounds. An open-shell transition metal like iron or copper is employed to interact with O2 and its derived intermediates such as hydrogen peroxide to afford a variety of metal–oxygen intermediates. These reactive intermediates, including metal-superoxo, -(hydro)peroxo, and high-valent metal–oxo species, are the basis for the various biological functions of O2-utilizing metalloproteins. Collectively, these processes are called oxygen activation. Much of our understanding of the reactivity of these reactive intermediates has come from the study of heme-containing proteins and related metalloporphyrin compounds. These studies not only have deepened our understanding of various functions of heme proteins, such as O2 storage and transport, degradation of reactive oxygen species, redox signaling, and biological oxygenation, etc., but also have driven the development of bioinorganic chemistry and biomimetic catalysis. In this review, we survey the range of O2 activation processes mediated by heme proteins and model compounds with a focus on recent progress in the characterization and reactivity of important iron–oxygen intermediates. Representative reactions initiated by these reactive intermediates as well as some context from prior decades will also be presented. We will discuss the fundamental mechanistic features of these transformations and delineate the underlying structural and electronic factors that contribute to the spectrum of reactivities that has been observed in nature as well as those that have been invented using these paradigms. Given the recent developments in biocatalysis for non-natural chemistries and the renaissance of radical chemistry in organic synthesis, we envision that new enzymatic and synthetic transformations will emerge based on the radical processes mediated by metalloproteins and their synthetic analogs.

Selective Catalytic Olefin Epoxidation with MnII-Exchanged MOF-5

Partial substitution of ZnII by MnII in Zn4O(terephthalate)3 (MOF-5) leads to a distorted all-oxygen ligand field supporting a single MnII site, whose structure was confirmed by Mn K-edge X-ray absorption spectroscopy. The MnII ion at the MOF-5 node engages in redox chemistry with a variety of oxidants. With tBuSO2PhIO, it produces a putative MnIV-oxo intermediate, which upon further reaction with adventitious hydrogen is trapped as a MnIII–OH species. Most intriguingly, the intermediacy of the high-spin MnIV–oxo species is likely responsible for catalytic activity of the MnII-MOF-5 precatalyst, which in the presence of tBuSO2PhIO catalyzes oxygen atom transfer reactivity to form epoxides from cyclic alkenes with >99% selectivity. These results demonstrate that MOF secondary building units serve as competent platforms for accessing terminal high-valent metal–oxo species that consequently engage in catalytic oxygen atom transfer chemistry owing to the relatively weak ligand fields provided by the SBU.

Aqueous Bismuth Titanium–Oxo Sulfate Cluster Speciation and Crystallization

AbstractInorganic aqueous metal–oxo clusters are both functional “molecular metal oxides” and intermediates to understand metal oxide growth from water. There has been a recent surge in discovery of aqueous Ti‐oxo clusters but without extensive solution characterization. We use small‐angle and total X‐ray scattering, dynamic light scattering, transmission electron microscopy, and a single‐crystal X‐ray structure to show that heterometals such as bismuth stabilize labile Ti–oxo sulfate clusters in aqueous solution.[Ti22Bi7O41(OH)(OH2)30(SO4)12]2+ features edge‐sharing between the Ti and Bi polyhedra, in contrast to the dominant corner‐linking of Ti‐oxo clusters. Bi stabilizes the Ti‐polyhedra, which are synergistically stabilized by the bidentate sulfates. Gained stability and potential functionality from heterometals is an incentive to develop more broadly the landscape of heterometallic Ti–oxo clusters.

Aqueous Bismuth Titanium–Oxo Sulfate Cluster Speciation and Crystallization

Inorganic aqueous metal-oxo clusters are both functional "molecular metal oxides" and intermediates to understand metal oxide growth from water. There has been a recent surge in discovery of aqueous Ti-oxo clusters but without extensive solution characterization. We use small-angle and total X-ray scattering, dynamic light scattering, transmission electron microscopy, and a single-crystal X-ray structure to show that heterometals such as bismuth stabilize labile Ti-oxo sulfate clusters in aqueous solution.[Ti22 Bi7 O41 (OH)(OH2 )30 (SO4 )12 ]2+ features edge-sharing between the Ti and Bi polyhedra, in contrast to the dominant corner-linking of Ti-oxo clusters. Bi stabilizes the Ti-polyhedra, which are synergistically stabilized by the bidentate sulfates. Gained stability and potential functionality from heterometals is an incentive to develop more broadly the landscape of heterometallic Ti-oxo clusters.

Coordinatively Unsaturated Metal-Organic Frameworks M3(btc)2 (M = Cr, Fe, Co, Ni, Cu, and Zn) Catalyzing the Oxidation of CO by N2O: Insight from DFT Calculations.

