Hybrid Uranium–Transition-Metal Oxide Cage Clusters

Electrocatalytic Hydrogen Evolution by a Uranium(VI) Polyoxometalate: an Environmental Toxin for Sustainable Energy Generation

The uranyl ion (UO2)2+, a uranium nuclear waste, is one of the serious contaminants in our ecosystem because of its radioactivity, relevant human activities, and highly mobile and complex nature of living cells. In this article, we have reported the synthesis and structural characterization of an uranyl cation-incorporated polyoxometalate (POM) compound, K10[{K4(H2O)6}{UO2}2(α-PW9O34)2]·13H2O (1), in which the uranyl cations are complexed with an in situ generated [α-PW9O34]9- cluster. Single-crystal X-ray diffraction (SCXRD) analysis of compound 1 reveals that the uranyl-potassium complex cationic species, [{K4(H2O)6}{UO2}2]8+, is sandwiched by two [α-PW9O34]9- clusters resulting in a Dawson type of POM. Compound 1 was further characterized by inductively coupled plasma optical emission spectroscopy (ICP-OES) analysis and infrared (IR), Raman, electronic absorption, and solid-state photoluminescence spectral studies. IR stretching vibrations at 895 and 856 cm-1 and the Raman signature peak at 792 cm-1 in the IR and Raman spectra of compound 1 primarily confirm the presence of a trans-[O═U═O]2+ ion. The solid-state photoluminescence spectrum of 1 exhibits a typical vibronic structure, resulting from symmetrical vibrations of [O═U═O]2+ bands, corresponding to the electronic transitions of S11 → S10 and S10 → S0υ (υ = 0-3). Interestingly, title compound 1 shows efficient electrocatalytic hydrogen evolution by water reduction with low Tafel slope values of 186.59 and 114.83 mV dec-1 at 1 mA cm-2 along with optimal Faradaic efficiency values of 82 and 87% at neutral pH and in acidic pH 3, respectively. Detailed electrochemical analyses reveal that the catalytic hydrogen evolution reaction (HER) activity mediated by compound 1 is associated with the UVI/UV redox couple of the POM. The microscopic as well as routine spectral analyses of postelectrode samples and controlled experiments have confirmed that compound 1 behaves like a true molecular electrocatalyst for the HER. To our knowledge, this is the first paradigm of a uranium-containing polyoxometalate that exhibits electrocatalytic water reduction to molecular H2. In a nutshell, an environmental toxin (a uranium-oxo compound) has been demonstrated to be utilized as an efficient electrocatalyst for hydrogen generation from water, a green approach of sustainable energy production.

Platonic and Archimedean solids in discrete metal-containing clusters

Metal-containing clusters have attracted increasing attention over the past 2-3 decades. This intense interest can be attributed to the fact that these discrete metal aggregates, whose atomically precise structures are resolved by single-crystal X-ray diffraction (SCXRD), often possess intriguing geometrical features (high symmetry, aesthetically pleasing shapes and architectures) and fascinating physical properties, providing invaluable opportunities for the intersection of different disciplines including chemistry, physics, mathematical geometry and materials science. In this review, we attempt to reinterpret and connect these fascinating clusters from the perspective of Platonic and Archimedean solid characteristics, focusing on highly symmetrical and complex metal-containing (metal = Al, Ti, V, Mo, W, U, Mn, Fe, Co, Ni, Pd, Pt, Cu, Ag, Au, lanthanoids (Ln), and actinoids) high-nuclearity clusters, including metal-oxo/hydroxide/chalcogenide clusters and metal clusters (with metal-metal binding) protected by surface organic ligands, such as thiolate, phosphine, alkynyl, carbonyl and nitrogen/oxygen donor ligands. Furthermore, we present the symmetrical beauty of metal cluster structures and the geometrical similarity of different types of clusters and provide a large number of examples to show how to accurately describe the metal clusters from the perspective of highly symmetrical polyhedra. Finally, knowledge and further insights into the design and synthesis of unknown metal clusters are put forward by summarizing these "star" molecules.

