Polynuclear Clusters Research Articles

Homogeneous Catalysis by Organometallic Polynuclear Clusters

Homogeneous polynuclear metal clusters constitute a broad class of coordination compounds with important applications in catalysis. The current interest of synthetic chemistry in this field demands the exploration of new strategies to develop catalytic methods that work under mild conditions and maximize atom utilization. This review covers the application of polynuclear clusters of nuclearity ≥ 3 in homogeneous catalytic processes, with focus on providing an array of examples of various reaction types within cluster catalysis.

A Histidine Residue and a Tetranuclear Cuprous-thiolate Cluster Dominate the Copper Loading Landscape of a Copper Storage Protein from Streptomyces lividans.

The chemical basis for protecting organisms against the toxic effect imposed by excess cuprous ions is to constrain this through high-affinity binding sites that use cuprous-thiolate coordination chemistry. In bacteria, a family of cysteine rich four-helix bundle proteins utilise thiolate chemistry to bind up to 80 cuprous ions. These proteins have been termed copper storage proteins (Csp). The present study investigates cuprous ion loading to the Csp from Streptomyces lividans (SlCsp) using a combination of X-ray crystallography, site-directed mutagenesis and stopped-flow reaction kinetics with either aquatic cuprous ions or a chelating donor. We illustrate that at low cuprous ion concentrations, copper is loaded exclusively into an outer core region of SlCsp via one end of the four-helix bundle, facilitated by a set of three histidine residues. X-ray crystallography reveals the existence of polynuclear cuprous-thiolate clusters culminating in the assembly of a tetranuclear [Cu4 (μ2 -S-Cys)4 (Νδ1 -His)] cluster in the outer core. As more cuprous ions are loaded, the cysteine lined inner core of SlCsp fills with cuprous ions but in a fluxional and dynamic manner with no evidence for the assembly of further intermediate polynuclear cuprous-thiolate clusters as observed in the outer core. Using site-directed mutagenesis a key role for His107 in the efficient loading of cuprous ions from a donor is established. A model of copper loading to SlCsp is proposed and discussed.

Infrared Spectroscopy and Density Functional Theory Investigations of PdTi Heterodimer Reactivity with Carbon Monoxide Isolated in Solid Argon.

The reactivity of diatomic palladium-titanium toward carbon monoxide has been studied in solid argon by infrared spectroscopy (Fourier transform infrared) in the carbonyl stretching frequency region. Our technique of sublimation of Ti and Pd atoms from two filaments heated separately allowed the identification of five new molecules. Small polynuclear carbonyl clusters, PdTi(CO) n ( n = 1-3), have been characterized on the basis of isotopic substitutions, metal atom and CO concentration variations, and irradiation effects. Selective irradiation in the visible region leads to conversion between two isomers distinguished by the stretching frequency of the diatomic CO: PdTiCO Ti-eo (νCO = 1908.4 cm-1) and PdTiCO Pd-eo (νCO = 2009.3 cm-1). The density functional theory calculations have been carried out to elucidate the geometrical and electronic structures and support the spectral assignments. The nature of the metal-carbon bonding has been discussed using the topology of the Laplacian of the electron density.

Mechanistically Driven Control over Cubane Oxo Cluster Catalysts.

