Aqueous Hafnium Sulfate Chemistry: Structures of Crystalline Precipitates

Crystalline precipitates resulting from the hydrolysis and subsequent condensation of Hf^IV aqueous acidic solutions at 60–95 °C are examined and compared. By varying the concentrations of the acid and sulfate source, a variety of complex hafnium-oxo-hydroxo-sulfate clusters are isolated and structures accessed. Four novel compounds were discovered, while the structures of two known compounds, an 18-mer and a planar hexamer, were updated. In total, the compounds described herein each contain one of four cluster architectures: 18-mer, 11-mer, nonamer, and planar hexamer. In addition, one compound contains small amounts of 19-mers together with 18-mers. As well as examining the individual structure of each complex cluster, we relate them to one another, as well as to the dense phases of HfO₂, to gain an understanding of their formation and stability. Finally, the solution conditions under which each cluster forms are identified by plotting the crystallization regions of each cluster against acidity and sulfate concentration. Most clusters form under slightly acidic conditions, in decreasing size as the sulfate concentration is raised. The flat hexamer is the single exception; it appears to require more acidic solutions. The degree of hydroxo- versus oxo-bridges with changing solution conditions is assessed within the broader context of the condensates. Of specific interest is the identification of these products as they relate to the use of hydrolysis reactions in designing new materials

Plutonium(IV) Cluster with a Hexanuclear [Pu₆(OH)₄O₄]¹²⁺ Core

A mixed hydroxo/oxo plutonium­(IV) carboxylate resulting from the hydrolysis and condensation of Pu^IV in an acidic aqueous solution has been isolated. The structure of Li₆[Pu₆(OH)₄O₄(H₂O)₆(HGly)₁₂]­Cl₁₈·10.5H₂O (<b>1</b>) consists of a cationic [Pu₆(OH)₄O₄]¹²⁺ core that is decorated by glycine ligands. The synthesis, structure, and characterization of the hexanuclear unit, which represents the first example of a Pu^IV polynuclear complex containing both hydroxo- and oxo-bridging ligands, are described herein

Comparative CHARMM and AMOEBA Simulations of Lanthanide Hydration Energetics and Experimental Aqueous-Solution Structures

The accurate understanding of metal ion hydration in solutions is a prerequisite for predicting stability, reactivity, and solubility. Herein, additive CHARMM force field parameters were developed to enable molecular dynamics simulations of lanthanide (Ln) speciation in water. Quantitatively similar to the much more resource-intensive polarizable AMOEBA potential, the CHARMM simulations reproduce the experimental hydration free energies and correlations in the first shell (Ln-oxygen distance and hydration number). Comparisons of difference pair-distribution functions obtained from the two simulation approaches with those from high-energy X-ray scattering experiments reveal good agreement of first-coordination sphere correlations for the Lu³⁺ ion (CHARMM only), but further improvement to both approaches is required to reproduce the broad, non-Gaussian distribution seen from the La³⁺ experiment. Second-coordination sphere comparisons demonstrate the importance of explicitly including an anion in the simulation. This work describes the usefulness of less resource-intensive additive potentials in some complex chemical systems such as solution environments where multiple interactions have similar energetics. In addition, 3-dimensional descriptions of the La³⁺ and Lu³⁺ coordination geometries are extracted from the CHARMM simulations and generally discussed in terms of potential improvements to solute-structure modeling within solution environments

Comparative CHARMM and AMOEBA Simulations of Lanthanide Hydration Energetics and Experimental Aqueous-Solution Structures

The accurate understanding of metal ion hydration in solutions is a prerequisite for predicting stability, reactivity, and solubility. Herein, additive CHARMM force field parameters were developed to enable molecular dynamics simulations of lanthanide (Ln) speciation in water. Quantitatively similar to the much more resource-intensive polarizable AMOEBA potential, the CHARMM simulations reproduce the experimental hydration free energies and correlations in the first shell (Ln-oxygen distance and hydration number). Comparisons of difference pair-distribution functions obtained from the two simulation approaches with those from high-energy X-ray scattering experiments reveal good agreement of first-coordination sphere correlations for the Lu³⁺ ion (CHARMM only), but further improvement to both approaches is required to reproduce the broad, non-Gaussian distribution seen from the La³⁺ experiment. Second-coordination sphere comparisons demonstrate the importance of explicitly including an anion in the simulation. This work describes the usefulness of less resource-intensive additive potentials in some complex chemical systems such as solution environments where multiple interactions have similar energetics. In addition, 3-dimensional descriptions of the La³⁺ and Lu³⁺ coordination geometries are extracted from the CHARMM simulations and generally discussed in terms of potential improvements to solute-structure modeling within solution environments

Th₃[Th₆(OH)₄O₄(H₂O)₆](SO₄)₁₂(H₂O)₁₃: A Self-Assembled Microporous Open-Framework Thorium Sulfate

