Chemical Stabilization and Electrochemical Destabilization of the Iron Keggin Ion in Water

Advancing Single-Particle Analysis in Synthetic Chemical Systems: A Forward-Looking Discussion

Single-particle analysis (SPA) is a fundamental method of cryo-electron microscopy developed to resolve the structures of biological macromolecules. This method has seen significant success in structural biology, yet its potential applications in synthetic chemical systems remain underexplored. In this perspective article, SPA and associated electron microscopy techniques are first briefly introduced. It is then proposed that SPA is well-suited for structural analysis of chemical systems where discrete, identical macromolecules can be readily obtained. Applicable systems include various clusters such as coinage metal clusters, metal-oxo/sulfur clusters, metal-organic clusters, and supramolecular compounds like coordination cages and metallo-supramolecular cages. When high-quality large single crystals are unattainable, SPA provides an alternative method for determining their structures. Beyond these end products, it is suggested that SPA can be instrumental in studying synthetic intermediates of materials with specific building units, such as metal-organic frameworks and zeolites. Given that various intermediates coexist in the reaction system, a purification step is necessary before conducting SPA, which can be facilitated by soft-landing electrospray ionization mass spectrometry.

pH-Driven Rotational Configuration of Keggin-Fe13 Clusters and Their Transformations

Keggin-Fe13 clusters are considered foundational building blocks or prenucleation precursors of ferrihydrite. Understanding the factors that influence the rotational configuration of these clusters, and their transformations in water, is vital for comprehending the formation mechanism of ferrihydrite. Here, we report syntheses and crystal structures of four lanthanide-iron-oxo clusters, namely, [Dy6Fe13(Gly)12(μ2-OH)6(μ3-OH)18(μ4-O)4(H2O)17]·13ClO4·19H2O (1), [Dy6Fe13(Gly)12(μ3-OH)24(μ4-O)4(H2O)18]·13ClO4·14H2O (2), [Pr8Fe34(Gly)24(μ3-OH)28(μ3-O)30(μ4-O)4(H2O)30]·6ClO4·20H2O (3), and [Pr6Fe13(Gly)12(μ3-OH)24(μ4-O)4(H2O)18]·13ClO4·22H2O (4, Gly = glycine). Single-crystal analyses reveal that 1 has a β-Keggin-Fe13 cluster, marking the first documented instance of such a cluster to date. Conversely, both 2 and 4 contain an α-Keggin-Fe13 cluster, while 3 is characterized by four hexavacant ε-Keggin-Fe13 clusters. Magnetic property investigations of 1 and 2 show that 2 exhibits ferromagnetic interactions, while 1 exhibits antiferromagnetic interactions. An exploration of the synthetic conditions for 1 and 2 indicates that a higher pH promotes the formation of α-Keggin-Fe13 clusters, while a lower pH favors β-Keggin-Fe13 clusters. A detailed analysis of the transition from 3 to 4 emphasizes that lacunary Keggin-Fe13 clusters can morph into Keggin-Fe13 clusters with a decrease in pH, accompanied by a significant change in their rotational configuration.

Computational study of the interactions of tetravalent actinides (An = Th-Pu) with the α-Fe13 Keggin cluster

In recent years, evidence has emerged that actinide (An) uptake at the enhanced actinide removal plant (EARP) at the UK's Sellafield nuclear site occurs via An interactions with an α-Fe13 Keggin molecular cluster during ferrihydrite formation. We here study theoretically the substitution of aquo complexes of the actinides Th-Pu onto a Na-decorated α-Fe13 Keggin cluster using DFT at the PBE0 level. The optimised Pu-O and Pu-Fe distances are in good agreement with experiment and Na/An substitutions are significantly favourable energetically, becoming more so across the early 5f series, with the smallest and largest ΔrG° being for Th and Pu at -335.7 kJ mol-1 and -396.0 kJ mol-1 respectively. There is strong correlation between the substitution reaction energy and the ionic radii of the actinides (Δrε0R2 = 0.97 and ΔrG° R2 = 0.91), suggesting that the principal An-Keggin binding mode is ionic. Notwithstanding this result, Mulliken and natural population analyses reveal that covalency increases from Th-Pu in these systems, supported by analysis of the occupied Kohn-Sham molecular orbitals where enhanced An(5f)-O(2p) overlap is observed in the Np and Pu systems. By contrast, quantum theory of atoms in molecules analysis shows that U-Keggin binding is the most covalent among the five actinides, in keeping with previous studies.

