Polyoxovanadate-alkoxide clusters are redox-active molecular oxides offering profound electronic tunability. The oxygen-deficient species [(V6O5)(μ6-O)(μ2-OCH3)12] is an ideal platform for probing the multisite localization and delocalization of redox states. Here, we introduce the redox topology modulation which is governed by the position of the central μ6-O oxygen and ligand coordination at the oxygen-deficient site as the mechanism controlling the stability of different electromers. The noncoordinated cluster exhibits a localized, Robin-Day class I/II hybrid ground state, featuring a V(III) center at the vacancy defect. We demonstrate computationally that ligand-field tuning inverts this behavior; coordination of a strong donor destabilizes the localized topology, stabilizing an electromer with all V(IV) topology as the new ground state. Time-dependent density functional theory calculations show that photoexcitation of species where centers are V(IV) triggers photoinduced intervalence charge transfer regenerating a valence-trapped class II excited state. This work establishes the redox topology modulation as a rational design principle for molecular switches, where the fundamental electronic topology can be toggled by chemical stimulus and/or by light. Furthermore, our results suggest that the Robin-Day classification should be revised and extended for multicenter systems, where valence behavior is better understood as excitation-specific rather than molecular-specific.

Polyoxovanadate-alkoxides are a class of polynuclear earth-abundant transition metal compounds with highly tunable catalytic, redox, and magnetic properties. These species are ideal electroactive molecular agents for environmentally friendly energy storage applications, such as non-aqueous redox flow batteries. As such, it is fundamental to understand structure-redox relationships in the multielectron redox profiles of polyoxovanadate-alkoxides species. Here, we studied the redox properties of monomeric vanadate-alkoxide species that control the formation of two distinct chemical spaces with mix-valent clusters and fully reduced cyclic species. The accuracy of various density functional theory approximations and basis sets choices was benchmarked against domain-based local pair natural orbital coupled cluster, specifically DLPNO-CCSD(T). To further confirm the broader applicability of density functional theory in higher nuclearity species, we studied the redox profile of [(VV6-nVIVnO6)(O)(OCH3)12]4-n, [(Nb = O)(VV5-nVIVnO5)(O)(OCH3)12]4-n, [(M-OCH3)(VV5-nVIVnO5)(O)(OCH3)12]4-n (M = Ti, Zr, or Hf), and [(Fe-Cl)(VV5-nVIVnO5)(O)(OCH3)12]3-n hexanuclear polyoxovanadate-alkoxide species. The calculated redox profiles of these species were in good agreement with experimental data with an average absolute error of 0.11 V.

Abstract Naturally occurring Zr with ∼2% Hf must be separated to high levels of purity for both nuclear and semiconductor industries. Since the dawn of the nuclear age, this process is executed on an industrial scale by selective Hf-isothiocyanate (N═C═S) extraction into methyl isobutyl ketone (MIBK). Yet SCN-MIBK mixtures yield toxic, noxious byproducts, also giving rise to performance variability. Here, we eliminate the MIBK and instead precipitate Hf-rich species via inclusion of choline, a common food additive, in the aqueous phase. Augmentation with sulfate, acid, and ammonium finely tunes Zr/Hf nuclearity and coordination, increasing separation factors (SF) from the industrial standard of 6/7 to 33, with Zr(Hf)(N═C═S)6/72/3– and Zr(Hf)(N═C═S)5(SO4)3 – as key Hf-rich coordination complexes. Solid-state (powder and single-crystal X-ray diffraction) and solution (Raman spectroscopy and small-angle X-ray scattering) characterization show increasing acid decreases nuclearity from tetrameric to trimeric to dimeric to monomeric Zr/Hf complexes by simultaneously decreasing oxyhydroxide ligation and increasing N═C═S ligation. In agreement with experiment, computation predicts increasing thermodynamic favorability for lower-nuclearity Hf species (monomer) compared to Zr species. This study demonstrates that precise atomic-level understanding and manipulation of both metal coordination and noncovalent interactions can yield competitive precipitation-based separations that have been notoriously inferior to their solvent extraction counterparts.

