Oxo-Cluster-Based Zr/HfIV Separation: Shedding Light on a 70-Year-Old Process.

Zirconium and hafnium in the tetravalent oxidation state are considered the two most similar elements on the periodic table, based on their coexistence in nature and their identical solid-state chemistry. However, differentiating solution phase chemistry is crucial for their separation for nuclear applications that exploit the neutron capture of Hf and neutron transparency of Zr. Here we provide molecular level detail of the multiple factors that influence Zr/Hf separation in a long-exploited, empirically designed industrial solvent-extraction process that favors Hf extraction into an organic phase. In the aqueous solution, both Hf and Zr form an oxo-centered tetramer cluster with a core formula of [OM4(OH)6(NCS)12]4- (OM4-NCS, M = Hf, Zr). This was identified by single-crystal X-ray diffraction, as well as small-angle X-ray scattering (SAXS), of both the aqueous and organic phase. In addition to this phase, Zr also forms (1) a large oxo-cluster formulated [Zr48O30(OH)92(NCS)40(H2O)40] (Zr48) and (2) NCS adducts of OZr4-NCS. Zr48 was identified first by SAXS and then crystallized by exploiting favorable soft-metal bonding to the sulfur of NCS. While the large Zr48 likely cannot be extracted due to its larger size, the NCS adducts of OZr4-NCS are also less favorable to extraction due to the extra negative charge, which necessitates coextraction of an additional countercation (NH4+) per extra NCS ligand. Differentiating Zr and Hf coordination and hydrolysis chemistry adds to our growing understanding that these two elements, beyond simple solid-state chemistry, have notable differences in chemical reactivity.

Nb2O5, LiNbO3, and (Na, K)NbO3 Thin Films from High-Concentration Aqueous Nb-Polyoxometalates.

Synthesizing functional materials from water contributes to a sustainable energy future. On the atomic level, water drives complex metal hydrolysis/condensation/speciation, acid-base, ion pairing, and solvation reactions that ultimately direct material assembly pathways. Here, we demonstrate the importance of Nb-polyoxometalate (Nb-POM) speciation in enabling deposition of Nb2O5, LiNbO3, and (Na, K)NbO3 (KNN) from high-concentration solutions, up to 2.5 M Nb for Nb2O5 and ∼1 M Nb for LiNbO3 and KNN. Deposition of KNN from 1 M Nb concentration represents a potentially important advancment in lead-free piezoelectrics, an application that requires thick films. Solution characterization via small-angle X-ray scattering and Raman spectroscopy described the speciation for all precursor solutions as the [HxNb24O72](x-24) POM, as did total pair distribution function analyses of X-ray scattering of amorphous gels prior to conversion to oxides. The tendency of the Nb24-POM to form extended networks without crystallization leads to conformal and well-adhered films. The films were characterized by X-ray diffraction, atomic force microscopy, scanning electron microscopy, ellipsometry, and X-ray photoelectron spectroscopy. As a strategy to convert aqueous deposition solutions from {Nb10}-POMs to {Nb24}-POMs, we devised a general procedure to produce doped Nb2O5 thin films including Ca, Ag, and Cu doping.

Solvent-Driven Transformation of Zn/Cd2+-Deoxycholate Assemblies.