Spatiotemporal Studies of Soluble Inorganic Nanostructures with X-rays and Neutrons

This Review addresses the use of X-ray and neutron scattering as well as X-ray absorption to describe how inorganic nanostructured materials assemble, evolve, and function in solution. We first provide an overview of techniques and instrumentation (both large user facilities and benchtop). We review recent studies of soluble inorganic nanostructure assembly, covering the disciplines of materials synthesis, processes in nature, nuclear materials, and the widely applicable fundamental processes of hydrophobic interactions and ion pairing. Reviewed studies cover size regimes and length scales ranging from sub-Ångström (coordination chemistry and ion pairing) to several nanometers (molecular clusters, i.e. polyoxometalates, polyoxocations, and metal-organic polyhedra), to the mesoscale (supramolecular assembly processes). Reviewed studies predominantly exploit 1) SAXS/WAXS/SANS (small- and wide-angle X-ray or neutron scattering), 2) PDF (pair-distribution function analysis of X-ray total scattering), and 3) XANES and EXAFS (X-ray absorption near-edge structure and extended X-ray absorption fine structure, respectively). While the scattering techniques provide structural information, X-ray absorption yields the oxidation state in addition to the local coordination. Our goal for this Review is to provide information and inspiration for the inorganic/materials science communities that may benefit from elucidating the role of solution speciation in natural and synthetic processes.

Bismuth-Polyoxocation Coordination Networks: Controlling Nuclearity and Dimension-Dependent Photocatalysis

Bismuth-oxocluster nodes for metal-organic frameworks (MOFs) and coordination networks/polymers are less prolific than other families featuring zinc, zirconium, titanium, lanthanides, etc. However, Bi³⁺ is non-toxic, it readily forms polyoxocations, and its oxides are exploited in photocatalysis. This family of compounds provides opportunity in medicinal and energy applications. Here, we show that Bi node nuclearity depends on solvent polarity, leading to a family of Bi_x-sulfonate/carboxylate coordination networks with x = 1-38. Larger nuclearity-node networks were obtained from polar and strongly coordinating solvents, and we attribute the solvent's ability to stabilize larger species in solution. The strong role of the solvent and the lesser role of the linker in defining node topologies differ from other MOF syntheses, and this is due to the Bi³⁺ intrinsic lone pair that leads to weak node-linker interactions. We describe this family by single-crystal X-ray diffraction (eleven structures), obtained in pure forms and high yields. Ditopic linkers include NDS (1,5-naphthalenedisulfonate), DDBS (2,2'-[biphenyl-4,4'-diylchethane-2,1-diyl] dibenzenesulphonate), and NH₂-benzendicarboxylate (BDC). While the BDC and NDS linkers yield more open-framework topologies that resemble those obtained by carboxylate linkers, topologies with DDBS linkers appear to be in part driven by association between DDBS molecules. An in situ small-angle X-ray scattering study of Bi₃₈-DDBS reveals stepwise formation, including Bi₃₈-assembly, pre-organization in solution, followed by crystallization, confirming the less important role of the linker. We demonstrate photocatalytic hydrogen (H₂) generation with select members of the synthesized materials without the benefit of a co-catalyst. Band gap determination from X-ray photoelectron spectroscopy (XPS) and UV-vis data suggest the DDBS linker effectively absorbs in the visible range with ligand-to-Bi-node charge transfer. In addition, materials containing more Bi (larger Bi₃₈-nodes or Bi₆ inorganic chains) exhibit strong UV absorption, also contributing to effective photocatalysis by a different mechanism. All tested materials became black with extensive UV-vis exposure, and XPS, transmission electron microscopy, and X-ray scattering of the black Bi₃₈-framework suggest that Bi⁰ is formed in situ, without phase segregation. This evolution leads to enhanced photocatalytic performance, perhaps due to increased light absorption.

