Reversible M-M Bonding by Alkaline Earth Metals (Mg, Ca, Sr, Ba) in Graphite Intercalation Compounds.

The alkaline earth metals (M=Mg, Ca, Sr, and Ba) exhibit a +2 oxidation state in nearly all known stable compounds, but MI dimeric complexes with M-M bonding, [M2 (en)2 ]2+ , (en=ethylenediamine) of all these metals can be stabilized within the galleries of donor-type graphite intercalation compounds (GICs). These metals can also form GICs with more conventional metal (II) ion complexes, [M(en)2 ]2+ . Here, the facile interconversion between dimeric-MI and monomeric-MII intercalates upon the addition/removal of en are reported. Thermogravimetry, powder X-ray diffraction, and pair distribution function analysis of total scattering data support the presence of either [M2 (en)2 ]2+ or [M(en)2 ]2+ guests. This phase conversion requires coupling graphene and metal redox centers, with associated reversible M-M bond formation within graphene galleries. This chemistry allows the facile isolation of unusual oxidation states, reveals M0 →M2+ reaction pathways, and present new opportunities in the design of hybrid conversion/intercalation materials for applications such as charge storage.

Discrete Hf18 Metal-oxo Cluster as a Heterogeneous Nanozyme for Site-Specific Proteolysis.

The selective hydrolysis of proteins by non-enzymatic catalysis is difficult to achieve, yet it is crucial for applications in biotechnology and proteomics. Herein, we report that discrete hafnium metal-oxo cluster [Hf18 O10 (OH)26 (SO4 )13 ⋅(H2 O)33 ] (Hf18 ), which is centred by the same hexamer motif found in many MOFs, acts as a heterogeneous catalyst for the efficient hydrolysis of horse heart myoglobin (HHM) in low buffer concentrations. Among 154 amino acids present in the sequence of HHM, strictly selective cleavage at only 6 solvent accessible aspartate residues was observed. Mechanistic experiments suggest that the hydrolytic activity is likely derived from the actuation of HfIV Lewis acidic sites and the Brønsted acidic surface of Hf18 . X-ray scattering and ESI-MS revealed that Hf18 is completely insoluble in these conditions, confirming the HHM hydrolysis is caused by a heterogeneous reaction of the solid Hf18 cluster, and not from smaller, soluble Hf species that could leach into solution.

Beyond Charge Balance: Counter-Cations in Polyoxometalate Chemistry.

Polyoxometalates (POMs) are molecular metal-oxide anions applied in energy conversion and storage, manipulation of biomolecules, catalysis, as well as materials design and assembly. Although often overlooked, the interplay of intrinsically anionic POMs with organic and inorganic cations is crucial to control POM self-assembly, stabilization, solubility, and function. Beyond simple alkali metals and ammonium, chemically diverse cations including dendrimers, polyvalent metals, metal complexes, amphiphiles, and alkaloids allow tailoring properties for known applications, and those yet to be discovered. This review provides an overview of fundamental POM-cation interactions in solution, the resulting solid-state compounds, and behavior and properties that emerge from these POM-cation interactions. We will explore how application-inspired research has exploited cation-controlled design to discover new POM materials, which in turn has led to the quest for fundamental understanding of POM-cation interactions.

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.

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.

Thorium Oxo-Clusters as Building Blocks for Open Frameworks.

Fundamental understanding of tetravalent actinide chemistry is essential to optimize many steps of the nuclear fuel cycle, as well as to predict actinide speciation in the environment. Herein, we report synthesis and structure of three open inorganic frameworks composed of Th4+ -oligomers and sulfate anions. The compounds [Th10 O4 (OH)8 (SO4 )12 (H2 O)18 ]⋅28 H2 O (1), [Th9 O4 (OH)5 (SO4 )12 (H2 O)18 ]⋅ 1 TMA⋅18 H2 O (2), and [Th8.5 O4 (OH)4 (SO4 )12 (H2 O)18 ]⋅2 TMA⋅n H2 O (3) were obtained by slow evaporation of aqueous Th4+ -SO42- solutions, with variable SO42- :Th4+ ratios (SO42- :Th4+ =1:1, 1.5:1 and 2:1 for 1, 2, and 3; TMA=tetramethylammonium). The structure of 1 features a neutral open framework architecture with two interpenetrating networks, built from Th6 O4 (OH)412+ hexamers and Th2 (OH)26+ dimers, linked by sulfate anions. The complex anionic framework of 2 is built from hexamers, dimers, and monomers, with charge-balancing TMA cations. Compound 3 is very similar to 2, but it is constructed only from hexamers and monomers. Small angle X-ray scattering (SAXS) of the reaction solutions reveals pre-assembled hexamer building units in the presence of sulfate, and underlines the key role of the sulfate-oxoanion ligand on thorium polymerization processes.