Polyoxovanadate clusters offer atomically defined platforms for studying how electronic structure influences reactivity at molecular metal-oxide surfaces. In this computational study, we investigated the coupling between reduction state and alkylation reactivity in the hexavanadate cluster [V6O13(TRIOLX)2]2- (TRIOL = tris(hydroxymethyl)methane; X = -CH3, -NO2) through a systematic analysis of successive methylation and reduction pathways using density functional theory and microkinetic modeling. The calculations show that changes in electron count redistribute the nucleophilicity of bridging μ2-oxo ligands, leading to redox-dependent regioselectivity during sequential methylation. Microkinetic simulations further indicate that several thermodynamically accessible intermediates do not accumulate because they are rapidly consumed in subsequent transformations, consistent with the need for stepwise synthetic strategies to isolate certain dimethylated species. In addition, substituent effects on the TRIOL ligands follow Hammett-type correlations, allowing rapid estimation of electronic trends without additional quantum chemical calculations. Overall, the results provide a detailed picture of how reduction state influences accessibility and selectivity during polyoxovanadate methylation.

Abstract There has been a renaissance in high valent lanthanide chemistry, which has resulted in the first examples of tetravalent praseodymium (Pr) and terbium (Tb) in molecular systems. These feats have been achieved with tailored ligands that facilitate tetravalent lanthanide stability in non-aqueous conditions. The next step in realizing the potential of high valent lanthanide chemistry is moving towards complexes that are stable in ambient and aqueous conditions. In this investigation, we advance this paradigm by obtaining definitive evidence of Tb(IV) in a molecular system under aqueous conditions utilizing the lacunary Wells-Dawson polyoxometalate, K 10 P 2 W 17 O 61 •20H 2 O. Herein we present the single crystal structure of K 16 Tb(IV)(P 2 W 17 O 61 ) 2 •39.85H 2 O (Tb(IV)W 34 ) as well as extensive spectroscopic evidence obtained via UV–Vis-NIR, X-ray absorption near edge spectroscopy, continuous wave X-band electron paramagnetic resonance spectroscopy and SQUID magnetometry measurements to confirm the oxidation state of Tb in W 34 complexes as +4.

Polyoxovanadate-alkoxides are a growing family of earth-abundant first-row transition metal polynuclear species highly promising for their tunable redox properties. The speciation and nucleation chemical space of these species is divided into two groups: 1) fully oxidized V(V) monomeric precursors that aggregate into Lindqvist-type clusters and 2) reduced V(IV) precursors forming cyclic structures. The nucleation of cyclic polyoxovanadate-alkoxides with varying alkyl chain lengths, the impact of the presence of templating anions, and their subsequent evolution to the Lindqvist-type congener were studied by using density functional theory. The evolution of cyclic polyoxovanadate-alkoxides to oxygen-vacant cluster structures was found to be endergonic, in agreement with previous experimental work. Moreover, the reactivity with traces of water in alcohol solvents was confirmed to be the main thermodynamic driving force toward the formation of the mixed-valent Lindqvist-type polyoxovanadate species.

Polyoxopalladates (POPs) are an extension of classic polyoxometalates. The two most frequently observed POP topologies are the nanostar (Pd15) and the cube (Pd12), where the heterometal cation in the central cavity may influence topology selection. Here, we crystallize ten lanthanoid-centered, phenylphosphonate-capped POPs (YIII, LaIII, CeIII, PrIII, NdIII, SmIII, EuIII, GdIII, YbIII, and LuIII) and one actinide-centered (ThIV) POP. Single-crystal X-ray analysis reveals that most LnIII favors the Pd15 nanostar, while Yb and Lu favor the Pd12 cube. Between Tb and Tm, no single-crystal growth occurred, challenging our initial hypothesis that topology selection was entirely based on the central metal cation size. Computation supported this hypothesis; the transition from Pd15 to Pd12 as the favored topology occurs in the third quarter of the lanthanide series. On the other hand, PXRD of all the crystallized/precipitated bulk materials suggested Pd15 is always the favored topology, and inspection of the lattices reveals that Pd15-Na interactions likely promote incipient crystallization, while no such interactions are seen in the Pd12 lattices. Electrospray ionization mass spectrometry confirmed the presence of both Pd12 and Pd15 for the smaller lanthanides (Yb), only Pd15 for the larger lanthanides (Ce), and no clusters were observed between Tb and Tm, where crystal growth was also challenging.