Deoxycholic acid (DOC) is a unique, biologically derived surfactant with facial amphiphilicity that has been exploited, albeit minimally, in supramolecular assembly of materials. Here, we present the synthesis and structural characterization of three hybrid metal (Zn2+ and Cd2+)-DOC compounds. Analysis by single-crystal X-ray diffraction reveals the many interactions that are possible between these facial surfactants and the influence of solvent molecules that drive the assembly of materials. These structures are the first metal-DOC complexes besides those obtained from alkali and alkaline earth metals. We isolated polymeric chains of both Cd and Zn (Znpoly-DOC and Cdpoly-DOC) from water. Major interactions between DOC molecules in these phases are hydrophobic in nature. Cdpoly-DOC exhibits unique P1 symmetry, with complete interdigitation of the amphiphiles between neighboring polymeric chains. Zn4-DOC, obtained from methanol dissolution of Znpoly-DOC, features the OZn4 tetrahedron, widely known in basic zinc acetate and MOF-5 (metal organic framework). We document a solvent-driven, room-temperature transition between Znpoly-DOC and Zn4-DOC (in both directions) by scanning and transmission electron microscopies in addition to small-angle X-ray scattering, powder X-ray diffraction, and infrared spectroscopy. These studies show the methanol-driven transition of Znpoly-DOC to Zn4-DOC occurs via an intermediate with no long-range order of the Zn4 clusters, indicating the strongest interactions driving assembly are intramolecular. On the contrary, water-driven solid-to-solid transformation from Zn4-DOC to Znpoly-DOC exhibits crystal-to-crystal transformation. Znpoly-DOC is robust, easy to synthesize, and comprised of biologically benign components, so we demonstrate dye absorption as a proxy for water treatment applications. It favors absorption of positively charged dyes. These studies advance molecular level knowledge of the supramolecular assembly of facial surfactants that can be exploited in the design of organic-inorganic hybrid materials. This work also highlights the potential of solvent for tuning supramolecular assembly processes, leading to new hybrid materials featuring facial surfactants.

Predicting the Solubility of Inorganic Ion Pairs in Water.

Polyoxometalates (POMs), ranging in size from 1 to 10's of nanometers, resemble building blocks of inorganic materials. Elucidating their complex solubility behavior with alkali-counterions can inform natural and synthetic aqueous processes. In the study of POMs ([Nb24 O72 H9 ]15- , Nb24 ) we discovered an unusual solubility trend (termed anomalous solubility) of alkali-POMs, in which Nb24 is most soluble with the smallest (Li+ ) and largest (Rb/Cs+ ) alkalis, and least soluble with Na/K+ . Via computation, we define a descriptor (σ-profile) and use an artificial neural network (ANN) to predict all three described alkali-anion solubility trends: amphoteric, normal (Li+ >Na+ >K+ >Rb+ >Cs+ ), and anomalous (Cs+ >Rb+ >K+ >Na+ >Li+ ). Testing predicted amphoteric solubility affirmed the accuracy of the descriptor, provided solution-phase snapshots of alkali-POM interactions, yielded a new POM formulated [Ti6 Nb14 O54 ]14- , and provides guidelines to exploit alkali-POM interactions for new POMs discovery.

Differentiating Zr/HfIV Aqueous Polyoxocation Chemistry with Peroxide Ligation.

This work complements our recent discovery of new phases derived from zirconium perchlorate by addition of hydrogen peroxide. Here, we investigate analogous reactions with hafnium perchlorate, which is found to have modifications of the Clearfield-Vaughan tetramer (CVT). For hafnium perchlorate derivatives, we find distorted versions of CVT by X-ray diffraction and study the reaction solutions by SAXS, Raman spectroscopy, and ESI-MS. Furthermore, we investigate mixed Hf-Zr solution and solid phases and find the latter resemble the zirconium family at low Hf concentrations and the hafnium family at higher hafnium contents.

Directional Bonding in Decaniobate Inorganic Frameworks.

Metal-oxo clusters offer an opportunity to assemble inorganic and metal-organic frameworks (MOFs) by a controlled building-block approach, which led to the revolutionary discoveries of zeolites and MOFs. Polyoxometalate clusters are soluble in water, but more challenging to link into frameworks; the inert oxo-caps that provide solubility are resistant to replacement or further connectivity. We demonstrate how the unique directional bonding and varying basicity of the decaniobate ([Nb10 ]) oxo-caps can be exploited to build 1D, 2D, and 3D inorganic frameworks. In nine structures, A+ (A=Li, Na, K, Rb and Cs), AE2+ (AE=Ca, Sr, Ba) and Mn2+ demonstrate that the dimensionality of the obtained material is controlled by cation charge and size. Increased cation charge decreases selectivity for oxo-site bonding, leading to higher dimensional linking. Larger cation radii also decreases bonding selectivity, yielding higher dimensional materials. Ion-exchange studies of the A+ -Nb10 family shows exclusive selectivity for Cs+ over other alkalis, which is important for radioactive Cs removal and sequestration.

