Cerium(IV) Hexanuclear Clusters from Cerium(III) Precursors: Molecular Models for Oxidative Growth of Ceria Nanoparticles

Isolation of a chloride-capped cerium polyoxo nanocluster built from 52 metal ions

Four cerium compounds - (HPy)2[CeCl6]·2(HPyCl) (Ce1-1), (HPy)2[CeCl6] (Ce1-2), (HPy)m[Ce38O56-x(OH)xCl50(H2O)12]·nH2O (Ce38), and (HPy)m[Ce52O80-x(OH)xCl59(H2O)17]·nH2O (Ce52) - were crystallized from acidic aqueous solutions using pyridinium (HPy) counterions. The latter consists of two unique cerium oxide nanoclusters that are built from 52 metal ions and represents the largest chloride capped {CeIII/IVO} and/or {MIVO} (M = Ce, Th, U, Np, Pu) nanocluster that adopts the fluorite-type structure of MO2 that has been reported.

Expanding the Coordination of f-Block Metals with Tris[2-(2-methoxyethoxy)ethyl]amine: From Molecular Complexes to Cage-like Structures

Low-valent f-block metals have intrinsic luminescence, electrochemical, and magnetic properties that are modulated with ligands, causing the coordination chemistry of these metals to be imperative to generating critical insights needed to impact modern applications. To this end, we synthesized and characterized a series of twenty-seven complexes of f-metal ions including EuII, YbII, SmII, and UIII and hexanuclear clusters of LaIII and CeIII to study the impact of tris[2-(2-methoxyethoxy)ethyl]amine, a flexible acyclic analogue of the extensively studied 2.2.2-cryptand, on the coordination chemistry and photophysical properties of low-valent f-block metals. We demonstrate that the flexibility of the ligand enables luminescence tunability over a greater range than analogous cryptates of EuII in solution. Furthermore, the ligand also displays a variety of binding modes to f-block metals in the solid state that are inaccessible to cryptates of low-valent f-block metals. In addition to serving as a ligand for f-block metals of various sizes and oxidation states, tris[2-(2-methoxyethoxy)ethyl]amine also deprotonates water molecules coordinated to trivalent triflate salts of f-block metal ions, enabling the isolation of hexanuclear clusters containing either LaIII or CeIII. The ligand was also found to bind more tightly to YbII and UIII in the solid state compared to 2.2.2-cryptand, suggesting that it can play a role in the isolation of other low-valent f-block metals such CfII, NpIII, and PuIII. We expect that our findings will inspire applications of tris[2-(2-methoxyethoxy)ethyl]amine in the design of light-emitting diodes and the synthesis of extremely reducing divalent f-block metal complexes that are of interest for a wide range of applications.

Synthesis and Characterization of Cerium-Oxo Clusters Capped by Acetylacetonate

Cerium-oxo clusters have applications in fields ranging from catalysis to electronics and also hold the potential to inform on aspects of actinide chemistry. Toward this end, a cerium-acetylacetonate (acac1-) monomeric molecule, Ce(acac)4 (Ce-1), and two acac1--decorated cerium-oxo clusters, [Ce10O8(acac)14(CH3O)6(CH3OH)2]·10.5MeOH (Ce-10) and [Ce12O12(OH)4(acac)16(CH3COO)2]·6(CH3CN) (Ce-12), were prepared and structurally characterized. The Ce(acac)4 monomer contains CeIV. Crystallographic data and bond valence summation values for the Ce-10 and Ce-12 clusters are consistent with both clusters having a mixture of CeIII and CeIV cations. Ce L3-edge X-ray absorption spectroscopy, performed on Ce-10, showed contributions from both CeIII and CeIV. The Ce-10 cluster is built from a hexameric cluster, with six CeIV sites, that is capped by two dimeric CeIII units. By comparison, Ce-12, which formed upon dissolution of Ce-10 in acetonitrile, consists of a central decamer built from edge sharing CeIV hexameric units, and two monomeric CeIII sites that are bound on the outer corners of the inner Ce10 core. Electrospray ionization mass spectrometry data for solutions prepared by dissolving Ce-10 in acetonitrile showed that the major ions could be attributed to Ce10 clusters that differed primarily in the number of acac1-, OH1-, MeO1-, and O2- ligands. Small angle X-ray scattering measurements for Ce-10 dissolved in acetonitrile showed structural units slightly larger than either Ce10 or Ce12 in solution, likely due to aggregation. Taken together, these results suggest that the acetylacetonate supported clusters can support diverse solution-phase speciation in organic solutions that could lead to stabilization of higher order cerium containing clusters, such as cluster sizes that are greater than the Ce10 and Ce12 reported herein.