High-Valence Metal-Organic Framework Materials Constructed from Metal-Oxo Clusters: Opportunities and Challenges

Metal-organic framework (MOF), which possesses stable framework structure constructed by highly connected metal-oxo cluster nodes and organic linkers, has shown great promise in gas storage, adsorption, and separation, owing to the high surface areas, tunable pore aperture, and rich functional groups. In this review article, we summarized recent progress made in synthesizing high-valence MOF (e. g., UiO-66, MIL-125, PCN-22, and MIP-207) with metal-oxo cluster as metal source. Of particular note, recent breakthroughs in the preparation of UiO-66 and MIL-125 membranes with the corresponding Zr₆ -oxo and Ti₈ -oxo cluster sources (e. g., Zr₆ O₄ (OH)₄ (OAc)₁₂ and Ti₈ O₈ (OOCR)₁₆ clusters) possessing superior separation performance were highlighted. In the end, an outlook on the preparation of versatile high-valence MOF membranes with the corresponding metal-oxo clusters as metal sources was highlighted.

Effects of electrostatic neutralization of Keggin Fe13 on the removal of micro and nano plastic

The successful preparation and identification of Keggin-structure Fe₁₃ clusters in recent years further enriched the potential application scenarios of ferric coagulants. Comparing the coagulation efficiencies and mechanisms of Fe₁₃ in the removal of nano/microplastics with conventional polymeric Al₁₃ and monomeric Al/Fe, this work aimed to elucidate the coagulation behaviour of Fe₁₃ compared with the traditional mono ferric coagulant, which has the coagulation applied bottleneck of quick and violet hydrolysis. The results showed that Fe₁₃ has a similar electrostatic neutralization potential to Al₁₃, which could keep a positively charged species, especially in acid conditions. The Fe₁₃ species has a selective removal potential toward the microplastics with a polar functional group like ester. Moreover, Fe₁₃ could hydrolyze to form active sol-gel hydroxides in neutral and alkalinity conditions, which is like the behaviour of traditional monomeric Fe coagulants but seldom restabilization. The electrostatic neutralization of Fe₁₃ could enhance the removal of nano plastic from - 25-75% compared with monomeric Fe at pH 4. The higher floc density as a monomeric Fe coagulant and better electrostatic neutralization potential of Keggin Fe₁₃ posed a good prospect for Fe₁₃ to replace the monomeric Fe coagulants in conventional coagulation.

Ionic γ-FeO(OH) Nanocrystal Stabilized by Small Isopolymolybdate Clusters as Reactive Core for Water Oxidation