Predictive and mechanistically driven access to polynuclear oxo clusters and related materials remains a grand challenge of inorganic chemistry. We here introduce a novel strategy for synthetic control over highly sought-after transition metal {M4O4} cubanes. They attract interest as molecular water oxidation catalysts that combine features of both heterogeneous oxide catalysts and nature's cuboidal {CaMn4O5} center of photosystem II. For the first time, we demonstrate the outstanding structure-directing effect of straightforward inorganic counteranions in solution on the self-assembly of oxo clusters. We introduce a selective counteranion toolbox for the controlled assembly of di(2-pyridyl) ketone (dpk) with M(OAc)2 (M = Co, Ni) precursors into different cubane types. Perchlorate anions provide selective access to type 2 cubanes with the characteristic {H2O-M2(OR)2-OH2} edge-site, such as [Co4(dpy-C{OH}O)4(OAc)2(H2O)2](ClO4)2. Type 1 cubanes with separated polar faces [Co4(dpy-C{OH}O)4(L2)4] n+ (L2 = OAc, Cl, or OAc and H2O) can be tuned with a wide range of other counteranions. The combination of these counteranion sets with Ni(OAc)2 as precursor selectively produces type 2 Co/Ni-mixed or {Ni4O4} cubanes. Systematic mechanistic experiments in combination with computational studies provide strong evidence for type 2 cubane formation through reaction of the key dimeric building block [M2(dpy-C{OH}O)2(H2O)4]2+ with monomers, such as [Co(dpy-C{OH}O)(OAc)(H2O)3]. Furthermore, both experiments and DFT calculations support an energetically favorable type 1 cubane formation pathway via direct head-to-head combination of two [Co2(dpy-C{OH}O)2(OAc)2(H2O)2] dimers. Finally, the visible-light-driven water oxidation activity of type 1 and 2 cubanes with tuned ligand environments was assessed. We pave the way to efficient design concepts in coordination chemistry through ionic control over cluster assembly pathways. Our comprehensive strategy demonstrates how retrosynthetic analyses can be implemented with readily available assembly directing counteranions to provide rapid access to tuned molecular materials.

Ligand-Based Control of Single-Site vs. Multi-Site Reactivity by a Trichromium Cluster.

The trichromium cluster (tbs L)Cr3 (thf) ([tbs L]6- =[1,3,5-C6 H9 (NC6 H4 -o-NSit BuMe2 )3 ]6- ) exhibits steric- and solvation-controlled reactivity with organic azides to form three distinct products: reaction of (tbs L)Cr3 (thf) with benzyl azide forms a symmetrized bridging imido complex (tbs L)Cr3 (μ3 -NBn); reaction with mesityl azide in benzene affords a terminally bound imido complex (tbs L)Cr3 (μ1 -NMes); whereas the reaction with mesityl azide in THF leads to terminal N-atom excision from the azide to yield the nitride complex (tbs L)Cr3 (μ3 -N). The reactivity of this complex demonstrates the ability of the cluster-templating ligand to produce a well-defined polynuclear transition metal cluster that can access distinct single-site and cooperative reactivity controlled by either substrate steric demands or reaction media.

Co(II, III) Hydroxides Supported on Zeolite Acting as an Efficient and Robust Catalyst for Catalytic Water Oxidation with Ru(bpy)33+

A novel highly efficient and stable for many cycles catalyst (1% Со-ZSM-5(17)) for water oxidation was developed using the method of polycondensation for stabilization of oxo/hydroxo complexes of cobalt (II, III) and nanosize Co3O4 in zeolite channels. In a weak-alkaline medium (pH 8.0, 9.2, 10.0) in the presence of a one-electron oxidant (Ru(bpy)33+), the catalyst provided the yield of oxygen as high as 56, 73 и 78% of the stoichiometric quantity, respectively. Catalysts based on ZSM-5 zeolite exhibited higher catalytic activity as compared to the catalysts based on the MOR, BEA, Y. Inspection of the electron states of cobalt in the Co-containing zeolite-based catalysts using TPR-H2 and UV–Vis DR techniques revealed that α-Co(OH)2-like polynuclear clusters and hydrocomplexes stabilized in the zeolite channels were most active to catalytic oxidation of water, while their transformation to Co3O4-like clusters/nanoparticles and Co2+ oxocomplexes, respectively, during thermal treatment led to some decrease in the catalyst efficiency. Coarsening of Co3O4-like clusters/nanoparticles caused the further decrease in the system efficiency. The minimal activity was observed with the catalysts containing predominantly isolated Co2 +Oh ions. The result obtained indicates a kind of at least polynuclear structure of the catalytically active center ConOx, where n > 2.

Self-Assembly of Polynuclear Complexes Based on Spacer- Armed Pyridylazoles

The paper reviews literature data on the structural features of coordination compounds based on spacer-armed pyridylazoles (pyrazoles, 1,2,4- and 1,2,3-triazoles, tetrazoles). It is shown that the realization of the “maximum site occupancy” (MSO) principle for the metal:ligand ratio of 2:3 leads to the self-assembly of polynuclear clusters, their metal cores having highly symmetric architectures of tetrahedron, trigonal prism, cube, truncated tetrahedron, and cuboctahedron.