Aqueous Bismuth Titanium-Oxo Sulfate Cluster Speciation and Crystallization.

Inorganic aqueous metal-oxo clusters are both functional "molecular metal oxides" and intermediates to understand metal oxide growth from water. There has been a recent surge in discovery of aqueous Ti-oxo clusters but without extensive solution characterization. We use small-angle and total X-ray scattering, dynamic light scattering, transmission electron microscopy, and a single-crystal X-ray structure to show that heterometals such as bismuth stabilize labile Ti-oxo sulfate clusters in aqueous solution.[Ti22 Bi7 O41 (OH)(OH2 )30 (SO4 )12 ]2+ features edge-sharing between the Ti and Bi polyhedra, in contrast to the dominant corner-linking of Ti-oxo clusters. Bi stabilizes the Ti-polyhedra, which are synergistically stabilized by the bidentate sulfates. Gained stability and potential functionality from heterometals is an incentive to develop more broadly the landscape of heterometallic Ti-oxo clusters.

Electronic and relativistic contributions to ion-pairing in polyoxometalate model systems.

Ion pairs and solubility related to ion-pairing in water influence many processes in nature and in synthesis including efficient drug delivery, contaminant transport in the environment, and self-assembly of materials in water. Ion pairs are difficult to observe spectroscopically because they generally do not persist unless extreme solution conditions are applied. Here we demonstrate two advanced techniques coupled with computational studies that quantify the persistence of ion pairs in simple solutions and offer explanations for observed solubility trends. The system of study, ([(CH3)4N]+,Cs)8[M6O19] (M = Nb,Ta), is a set of unique polyoxometalate salts whose water solubility increases with increasing ion-pairing, contrary to most ionic salts. The techniques employed to characterize Cs+ association with [M6O19]8- and related clusters in simple aqueous media are 133Cs NMR (nuclear magnetic resonance) quadrupolar relaxation rate and PDF (pair distribution function) from X-ray scattering. The NMR measurements consistently showed more extensive ion-pairing of Cs+ with the Ta-analogue than the Nb-analogue, although the electrostatics of the ions should be identical. Computational studies also ascertained more persistent Cs+-[Ta6O19] ion pairs than Cs+-[Nb6O19] ion pairs, and bond energy decomposition analyses determined relativistic effects to be the differentiating factor between the two. These distinctions are likely responsible for many of the unexplained differences between aqueous Nb and Ta chemistry, while they are so similar in the solid state. The X-ray scattering studies show atomic level detail of this ion association that has not been prior observed, enabling confidence in our structures for calculations of Cs-cluster association energies. Moreover, detailed NMR studies allow quantification of the number of Cs+ associated with a single [Nb6O19]8- or [Ta6O19]8- anion which agrees with the PDF analyses.

Closing Uranyl Polyoxometalate Capsules with Bismuth and Lead Polyoxocations.

Uranyl polyoxometalate clusters are both fundamentally fascinating and potentially relevant to nuclear energy applications. With only ten years of development, there is still much to be discovered about heterometal derivatives and aqueous speciation and behavior. Herein, we show that it is possible to encapsulate the polyoxocations [Bi6 O8 ]2+ and [Pb8 O6 ]4+ in [(UO2 )(O2 )(OH)]2424- (denoted Bi@U24 and Pb@U24 ) in pure form and high yields despite the fact that under aqueous conditions, these compounds are stable on opposite ends of the pH scale. Moreover, [Pb8 O6 ]4+ is a formerly unknown PbII polynuclear species, both in solution and in the solid state. Raman spectroscopic and mass spectrometric analysis of the reaction solutions revealed the very rapid assembly of the nested clusters, driven by bismuth- or lead-promoted decomposition of excess peroxide, which inhibits U24 formation. Experimental and simulated small-angle X-ray scattering data of Bi@U24 and Pb@U24 solutions revealed that this technique is very sensitive not only to the size and shape of the clusters, but also to the encapsulated species.