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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 [Nb10O28]6− reassembles into a new decaniobate, [Nb10O25(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. [ABSTRACT FROM AUTHOR]

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 [Y12Fe33(TEOA)12(Hyp)6(μ3‐OH)20(μ4‐O)28(H2O)12](ClO4)23⋅50 H2O (1, Y12Fe33), [Gd12Fe33(TEOA)12(Hyp)6(μ3‐OH)20(μ4‐O)32(H2O)12](ClO4)15⋅50 H2O (2, Gd12Fe33) and [Ln16Fe29(TEOA)12(Hyp)6(μ3‐OH)24(μ4‐O)28(H2O)16](ClO4)16(NO3)3⋅n H2O (Ln=Y for 3, Y16Fe29, n=37 and Ln=Gd for 4, Gd16Fe29n=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. [ABSTRACT FROM AUTHOR]

Abstract: 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, Fe3BiO2(CCl3COO)8(THF)(H2O)2, and demonstrated its conversion into an iron Keggin ion capped by six Bi3+ irons (Bi6Fe13). 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. [ABSTRACT FROM AUTHOR]