Synthesis and characterization of the GeO(4)Al(12)(OH)(24)(OH2)(12)(8+) polyoxocation

ClusterFinder: a fast tool to find cluster structures from pair distribution function data

A novel automated high-throughput screening approach, ClusterFinder, is reported for finding candidate structures for atomic pair distribution function (PDF) structural refinements. Finding starting models for PDF refinements is notoriously difficult when the PDF originates from nanoclusters or small nanoparticles. The reported ClusterFinder algorithm can screen 104 to 105 candidate structures from structural databases such as the Inorganic Crystal Structure Database (ICSD) in minutes, using the crystal structures as templates in which it looks for atomic clusters that result in a PDF similar to the target measured PDF. The algorithm returns a rank-ordered list of clusters for further assessment by the user. The algorithm has performed well for simulated and measured PDFs of metal-oxido clusters such as Keggin clusters. This is therefore a powerful approach to finding structural cluster candidates in a modelling campaign for PDFs of nanoparticles and nanoclusters.

Formation of Nanoscale [Ge4 O16 Al48 (OH)108 (H2 O)24 ]20+ from Condensation of ϵ-GeAl12 8+ Keggin Polycations*

Keggin-type polyaluminum cations belong to a unique class of compounds with their large positive charge, hydroxo bridges, and divergent isomerization/oligomerization. Previous reports indicated that oligomerization of this species can only occur through one isomer (δ), but herein we report the isolation of largest Keggin-type cluster that occurs through self-condensation of four ϵ-isomers ϵ-GeAl₁₂ ⁸⁺ to form [Ge₄ O₁₆ Al₄₈ (OH)₁₀₈ (H₂ O)₂₄ ]²⁰⁺ cluster (Ge₄ Al₄₈ ). The cluster was crystallized and structurally characterized by single-crystal X-ray diffraction (SCXRD) and the elemental composition was confirmed by ICP-MS and SEM-EDS. Additional dynamic light scattering experiments confirms the presence of the Ge₄ Al₄₈ in thermally aged solutions. DFT calculations reveal that a single atom Ge substitution in tetrahedral site of ϵ-isomer is the key for the formation of Ge₄ Al₄₈ because it activates deprotonation at key surface sites that control the self-condensation process.

Density functional theory and thermodynamics analysis of MAl12 Keggin substitution reactions: Insights into ion incorporation and experimental confirmation

Polyaluminum cations, such as the MAl₁₂ Keggin, undergo atomic substitutions at the heteroatom site (M), where nanoclusters with M = Al³⁺, Ga³⁺, and Ge⁴⁺ have been experimentally studied. The identity of the heteroatom M has been shown to influence the structural and electronic properties of the nanocluster and the kinetics of ligand exchange reactions. To date, only three ε-analogs have been identified, and there is a need for a predictive model to guide experiment to the discovery of new MAl₁₂ species. Here, we present a density functional theory (DFT) and thermodynamics approach to predicting favorable heteroatom substitution reactions, alongside structural analyses on hypothetical ε-MAl₁₂ nanocluster models. We delineate trends in energetics and geometry based on heteroatom cation properties, finding that Al³⁺-O bond lengths are related to heteroatom cation size, charge, and speciation. Our analyses also enable us to identify potentially isolable new ε-MAl₁₂ species, such as FeAl₁₂ ⁷⁺. Based upon these results, we evaluated the Al³⁺/Zn²⁺/Cr³⁺ system and determined that substitution of Cr³⁺ is unfavorable in the heteroatom site but is preferred for Zn²⁺, in agreement with the experimental structures. Complimentary experimental studies resulted in the isolation of Cr³⁺-substituted δ-Keggin species where Cr³⁺ substitution occurs only in the octahedral positions. The isolated structures Na[AlO₄Al_9.6Cr_2.4(OH)₂₄(H₂O)₁₂](2,6-NDS)₄(H₂O)₂₂ (δ-Cr_nAl_13-n-1) and Na[AlO₄Al_9.5Cr_2.5(OH)₂₄(H₂O)₁₂](2,7-NDS)₄(H₂O)_18.5 (δ-Cr_nAl_13-n-2) are the first pieces of evidence of mixed Al³⁺/Cr³⁺ Keggin-type nanoclusters that prefer substitution at the octahedral sites. The δ-Cr_nAl_13-n-2 structure also exhibits a unique placement of the bound Na⁺ cation, which may indicate that Cr³⁺ substitution can alter the surface reactivity of Keggin-type species.