A neutral-framework thorium oxohydroxosulfate hydrate has been isolated from aqueous solution. This microporous structure, which self-assembles without a templating agent, is built from [Th₆(OH)₄O₄(H₂O)₆]¹²⁺ hexamers and thorium­(IV) monomers linked through bridging sulfates. Solution conditions were chosen to enable an active competition between sulfate and hydroxide for thorium coordination. Synthetic requirements are discussed for this rare example of a thorium­(IV) polynuclear complex containing mixed oxo-, hydroxo-, and sulfato-bridging moieties

Three New Sodium Neptunyl(V) Selenate Hydrates: Structures, Raman Spectroscopy, and Magnetism

Green crystals of Na­(NpO₂)­(SeO₄)­(H₂O) (<b>1</b>), Na₃(NpO₂)­(SeO₄)₂(H₂O) (<b>2</b>), and Na₃(NpO₂)­(SeO₄)₂(H₂O)₂ (<b>3</b>) have been prepared by a hydrothermal method for <b>1</b> or evaporation from aqueous solutions for <b>2</b> and <b>3</b>. The structures of these compounds have been characterized by single-crystal X-ray diffraction. Compound <b>1</b> is isostructural with Na­(NpO₂)­(SO₄)­(H₂O) (<b>4</b>). The structure of <b>1</b> consists of ribbons of neptunyl­(V) pentagonal bipyramids, which are decorated and further connected by selenate tetrahedra to form a three-dimensional framework. The resulting open channels are filled by Na⁺ cations and H₂O molecules. Within the ribbon, each neptunyl polyhedron shares corners with each other solely through cation–cation interactions (CCIs). The structure of <b>2</b> adopts one-dimensional [(NpO₂)­(SeO₄)₂(H₂O)]^3– chains connected by Na⁺ cations. Each NpO₂⁺ cation is coordinated by four monodentate SeO₄^2– anions and one H₂O molecule to form a pentagonal bipyramid. The structure of <b>3</b> is constructed by one-dimensional [(NpO₂)­(SeO₄)₂]^3– chains separated by Na⁺ cations and H₂O molecules. These chains have two configurations resulting in two disordered orientations of the Se(2)­O₄^2– tetrahedra. Each NpO₂⁺ cation is coordinated by one bidentate Se(1)­O₄^2– and three monodentate Se(2)­O₄^2– anions to form a pentagonal bipyramid. Raman spectra of <b>1</b>, <b>2</b>, and <b>4</b> were collected on powder samples. For <b>1</b> and <b>4</b>, the neptunyl symmetric stretch modes (670, 676, 730, and 739 cm^–1) shift significantly toward lower frequencies compared to that in <b>2</b> (773 cm^–1), and there are several asymmetric neptunyl stretch bands in the region of 760–820 cm^–1. Magnetic measurements obtained from crushed crystals of <b>1</b> are consistent with a ferromagnetic ordering of the neptunyl­(V) spins at 6.5(2) K, with an average low temperature saturation moment of 2.2(1) μ_B per Np. Well above the ordering temperature, the susceptibility follows Curie–Weiss behavior, with an average effective moment of 3.65(10) μ_B per Np and a Weiss constant of 14(1) K. Correlations between lattice dimensionality and magnetic behavior are discussed

Two Dihydroxo-Bridged Plutonium(IV) Nitrate Dimers and Their Relevance to Trends in Tetravalent Ion Hydrolysis and Condensation

We report the room temperature synthesis and structural characterization of a μ₂-hydroxo-bridged Pu^IV dimer obtained from an acidic nitric acid solution. The discrete Pu₂(OH)₂(NO₃)₆(H₂O)₄ moiety crystallized with two distinct crystal structures, [Pu₂(OH)₂(NO₃)₆(H₂O)₄]₂·11H₂O (<b>1</b>) and Pu₂(OH)₂(NO₃)₆(H₂O)₄·2H₂O (<b>2</b>), which differ primarily in the number of incorporated water molecules. High-energy X-ray scattering (HEXS) data obtained from the mother liquor showed evidence of a correlation at 3.7(1) Å but only after concentration of the stock solution. This distance is consistent with the dihydroxo-bridged distance of 3.799(1) Å seen in the solid-state structure as well as with the known Pu–Pu distance in PuO₂. The structural characterization of a dihydroxo-bridged Pu moiety is discussed in terms of its relevance to the underlying mechanisms of tetravalent metal-ion condensation