Cavity Formation in Uranyl Ion Complexes with Kemp's Tricarboxylate: Grooved Diperiodic Nets and Polynuclear Cages

Kemp's triacid (cis,cis-1,3,5-trimethylcyclohexane-1,3,5-tricarboxylic acid, H₃kta) was reacted with uranyl nitrate under solvo-hydrothermal conditions in the presence of diverse counterions or additional metal cations to give eight zero- or diperiodic complexes. All the coordination polymers in the series, [PPh₃Me][UO₂(kta)]·0.5H₂O (1), [PPh₄][UO₂(kta)] (2), [C(NH₂)₃][UO₂(kta)] (3), [Cd(bipy)₃][UO₂(kta)]₂ (4), and [Zn(phen)₃][UO₂(kta)]₂·2H₂O (5) (bipy = 2,2'-bipyridine, phen = 1,10-phenanthroline) crystallize as networks with the hcb topology, the ligand being in the chair conformation with the three carboxylate groups equatorial, except in 3, in which the axial/diequatorial boat conformation is present. Various degrees of corrugation and different arrangements of neighboring layers are observed depending on the counterion, with complexes 4 and 5, in particular, displaying cavities containing the bulky cations. [Co(en)₃]₂[(UO₂)₂(kta)(Hkta)₂]₂·2NMP·10H₂O (6) (en = 1,2-ethanediamine; NMP = N-methyl-2-pyrrolidone) contains a metallatricyclic, tetranuclear anionic species, displaying two clefts in which the cations are held by extensive hydrogen bonding, and with the ligands in both triaxial chair and axial/diequatorial boat conformations. [(UO₂)₃Pb(kta)₂(Hkta)(H₂O)]₂·1.5THF (7) (THF = tetrahydrofuran) and [(UO₂)₂Pb₂(kta)₂(Hkta)(NMP)]₂ (8) are two heterometallic cage compounds containing only the convergent, triaxial chair form of the ligand, which have the same topology in spite of the different U/Pb ratio. These complexes are compared to previous ones also involving Kemp's triacid anions, and the roles of ligand conformation and of counterions in the formation of cavities, either in cage-like species or as grooves in diperiodic networks, is discussed.

Trends and new directions in the crystal chemistry of actinide oxo-clusters incorporated in polyoxometalates

This highlight article reports the synthetic methods, the structural topologies and the behaviour of actinide-polyoxometalate species in aqueous solution.

Closed Uranyl-Dicarboxylate Oligomers: A Tetranuclear Metallatricycle with Uranyl Bridgeheads and 1,3-Adamantanediacetate Linkers

In the presence of NH₄⁺ and either PPh₄⁺ or PPh₃Me⁺ cations, 1,3-adamantanediacetic acid (H₂ADA) reacts with uranyl ions under solvo-hydrothermal conditions to give the complexes [NH₄]₂[PPh₄]₂[(UO₂)₄(ADA)₆] (1) and [NH₄]₂[PPh₃Me]₂[(UO₂)₄(ADA)₆] (2), both of which contain a tetranuclear metallatricycle built from two 2:2 rings including convergent ligands, linked by two additional ligands in an extended conformation defining a third, larger ring. While the ammonium cations are closely associated with the 2:2 rings through triple hydrogen bonding, the large PPh₄⁺ or PPh₃Me⁺ cations are more loosely bound to each of the two faces of the larger ring. In contrast, the complex [H₂NMe₂][PPh₃Me][(UO₂)₂(ADA)₃]·H₂O (3), in which dimethylammonium replaces ammonium cations, crystallizes as a two-dimensional network with honeycomb {6³} topology, albeit with very distorted, elongated hexagonal cells. These and previous results show that both NH₄⁺ and PPh₄⁺ or PPh₃Me⁺ cations are essential to the formation of the metallatricycle. The role of the flexibility imparted to ADA^2- by the acetate arms, in comparison to the more rigid 1,3-adamantanedicarboxylate (ADC^2-), is also discussed. All three complexes are luminescent, with quantum yields of 0.06, 0.06, and 0.09 for 1-3, respectively. The vibronic fine structure apparent on the emission spectra gives peak positions typical of species in which the uranyl ion is chelated by three carboxylate groups.

Captivation with encapsulation: a dozen years of exploring uranyl peroxide capsules

Uranyl peroxide cages are an extensive family of topologically varied self-assembling nanoscale clusters with fascinating properties and applications.