Ga3+ Incorporation into Al13 Keggin Polyoxometalates and the Formation of δ-(GaAl12)7+ and (Ga2.5Al28.5)19+ Polycations

Keggin-type polyaluminum species (ε-Al₁₃, δ-Al₁₃, Al₂₆, Al₃₀, Al₃₂) can form upon partial hydrolysis of Al³⁺-bearing solutions and are important species for water purification and contaminant transport. While the structural features for the major Al³⁺ polyaluminum species have been delineated, much less is known regarding heteroatom substitution and resultant structures other than the previously identified ε-GaAl₁₂⁷⁺ and ε-GeAl₁₂⁸⁺ cations. Single-atom substitution within polyaluminum species can change the surface reactivity within water treatment scenarios; thus, it is important to understand heteroatom incorporation within this system. The present work describes the synthesis and characterization of two novel Ga³⁺-substituted Keggin-type polyaluminum species. Na[GaO₄Al₁₂(OH)₂₄(H₂O)₁₂](2,6-NDS)₄(H₂O)_20.5 (δ-GaAl₁₂) and [Ga₂O₈Al_28.5Ga_0.5(OH)₅₈(H₂O)₂₇(SO₄)₂](SO₄)₄Cl₇(H₂O)_8.5 (Ga_2.5Al_28.5) were crystallized from a thermally aged, partially hydrolyzed Ga³⁺/Al³⁺ solution. Structural refinement from single-crystal X-ray diffraction indicated fully occupied Ga³⁺ within tetrahedral site(s) of both isolated species. Partial substitution was observed for octahedral sites for the larger Ga_2.5Al_28.5 cluster. The chemical compositions of both clusters were confirmed by inductively coupled plasma mass spectrometry (ICP-MS). Density functional theory (DFT) calculations corroborated the structural refinement, with the energetics of Ga³⁺ substitution suggesting preferential substitution within tetrahedral sites for both species. Additional theoretical work suggests that the rotated trimer in δ-GaAl₁₂ is highly reactive, which can serve as the driving force in the formation of the Ga_2.5Al_28.5 cluster.

Synthesis and structural characterization of heterometallic thorium aluminum polynuclear molecular clusters

Aluminum can undergo hydrolysis in aqueous solutions leading to the formation of soluble molecular clusters, including polynuclear species that range from 1 to 2 nm in diameter. While the behavior of aluminum has been extensively investigated, much less is known about the hydrolysis of more complex mixed-metal systems. This study focuses on the structural characteristics of heterometallic thorium-aluminum molecular species that may have important implications for the speciation of tetravalent actinides in radioactive waste streams and environmental systems. Two mixed metal (Th(4+)/Al(3+)) polynuclear species have been synthesized under ambient conditions and structurally characterized by single-crystal X-ray diffraction. [Th(2)Al(6)(OH)(14)(H(2)O)(12)(hedta)(2)](NO(3))(6)(H(2)O)(12) (ThAl1) crystallizes in space group P2(1)/c with unit cell parameters of a = 11.198(1) Å, b = 14.210(2) Å, c = 23.115(3) Å, and β = 96.375° and [Th(2)Al(8)(OH)(12)(H(2)O)(10)(hdpta)(4)](H(2)O)(21) (ThAl2) was modeled in P1 with a = 13.136(4) Å, b = 14.481(4) Å, c = 15.819(4) Å, α = 78.480(9)°, β = 65.666(8)°, γ = 78.272(8)°. Infrared spectra were collected on both compounds, confirming complexation of the ligand to the metal center, and thermogravimetric analysis indicated that the thermal degradation of these compounds resulted in the formation of an amorphous product at high temperatures. These mixed metal species have topological relationships to previously characterized aluminum-based polynuclear species and may provide insights into the adsorption of tetravalent actinides on colloidal or mineral surfaces.

Hydrophilic macroionic solutions: what happens when soluble ions reach the size of nanometer scale?