The synthesis of a niobium-(V) substituted polyoxovanadate-alkoxide (NbPOV-alkoxide; [NbV5O7(OCH3)12]) is reported. Addition of 5,10-dihydrophenazine to [NbV5O7(OCH3)12] results in formation of the 2 H+/e- reduced assembly, [NbV5O6(OH2)-(OCH3)12], via proton-coupled electron transfer. [NbV5O6(OH2)-(OCH3)12] has a bond dissociation free energy (BDFE-(O-H)avg) of 62.3 kcal mol-1, resembling that of its homometallic congener, [V6O6(OH2)-(OCH3)12] (BDFE-(O-H)avg = 62.3 kcal mol-1). Single-crystal X-ray diffraction reveals that [NbV5O6(OH2)-(OCH3)12] exists as a mixture of two structural isomers, with the vanadium-aquo moiety formed in either the trans- or cis- positions relative to the Nb-(V) dopant. The formation of two regioisomers is a departure from prior observations of H-atom uptake at the surface of heterometal-doped polyoxovanadate-alkoxides, and is credited to distortions in intercluster metal oxygen bond lengths. Improved selectivity for the trans- isomer is achieved by decreasing the dielectric constant of the reaction solvent. Computational analysis predicts the preferential formation of trans-[NbV5O6(OH2)-(OCH3)12] in solvents with low dielectric constants as a result of changes to the dispersed charge across the assembly.

The metallation, bonding, and electronic structure of redox active incomplete-cubane M3(μ3-Q)(μ2-Q)3 chalcogenide clusters with early actinides were studied using density functional theory and multireference methodologies. We confirmed that the incorporation of low-valent U(III) leads to its oxidation to U(IV) due to an intramolecular redox reaction with the molybdenum sulfide core. On the contrary, open questions remain with respect to the oxidation state of low-valent transuranic elements upon their coordination to the molybdenum sulfide core. Density functional theory calculations indicate that the transuranic center remains An(III), while complete active space calculations suggest an actinide oxidation analogous to the one observed in the uranium species. The predominantly electrostatic bonding between the actinides and the molybdenum sulfide cluster was assigned using the quantum theory of atoms in molecules. Our results on clusters with harder and softer chalcogenides are in agreement with a preferential soft-soft interaction between the actinide and its support.

Hexameric tetravalent zirconium and hafnium molecular metal oxides clusters are key building blocks of many metal-organic frameworks; however, the chemical space to form all possible MOF nodes is vast, containing many potential new clusters. Our computational study provides a complete picture of the structure, properties, and reactivity of two tetrameric zirconium and hafnium [M4(μ2-η2:η2-O2)x(μ2-OH)8-2x(H2O)16]8+ polycationic clusters. The electronic structure of the studied species has the characteristic polyoxometalate oxygen-based and metal-based bands in the valence region. The energetics for the evolution of pure metal clusters into mixed-metal clusters revealed that only the incorporation of zirconium into hafnium clusters is thermodynamically favorable. We confirmed that the incorporation of up to four peroxide ligands is thermodynamically favorable; however, the experimental absence of rich peroxide species with three or more peroxides is attributed to their thermal degradation. The mechanism for peroxide incorporation involves the partial dissociation of the cluster rather than complete dissociation.

Innovating robust separation methods for critical elements extraction and reuse is important to sustain the global consumption of microelectronics, energy, and pharmaceuticals. Effective separations via liquid-liquid extraction (LLE) requires molecular-level understanding and optimization of solution speciation. Here, we design LLE for palladium based on polyoxopalladates (POPs), where Pd is critical for microelectronics, catalysts, and drug production. Mild solution conditions are designed to evaluate the role of templating heterometals and ligands (arsenate, phenylarsonate, phenylphosphonate, acetate, phosphate) that drive POP assembly. Small-angle X-ray scattering, electrospray ionization mass spectrometry, UV-vis spectroscopy, along with compositional analysis, respectively described speciation and extraction efficiency (including separation factors for competitive Pd-Ni separation). Most effective LLE of Pd (>99.9%) are arsenate/phenylphosphonate/acetate capped hexamers or heptamers without templating metals. These fragments of the larger, prior-reported Pd12/Pd15/Pd84 POPs represent simple formulations, important for translation to scaled-up processes. Comparing alkali-acetate buffers highlight that potassium is more effective than lithium or sodium, presumably due to strong ion-pairing between Pd-oxoanions and the larger alkali, facilitating transport across the aqueous-organic interface. Pd and Au-Pd LLE studies yielded the first K+-charge balanced and K+-templated POPs, illustrating simply swapping the alkali from Na+ (usually employed) to K+ can enable isolation of new topologies and inspire new applications.