Tetraalkylammonium Uranyl Isothiocyanates

Three tetraalkylammonium uranyl isothiocyanates, [(CH₃)₄N]₃UO₂(NCS)₅ (<b>1</b>), [(C₂H₅)₄N]₃UO₂(NCS)₅ (<b>2</b>), and [(C₃H₇)₄N]₃UO₂(NCS)₅ (<b>3</b>), have been synthesized from aqueous solution and their structures determined by single-crystal X-ray diffraction. All of the compounds consist of the uranyl cation equatorially coordinated to five N-bound thiocyanate ligands, UO₂(NCS)₅^3–, and charge-balanced by three tetraalkylammonium cations. Raman spectroscopy data have been collected on compounds <b>1</b>–<b>3</b>, as well as on solutions of uranyl nitrate with increasing levels of sodium thiocyanate. By tracking the Raman signatures of thiocyanate, the presence of both free and bound thiocyanate is confirmed in solution. The shift in the Raman signal of the uranyl symmetric stretching mode suggests the formation of higher-order uranyl thiocyanate complexes in solution, while the solid-state Raman data support homoleptic isothiocyanate coordination about the uranyl cation. Presented here are the syntheses and crystal structures of <b>1</b>–<b>3</b>, pertinent Raman spectra, and a discussion regarding the relationship of these isothiocyanates to previously described uranyl halide phases, UO₂X₄^2–

Tetraalkylammonium Uranyl Isothiocyanates

Three tetraalkylammonium uranyl isothiocyanates, [(CH₃)₄N]₃UO₂(NCS)₅ (<b>1</b>), [(C₂H₅)₄N]₃UO₂(NCS)₅ (<b>2</b>), and [(C₃H₇)₄N]₃UO₂(NCS)₅ (<b>3</b>), have been synthesized from aqueous solution and their structures determined by single-crystal X-ray diffraction. All of the compounds consist of the uranyl cation equatorially coordinated to five N-bound thiocyanate ligands, UO₂(NCS)₅^3–, and charge-balanced by three tetraalkylammonium cations. Raman spectroscopy data have been collected on compounds <b>1</b>–<b>3</b>, as well as on solutions of uranyl nitrate with increasing levels of sodium thiocyanate. By tracking the Raman signatures of thiocyanate, the presence of both free and bound thiocyanate is confirmed in solution. The shift in the Raman signal of the uranyl symmetric stretching mode suggests the formation of higher-order uranyl thiocyanate complexes in solution, while the solid-state Raman data support homoleptic isothiocyanate coordination about the uranyl cation. Presented here are the syntheses and crystal structures of <b>1</b>–<b>3</b>, pertinent Raman spectra, and a discussion regarding the relationship of these isothiocyanates to previously described uranyl halide phases, UO₂X₄^2–

Thorium(IV)–Selenate Clusters Containing an Octanuclear Th(IV) Hydroxide/Oxide Core

Four Th­(IV) hydroxide/oxide clusters have been synthesized from aqueous solution. The structures of [Th₈(μ₃-O)₄(μ₂-OH)₈(H₂O)₁₅(SeO₄)₈·7.5H₂O] (<b>1</b>), [Th₈(μ₃-O)₄(μ₂-OH)₈(H₂O)₁₇(SeO₄)₈·<i>n</i>H₂O] (<b>2</b>), [Th₉(μ₃-O)₄(μ₂-OH)₈(H₂O)₂₁(SeO₄)₁₀] (<b>3</b>), and Th₉(μ₃-O)₄(μ₂-OH)₈(H₂O)₂₁(SeO₄)₁₀·<i>n</i>H₂O (<b>4</b>) were determined using single crystal X-ray diffraction. Each structure consists of an octanuclear core, [Th₈O₄(OH)₈]¹⁶⁺, that is built from eight Th­(IV) atoms (four Th in a plane and two up and two down) linked by four “inner” μ₃-O and eight “outer” μ₂-OH groups. Compounds <b>3</b> and <b>4</b> additionally contain mononuclear [Th­(H₂O)₅(SeO₄)₄]^4– units that link the octamers into an extended structure. The octanuclear units are invariably complexed by two selenate anions that sit in two cavities formed by four planar Th­(IV) and four extra-planar Th­(IV) atoms, thus making [Th₈O₄(OH)₈(SeO₄)₂]¹²⁺ a common building block in <b>1</b>–<b>4</b>. However, changes in hydration as well selenate coordination give rise to structural differences that are observed in the extended structures of <b>1</b>–<b>4</b>. The compounds were also characterized by Raman spectroscopy. Density functional theory calculations were performed to predict the geometries, vibrational frequencies, and relative energies of different structures. Details of the calculated structures are in good agreement with experimental results, and the calculated frequencies were used to assign the experimental Raman spectra. On the basis of an analysis of the DFT results, the compound Th₈O₈(OH)₄(SeO₄)₆ was predicted to be a strong gas phase acid but is reduced to a weak acid in aqueous solution. Of the species studied computationally, the dication Th₈O₆(OH)₆(SeO₆)₆²⁺ is predicted to be the most stable in aqueous solution at 298 K followed by the monocation Th₈O₇(OH)₅(SeO₆)₆⁺