Synthesis and structures of cobalt-expanded zirconium- and cerium-oxo clusters as precursors for mixed-metal oxide thin films

Transforming current complementary metal-oxide-semiconductor (CMOS) technology to fabricate memory chips and microprocessors into environmentally friendlier electronics requires the development of new approaches to resource- and energy-efficient electron transport and switching materials. Metal and multi-metal oxide layers play a key role in high-end technical applications. However, these layers are commonly produced through high-energy and high-temperature procedures. Herein, we demonstrate our first attempts to obtain stimuli-responsive mixed-metal oxide thin films from solution-processed molecular precursors under milder conditions. The molecular compounds of interest were prepared by one-pot reactions of a CoII carboxylate complex, triethylamine (Et3N), N-butyldiethanolamine (H2bda), and a hexanuclear complex [Ce6O4(OH)4(piv)12] (Hpiv = pivalic acid) or [Zr6O4(OH)4(ib)12(H2O)]·3Hib (Hib = isobutyric acid) in acetonitrile solution. The resulting charge-neutral, heterometallic coordination compounds display a ligand-supported pentanuclear {CeIV3CoIII2} core (in 1) and a dodecanuclear {ZrIV6CoII6} core (in 2), exhibiting thermal stability up to ca. 100 °C in air. Compound 2 was deposited and analyzed on Au(111) and SiO2/Si(100) surfaces to explore its potential as a single-molecule precursor for the preparation of atomically precise, complex mixed-metal oxide thin films. The adsorption characteristics of it demonstrate the ability to form stable agglomerates on the investigated surfaces.

Siloxide tripodal ligands as a scaffold for stabilizing lanthanides in the +4 oxidation state

Synthetic strategies to isolate molecular complexes of lanthanides, other than cerium, in the +4 oxidation state remain elusive, with only four complexes of Tb(iv) isolated so far. Herein, we present a new approach for the stabilization of Tb(iv) using a siloxide tripodal trianionic ligand, which allows the control of unwanted ligand rearrangements, while tuning the Ln(iii)/Ln(iv) redox-couple. The Ln(iii) complexes, [LnIII((OSiPh2Ar)3-arene)(THF)3] (1-LnPh) and [K(toluene){LnIII((OSiPh2Ar)3-arene)(OSiPh3)}] (2-LnPh) (Ln = Ce, Tb, Pr), of the (HOSiPh2Ar)3-arene ligand were prepared. The redox properties of these complexes were compared to those of the Ln(iii) analogue complexes, [LnIII((OSi(OtBu)2Ar)3-arene)(THF)] (1-LnOtBu) and [K(THF)6][LnIII((OSi(OtBu)2Ar)3-arene)(OSiPh3)] (2-LnOtBu) (Ln = Ce, Tb), of the less electron-donating siloxide trianionic ligand, (HOSi(OtBu)2Ar)3-arene. The cyclic voltammetry studies showed a cathodic shift in the oxidation potential for the cerium and terbium complexes of the more electron-donating phenyl substituted scaffold (1-LnPh) compared to those of the tert-butoxy (1-LnOtBu) ligand. Furthermore, the addition of the -OSiPh3 ligand further shifts the potential cathodically, making the Ln(iv) ion even more accessible. Notably, the Ce(iv) complexes, [CeIV((OSi(OtBu)2Ar)3-arene)(OSiPh3)] (3-CeOtBu) and [CeIV((OSiPh2Ar)3-arene)(OSiPh3)(THF)2] (3-CePh), were prepared by chemical oxidation of the Ce(iii) analogues. Chemical oxidation of the Tb(iii) and Pr(iii) complexes (2-LnPh) was also possible, in which the Tb(iv) complex, [TbIV((OSiPh2Ar)3-arene)(OSiPh3)(MeCN)2] (3-TbPh), was isolated and crystallographically characterized, yielding the first example of a Tb(iv) supported by a polydentate ligand. The versatility and robustness of these siloxide arene-anchored platforms will allow further development in the isolation of more oxidizing Ln(iv) ions, widening the breadth of high-valent Ln chemistry.