At near neutral to basic pH, hydrolysis-induced aggregation to insoluble bulk iron-oxide is often regarded as the pitfalls of molecular iron clusters. Iron-oxide nanocrystals are encouragingly active over the molecular clusters and/or bulk oxides albeit, stabilizing such nanostructures in aqueous pH and under turnover condition remain a perdurable challenge. Herein, an Anderson-type [Mo₇ O₂₄ ]^6- isopolyanion, a small (dimension ca. 0.85 nm) isolable polyoxometalate (POM) possessing only {31} atoms, has been introduced for the first time as a covalent linker to stabilize an infinitely stable and aqueous-soluble γ-FeO(OH) nanocore. During the hydrothermal isolation of the material, a partial dissociation of the parent [Mo₇ O₂₄ ]^6- may lead to the in situ generation of few analogous [Mo_x O_y ]^n- clusters, proved by Raman study, which can also participate in stabilizing the γ-FeO(OH) nanocore, Mo_x O_y @FeO(OH). However, due to high ionic charge on {Mo=O} terminals of the [Mo_x O_y ]^n- , they are covalently linked via Mo^VI -μ₂ O-Fe^III bridging to γ-FeO(OH) core in Mo_x O_y @FeO(OH), established by numerous spectroscopic and microscopic evidence. Such bonding mode is more likely as precedent from the coordination motif documented in the transition metal clusters stabilized by this POM. The γ-FeO(OH) nanocore of Mo_x O_y @FeO(OH) behaves as potent active center for electrochemical water oxidation with a overpotential, 263 mV @ 10 mA cm^-2 , lower than that observed for bare γ-FeO(OH). Despite of some molybdenum dissolution from the POM ligands to the electrolyte, residual anionic POM fragments covalently bound to the OER active γ-FeO(OH) core of the Mo_x O_y @FeO(OH) makes the surface predominantly ionic that results in an ordered electrical double layer to promote a better charge transport across the electrode-electrolyte junction, less likely in bulk γ-FeO(OH).

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.

Molecular Iron Oxide Clusters Boost the Oxygen Reduction Reaction of Platinum Electrocatalysts at Near‐Neutral pH

The oxygen reduction reaction (ORR) is a key energy conversion process, which is critical for the efficient operation of fuel cells and metal–air batteries. Here, we report the significant enhancement of the ORR‐performance of commercial platinum‐on‐carbon electrocatalysts when operated in aqueous electrolyte solutions (pH 5.6), containing the polyoxoanion [Fe₂₈(μ₃‐O)₈(L‐(−)‐tart)₁₆(CH₃COO)₂₄]²⁰⁻. Mechanistic studies provide initial insights into the performance‐improving role of the iron oxide cluster during ORR. Technological deployment of the system is demonstrated by incorporation into a direct formate microfluidic fuel cell (DFMFC), where major performance increases are observed when compared with reference electrolytes. The study provides the first examples of iron oxide clusters in electrochemical energy conversion and storage.

An [FeIII8] molecular oxyhydroxide

An [Fe^III₈] hexagonal bipyramid displays antiferromagnetic exchange between the two capping tetrahedral ions and the six ring octahedral ions resulting in a spin ground state of S = 10.

What are the inorganic nanozymes? Artificial or inorganic enzymes!

The research on inorganic nanozymes remains very active since the first paper on the “intrinsic peroxidase-like properties of ferromagnetic nanoparticles” was published in Nature Nanotechnology in 2007. However, there is...

An [FeIII30] molecular metal oxide

Dissolution of FeBr₃ in a mixture of acetonitrile and 3,4-lutidine in the presence of an amine results in the formation of an [Fe₃₀] molecular metal oxide containing alternating layers of tetrahedral and octahedral Fe^III ions. Mass spectrometry suggests the cluster is formed quickly and remains stable in solution, while magnetic measurements and DFT calculations reveal competing antiferromagnetic exchange interactions.

Aqueous Stability of Zirconium Clusters, Including the Zr(IV) Hexanuclear Hydrolysis Complex [Zr6O4(OH)4(H2O)24]12+, from Density Functional Theory