Deliberate Construction of Polyoxoniobates Exploiting the Carbonate Ligand.

Polyoxometalates (POMs, metals=VV , NbV , TaV , MoVI , WVI ) are molecular metal oxides that can be isolated without capping ligands. The high negative charge of polyoxoniobates (PONb) provides strong interactions with heterocations, advantageous for electrostatic assembly of modular materials. In four single-crystal X-ray structures, we demonstrate that carbonate combined with the very reactive decaniobate [Nb10 O28 ]6- reassembles into a new decaniobate, [Nb10 O25 (CO3 )6 ]12- , featuring three carbonate-ligated Nb-polyhedra. These Nb-sites can be replaced by heterometals (lanthanides), and the tridentate carbonate can serve as an anchor point to build niobate-frameworks. Small-angle X-ray scattering and two additional X-ray structures reveal that the reaction pathway proceeds through a Nb24 -PONb intermediate, and the obtained PONb (with or without carbonate) is counterion, temperature, and solvent-dependent (water or mixed water-methanol). This provides an uncommon level of control for PONb chemistry.

Bismuth for Controlled Assembly/Disassembly of Transition-Metal Oxo Clusters, Defining Reaction Pathways in Inorganic Synthesis and Nature.

Trivalent bismuth is a unique heavy p-block ion. It is highly insoluble in water, due to strong hydrolysis tendencies, and known for low toxicity. Its lone pair is structure-directing, providing framework materials with structural flexibility, leading to piezoelectric and multiferroic function. The flexibility it provides is also advantageous for dopants and vacancies, giving rise to conductivity, luminescence, color, and catalytic properties. We are exploiting Bi3+ in a completely different way, as a knob to "tune" the solubility and stability of transition-metal oxo clusters. The lone pair allows capping and isolation of metastable cluster forms for solid-state and solution characterization. With controlled release of the bismuth (via bismuth oxyhalide metathesis), the metal oxo clusters can be retained in aqueous solution, and we can track their reaction pathways and conversion to related metal oxyhydroxides. Here we present isolation of a bismuth-stabilized MnIV cluster, fully formulated [MnIV6Bi2KO9(CH3COO)10(H2O)3(NO3)2] (Mn6Bi2). In addition to characterization by single-crystal X-ray diffraction, solution characterization in acetonitrile and acetonitrile-acetic acid by small-angle X-ray scattering (SAXS) and electrospray ionization mass spectrometry shows high stability and the tendency of Mn6Bi2 to link into chains by bridging the bismuth (and potassium) caps with nitrate and acetate ligands. On the other hand, the dissolution of Mn6Bi2 in water, with and without metathesis of the bismuth, leads to the precipitation of related oxyhydroxide phases, which we characterized by transmission electron microscopy (TEM), electron diffraction, and energy-dispersive spectroscopy, and the conversion pathway by SAXS. Without removal of bismuth, amorphous manganese/bismuth oxyhydroxides precipitate within a day. On the other hand, metathesis of BiOBr yields a solution containing soluble manganese oxyhydroxide prenucleation clusters that assemble and precipitate over 10 days. This allows tracking of the reaction pathway via SAXS. We observe one-dimensional growth of species, followed by the precipitation of nanocrystalline hollandite (identified by TEM). The hollandite is presumably templated by the K+, originally in the crystalline lattice of Mn6Bi2. In this Forum Article that combines new results and prospective, we compare these results to prior studies in which we first introduced the use of capping Bi3+ to stabilize reactive clusters, followed by destabilization to understand reaction pathways in synthesis and low-temperature geochemistry.