Crystal Engineering and Magnetostructural Properties of Newly Designed Azide/Acetate-Bridged Mn12 Coordination Polymers

Crystal engineering of the coordination polymers where polynuclear clusters are building blocks constitutes an emerging class of chemistry. The fine-tuning of the structural motifs leads to interesting and varying magnetic properties. Owing to such properties, two rare μ6-oxo centered mixed-valent, azide or acetate-bridged coordination polymers viz, [{MnII2MnIII10Na2(μ6-O)2(N3)10(NO3)(H2O)4(thme)8}·3(Et3NH)]n (1) and [{MnII3MnIII9Na7(μ2-O)2(μ6-O)2(O)5(CH3O)(CH3CO2)11(thmp)8}·4(O)]n (2), with retention of a Mn12 metallic core in both polymers are obtained using tripodal polyalcohol 1,1,1-tris (hydroxymethyl)ethane (H3thme) and 1,1,1-tris (hydroxymethyl)propane (H3thmp) ligands, respectively. X-ray analysis shows that 1 is a one-dimensional coordination polymer where Mn12 units are propagating by bridging azide function. 1 shows the underlying net of 2,2,3C6 topological type. 2 forms a cyclic ring as a result of repeating Mn12 zigzag chains bridging by sodium and H2O. The topology of 2 results in a 31-nodal...

Theoretical Studies on Hexanuclear [M3(μ3-O/OH)]2 (M = Fe(III), Mn(III), and Ni(II)) Clusters: Magnetic Exchange, Magnetic Anisotropy, and Magneto-Structural Correlations.

Controlling spin Hamiltonian parameters such as magnetic exchange and magnetic anisotropy of polynuclear clusters is of great interest in the area of single molecule magnets (SMMs). Among large polynuclear clusters, hexanuclear clusters offer the best compromise in terms of size as they are often rigid, solution stable, and chemically amenable. The {M6O2} core is one of the common architectures known for many hexanuclear clusters and generally reported to possess a diamagnetic ST = 0 spin ground state, barring a few exceptions. In these clusters, there are several open questions that are poorly understood: (a) What controls the nature of magnetic exchange, which in turn dictates the ground state spin values? (b) For clusters possessing a nonzero spin ground state, what dictates the magnetic anisotropy? Here, using density functional methods, we have attempted to shed light on these two question by evaluating the exchange coupling constants in [Fe6IIIO2(OH)2{(C4N2H2SMe)2C(OH)O}2( tBuCO2)10] (1), [Fe6III(O)2(O2CH2)(O2CCH2 tBu)12(py)2] (2), [Fe6III(O2)(O)2(O2CCMe3)12(py)2] (3), [FeIII6O3(O2CMe)9(OEt)2(bpy)2]ClO4 (4), [MnIII6O2(O2CH2)(O2CPe t)11(HO2CPe t)2(O2CMe)] (5), and [NiII6(OH)4(O2C tBu)8( tBuCO2H)4] (6) complexes. We have estimated all the eight near-neighbor exchange coupling constants in these clusters. Our calculations not only agree with the experimental results but also offer insight on the origin of the spin ground state. Extensive magneto-structural correlations developed by varying M-O-M angles and M-O distances reveal that J values are extremely sensitive to small structural distortions. Correlations developed indicate that both the parameters are important for Fe(III), but for Mn(III) and Ni(II), the angles were found to play a dominant role. Quite interestingly, the computed zero-field splitting parameter D S=5 of complex 1 reveals that the exchange contribution to the anisotropy controls the sign of the ground state D value-an observation which differs from the general perception that the ground state D is controlled by the single-ion zero-field splitting parameter.