Framework materials constitute a broad family of solids that range from zeolites and metal-organic frameworks (MOFs) to coordination polymers. The synthesis of such network structures typically rely on precursor molecular building blocks. As an example, the UiO-66 MOF series is constructed of hexanuclear [Zr₆O₄(OH)₄(CO₂)₁₂] cluster nodes and linear carboxylate linkers. Unfortunately, these Zr MOF cluster nodes cannot currently be manufactured in a sustainable way, motivating a search for "green" alternative synthesis methods. Stabilizing the hexanuclear Zr(IV) cluster (i.e., the hexamer, {Zr₆¹²⁺}) without the use of organic ligation would enable the use of environmentally friendly solvents such as water. The Zr(IV) tetranuclear cluster (i.e., the tetramer, {Zr₄⁸⁺}) can be stabilized in solution with or without organic ligands, yet the hexamer has yet to be synthesized without supporting ligands. The reasons why certain zirconium clusters are favored in aqueous solution over others are not well understood. This study reports the relative thermodynamic instability of the hypothetical hexamer {Zr₆¹²⁺} compared to the ubiquitous {Zr₄⁸⁺} tetramer. Density functional theory calculations were performed to obtain the hydrolysis Gibbs free energy of these species and used to construct Zr Pourbaix diagrams that illustrate the effects of electrochemical potential, pH, and Zr(IV) concentration. It was found that the aqueous {Zr₆¹²⁺} hexamer is ∼17.8 kcal/mol less stable than the aqueous {Zr₄⁸⁺} tetramer at pH = 0, V = 0, and [Zr(IV)] = 1 M, which is an energy difference on the order of counterion interactions. Electronic structure analyses were used to explore trends in the highest occupied molecular orbital-lowest unoccupied molecular orbital gap, frontier molecular orbitals, and electrostatic potential distribution of these clusters. The evidence suggests that the aqueous {Zr₆¹²⁺} hexamer may be promoted with more strategic syntheses incorporating minimal ligands and counterions.

Exploring the Geometric Space of Metal-Organic Polyhedrons (MOPs) of Metal-Oxo Clusters

Metal organic polyhedra (MOPs) such as coordination cages and clusters are increasingly utilized across many fields, but their geometrically selective assembly during synthesis is nontrivial. When ligand coordination along these polyhedral edges is arranged in an unsymmetrical mode or the bridging ligand itself is nonsymmetric, a vast combinatorial space of potential isomers exists complicating formation and isolation. Here we describe two generalizable combinatorial methodologies to explore the geometrical space and enumerate the configurational isomers of MOPs with discrimination of the chiral and achiral structures. The methodology has been applied to the case of the octahedron {Bi₆Fe₁₃L₁₂} which has unsymmetrical coordination of a carboxylate ligand (L) along its edges. For these polyhedra, the enumeration methodology revealed 186 distinct isomers, including 74 chiral pairs and 38 achiral. To explore the programming of these, we then used a range of ligands to synthesize several configurational isomers. Our analysis demonstrates that ligand halo-substituents influence isomer symmetry and suggests that more symmetric halo-substituted ligands counterintuitively yield lower symmetry isomers. We performed mass spectrometry studies of these {Bi₆Fe₁₃L₁₂} clusters to evaluate their stability and aggregation behavior in solution and the gas phase showing that various isomers have different levels of aggregation in solution.

We describe combinatorial methodologies to explore the geometrical space and enumerate the configurational isomers of metal organic polyhedra with discrimination of the chiral and achiral structures. The methodology was applied to the octahedral {Bi₆Fe₁₃L₁₂} which has an unsymmetrical coordination of a carboxylate ligands (L) along its edges. For these polyhedra, the enumeration methodology revealed 186 distinct isomers, including 74 chiral pairs and 38 achiral. We used a range of ligands to synthesize several configurational isomers.

Capturing Lacunary Iron-Oxo Keggin Clusters and Insight Into the Keggin-Fe13 Cluster Rotational Isomerization

The formation mechanism of ferrihydrite is the key to understand its treatment of pollutants in waste water and purification of surface water and groundwater. Although emerging evidence suggests that formation of the ferrihydrite occurs through the aggregation of prenucleation clusters, rather than classical atom-by-atom growth, its formation mechanism remains unclear. Herein, an iron-oxo anionic cluster of [Fe₂₂ (μ₄ -O)₈ (μ₃ -OH)₂₀ (μ₂ -OH)₁₈ (CH₃ COO)₁₆ (H₂ O)₂ ]^4- viewed as a dimer of bivacant β-Keggin-Fe₁₃ clusters was for the first time obtained by using lanthanide ions as stabilizers. Upon dissolution in a mixed solution of isopropanol and water, the lacunary β-Keggin-Fe₁₃ cluster can transform into an α-Keggin-Fe₁₃ cluster, distinctly demonstrating that the Keggin-Fe₁₃ cluster rotational isomerization can be realized through the vacant Keggin-Fe₁₃ cluster.