The Role of the Organic Solvent Polarity in Isolating Uranyl Peroxide Capsule Fragments.

Uranyl peroxide capsules are a fascinating class of polyoxometalates (POMs), discovered only in the 21st century. Understanding the reactivity between peroxide, alkali cations, and uranyl in alkaline solutions is important in nuclear science disciplines including mineralogy, nuclear energy, and legacy nuclear wastes. Here, we have developed a general procedure to isolate different fragments of the uranyl-peroxide POM capsules, using organic solvents to partially remove K+ salts from crude solids of the monomer building block UO2(O2)34- (K-U1), leading to stabilization of these reactive fragments. Higher polarity organic solvents remove more K+ salts from the crude solid, owed to higher solubility, resulting in more extensive linking of uranyl peroxide building units. By this strategy we have isolated and structurally characterized a dimer K6[(UO2)2(O2)4(OH)2]·7H2O (K-U2) and a hexamer face frequently observed in the capsules, K12[(UO2)6(O2)9(OH)6]·xH2O (K-U6). Comparing experimental and computed Raman spectra shows that these intermediates crystallize by a solid-to-solid transformation, via polymerization of the monomer building block. By small-angle X-ray scattering (SAXS), we track the conversion of the fragments to POM capsules; the reaction rate increases from K-U1 (days) < K-U2 (hours) < K-U6 (instantaneous). This study provides a general synthetic procedure to isolate metastable uranyl peroxide oligomers and control the oligomerization, which will be later applied to systems with the heavier alkalis that are even less stable.

Peroxide-Promoted Disassembly Reassembly of Zr-Polyoxocations.

Zr/Hf aqueous-acid clusters are relevant to inorganic nanolithography, metal-organic frameworks (MOFs), catalysis, and nuclear fuel reprocessing, but only two topologies have been identified. The (Zr4) polyoxocation is the ubiquitous square aqueous Zr/Hf-oxysalt of all halides (except fluoride), and prior-debated for perchlorate. Simply adding peroxide to a Zr oxyperchlorate solution leads to a striking modification of Zr4, yielding two structures identified by single-crystal X-ray diffraction. Zr25, isolated from a reaction solution of 1:1 peroxide/Zr, is fully formulated [Zr25O10(OH)50(O2)5(H2O)40](ClO4)10·xH2O. Zr25 is a pentagonal assembly of 25 Zr-oxy/peroxo/hydroxyl polyhedra and is the largest Zr/Hf cluster topology identified to date. Yet it is completely soluble in common organic solvents. ZrTd, an oxo-centered tetrahedron fully formulated [Zr4(OH)4(µ-O2)2(µ4-O)(H2O)12](ClO4)6·xH2O, is isolated from a 10:1 peroxide/Zr reaction solution. The formation pathways of ZrTd and Zr25 in water were described by small-angle X-ray scattering (SAXS), pair distribution function (PDF), and electrospray ionization mass spectrometry (ESI-MS). Zr4 undergoes disassembly by 1 equiv of peroxide (per Zr) to yield small oligomers of Zr25 that assemble predominantly in the solid state, an unusual crystal growth mechanism. The self-buffering acidity of the Zr-center prevents Zr25 from remaining intact in water. Identical species distribution and cluster fragments are observed in the assembly of Zr25 and upon redissolution of Zr25. On the other hand, the 10:1 peroxide/Zr ratio of the ZrTd reaction solution yields larger prenucleation clusters before undergoing peroxide-promote disassembly into smaller fragments. Neither these larger cluster intermediates of ZrTd nor the smaller intermediates of Zr25 have yet been isolated and structurally characterized, and they represent an opportunity to expand this new class of group IV polycations, obtained by peroxide reactivity and ligation.

Strategic Capture of the {Nb7 } Polyoxometalate.