Modulating single-molecule magnet behaviors of Dy4III clusters through utilizing two different β-diketonate coligands

In search of simple approaches to rationally modulate the single-molecule magnet behaviors in polynuclear lanthanide clusters, a new system containing two structurally closely related rhombus-shaped Dy4 clusters, Dy4(tmhd)4L6(μ3-OH)2·5CH3CN (1) and Dy4(acac)4L6(μ3-OH)2 (2) (HL = 5-(4-ethyoxylbenzylidene)-8-hydroxylquinoline, Htmhd = 2,2,6,6-tetramethyl-3,5-heptanedione and Hacac = acetylacetonate), have been obtained by employing two dysprosium β-diketonate salts to react with a bidentate Schiff base ligand (HL). The two Dy4 clusters have similar structures and possess a structure in which four Dy3+ ions remain in a perfect plane with a rhombus-shaped geometric construction. Dynamic magnetic analysis revealed different single-molecule magnet (SMM) behaviors occurs in 1 and 2. Under zero direct-current (dc) field, 1 displays typical SMMs behavior with ΔE/kB = 39.6 K and τ0 = 2.1 × 10−6 s. However, for 2, a peculiar feature of the χ″ versus temperature curves is that more than two peaks that are frequency-dependent are revealed, indicating the occurrence of multifold relaxation processes that lead to two ΔE/kB (31.3 K and 81.6 K) and pre-exponential factors (τ0 = 3.1 × 10−6 s, τ0 = 2.2 × 10−7 s).

Самоорганизация полиядерных комплексов на основе спейсерированных пиридилазолов

Проанализированы литературные данные о структурных особенностях координационных соединений на основе спейсерированных пиридилазолов (пиразолы, 1,2,4- и 1,2,3-триазолы, тетразолы). Показано, что реализация принципа MSO — "maximum site occupancy" при соотношении металл:лиганд = 2:3 приводит к самоорганизации полиядерных кластеров с геометрией металлоостова в виде высокосимметричных архитектур, таких как тетраэдр, тригональная призма, куб, усеченный тетраэдр и кубооктаэдр.

Thermo-sensitive luminescence of lanthanide complexes, clusters, coordination polymers and metal–organic frameworks with organic photosensitizers

Historical and recent advances of lanthanide mononuclear complexes, polynuclear clusters, coordination polymers (CPs) and metal–organic frameworks (MOFs) with temperature-dependent luminescence are reviewed for future thermo-sensitive paints.

Activation of thiosaccharin at a polynuclear osmium cluster

The reaction of thiosaccharin (tsacH) with the triosmium cluster [Os3(CO)10(NCMe)2] furnishes the decacarbonyl isomers [HOs3(CO)10(μ-S-tsac)] (1) and [HOs3(CO)10(μ-N,S-1,3-tsac)] (2) in a 3:1 ratio at room temperature. These isomers differ by the coordination mode displayed by tsac ligand. The tsac moiety functions as an edge-bridging ligand via the sulfur atom in 1 while in 2 the bridging of adjacent osmium centers is achieved through the sulfur and nitrogen groups. The ancillary hydride in both products shares the OsOs edge that is bridged by the heterocyclic ligand. Heating 1 at 80 °C affords 2 and demonstrates that the former cluster is the product of kinetic control. The conversion of 1 → 2 has been investigated by DFT and the isomerization pathway elucidated. The DFT calculations confirm cluster 2 as the thermodynamically preferred isomer in this pair of products. Thermolysis of 2 in refluxing toluene affords the hexanuclear cluster [H2Os6(CO)17(μ-C,N-1,2-C6H4CNSO2)2(μ3-S)(μ4-S)] (3) via carbon-sulfur bond scission and subsequent capture of the extruded sulfur by the cluster core. The molecular structures for the three new clusters have been determined by single-crystal X-ray diffraction analyses.