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 Mn^IV cluster, fully formulated [Mn^IV₆Bi₂KO₉(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.

Butyltin Keggin Ion with a Rare Four-Coordinate Ca Center

Alkyltin clusters are exploited in nanolithography for the fabrication of microelectronics. The alkyltin Keggin family is unique among Keggin clusters across the periodic table; its members appear to favor the lower-symmetry β and γ isomers rather than the highly symmetrical α and ε isomers. Therefore, the alkyltin Keggin family may provide important fundamental information about the formation and isomerization of Keggin clusters. We have synthesized and structurally characterized a new butyltin Keggin cluster with a tetrahedral Ca²⁺ center, fully formulated [(BuSn)₁₂(CaO₄)(OCH₃)₁₂(O)₄(OH)₈]²⁺ (β-CaSn₁₂). The synthesis is a simple one-step process. Extensive solution characterization including electrospray ionization mass spectrometry, small-angle X-ray scattering, and multinuclear (¹H, ¹³C, and ¹¹⁹Sn) nuclear magnetic resonance shows β-CaSn₁₂ is essentially phase-pure and stable. This differs from the previously reported Na-centered analogues that always form a mixture of β and γ isomers, with facile interconversion. Therefore, this study has clarified prior confusion over complex spectroscopic and crystallographic characterization of the Na-centered analogues. Density functional theory calculations showed the following stability order: γ-CaSn₁₂ < γ-NaSn₁₂ < β-CaSn₁₂ < β-NaSn₁₂. The β analogue is always more stable than the γ analogue, consistent with experiment. Notable outcomes of this study include a rare tetrahedral Ca coordination, a Na-free alkyltin cluster (important for microelectronics manufacturing), and a better understanding of Keggin families built of different metal cations.

Atomically Precise Lanthanide-Iron-Oxo Clusters Featuring the ϵ-Keggin Ion

Atomically precise molecular metal-oxo clusters provide ideal models to understand metal oxide surfaces, self-assembly, and form-function relationships. Devising strategies for synthesis and isolation of these molecular forms remains a challenge. Here, the synthesis of four Ln-Fe oxo clusters that feature the ϵ-{Fe₁₃ } Keggin cluster in their core is reported. The {Fe₁₃ } metal-oxo cluster motif is the building block of two important iron oxyhydroxyide phases in nature and technology, ferrihydrite (as the δ-isomer) and magnetite (the ϵ-isomer). The reported ϵ-{Fe₁₃ } Keggin isomer as an isolated molecule provides the opportunity to study the formation of ferrihydrite and magnetite from this building unit. The four currently reported isostructural lanthanide-iron-oxo clusters are fully formulated [Y₁₂ Fe₃₃ (TEOA)₁₂ (Hyp)₆ (μ₃ -OH)₂₀ (μ₄ -O)₂₈ (H₂ O)₁₂ ](ClO₄ )₂₃ ⋅50 H₂ O (1, Y₁₂ Fe₃₃ ), [Gd₁₂ Fe₃₃ (TEOA)₁₂ (Hyp)₆ (μ₃ -OH)₂₀ (μ₄ -O)₃₂ (H₂ O)₁₂ ](ClO₄ )₁₅ ⋅50 H₂ O (2, Gd₁₂ Fe₃₃ ) and [Ln₁₆ Fe₂₉ (TEOA)₁₂ (Hyp)₆ (μ₃ -OH)₂₄ (μ₄ -O)₂₈ (H₂ O)₁₆ ](ClO₄ )₁₆ (NO₃ )₃ ⋅n H₂ O (Ln=Y for 3, Y₁₆ Fe₂₉ , n=37 and Ln=Gd for 4, Gd₁₆ Fe₂₉ n=25; Hyp=trans-4-Hydroxyl-l-proline and TEOA=triethanolamine). The next metal layer surrounding the ϵ-{Fe₁₃ } core within these clusters exhibits a similar arrangement as the magnetite lattice, and Fe and Ln can occupy the same positions. This provides the opportunity to construct a family of compounds and optimize magnetic exchange in these molecules through composition tuning. Small-angle X-ray scattering (SAXS) and high-resolution electrospray ionization mass spectrometry (HRESI-MS) show that these clusters are stable upon dissolution in both water and organic solvents, as a first step to performing further chemistry towards building magnetic arrays or investigating ferrihydrite and magnetite assembly from pre-nucleation clusters.