Group V Nb-polyoxometalate (Nb-POM) chemistry generally lacks the elegant pH-controlled speciation exhibited by group VI (Mo, W) POM chemistry. Here three Nb-POM clusters were isolated and structurally characterized; [Nb14 O40 (O2 )2 H3 ]14- , [((UO2 )(H2 O))3 Nb46 (UO2 )2 O136 H8 (H2 O)4 ]24- , and [(Nb7 O22 H2 )4 (UO2 )7 (H2 O)6 ]22- , that effectively capture the aqueous Nb-POM species from pH 7 to pH 10. These Nb-POMs illustrate a reaction pathway for control over speciation that is driven by counter-cations (Li+ ) rather than pH. The two reported heterometallic POMs (with UO2 2+ moieties) are stabilized by replacing labile H2 O/HO-Nb=O with very stable O=U=O. The third isolated Nb-POM features cis-yl-oxos, prior observed only in group VI POM chemistry. Moreover, with these actinide-heterometal contributions to the burgeoning Nb-POM family, it now transects all major metal groups of the periodic table.

Energetic Trends in Monomer Building Blocks for Uranyl Peroxide Clusters.

The uranyl triperoxide anionic monomer is a fundamental building block for uranyl peroxide polyoxometalate capsules. The reaction pathway from the monomer to the capsule can be greatly altered by the counterion: both the reaction rate and the resulting capsule structure. We synthesized and characterized uranyl triperoxides Mg2UO2(O2)3·13H2O (MgUT), Ca2UO2(O2)3·9H2O (CaUT), Sr2UO2(O2)3·9H2O (SrUT), and K4UO2(O2)3·3H2O (KUT) and compared their thermodynamic stabilities. The enthalpies of formation from oxides and elements of these compounds were calculated by thermochemical cycles from measurements by high temperature oxide melt drop solution calorimetry. Their formation enthalpies from oxides become more negative linearly as a function of the increasing basicity of the respective oxides on the Smith scale. This relationship holds for previously Li and Na analogues. Further affirming the trend, Δ Hf,ox of MgUT departs from linearity, due to the distinct bonding environment of Mg2+, as compared to the other alkalis and alkaline earths in the series.

Synthesis of Coordination Polymers of Tetravalent Actinides (Uranium and Neptunium) with a Phthalate or Mellitate Ligand in an Aqueous Medium.

Four metal-organic coordination polymers bearing uranium or neptunium have been hydrothermally synthesized from a tetravalent actinide chloride (AnCl4) and phthalic (1,2-H2bdc) or mellitic (H6mel) acid in aqueous media at 130 °C. With the phthalate ligand, two analogous assemblies ([AnO(H2O)(1,2-bdc)]2·H2O; An = U4+ (1) or Np4+ (2)) have been isolated, in which the square-antiprismatic polyhedra of AnO8 are linked to each other via µ3-oxo groups with an edge-sharing mode to materialize infinite zigzag ribbons. The phthalate molecules play a role in connecting the adjacent zigzag chains to build a two-dimensional (2D) network. Water molecules are bonded to the actinide center or found intercalated between the layers. With the mellitate ligand, two distinct structures have been identified. The uranium-based compound [U2(OH)2(H2O)2(mel)] (3) exhibits a three-dimensional (3D) structure composed of the dinuclear units of UO8 polyhedra (square antiprism), which are further linked via the µ2-hydroxo groups. The mellitate linkers use their carboxylate groups to connect the dinuclear units, eventually building a 3D framework. The compound obtained for the neptunium mellitate ([(NpO2)10(H2O)14(Hmel)2]·12H2O (4)) reveals oxidation of the initial NpIV to NpV under the applied hydrothermal synthetic conditions, yielding the neptunyl(V) (NpO2+) unit with a pentagonal-bipyramidal NpO7 environment. This further leads to the formation of a layered assembly of the square-frame NpO7 sheets via the bridging oxygen atoms from the neptunyl oxo groups, which further coordinate to the pentagonal equatorial coordination plane of the adjacent neptunium unit (i.e., cation-cation interactions). In compound 4, the mellitate molecules act as bridging linkers between the NpO7 sheets by using four of their carboxylage groups, eventually building up a 3D structure.