Large, hydrophilic inorganic ions (mostly polyoxometalate macroions and cationic metal-organic hybrid nanocages) with high solubility in water and/or other polar solvents demonstrate unique solution behaviors. In dilute solutions, they behave significantly different from small simple ions (as described by the Debye-Hückel theory) because the macroions cannot be treated as point charges or large, insoluble colloidal suspensions (usually described by the DLVO theory) because the macroions form homogeneous, stable "real solutions". The size disparity between the macroions and their counterions results in complex macroion-counterion interaction and leads to the self-assembly of macroions into single-layered, hollow, spherical "blackberry" structures. The blackberries, with robust and very stable structures mimicking biological membranes, can adjust their size accurately and reversibly in response to the change of solvent content, charge density on the macroions, or in some cases merely solution pH. The blackberry membrane is permeable to small cations. The inorganic macroions with well-defined size, shape, mass, charge density (even accurately tunable within certain range), and no intramolecular interaction can be treated as simple model systems to understand the intermolecular interaction in polyelectrolyte solutions. The blackberry structures show certain similarities to the spherical virus capsids, from the overall structure to the kinetic properties of formation.

Enhanced water purification: a single atom makes a difference

The aluminum Keggin polycation (Al13) has been identified as an effective specie for neutralization and coagulation of anionic contaminants in water. In this study, we compare efficacy of the aluminum Keggin-ion to the analogues containing a single Ga-atom or single Ge-atom (GaAl12 and GeAl12, respectively) substituted into the center of the polycation in water-treatment studies. We investigated removal of bacteriophage (model viruses), Cryptosporidium, dissolved organic carbon (DOC), and turbidity. In every study, the order of contaminant removal efficacy trends GaAl12 > Al13 > GeAl12. By ESI MS (electrospray ionization mass spectrometry), we noted the GaAl12 deprotonates least of the three aluminum polycations, and thus probably carries the highest charge, and also optimal contaminant-neutralization ability. The ESI MS studies of the aluminum polycation solutions, as well as solid-state characterization of their resulting precipitates both reveal some conversion of Al13 to larger polycations, Al30 for instance. The GaAl12 does not show any evidence for this alteration that is responsible for poor shelf life of commercial prehydrolyzed aluminum coagulants such as polyaluminum chloride. Based on these studies, we conclude that substitution of a single Ga-atom in the center of the aluminum Keggin polycation produces an optimal water-treatment product due to enhanced shelf life and efficacy in neutralization of anionic contaminants.

A Correlation for Establishing Solvolysis Rates of Aqueous Al(III) Complexes: A Possible Strategy for Colloids and Nanoparticles

We here examine whether rates of solvolysis for a range of aluminum complexes can be predicted semiempirically by correlating calculated values of Al-O bond lengths with rate coefficients. We focus on a series of mono- and bis-ligated aqueous aluminum monomers and three epsilon-Keggin-like aluminum polyoxocations, and we make no attempt to simulate transition states. The Al-O bond lengths were calculated by performing ab initio geometry optimizations using the polarizable-continuum model to estimate solvation effects. Both Hartree-Fock and density functional methods (B3LYP) were tested using several basis sets up to 6-31+G(d). We find a strong correlation between rate coefficients for mono-ligated aluminum monomers and bond lengths to hydration waters. We cannot extrapolate the correlation, however, to large epsilon-Keggin-like multimers or to bis-ligated complexes, which suggests that the activated equilibrium for exchange of a water molecule in these molecules is different than that in the simple monomers.

On the Acid–Base Chemistry of the Keggin Polymers: GaAl12 and GeAl12

The acid-base properties of GaAl12 and GeAl12 molecules have been investigated and compared to those of the Al13 molecule using alkalimetric titration at two concentrations of the Keggin complex. As in the case of Al13, the positive charge of both GaAl13 and GeAl12 is entirely lost within half a pH unit if the concentration of the Keggin molecules is low (1.25 microM). We find that GeAl12 is significantly more acidic than either the GaAl12 or the Al13 molecules. As positive charge is removed from the Keggin complexes by deprotonation, they tend to aggregate, resulting in titration curves that flatten out at higher Z values. Molecule-molecule interactions appear to be more distinct at elevated concentrations of the Keggin complexes (12.5 microM).