Isolation and redox reactivity of cerium complexes in four redox states

The chemistry of lanthanides is limited to one electron transfer reactions due to the difficulty of accessing multiple oxidation states. Here we report that a redox-active ligand combining three siloxides with an arene ring in a tripodal ligand can stabilize cerium complexes in four different redox states and can promote multielectron redox reactivity in cerium complexes. Ce(iii) and Ce(iv) complexes [(LO₃)Ce(THF)] (1) and [(LO₃)CeCl] (2) (LO₃ = 1,3,5-(2-OSi(O^tBu)₂C₆H₄)₃C₆H₃) were synthesized and fully characterized. Remarkably the one-electron reduction and the unprecedented two-electron reduction of the tripodal Ce(iii) complex are easily achieved to yield reduced complexes [K(2.2.2-cryptand)][(LO₃)Ce(THF)] (3) and [K₂{(LO₃)Ce(Et₂O)₃}] (5) that are formally "Ce(ii)" and "Ce(i)" analogues. Structural analysis, UV and EPR spectroscopy and computational studies indicate that in 3 the cerium oxidation state is in between +II and +III with a partially reduced arene. In 5 the arene is doubly reduced, but the removal of potassium results in a redistribution of electrons on the metal. The electrons in both 3 and 5 are stored onto δ-bonds allowing the reduced complexes to be described as masked "Ce(ii)" and "Ce(i)". Preliminary reactivity studies show that these complexes act as masked Ce(ii) and Ce(i) in redox reactions with oxidizing substrates such as Ag⁺, CO₂, I₂ and S₈ effecting both one- and two-electron transfers that are not accessible in classical cerium chemistry.

Interfacial Unit-Dependent Catalytic Activity for CO Oxidation over Cerium Oxysulfate Cluster Assemblies

Atomically precise cerium oxo clusters offer a platform to investigate structure-property relationships that are much more complex in the ill-defined bulk material cerium dioxide. We investigated the activity of the MCe70 torus family (M = Cd, Ce, Co, Cu, Fe, Ni, and Zn), a family of discrete oxysulfate-based Ce70 rings linked by monomeric cation units, for CO oxidation. CuCe70 emerged as the best performing MCe70 catalyst among those tested, prompting our exploration of the role of the interfacial unit on catalytic activity. Temperature-programmed reduction (TPR) studies of the catalysts indicated a lower temperature reduction in CuCe70 as compared to CeCe70. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) indicated that CuCe70 exhibited a faster formation of Ce3+ and contained CO bridging sites absent in CeCe70. Isothermal CO adsorption measurements demonstrated a greater uptake of CO by CuCe70 as compared to CeCe70. The calculated energies for the formation of a single oxygen defect in the structure significantly decreased with the presence of Cu at the linkage site as opposed to Ce. This study revealed that atomic-level changes in the interfacial unit can change the reducibility, CO binding/uptake, and oxygen vacancy defect formation energetics in the MCe70 family to thus tune their catalytic activity.

MnII/III and CeIII/IV Units Supported on an Octahedral Molecular Nanoparticle of CeO2

