Mechanochemical Polyoxometalate Super-Reduction with Lithium Metal

In this first systematic investigation of mechanochemical polyoxometalate (POM) reduction, (TBA)3[PMo12O40] was reacted with n equiv of lithium metal (n = 1-24) to generate PMo12/n products which were shown to be mixtures of electron-rich PMo12Lix species. FTIR analysis revealed the lengthening/weakening of terminal Mo═O bonds with increasing levels of reduction, while EXAFS spectra indicated the onset of Mo-Mo bond formation at n ∼ 8 and a significant structural change at n > 12. Successive MoVI reductions were monitored by XANES and XPS, and at n = 24, results were consistent with the formation of at least one MoIV-MoIV bonded {MoIV3} triad together with MoV. Upon dissolution, the PMo12Lix species present in the solid PMo12/n products undergo electron exchange and single-peak 31P NMR spectra were observed for n = 1-12. For n ≥ 16, changes in solid state and solution 31P NMR spectra coincided with the emergence of features in the UV-vis spectra associated with MoV-MoV and {MoIV3} bonding in an ε-Keggin structure. Bonding between {Li(NCMe)}+ and 2-electron-reduced PMo12 in (TBA)4[PMo12O40{Li(NCMe)}] suggests that super-reduction gives rise to more extensive Li-O bonding that ultimately causes lithium-oxide-promoted TBA cation decomposition and POM degradation, which might explain the appearance of XPS peaks for Mo2C at n ≥ 16. This work has revealed some of the complex, unexplored chemistry of super-reduced POMs and establishes a new, solvent-free approach in the search for a better fundamental understanding of the electronic properties and reactivity of electron-rich nanoscale metal oxides.

Deeply Reduced Chiral MoIV6-Polyoxometalates with Highly Enhanced Electronic Conductivity

A series of deeply reduced MoIV6-polyoxometalates (POMs), [MoIV6(bpy)6MoVI2O16S2]·10H2O (1) (bpy = 2,2'-bipyridine), [MoIV6(bpy)4(L-TrA)2MoV4(bpy)2O14S4]2·36.5H2O (2), and [MoIV6(bpy)4(D-TrA)2MoV4(bpy)2O14S4]2 (3) (TrA = tartaric acid), were obtained from one-pot hydrothermal syntheses. They feature the bioxo-bridged [MoIV3O3S]4+ incomplete cuboidal cores stabilized by strong triangular metal-metal bonds and datively chelated bpy π-ligands. The two apical MoVIO4 in 12e-Mo8 (1) and two [MoV2O3STrAbpy] in 16e-Mo10 (2, 3) complete the octahedral coordination geometry of the deeply reduced MoIV, not counting the MoIV-MoIV d2-d2 bonds. The aromatic planar bpy π ligands form a two-dimensional face-to-face π stacking supramolecular structure, which are further strengthened by point-to-face C-H···π interactions in the π stacked layers. 16e-Mo10 (2, 3) has stronger intramolecular face-to-face π stacking interactions, which are interconnected by intermolecular C-H···π into a three-dimensional framework structure. These structural features account for their highly enhanced electronic conductivities (1, 6.19 × 10-8 S cm-1; 2, 2.18 × 10-7 S cm-1), providing a new way of thinking to improve electronic conductivity of POMs. 1-3 have been described by first-principles density function theory (DFT) calculations and also characterized by elemental analyses, powder X-ray diffraction (PXRD), X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA), solid UV-visible spectra, and circular dichroic (CD) spectra.

Aqueous solutions of super reduced polyoxotungstates as electron storage systems

Due to the increasing energy density demands of battery technology, it is vital to develop electrolytes with high electron storage capacity. Polyoxometalate (POM) clusters can act as electron sponges, storing and releasing multiple electrons and have potential as electron storage electrolytes for flow batteries. Despite this rational design of clusters for high storage ability can not yet be achieved as little is known about the features influencing storage ability. Here we report that the large POM clusters, {P5W30} and {P8W48}, can store up to 23 e- and 28 e- per cluster in acidic aqueous solution, respectively. Our investigations reveal key structural and speciation factors influencing the improved behaviour of these POMs over those previously reported (P2W18). We show, using NMR and MS, that for these polyoxotungstates hydrolysis equilibria for the different tungstate salts is key to explaining unexpected storage trends while the performance limit for {P5W30} and {P8W48}, can be attributed to unavoidable hydrogen generation, evidenced by GC. NMR spectroscopy, in combination with the MS analysis, provided experimental evidence for a cation/proton exchange process during the reduction/reoxidation process of {P5W30} which likely occurs due to this hydrogen generation. Our study offers a deeper understanding of the factors affecting the electron storage ability of POMs and provides insights allowing for further development of these materials for energy storage.