The lanthanides form a chemically cohesive series, and identifying differences that can be leveraged to separate near neighbors remains an ongoing need. Here, the synthesis, structural chemistry, and spectroscopic properties of a neodymium-acetylacetonate monomer, [Nd(acac)3(H2O)2] (Nd-1), and an acac-bridged Nd-dimer, [Nd2(acac)6(H2O)2] (Nd-2), are described. Interestingly, the Nd-dimer readily precipitates from solution; however, efforts to crystallize the Dy analog using the same synthetic approach are unsuccessful. Yet heterometal dimers, [Nd2-xDyx(acac)6(H2O)2] (NdDy-2), can be prepared from mixed-metal solutions. To understand these differences in crystallization behavior of Nd- and Dy-acac complexes, solution speciation of Ln-acac systems is examined using nano-electrospray ionization-mass spectrometry (nESI-MS). Both homometal and heterometal solutions are prepared (Ln = Nd, Dy, Nd/Dy). nESI-MS of the solutions showed presence of homometallic dimeric species for Nd and Dy and peaks consistent with both homometallic and heterometallic complexes in mixed-metal solutions. Density functional theory is used to further understand the intrinsic differences in nucleation energetics in dimeric homo- and heterometallic species. Overall, this work provides fundamental insight into the correlation between solution- and solid-state structural units and differences in the crystallization behavior of Ln complexes; the latter is particularly relevant to separating metal ions with otherwise very similar chemical behavior.

We report the synthesis and characterization of pseudo-tetrahedral f-element complexes bearing the sterically demanding bidentate di(silylamido)silane ligand, {Me2Si(NSiiPr3)2}2- (Me2SiN†N†). Two equivalents of the dipotassium salt, [Me2SiN†N†K2] (1), react with either MIIII3(THF)n (M = Ce, Pr, n = 4; M = Tb, n = 3.5) or [UIV(BH4)4]n affording trivalent [MIII(Me2SiN†N†)2{K(toluene)2}]n (2M; M = Ce, Pr, Tb) and tetravalent [UIV(Me2SiN†N†)2] (3U), respectively. Complex 2Ce was oxidized to tetravalent [CeIV(Me2SiN†N†)2] (3Ce) with AgII, but 2Pr and 2Tb were unreactive under these conditions. The electrochemical accessibility of M(IV/III) couples in 2M (M = Ce, Pr, Tb) and 3M (M = Ce, U) was examined by cyclic voltammetry. For 2Ce and 3Ce, a quasi-reversible Ce(IV/III) couple was observed (Epc = -1.19 V vs [Fc]+/0), but complexes 2Pr and 2Tb showed anodic waves attributed to ligand oxidation only, which is supported by computational analyses. For complex 3U, U(IV/III), U(V/IV), and U(VI/V) redox couples with E1/2 values of -2.38, -0.89, and -0.11 V vs [Fc]+/0, respectively, were found. All complexes have been characterized variously by single-crystal X-ray diffraction, multinuclear NMR, UV-vis-NIR, and IR spectroscopies, SQUID magnetometry, and elemental analyses as appropriate. Electronic structure calculations were employed to rationalize electrochemical results for 2Pr, 2Tb, and 3Ce.

The mechanism of proton-coupled electron transfer at the surface of titanium-substituted polyoxovanadate-alkoxide clusters can be tuned by judicious selection of substrate.