Homo‐ and Heteroleptic Coordination Polymers and Oxido Clusters of Bismuth(III) Vinylsulfonates

The synthesis and characterization of six homo- and heteroleptic coordination polymers and oxido clusters of bismuth(III) vinylsulfonates are reported. The solvent-mediated reaction of BiPh3 and vinylsulfonic acid in ethanol produces [{Ph2 Bi(O3 SCH=CH2 )}n ] (1), which crystallizes as a one-dimensional coordination polymer as a result of bridging sulfonato ligands accompanied by intermolecular Bi⋅⋅⋅ π(arene) London dispersion interactions. In solution, compound 1 equilibrates to give [{PhBi(O3 SCH=CH2 )2 }n ] (2) and BiPh3 . Compound 2 is obtained as a single product by the reaction of BiPh3 with vinylsulfonic acid in acetonitrile and crystallizes as a one-dimensional coordination polymer. The homoleptic vinylsulfonate [{Bi(O3 SCH=CH2 )3 }n ] (3) was isolated as a two-dimensional coordination polymer, which is quite moisture sensitive, but did not provide a distinct polynuclear bismuth oxido cluster upon hydrolysis. However, by treatment of [Bi6 O4 (OH)4 (NO3 )6 ]⋅H2 O or [Bi38 O45 (OMc)24 (dmso)9 (H2 O)2 ]⋅2 DMSO⋅5 H2 O (OMc=methacrylate) with vinylsulfonic acid, such a cluster, namely, [Bi9 O7 (OH)(O3 SCH=CH2 )11 (dmso)11 ](O3 SCH=CH2 )⋅3 DMSO (4), is available as the main product. Starting from the hexanuclear bismuth oxido nitrate, another cluster, [Bi38 O45 (NO3 )8 (O3 SCH=CH2 )14 (dmso)18 ](O3 SCH=CH2 )2 ⋅2 DMSO (5), was observed as a co-crystallizing side product, which upon further hydrolysis afforded [Bi38 O45 (NO3 )6 (OH)4 (O3 SCH=CH2 )12 (dmso)23 (H2 O)2 ](O3 SCH=CH2 )2 ⋅2 H2 O (6).

Recent progresses on synthesis, structures and applications of reduced polyoxometalates with low-nuclearity

Polyoxometalate (POMs) are large group of discrete, mostly anionic polynuclear metal-oxo clusters amenable to a variety of chemical transformations. Because of high oxidation of the central metal, the original structure of POMs did not change after electron implantation. The electrons introduced during the reduction process can be immobilized on a certain metal or between the metal center and the titanium oxide metal ions. Compared with fully oxidized POMs, reduced POMs have different electron densities of oxygen atoms, so their coordination ability is different, which affects their physical and chemical properties. When synthesizing reduced POMs, some basic principle must be considered, such as whether the POMs system contains diversified structure points, whether the intermediate product has high enthalpy value, whether it has isomerization, whether the system can cover other metal ions should be considered, etc. According to the stability of reduced POMs, they can divided into three types: firstly each center metal contains only one end oxygen atom, second each center metal contains two end oxygen atoms, last the combination of the first two conditions. There are significant differences in the preparation methods of different types of reduced POMs. There are three kinds of common methods: photochemical reduction, control potential electrolysis and chemical oxidation reduction. Besides, hydrothermal synthesis, microwave synthesis and one pot synthesis were new approaches without specific reaction mechanism. With the injection of electrons in the system, different degrees of reduction and the way will be derived from new derivatives. The reduced POMs of various types of structures and corresponding typical examples are briefly introduced, and the application of reduced POMs in many aspects is discussed. In the field of traditional catalysis, reduced POMs in molecular devices, biological reagents and new application of energy materials has gradually begun to attract peoples attention. Chemists have long realized that the delocalized electrons of reduced POMs can produce magnetism, but the magnetic utilization work has just started, although there are a large number of heteropoly blue molecules. It has been reported, but only a few molecules have been studied as functional elements of magnet devices or spins. Scientists understanding of the biological activity of POMs is not very thorough, but research in this field is gradually deepening and systematic. Studies have shown that reduced POMs can exhibit higher biological activity relative to their oxidation states. There are many challenges in the study of reduced POMs from basic chemistry to molecular applications. However, the discovery and characterization of multi-molecular molecules has led to the refinement of POMs in the structural model, the clear relationship between structure and properties. The advantages are more obvious. Furthermore, the research on related materials will be more and more worthy of attention. It is believed that through unremitting efforts, the reduced POMs in future new material systems, such as molecular-based magnetic materials, nanotechnology, catalyst molecular-based energy storage devices and life. In areas such as learning, it will show its revolutionary potential and prospects.