The preparation of three new heterometallic clusters [Ce₆Mn₁₂O₁₇(O₂CPh)₂₆] (1), [Ce₁₀Mn₁₄O₂₄(O₂CPh)₃₂] (2), and [Ce₂₃Mn₂₀O₄₈(OH)₂(tbb)₄₆(H₂O)₄](NO₃)₂ (3; tbb^- = 4-^tBu-benzoate) is reported. They all possess unprecedented structures with a common feature being the presence of an octahedral Ce^IV-oxo core: a Ce₆ in 1, two edge-fused Ce₆ giving a Ce₁₀ bioctahedron in 2, or a larger Ce₁₉ octahedron in 3. Complex 1 is the first Ce₆ cluster with a central μ₆-O^2-. 2 and the cation of 3 are molecular nanoparticles of CeO₂ (ceria) because they possess the fluorite structure of bulk ceria and are thus ultrasmall ceria nanoparticles in molecular form. The {Ce₁₉O₃₂} octahedral subunit of the cation of 3 had been predicted from density functional theory studies to be one of the stable fragments of the CeO₂ lattice, but has never been previously synthesized in molecular chemistry. Around the Ce/O core of 1-3 is an incomplete monolayer of Mn_n ions disposed as four Mn₃, two Mn₇, and four Mn₅ units, respectively. This represents a clear structural similarity with composite (phase-separated) CeO₂/MnO_x mixtures where at high Ce:Mn ratios the Mn atoms segregate on the surface of CeO₂ phases. Variable-temperature dc and ac magnetic susceptibility studies have revealed S = 2, S = ¹/₂, and S = ³/₂ ground states for 1-3, respectively. Fitting of the 5.0-300 K dc data for 1 to a two-J model for an asymmetrical V-shaped Mn₃ unit with no interaction between the end Mn^III ions gave an excellent fit with the following values: J₁ = 5.2(3) cm^-1, J₂ = -7.4(3) cm^-1, and g = 1.96(2).

Reactivity of a Chloride Decorated, Mixed Valent CeIII/IV38-Oxo Cluster

A cerium-oxo nanocluster capped by chloride ligands, [Ce^IV_38-nCe^III_nO_56-(n+1)(OH)_n+1Cl₅₁(H₂O)₁₁]^10- (n = 1-24), has been isolated from acidic chloride solutions by using potassium counterions. The crystal structure was elucidated using single crystal X-ray diffraction. At the center of the cluster is a {Ce₁₄} core that exhibits the same fluorite-type structure as bulk CeO₂, with eight-coordinate Ce sites bridged by tetrahedral oxo anions. The {Ce₁₄} is further surrounded by a peripheral shell of six tetranuclear {Ce₄} subunits that are located on each of the faces of the core to yield the {Ce₃₈} cluster. The surface of the cluster is capped by 51 bridging/terminal chloride ligands and 11 water molecules; the anionic cluster is charge balanced by potassium counterions that exist in the outer coordination sphere. While assignment of the Ce oxidation state by bond valence summation was ambiguous, Ce L₃-edge X-ray absorption, X-ray photoelectron, and UV-vis-NIR absorption results were consistent with a Ce^III/Ce^IV cluster. Systematic changes in the XANES and UV-vis-NIR absorption spectra over time pointed to reactivity of the cluster upon exposure to air. These changes were examined using single crystal X-ray diffraction, and a clear single-crystal-to-single-crystal transformation was captured; an overall loss of surface-bound chlorides and water molecules as well as new μ₂-OH sites was observed on the cluster surface. This work provides a rare snapshot of metal oxide cluster reactivity. The results may hold implications for understanding the physical and chemical properties of ceria nanoparticles and provide insight into the behavior of other metal-oxo clusters of significant technological and environmental interest.

Hydrolysis-Promoted Building Block Assembly: Structure Transformation from Wheel and Ship to Cage

Accurately controlling the hydrolysis of metal ions can not only yield the desired structure of metal hydroxide clusters but also provide a deeper understanding of the formation process of natural hydroxide minerals. However, the capture of hydrolysis intermediates remains a significant challenge, and metal hydroxide clusters are mainly obtained by employing adventitious hydrolysis. In this study, we realized a hierarchical building block assembly from Y³⁺ ions to large Y₁₂, Y₃₄, and Y₆₀ clusters by controlling the hydrolysis process of lanthanide ions under different pH conditions. Single-crystal structural analysis showed that the Y₁₂ wheel, Y₃₄ ship, and Y₆₀ sodalite cage contain 4, 12, and 24 cubane-like [Y₄(μ₃-OH)₄]⁸⁺ units, respectively. The structure of the Y₆₀ cluster can be attributed to two Y₃₄ clusters or six Y₁₂ clusters linked by vertices. These clusters can be synthesized through the hydrolysis of Y³⁺ under different pH conditions, and Y₆₀ can be prepared from the obtained Y₁₂ or Y₃₄ crystals by the simple addition of Y³⁺ ions. The capture and conversion of the intermediates of lanthanide series hydroxide clusters, Y₁₂ or Y₃₄, during the assembly from Y³⁺ ions to Y₆₀ can facilitate an understanding of the formation process of high-nuclearity lanthanide clusters.