Dicyanoaurate-based heterobimetallic uranyl coordination polymers

The first series of uranyl ([UO₂]²⁺)-dicyanoaurate coordination polymers and molecular complexes has been synthesized. Reactions of [A][Au(CN)₂] (A = [ⁿBu₄N]⁺ or [(Ph₃P)₂N]⁺ ([PPN])) and uranyl nitrate in alcoholic solvents in ambient light led to [A]₂[(UO₂)₂(μ-η²:η²-O₂)(NO₃)₂(μ-Au(CN)₂)₂], which incorporates peroxo ligands into a one-dimensional ladder topology with alternating aurophilic and peroxo rungs. Conducting the reaction with non-alcoholic solvents formed two polymorphs of a one-dimensional chain, [PPN][UO₂(NO₃)₂Au(CN)₂], from acetone, and a molecular analogue, [PPN]₂[UO₂(NO₃)₂(Au(CN)₂)₂], from acetonitrile, none of which exhibited aurophilic interactions. The addition of 2,2'-bipyridine to the initial reaction resulted in [UO₂(bipy)(MeO)(MeOH)]₂[(μ-Au(CN)₂)(Au(CN)₂)], a one-dimensional structure which propagates via a series of linear aurophilic bonds with pendant uranyl complexes; methanol and methoxy ligands provide additional connections through hydrogen bonding. The addition of 5,5'-dimethyl-2,2'-bipyridine using solvothermal conditions resulted in the one-dimensional ladder [UO₂(Me₂bipy)Au(CN)₂]₂[(μ-OH)₂], generated through aurophilic bonds and hydroxide ligands. The incorporation of 2,2':6',2''-terpyridine (terpy) using solvothermal conditions resulted in [[UO₂(terpy)]₂(μ-NO₃)(μ-O)][Au(CN)₂], a molecular salt with no aurophilic interactions. Emission spectra attributable to aurophilic interactions are observed in [ⁿBu₄N]₂[(UO₂)₂(μ-η²:η²-O₂)(NO₃)₂(μ-Au(CN)₂)₂], while all others only show emission typical of the uranyl cation.

Insights into the structure and thermal stability of uranyl aluminate nanoparticles

Synthesis and characterization by HRTEM, XAFS and HTXRD of uranyl aluminate nanocrystals prepared with various uranium loadings and at different calcination temperatures.

Recent advances in structural studies of heterometallic uranyl-containing coordination polymers and polynuclear closed species

A survey is given of recent original structural results on heterometallic species incorporating uranyl ions, particularly with carboxylate ligands.

Hybrid Lanthanide-Actinide Peroxide Cage Clusters

A cage cluster consisting of 31 uranyl and 9 Sm(3+) polyhedra self-assembles in an alkaline aqueous peroxide solution and crystallizes (U31Sm9). Trimers of Sm(3+) polyhedra are templated by μ3-η(2):η(2):η(2)-peroxide groups and link to oxo atoms of uranyl ions. Three such trimers link into a ring through uranyl hexagonal bipyramids, and these are attached through six polyhedra to a unit consisting of 21 uranyl hexagonal bipyramids to complete the cage. Luminescence spectra collected with an excitation wavelength of 420 nm reveal fine structure, which is not observed for a cluster containing only uranyl polyhedra.

Structure and Solution Speciation of U(IV) Linked Phosphomolybdate (Mo(V)) Clusters

Crystals of [NaU(Mo6P4O31H7)2]·5Na·(H2O)n (NaUMo6) have been synthesized by slow evaporation of an aqueous mixture containing uranyl nitrate, sodium molybdate, phosphoric acid, and sodium dithionate. Single crystal diffraction of NaUMo6 reveals the assembly of {Mo6P4} clusters linked into one-dimensional chains with alternating Na(+) and U(4+) cations. To our knowledge, NaUMo6 is a unique example of Mo(5+) based polyoxometalate associated with actinides. With the use of similar synthesis conditions but without uranium in the aqueous solution, [Na(Mo6P4O31H10)2]·5Na·(H2PO4)·(H2O)n (NaMo6) is obtained. NaMo6 is a sandwich type cluster which is built on the assemblage of two {Mo6P4} units linked by one sodium cation. Using small-angle X-ray scattering techniques and aqueous electrolyte based dissolution strategies, we can accurately observe chains of [UNa(Mo6P4O31H7)2]n(5n), where n = 7 is the dominant soluble specie. Likewise, the dimeric form of [Na(Mo6P4O31H10)2](5-) dominates the aqueous solution, revealing the structural units observed in the crystal structure are also stable in solution, under appropriate dissolution conditions.

Hybrid uranyl–vanadium nano-wheels

Uranyl vanadate sulfate clusters isolated from a mixture of water and ionic liquid present a structure intermediate between those typical of transition metal polyoxometalates and uranyl peroxide cage clusters.