Boron triiodide (BI3) is a strong Lewis acid that has underexplored reactivity. Herein we detail a new reaction utilizing BI3 to reduce acenes to their monohydrogenated products while investigating the mechanistic features of these reductions. Both pure BI3 or BI3 generated in situ from KBH4 and I2 are competent in the reduction, which is performed at room temperature and results in isolated yields up to 99%. Acenes with substitution on terminal and central rings were explored, with heteroatoms on the central ring being cleaved in the reaction. Mechanistic experiments include deuterium labeling, reaction time course monitoring, radical trap experiments, and control experiments. We observed that water is a necessary additive for the reduction using pure BI3. Anhydrous HI was found to be insufficient to reduce acenes under these conditions, with or without BI3; however, HI formation was observed and subsequently consumed at rates that indicate its involvement in product formation. Ultimately, experimental evidence suggests that either a BI3•OH2 adduct protonates the acene to initiate the reaction, or a hydrolysis product of BI3 catalyzes HI addition to the acene. Subsequently, three possible pathways utilizing HI are outlined to generate the final dihydro products and iodine as the oxidized byproduct.

We report the synthesis, the magnetic, electronic and spatial characterization of the novel compound [Dy (HDPA)(2)(H(2)DPA)]center dot 1.5 H2O (H(2)DPA = dipicolinic acid), 1. The reaction yields ca. 76 % of a nonacoordinated formally Dy3+ compound possessing a rare tricapped trigonal prismatic geometry whose spectroscopic analyses by X-ray crystallography, H-1 NMR, UV-Vis, and ESR are presented herein. The bond distances and angles coincide with most Dy3+ reports. DFT description of 1 suggests a non-innocent Dy3+ coordinated to a radical anion ligand, and other 2 non radical ligands and CASSCF/CASPT2 description support this description. Specifically, ESR confirms the presence of such a free radical system. At low temperatures (T approximate to 2 K), the ESR spectrum shows a similar free radical signal with a g value of 2.006, suggesting the delocalization of the electron in the pi system. Moreover, the comparison of results with the equivalent Dy3+ with no free radical interaction is also included as a Dy3+ counterpart, [Dy3+(HDPA)(3)] 2.

Tetravalent zirconium and hafnium molecular metal oxides clusters are key building blocks of many metal-organic frameworks; however, the chemical space to form all possible MOF nodes is vast, containing many potential new clusters. Our computational study provides a complete picture of the structure, properties, and reactivity of two tetrameric zirconium and hafnium [M4(μ2-η2:η2-O2)x(μ2-OH)8-2x(H2O)16]8+ polycationic clusters. The electronic structure of the studied species has characteristic polyoxometalate oxygen-based and metal-based bands in the valence region. The energetics for the evolution of pure metal clusters into mixed-metal clusters revealed that only the incorporation of zirconium into hafnium clusters is thermodynamically favorable. We confirmed that the incorporation of up to four peroxide ligands is thermodynamically favorable; however, the experimental absence of rich peroxide species with three or more peroxides is attributed to their thermal degradation. The mechanism for peroxide incorporation involves the partial dissociation of the cluster rather than complete dissociation.

Low-valent uranium(III) primary phosphido complexes supported by hydrotris(3,5-dimethylpyrazolyl)borate (Tp*) were synthesized with phosphines of varying steric and electronic profiles. Compounds were characterized by multinuclear NMR spectroscopy (1H, 11B, 31P NMR), infrared spectroscopy, electronic absorption spectroscopy, X-ray crystallography, and quantum chemical calculations.

An unprecedented sandwich complex of the actinides is synthesized from the treatment of [UI2(HMPA)4]I (HMPA = OP(NMe2)3) (2) with 3 equiv. of K(C14H10) to give the neutral, bis(arenide) species U(η6-C14H10)(η4-C14H10)(HMPA)2 (1). Solid-state X-ray, SQUID magnetometry, and XANES analyses are consistent with tetravalent uranium supported by [C14H10]2- ligands. In one case, treatment of 1 with an equiv. of AgOTf led to the isolation of U(η6-C14H10)2(HMPA)(THF) (3), formed from ring migration and haptotropic rearrangement. Complete active space (CASSCF) calculations indicate the U-C bonding to solely consist of π-interactions, presenting a unique electronic structure distinct from classic actinide sandwich compounds.