The oxidation of CO by N2O over metal-organic framework (MOF) M3(btc)2 (M = Fe, Cr, Co, Ni, Cu, and Zn) catalysts that contain coordinatively unsaturated sites has been investigated by means of density functional theory calculations. The reaction proceeds in two steps. First, the N-O bond of N2O is broken to form a metal oxo intermediate. Second, a CO molecule reacts with the oxygen atom of the metal oxo site, forming one C-O bond of CO2. The first step is a rate-determining step for both Cu3(btc)2 and Fe3(btc)2, where it requires the highest activation energy (67.3 and 19.6 kcal/mol, respectively). The lower value for the iron compound compared to the copper one can be explained by the larger amount of electron density transferred from the catalytic site to the antibonding of N2O molecules. This, in turn, is due to the smaller gap between the highest occupied molecular orbital (HOMO) of the MOF and the lowest unoccupied molecular orbital (LUMO) of N2O for Fe3(btc)2 compared to Cu3(btc)2. The results indicate the important role of charge transfer for the N-O bond breaking in N2O. We computationally screened other MOF M3(btc)2 (M = Cr, Fe, Co, Ni, Cu, and Zn) compounds in this respect and show some relationships between the activation energy and orbital properties like HOMO energies and the spin densities of the metals at the active sites of the MOFs.

Alkoxides of Trivacant Lacunary Polyoxometalates

Lacunary polyoxometalates (POMs) are useful rigid multidentate inorganic ligands to construct multinuclear metal oxo clusters and inorganic-organic hybrid materials. However, the use of multivacant lacunary POMs often suffers from their undesired condensation reactions at the highly reactive vacant sites. Herein, the introduction of alkoxy protecting groups into the vacant sites of lacunary POMs in organic media is reported for the first time. By reacting organic solvent-soluble trivacant lacunary Keggin-type POMs with methanol, six methoxy groups were successfully introduced into the vacant sites, resulting in the formation of the alkoxides TBA4 [A-α-XW9 O28 (OCH3 )6 ] (Ix -OMe; X=Si, Ge). These methoxy groups could protect the reactive vacant sites and suppress the undesired dimerization reaction into the corresponding α-Dawson-type POMs. Since the introduced monodentate methoxy groups could easily be dissociated by hydrolysis, ISi -OMe could be utilized as the precursor for the synthesis of metal-substituted POMs. In addition, by using pentaerythritol as the multidentate alkoxy ligand, the monomeric alkoxide TBA4 [A-α-SiW9 O28 {(OCH2 )2 C(CH2 OH)2 }3 ] (ISi -PE) was successfully synthesized. In particular, these multidentate alkoxy groups showed high resistance against hydrolysis. Moreover, by reacting ISi -PE with trivacant lacunary Keggin-type POMs, the dimeric inorganic-organic-inorganic hybrid structures TBAn H8-n [(A-α-SiW9 O28 ){(OCH2 )2 C(CH2 O)2 }3 (A-α-XW9 O28 )] (IISiX -PE; X=Si, Ge) with the same or different heteroatoms (Si and Ge) were successfully synthesized.

Electrocatalytic Water Oxidation by a Homogeneous Copper Catalyst Disfavors Single-Site Mechanisms.

Deployment of solar fuels derived from water requires robust oxygen-evolving catalysts made from earth abundant materials. Copper has recently received much attention in this regard. Mechanistic parallels between Cu and single-site Ru/Ir/Mn water oxidation catalysts, including intermediacy of terminal Cu oxo/oxyl species, are prevalent in the literature; however, intermediacy of late transition metal oxo species would be remarkable given the high d-electron count would fill antibonding orbitals, making these species high in energy. This may suggest alternate pathways are at work in copper-based water oxidation. This report characterizes a dinuclear copper water oxidation catalyst, {[(L)Cu(II)]2-(μ-OH)2}(OTf)2 (L = Me2TMPA = bis((6-methyl-2-pyridyl)methyl)(2-pyridylmethyl)amine) in which water oxidation proceeds with high Faradaic efficiency (>90%) and moderate rates (33 s-1 at ∼1 V overpotential, pH 12.5). A large kinetic isotope effect (kH/kD = 20) suggests proton coupled electron transfer in the initial oxidation as the rate-determining step. This species partially dissociates in aqueous solution at pH 12.5 to generate a mononuclear {[(L)Cu(II)(OH)]}+ adduct (Keq = 0.0041). Calculations that reproduce the experimental findings reveal that oxidation of either the mononuclear or dinuclear species results in a common dinuclear intermediate, {[LCu(III)]2-(μ-O)2}2+, which avoids formation of terminal Cu(IV)═O/Cu(III)-O• intermediates. Calculations further reveal that both intermolecular water nucleophilic attack and redox isomerization of {[LCu(III)]2-(μ-O)2}2+ are energetically accessible pathways for O-O bond formation. The consequences of these findings are discussed in relation to differences in water oxidation pathways between Cu catalysts and catalysts based on Ru, Ir, and Mn.