Hydrothermal Crystallization of Uranyl Coordination Polymers Involving an Imidazolium Dicarboxylate Ligand: Effect of pH on the Nuclearity of Uranyl-Centered Subunits.

Four uranyl-bearing coordination polymers (1-4) have been hydrothermally synthesized in the presence of the zwitterionic 1,3-bis(carboxymethyl)imidazolium (= imdc) anion as organic linkers after reaction at 150 °C. At low pH (0.8-3.1), the form 1 ((UO2)2(imdc)2(ox)·3H2O; ox stands for oxalate group) has been identified. Its crystal structure (XRD analysis) consists of the 8-fold-coordinated uranyl centers linked to each other through the imdc ligand together with oxalate species coming from the partial decomposition of the imdc molecule. The resulting structure is based on one-dimensional infinite ribbons intercalated by free water molecules. By adding NaOH solution, a second form 2 is observed for pH 1.9-3.9 but in a mixture with phase 1. The pure phase of 2 is obtained after a hydrothermal treatment at 120 °C. It corresponds to a double-layered network (UO2(imdc)2) composed of 7-fold-coordinated uranyl cations linked via the imdc ligands. In the same pH range, a third phase ((UO2)3O2(H2O)(imdc)·H2O, 3) is formed: it is composed of hexanuclear units of 7-fold- and 8-fold-coordinated uranyl cations, connected via the imdc molecules in a layered assembly. At higher pH, the chain-like solid (UO2)3O(OH)3(imdc)·2H2O (4) is observed and composed of the infinite edge-sharing uranyl-centered pentagonal bipyramidal polyhedra. As a function of pH, uranyl nuclearity increases from discrete 8- or 7-fold uranyl centers (1, 2) to hexanuclear bricks (3) and then infinite chains in 4 (built up from the hexameric fragments found in 3). This observation emphasized the influence of the hydrolysis reaction occurring between uranyl centers. The compounds have been further characterized by thermogravimetric analysis, infrared, and luminescence spectroscopy.

Time-controlled synthesis of the 3D coordination polymer U(1,2,3-Hbtc)2 followed by the formation of molecular poly-oxo cluster {U14} containing hemimellitate uranium(iv).

Two coordination compounds bearing tetravalent uranium were synthesized in the presence of tritopic hemimellitic acid in acetonitrile with a controlled amount of water (H2O/U ≈ 8) and structurally characterized. Compound 1, [U(1,2,3-Hbtc)2]·0.5CH3CN is constructed around an eight-fold coordinated uranium cationic unit [UO8] linked by the poly-carboxylate ligands to form dimeric subunits, which are further connected to form infinite corrugated ribbons and a three-dimensional framework. Compound 2, [U14O8(OH)4Cl8(H2O)16(1,2,3-Hbtc)8(ox)4(ac)4] ({U14}) exhibits an unprecedented polynuclear {U14} poly-oxo uranium cluster surrounded by O-donor and chloride ligands. It is based on a central core of [U6O8] type surrounded by four dinuclear uranium-subunits {U2}. Compound 1 was synthesized by a direct reaction of hemimellitic acid with uranium tetrachloride in acetonitrile (+H2O), while the molecular species ({U14} (2)) crystallized from the supernatant solution after one month. The slow hydrolysis reaction together with the partial decomposition of the starting organic reactants into oxalate and acetate molecules induces the generation of such a large poly-oxo cluster with fourteen uranium centers. Structural comparisons with other closely related uranium-containing clusters, such as the {U12} cluster based on the association of inner core [U6O8] with three dinuclear sub-units {U2}, were performed.

The role of titanium-oxo clusters in the sulfate process for TiO2 production.