Phosphorus-based ligand effects on the structure and radical scavenging ability of molecular nanoparticles of CeO2

Two new Ce^IV/O^2- clusters, (pyH)₈[Ce₁₀O₄(OH)₄(O₃PPh)₁₂(NO₃)₁₂] (1) and [Ce₆O₄(OH)₄(O₂PPh₂)₄(O₂C^tBu)₈] (2), have been prepared that contain P-based ligands for the first time. They were obtained from the reaction of (NH₄)₂[Ce(NO₃)₆], PhPO₃H₂ or Ph₂PO₂H, and ^tBuCO₂H in a 2 : 1 : 2 molar ratio in pyridine/MeOH (10 : 1 mL). Both compounds contain a {Ce₆O₄(OH)₄} face-capped octahedral core, with 1 containing an additional four Ce^IV on the outside to give a supertetrahedral Ce₁₀ topology; the {Ce₆O₈} unit is the smallest recognizable fragment of the fluorite structure of CeO₂. The HO˙ radical scavenging activities of 1 and 2 were measured by UV/vis spectral monitoring of methylene blue oxidation by HO˙ radicals in the presence and absence of the Ce/O clusters, and the results compared with those for larger Ce₂₄ and Ce₃₈ molecular nanoparticles of CeO₂ prepared in previous work. 1 and 2 are both very poor HO˙ radical scavengers compared with Ce₂₄ and Ce₃₈, a result that is consistent with reports in the literature that PO₄^3- ions inhibit the radical scavenging ability of traditional CeO₂ nanoparticles and putatively assigned to PO₄^3- binding to the surface.

CeIV 70 Oxosulfate Rings, Frameworks, Supramolecular Assembly, and Redox Activity*

M^IV molecular oxo-clusters (M=Zr, Hf, Ce, Th, U, Np, Pu) are prolific in bottoms-up material design, catalysis, and elucidating reaction pathways in nature and in synthesis. Here we introduce Ce₇₀ , a wheel-shaped oxo-cluster, [Ce^IV ₇₀ (OH)₃₆ (O)₆₄ (SO₄ )₆₀ (H₂ O)₁₀ ]^4- . Ce₇₀ crystallizes into intricate high pore volume frameworks with divalent transition metals and Ce-monomer linkers. Eight crystal-structures feature four framework types in which the Ce₇₀ -rings are linked as propellers, in offset-stacks, in a tartan pattern, and as isolated rings. Small-angle X-ray scattering of Ce₇₀ dissolved in butylamine, with and without added cations (Ce^IV , alkaline earths, Mn^II ), shows the metals' differentiating roles in ring linking, leading to supramolecular assemblies. The large acidic pores and abundant terminal sulfates provide ion-exchange behavior, demonstrated with U^IV and Nd^III . Frameworks featuring Ce^III/IV -monomer linkers demonstrate both oxidation and reduction. This study opens the door to mixed-metal, highly porous framework catalysts, and new clusters for metal-organic framework design.

The chemistry and applications of hafnium and cerium(iv) metal-organic frameworks

The coordination connection of organic linkers to the metal clusters leads to the formation of metal-organic frameworks (MOFs), where the metal clusters and ligands are spatially entangled in a periodic manner. The immense availability of tuneable ligands of different length and functionalities gives rise to robust molecular porosity ranging from several angstroms to nanometres. Among the large family of MOFs, hafnium (Hf) based MOFs have been demonstrated to be highly promising for practical applications due to their unique and outstanding characteristics such as chemical, thermal, and mechanical stability, and acidic nature. Since the report of UiO-66(Hf) and DUT-51(Hf) in 2012, less than 200 Hf-MOFs (ca. 50 types of structures) have been reported. Besides, tetravalent cerium [Ce(iv)] has been proven to be capable of forming similar topological MOF structures to Zr and Hf since its first discovery in 2015. So far, ca. 40 Ce(iv) MOFs with 60% having UiO-66-type structure have been reported. This review will offer a holistic summary of the chemistry, uniqueness, synthesis, and applications of Hf/Ce(iv)-MOFs with a focus on presenting the development in the Hf/Ce(iv)-clusters, topologies, ligand structures, synthetic strategies, and practical applications of Hf/Ce(iv)-MOFs. In the end, we will present the research outlook for the development of Hf/Ce(iv)-MOFs in the future, including fundamental design of Hf/Ce(iv)-clusters, defect engineering, and various applications including membrane development, diversified types of catalytic reactions, irradiation absorption in nuclear waste treatment, water production and wastewater treatment, etc. We will also present the emerging computational approaches coupled with machine-learning algorithms that can be applied in screening Hf and Ce(iv) based MOF structures and identifying the best-performing MOFs for tailor-made applications in future practice.