Liquid cell transmission electron microscopy and its applications

Transmission electron microscopy (TEM) has long been an essential tool for understanding the structure of materials. Over the past couple of decades, this venerable technique has undergone a number of revolutions, such as the development of aberration correction for atomic level imaging, the realization of cryogenic TEM for imaging biological specimens, and new instrumentation permitting the observation of dynamic systems in situ. Research in the latter has rapidly accelerated in recent years, based on a silicon-chip architecture that permits a versatile array of experiments to be performed under the high vacuum of the TEM. Of particular interest is using these silicon chips to enclose fluids safely inside the TEM, allowing us to observe liquid dynamics at the nanoscale. In situ imaging of liquid phase reactions under TEM can greatly enhance our understanding of fundamental processes in fields from electrochemistry to cell biology. Here, we review how in situ TEM experiments of liquids can be performed, with a particular focus on microchip-encapsulated liquid cell TEM. We will cover the basics of the technique, and its strengths and weaknesses with respect to related in situ TEM methods for characterizing liquid systems. We will show how this technique has provided unique insights into nanomaterial synthesis and manipulation, battery science and biological cells. A discussion on the main challenges of the technique, and potential means to mitigate and overcome them, will also be presented.

Energetics of paramagnetic oxide clusters: the Fe(iii) oxyhydroxy Keggin ion

The full energy landscape of the iron(iii) oxyhydroxy Keggin ion is explored through a combination of computation and predictive fitting.

An [FeIII 34 ] Molecular Metal Oxide

The dissolution of anhydrous iron bromide in a mixture of pyridine and acetonitrile, in the presence of an organic amine, results in the formation of an [Fe₃₄] metal oxide molecule, structurally characterised by alternate layers of tetrahedral and octahedral Fe^III ions connected by oxide and hydroxide ions. The outer shell of the complex is capped by a combination of pyridine molecules and bromide ions. Magnetic data, measured at temperatures as low as 0.4 K and fields up to 35 T, reveal competing antiferromagnetic exchange interactions; DFT calculations showing that the magnitudes of the coupling constants are highly dependent on both the Fe‐O‐Fe angles and Fe−O distances. The simplicity of the synthetic methodology, and the structural similarity between [Fe₃₄], bulk iron oxides, previous Fe^III–oxo cages, and polyoxometalates (POMs), hints that much larger molecular Fe^III oxides can be made.