Macrocycle-Encircled Polynuclear Metal Clusters: Controllable Synthesis, Reactivity Studies, and Applications.

Macrocyclic ligands have been extensively applied to recognize single metal ions with high selectivity and good affinity based on the size-match principle. The resulting metal-macrocycle complexes play a significant role in mimicking the function of natural metal ion carriers and understanding and reproducing the catalytic activity of metalloenzymes. Because of the known macrocyclic effect, those single metal-macrocycle adducts often show an enhanced kinetic and thermodynamic stability in comparison with their open-chain analogues. By virtue of such extraordinary coordination properties of macrocyclic ligands, it is expected that larger macrocycles with multiple coordination sites could properly act as an outer scaffold to direct the formation of multiatom species inside, such as polynuclear metal cluster aggregates, whose assembly may largely depend on the template positioning of coordinative atoms in the macrocyclic ring. Thus, the employment of polydentate macrocyclic ligands may provide a convenient tool to access polynuclear metal clusters in a controllable way. In this Account, we review our studies of the metal ion binding process of a class of polydentate macrocyclic ligands, azacalixpyridines (Py[ n]s), and the application of Py[ n]s as an outer template to direct the controllable synthesis of polynuclear metal clusters. Our investigations revealed that Py[ n]s show a significant cooperative coordination effect in the metal ion binding process that facilitated the easy formation of a polymetallic assembled structure. Taking advantage of the cooperative coordination effect and the tunable and highly fluxional conformation of Py[ n]s, we laid our focus on control of the nuclearity number by tuning the size of Py[ n]s and the adoption of Py[ n]s with different anionic centers in metal cluster synthesis. As an important example for application, this new established macrocycle-directed method has been employed to achieve a variety of metal-cluster-centered capsule, rotaxane, catenane, polygon, and other supramolecular assemblies. Furthermore, a cluster-to-cluster transformation inside the cavity of Py[ n]s is presented to showcase the use of the acquired metal cluster-macrocycle complexes to achieve unconventional metal cluster entities. With regard to the application of the newly synthesized macrocycle-encircled metal clusters, examples of the fabrication of functional materials and catalysts are presented. With the assistance of Py[ n]s, a bulk-to-cluster-to-nanoparticle transformation of silver sulfide (Ag2S) and silver halides (AgX) has been conducted to produce a series of nonstoichiometric silver sulfide and halide nanoparticles. The resulting Ag-S nanoparticle material with a high Ag/S ratio, which is inherited from the Py[ n]-protected polysilver sulfide clusters, has a large energy gap relative to conventional Ag2S nanoparticles. Moreover, the nonstoichiometric silver halide nanoparticles can act as a new kind of electrocatalyst for the chlorine evolution reaction, showing excellent selectivity and high catalytic efficiency. Overall, in this Account we try to highlight the application of polydentate macrocycles as an outer template to guide the synthesis of polynuclear metal clusters in a controllable manner. This unique synthesis will provide a new avenue to access unconventional metal clusters of different metal kinds and diverse anionic centers, which are expected to have promising and significant applications in many interdisciplinary areas of chemistry.

Microscopic Approach to the Problem of Cooperative Spin Crossover in Polynuclear Cluster Compounds: Application to Tetranuclear Iron(II) Square Complexes

A new microscopic approach to the problem of cooperative spin crossover in molecular crystals containing polynuclear complexes as structural units has been developed. The cooperative intercluster interaction in the crystal is shown to arise from the coupling of the molecular modes to the acoustic phonons. The interaction between ions, which belong to the same cluster, is also taken into account, that leads to interesting features in the temperature dependence of the magnetic susceptibility, which are not observed in mononuclear molecular crystals. The developed general approach is adapted to the case of tetranuclear square complexes. In the latter, the effect of intracluster interactions on the type of the spin transition is analyzed. The applicability of the developed approach is illustrated by the interpretation of the experimental data on the spin transition in the tetranuclear [Fe(tpa){N(CN)2}]4·(BF4)4·(H2O)2 cluster compound. For this compound, the theory well reproduces the observed two-step spin tr...