Bridging the Transuranics with Uranium(IV) Sulfate Aqueous Species and Solid Phases

Isolating isomorphic compounds of tetravalent actinides (i.e., Th^IV, U^IV, Np^IV, and Pu^IV) improve our understanding of the bonding behavior across the series, in addition to their relationship with tetravalent transition metals (Zr and Hf) and lanthanides (Ce). Similarities between these tetravalent metals are particularly illuminated in their hydrolysis and condensation behavior in aqueous systems, leading to polynuclear clusters typified by the hexamer [M^IV₆O₄(OH)₄]¹²⁺ building block. Prior studies have shown the predominance and coexistence of smaller species for Th^IV (monomers, dimers, and hexamers) and larger species for U^IV, Np^IV, and Pu^IV (including 38-mers and 70-mers). We show here that aqueous uranium(IV) sulfate also displays behavior similar to that of Th^IV (and Zr^IV) in its isolated solid-phase and solution speciation. Two single-crystal X-ray structures are described: a dihydroxide-bridged dimer (U₂) formulated as U₂(OH)₂(SO₄)₃(H₂O)₄ and a monomer-linked hexamer framework (U-U₆) as (U(H₂O)_3.5)₂U₆O₄(OH)₄(SO₄)₁₀(H₂O)₉. These structures are similar to those previously described for Th^IV. Moreover, cocrystallization of monomer and dimer and of dimer and monomer-hexamer phases for both Th^IV (prior) and U^IV (current) indicates the coexistence of these species in solution. Because it was not possible to effectively study the sulfate-rich solutions via X-ray scattering from which U₂ and U-U₆ crystallized, we provide a parallel solution speciation study in low sulfate conditions, as a function of the pH. Raman spectroscopy, UV-vis spectroscopy, and small-angle X-ray scattering of these show decreasing sulfate binding, increased hydrolysis, increased species size, and increased complexity, with increasing pH. This study describes a bridge across the first half the actinide series, highlighting U^IV similarities to Th^IV, in addition to the previously known similarities to the transuranic elements.

Molecular nanoparticles of cerium dioxide: structure-directing effect of halide ions

The use of halide ions in the synthesis of Ce/O clusters diverts the reaction to two halide-containing products: Cl- gives a new Ce20 nuclearity with both a high 1 : 1 Ce3+ : Ce4+ ratio and a high percentage of (100) facet coverage, whereas F- gives a known Ce6 nuclearity. Both products include bridging halide ions and are thus the first confirmation of non-oxo (OH-/O2-) anion incorporation onto the Ce/O cluster core.

From a Cerium-Doped Polynuclear Bismuth Oxido Cluster to β-Bi2O3:Ce

The simultaneous hydrolysis of Bi(NO₃)₃·5H₂O and Ce(NO₃)₃·6H₂O results in the formation of novel heterometallic bismuth oxido clusters with the general formula [Bi₃₈O₄₅(NO₃)₂₄(DMSO)_28+δ]:Ce (DMSO = dimethyl sulfoxide; cerium content <1.50%), which is demonstrated by single-crystal X-ray diffraction analysis. The incorporation of cerium into the cluster core is a result of the interplay of hydrolysis and condensation of the metal nitrates in the presence of oxygen. Diffuse-reflectance UV-vis and X-ray photoelectron spectroscopy reveal the presence of Ce^IV in the final bismuth oxido clusters as a result of oxidation of the cerium source. The cerium atoms are statistically distributed mainly on the bismuth atom positions of the central [Bi₆O₉] motif of the [Bi₃₈O₄₅] cluster core. Hydrolysis and subsequent annealing of the bismuth oxido clusters in the temperature range of 300-400 °C provides β-Bi₂O₃:Ce samples with slightly lowered band gaps of approximately 2.3 eV compared to the undoped β-Bi₂O₃ (approximately 2.4 eV). The sintering behavior of β-Bi₂O₃ is significantly affected by the cerium dopant. Finally, differences in the efficiency of the as-prepared β-Bi₂O₃:Ce and undoped β-Bi₂O₃ samples in the photocatalytic decomposition of the biocide triclosan in an aqueous solution under visible-light irradiation are demonstrated.