Alkyltin clusters: the less symmetric Keggin isomers

The Keggin structure is prevalent in nature and synthesis, self-assembled from many metals across the periodic table as both isolated clusters and building blocks of condensed framework oxides. Here we present a one-step synthesis to obtain the sodium-centered butyltin Keggin ion in high yield and high purity, important for mechanistic nanolithography studies. Extensive solution characterization (small-angle X-ray scattering, 1H, 13C and 119Sn nuclear magnetic resonance, electrospray mass spectrometry) also confirms solutions contain only the Na-centered dodecamers. We report three butyltin Keggin structures: the β-isomer (β-NaSn12), the γ-isomer (γ-NaSn12), and a γ-isomer capped with an additional butyltin (γ-NaSn13). All Keggin ions presented here have the general formula [NaO4BuSn12(OCH3)12(O,OH)12] (Bu = butyl), and are of neutral charge. The lack of counterions (OH-) facilitates mechanistic lithographic studies without inference from hydrolysis chemistry. The methanol reaction media enables solubility and ligates the cluster, both important to obtain high purity materials. Despite the monospecific nature of the NaSn12 solutions, NMR reveals both isomer interconversion and ligand exchange. DFT computational comparisons of our three isolated structures, the capped β-isomer (β-NaSn13), along with hypothetical α-isomers (α-NaSn12 and α-NaSn13), showed that the stability ranks β-NaSn12 > γ-NaSn12 > α-NaSn12, consistent with experimental observation. The uncapped isomers were computationally determined to be more stable than the respective capped analogues. These clusters provide a unique opportunity to investigate the lower-symmetry Keggin isomers, and to determine structural factors that control isomer selectivity as well as isomer labilization.

The Role of Bi3+ in Promoting and Stabilizing Iron Oxo Clusters in Strong Acid

Metal oxo clusters and metal oxides assemble and precipitate from water in processes that depend on pH, temperature, and concentration. Other parameters that influence the structure, composition, and nuclearity of "molecular" and bulk metal oxides are poorly understood, and have thus not been exploited. Herein, we show that Bi³⁺ drives the formation of aqueous Fe³⁺ clusters, usurping the role of pH. We isolated and structurally characterized a Bi/Fe cluster, Fe₃ BiO₂ (CCl₃ COO)₈ (THF)(H₂ O)₂ , and demonstrated its conversion into an iron Keggin ion capped by six Bi³⁺ irons (Bi₆ Fe₁₃ ). The reaction pathway was documented by X-ray scattering and mass spectrometry. Opposing the expected trend, increased cluster nuclearity required a pH decrease instead of a pH increase. We attribute this anomalous behavior of Bi/Fe(aq) solutions to Bi³⁺ , which drives hydrolysis and condensation. Likewise, Bi³⁺ stabilizes metal oxo clusters and metal oxides in strongly acidic conditions, which is important in applications such as water oxidation for energy storage.

Magneto-Structural Analysis of Iron(III) Keggin Polyoxometalates

A computational study and magnetic susceptibility measurements of three homonuclear Fe(III) Keggin structures are herein presented: the [FeO₄@Fe₁₂F₂₄(μ-OCH₃)₁₂]^5- anion (1), the [Bi₆{FeO₄@Fe₁₂O₁₂(OH)₁₂}(μ-O₂CCCl₃)₁₂]⁺ cation (2) and its polymorph [Bi₆{FeO₄@Fe₁₂O₁₂(OH)₁₀(H₂O)₂}(μ-O₂CCF₃)₁₀]³⁺ (3). These results are contrasted with the exchange interactions present in the previously characterized [Fe₆(OH)₃Ge₂W₁₈O₆₈(OH)₆]^11- and [H₁₂As₄Fe₈W₃₀O₁₂₀(H₂O)₂]^4- anions. The computational analysis shows that the most significant antiferromagnetic spin coupling takes place at the junction between each of the {Fe₃O₆(OH)₃}/{Fe₃F₆(OCH₃)₃} framework motifs, a possibility that had been previously discarded in the literature on the basis of the Fe-Fe distances. For all the examined iron(III) Keggin structures, it is found that the magnitude of the magnetic couplings within each structural subunit follows the same trend.

Frontiers of water oxidation: the quest for true catalysts

Development of advanced analytical techniques is essential for the identification of water oxidation catalysts together with mechanistic studies.