TiO2 is manufactured for white pigments, solar cells, self-cleaning surfaces and devices, and other photocatalytic applications. The industrial synthesis of TiO2 entails: (1) the dissolution of ilmenite ore (FeTiO3) in aqueous sulfuric acid which precipitates the Fe while retaining the Ti in solution, followed by (2) dilution or heating the Ti sulfate solution to precipitate the pure form of TiO2. The underlying chemistry of these processing steps remains poorly understood. Here we show that the dissolution of a simple TiIV-sulfate salt, representative of the industrial sulfate process for the production of TiO2, immediately self-assembles into a soluble Ti-octadecameric cluster, denoted as {Ti18}. We observed {Ti18} in solution by small-angle X-ray scattering and Ti extended X-ray absorption fine structure (Ti-EXAFS) analysis, and ultimately crystallized it for absolute identification. The {Ti18} metal-oxo cluster was previously reported as a polycation; but shown here, it can also be a polyanion, dependent on the number of sulfate ligands it carries. After immediate self-assembly, the {Ti18}-cluster persists until TiO2 precipitates, with no easily identified structural intermediates in the solution or solid state, despite the fact that the atomic arrangement of {Ti18} differs vastly from that of titania. The evolution from solution phase {Ti18} to precipitated TiO2 nanoparticles was detailed by X-ray scattering and Ti-EXAFS. We offer a hypothesis for the key mechanism of complete separation of Fe from Ti in the industrial sulfate process. These findings also highlight the emerging importance of the unusual Ti(Ti)5 pentagonal building unit, featured in {Ti18} as well as other early d0 transition metal-oxo clusters including Nb, Mo and W. Finally, this study presents an example of crystal growth mechanisms in which the observed "pre-nucleation cluster" does not necessarily predicate the structure of the precipitated solid.

{Np38} clusters: the missing link in the largest poly-oxo cluster series of tetravalent actinides.

Two poly-oxo cluster complexes of tetravalent neptunium (Np(iv)), Np38O56Cl18(bz)24(THF)8·nTHF and Np38O56Cl42(ipa)20·mipa (bz = benzoate, THF = tetrahydrofuran, and ipa = isopropanol), were obtained via solvothermal synthesis and structurally characterised by single-crystal X-ray diffraction. The {Np38} clusters are comparable to the analogous {U38} and {Pu38} motifs, filling the gap in this largest poly-oxo cluster series of tetravalent actinides.

Formation of a new type of uranium(iv) poly-oxo cluster {U38} based on a controlled release of water via esterification reaction.

A new strategy for the synthesis of large poly-oxo clusters bearing 38 tetravalent uranium atoms {U38} has been developed by controlling the water release from the esterification reaction between a carboxylic acid and an alcohol. The molecular entity [U38O56Cl40(H2O)2(ipa)20]·(ipa) x (ipa = isopropanol) was crystallized from the solvothermal reaction of a mixture of UCl4 and benzoic acid in isopropanol at temperature ranging from 70 to 130 °C. Its crystal structure reveals the molecular assembly of the UO2 fluorite-like inner core {U14} with oxo groups bridging the uranium centers. The {U14} core is further surrounded by six tetrameric sub-units of {U4} to form the {U38} cluster. Its surface is decorated by either bridging- and terminal chloride anions or terminal isopropanol molecules. Another synthesis using the same reactant mixture at room temperature resulted in the crystallization of a discrete dinuclear complex [U2Cl4(bz)4(ipa)4]·(ipa)0.5 (bz = benzoate), in which each uranium center is coordinated by two chlorine atoms, four oxygen atoms from carboxylate groups and two additional oxygen atoms from isopropanol. The slow production of water released from the esterification of isopropanol allows the formation of the giant cluster with oxo bridges linking the uranium atoms at a temperature above 70 °C, whereas no such oxo groups are present in the dinuclear complex formed at room temperature. The kinetics of {U38} crystallization as well as the ester formation are analyzed and discussed. SAXS experiments indicate that the {U38} species are not dominant in the supernatant, but hexanuclear entities which are closely related to the [U6O8] type are formed.