Cerium(IV) Carboxylate Photocatalyst for Catalytic Radical Formation from Carboxylic Acids: Decarboxylative Oxygenation of Aliphatic Carboxylic Acids and Lactonization of Aromatic Carboxylic Acids

We found that in situ generated cerium(IV) carboxylate generated by mixing the precursor Ce(O^tBu)₄ with the corresponding carboxylic acids served as efficient photocatalysts for the direct formation of carboxyl radicals from carboxylic acids under blue light-emitting diodes (blue LEDs) irradiation and air, resulting in catalytic decarboxylative oxygenation of aliphatic carboxylic acids to give C-O bond-forming products such as aldehydes and ketones. Control experiments revealed that hexanuclear Ce(IV) carboxylate clusters initially formed in the reaction mixture and the ligand-to-metal charge transfer nature of the Ce(IV) carboxylate clusters was responsible for the high catalytic performance to transform the carboxylate ligands to the carboxyl radical. In addition, the Ce(IV) carboxylate cluster catalyzed direct lactonization of 2-isopropylbenzoic acid to produce the corresponding peroxy lactone and γ-lactone via intramolecular 1,5-hydrogen atom transfer (1,5-HAT).

Ce-MIL-140: expanding the synthesis routes for cerium(iv) metal-organic frameworks

A microwave-assisted synthesis method for Ce(iv)-based MOFs crystallizing in the MIL-140 structure has been developed. Three different linker molecules, i.e. terephthalic acid (H2BDC), 2-chloroterephthalic acid (H2BDC-Cl) and 2,6-naphtalenedicarboxylic acid (H2NDC) that have previously been used for the synthesis of Ce-UiO-66 which contains hexanuclear Ce-O clusters as the inorganic building unit (IBU), were employed. Under solvothermal reaction conditions (140 °C) with acetonitrile as the solvent the compounds Ce-MIL-140-BDC, -BDC-Cl and -NDC, with the general composition [CeO(linker)] were obtained as microcrystalline products. For all three MOFs an extended purification process had to be carried out. The MOFs were fully characterized and the structure of Ce-MIL-140-BDC was refined against PXRD data using the Rietveld method. In contrast to Zr-MIL-140-BDC a symmetry reduction to the space group P1[combining macron] is observed. The MIL-140 structure type is built up by infinite CeO7 polyhedra that are interconnected by dicarboxylate ions to generate 1D pores. For Ce-MIL-140-BDC the highest specific surface area of asBET = 222 m2 g-1 is observed and the MOF is thermally stable up to 370 °C. This new synthetic route to Ce(iv)-MOFs avoids the formation of the previously extremely dominant hexanuclear IBU, and paves the way for higher IBU diversity in Ce(iv)-MOFs.

The chemistry of Ce-based metal-organic frameworks

Metal-organic frameworks (MOFs) have gained widespread attention due to their modular construction that allows the tuning of their properties. Within this vast class of compounds, metal carboxylates containing tri- and tetravalent metal ions have been in the focus of many studies due to their often high thermal and chemical stabilities. Cerium has a rich chemistry, which depends strongly on its oxidation state. Ce(iii) exhibits properties typically observed for rare earth elements, while Ce(iv) is mostly known for its oxidation behaviour. In MOF chemistry this is reflected in their unique optical and catalytic properties. The synthetic parameters for Ce(iii)- and Ce(iv)-MOFs also differ substantially and conditions must be chosen to prevent reduction of Ce(iv) for the formation of the latter. Ce(iii)-MOFs are usually reported in comprehensive studies together with those constructed with other RE elements and normally they are isostructural. They exhibit a greater structural diversity, which is reflected in the larger variety of inorganic building units. In contrast, the synthesis conditions of Ce(iv)-MOFs were only recently (2015) established. These lead selectively to hexanuclear Ce-O clusters that are well-known for Zr-MOFs and therefore very similar structural and isoreticluar chemistry is found. Hence Ce(iv)-MOFs exhibit often high porosity, while only a few porous Ce(iii)-MOFs have been described. Some of these show structural flexibility which makes them interesting for separation processes. For Ce(iv)-MOFs the redox properties are most relevant. Thus, they are intensively discussed for catalytic, photocatalytic and sensing applications. In this perspective, the synthesis, structural chemistry and properties of Ce-MOFs are summarized.