Iron-based trinuclear metal-organic nanostructures on a surface with local charge accumulation

Coordination chemistry relies on harnessing active metal sites within organic matrices. Polynuclear complexes—where organic ligands bind to several metal atoms—are relevant due to their electronic/magnetic properties and potential for functional reactivity pathways. However, their synthesis remains challenging; few geometries and configurations have been achieved. Here, we synthesise—via supramolecular chemistry on a noble metal surface—one-dimensional metal-organic nanostructures composed of terpyridine (tpy)-based molecules coordinated with well-defined polynuclear iron clusters. Combining low-temperature scanning probe microscopy and density functional theory, we demonstrate that the coordination motif consists of coplanar tpyʼs linked via a quasi-linear tri-iron node in a mixed (positive-)valence metal–metal bond configuration. This unusual linkage is stabilised by local accumulation of electrons between cations, ligand and surface. The latter, enabled by bottom-up on-surface synthesis, yields an electronic structure that hints at a chemically active polynuclear metal centre, paving the way for nanomaterials with novel catalytic/magnetic functionalities.

Oxidation of Water by Cerium(IV) Compounds in the Presence of Trinuclear Ruthenium Complexes

The oxidation of water by Ce(IV) compounds in the presence of ruthenium complex (NH3)5Ru–O–Ru(NH3)4–O–Ru(NH3)5Cl6 is studied. It is shown that the complex is a catalyst of water oxidation if one-electron oxidizing species Ce4+ are used. Four units of the indicated ruthenium complex then organize themselves into a reactive dodecanuclear cluster. It is established that the formation of bi-, tetra-, and polynuclear cerium clusters in the reacting system is accompanied by the four oxygen atoms in the catalyst molecule being replaced with 4NH3 ligands, resulting in a different catalytic system for biomimetic water oxidation.

γ-Bi2O3 - To Be or Not To Be? Comparison of the Sillenite γ-Bi2O3 and Isomorphous Sillenite-Type Bi12SiO20.

The "controlled" synthesis of metastable γ-Bi2O3 by solution based approaches was reported several times recently, but the formation of Bi12SiO20 in the presence of trace amounts of silicates renders the results to be questionable. Here, the preparation of the Sillenite γ-Bi2O3 and the Sillenite-type Bi12SiO20 starting from the polynuclear bismuth oxido cluster [Bi38O45(O2CC3H5)24(DMSO)9] is reported. γ-Bi2O3 crystallizes after calcination at 800 °C of the silicate-free hydrolysis product "[Bi38O45(OH)24]" on a silver sheet. Corrosion of the substrate causes contamination with silver, which is not incorporated into the Bi-O lattice, and was removed by treatment with an aqueous KCN-solution. Bi12SiO20 was obtained after hydrothermal treatment of the bismuth oxido cluster in the presence of NaOH in glass vessels or Na2SiO3 in a Teflon-lined reactor vessel followed by calcination at 600 °C. PXRD studies, scanning electron microscopy, nitrogen adsorption measurements, IR- and Raman spectroscopy, diffuse UV-vis spectroscopy, and DSC were used for characterization. The phase transition of γ-Bi2O3 to give α-Bi2O3 occurred slowly in the temperature range of 348-510 °C ( Δ Hγ→α = 6.57 kJ·mol-1). The silver-containing γ-Bi2O3 exhibits slightly increased Raman modes compared to the silver-free sample due to the SERS effect. In the diffuse UV-vis spectrum γ-Bi2O3 exhibits an absorption edge at λ = 485 nm ( E g = 2.76 eV), and the contamination with silver results in an additional absorption edge at λ = 572 nm. Silver-free γ-Bi2O3 exhibits an absorption edge at λ = 460 nm ( E g = 2.83 eV) and Bi12SiO20 at λ = 422 nm ( E g = 3.16 eV). The photocatalytic activity of the compounds was investigated in the decomposition of aqueous rhodamine B under visible light irradiation, showing silver-containing γ-Bi2O3 to be slightly more effective compared to Bi12SiO20 and significantly more effective than the silver-free γ-Bi2O3.