Unravelling a long-lived ligand-to-metal cluster charge transfer state in Ce–TCPP metal organic frameworks

We report the ultrafast charge separation dynamics in porphyrin-based Ce–TCPP MOFs using optical and X-ray transient absorption (XTA) spectroscopy.

A precursor method for the synthesis of new Ce(iv) MOFs with reactive tetracarboxylate linkers

A precursor method has been developed to synthesize Ce(iv) MOFs that could not be prepared directly from Ce(iv) salts. Starting from Ce₆ clusters, two Ce-UiO-66 analogues and four tetracarboxylate-based Ce(iv) MOFs could be synthesized. The applied method facilitates framework formation by evading reactive individual Ce(iv)-ions thereby paving the way for further development of Ce-MOFs.

Atomically-precise colloidal nanoparticles of cerium dioxide

Synthesis of truly monodisperse nanoparticles and their structural characterization to atomic precision are important challenges in nanoscience. Success has recently been achieved for metal nanoparticles, particularly Au, with diameters up to 3 nm, the size regime referred to as nanoclusters. In contrast, families of atomically precise metal oxide nanoparticles are currently lacking, but would have a major impact since metal oxides are of widespread importance for their magnetic, catalytic and other properties. One such material is colloidal CeO₂ (ceria), whose applications include catalysis, new energy technologies, photochemistry, and medicine, among others. Here we report a family of atomically precise ceria nanoclusters with ultra-small dimensions up to ~1.6 nm (~100 core atoms). X-ray crystallography confirms they have the fluorite structure of bulk CeO₂, and identifies surface features, H⁺ binding sites, Ce³⁺ locations, and O vacancies on (100) facets. Monodisperse ceria nanoclusters now permit investigation of their properties as a function of exact size, surface morphology, and Ce³⁺:Ce⁴⁺ composition.

A [Ce21] keplerate

The solvothermal reaction between Ce(NO₃)₃·6H₂O, 2-amino-isobutyric acid, 2-hydroxy-1-naphthaldehyde and 2-amino-2-methyl-1,3-propanediol in MeOH, in the presence of base, leads to the formation of a unique [CeCe ] keplerate cage.

Cerium(IV) Imido Complexes: Structural, Computational, and Reactivity Studies

A series of alkali metal capped cerium(IV) imido complexes, [M(solv)_x][Ce═N(3,5-(CF₃)₂C₆H₃)(TriNOx)] (M = Li, K, Rb, Cs; solv = TMEDA, THF, Et₂O, or DME), was isolated and fully characterized. An X-ray structural investigation of the cerium imido complexes demonstrated the impact of the alkali metal counterions on the geometry of the [Ce═N(3,5-(CF₃)₂C₆H₃)(TriNOx)]^- moiety. Substantial shortening of the Ce═N bond was observed with increasing size of the alkali metal cation. The first complex featuring an unsupported, terminal multiple bond between a Ce(IV) ion and a ligand fragment was also isolated by encapsulation of a Cs⁺ counterion with 2.2.2-cryptand. This complex shows the shortest recorded Ce═N bond length of 2.077(3) Å. Computational investigation of the cerium imido complexes using DFT methods showed a relatively larger contribution of the cerium 5d orbital than the 4f orbital to the Ce═N bonds. The [K(DME)₂][Ce═N(3,5-(CF₃)₂C₆H₃)(TriNOx)] complex cleaves the Si-O bond in (Me₃Si)₂O, yielding the [(Me₃SiO)Ce^IV(TriNOx)] adduct. The reaction of the rubidium capped imido complex with benzophenone resulted in the formation of a rare Ce(IV)-oxo complex, that was stabilized by a supramolecular, tetrameric oligomerization of the Ce═O units with rubidium cations.