Making Heterometallic Metal-Metal Bonds in Keggin-Type Polyoxometalates by a Six-Electron Reduction Process

Polyoxometalates (POMs) represent a promising class of molecular electron reservoirs. However, their multielectron reduction gives rise to intricate physical-chemical phenomena that must be fully understood for their future use in energy-storage devices. Herein, we show that bulk electrolysis of the archetypal Keggin-type POM [Si(W^VI₂Mo^VIO₁₀)(W^VI₃O₁₀)₃]^4- in aqueous solution leads to the six-electron-reduced derivative [Si(W^IV₂Mo^IVO₇(H₂O)₃)(W^VI₃O₁₀)₃]^4- (notated SiW₁₁Mo-VI') in which the mixed-metal triad acts as a storage unit for six electrons and six protons. X-ray diffraction analysis and multinuclear NMR (¹⁸³W and ⁹⁵Mo) studies reveal that this electron-rich species represents the first example of POMs containing heterometallic metal-metal bonds between addenda centers. This electron-rich POM can be further reduced through multielectronic events, while its full oxidation restores the structure of the oxidized parent ion. Remarkably, the formation of SiW₁₁Mo-VI' results from a fast clustering process compared to that observed for the entirely W-based analogue, revealing that the formation of metal-metal bonds in the mixed-metal Mo/W POM is facilitated because the reaction rate is not limited by a slow disproportionation step. Last, we evaluate the supramolecular properties of SiW₁₁Mo-VI' using a method based on the cloud-point measurement of a nonionic surfactant. This investigation demonstrates that the clustering process has dramatic consequences on the solution behavior of the POM, canceling its superchaotropic character due to a local structuring effect of the hydration shell. These fundamental results pave the way for applications using the massive electron-storage properties of mixed-metal POMs.

Extended Mo3IV-Polyoxometalates: 1D-[Mo6IVMo4VIGe2ZnO31py9 (H2O)]4- and 2D-[Mo3IVMo9VIZn6P7O56py8]+ (py = Pyridine)

The first examples of extended Mo₃^IV-polyoxometalates (Mo₃^IV-POMs), 1D-H₂ [Ge₂Mo₆^IVMo₄^VIO₃₁Zn(H₂O)py₉]·2Hpy·HOCH₂CH₂ OH^•4H₂O(H₂[1]·2Hpy^•glycol·H₂O) and 2D-[P@Mo₃^IVMo₉^VIZn₆(PO₄)₆O₃₂py₈]Cl·2py·7H₂O ([2]·Cl^•2py·7H₂O), were prepared through the solvothermal partial oxidation of [Mo₃^IVO₂(O₂CCH₃)₆(H₂O)₃]ZnCl₄ in py/H₂O containing glycol (for 1) or H₃PO₄ (for 2). They were characterized by X-ray crystallography and elemental analyses. Their electronic structure and bonding were discussed on the basis of density functional theory (DFT) theoretical calculations. X-ray photoelectron spectroscopy, IR/UV-vis spectra, powder X-ray diffraction, thermal gravimetric analysis, and mass spectrometry were also performed and discussed for 2D-2.

Metal-metal bonds in polyoxometalate chemistry

Half a century ago, F. Albert Cotton emphasized the relevance of metal-metal bonding in the constitution of cluster materials. Based on his description, nanoscale polyoxometalates (POMs) normally would not be regarded as cluster materials. One reason is that metal-metal bonding is typically associated with inorganic systems featuring metal centres in low oxidation states, a feature that is not common for POMs. However, over the past decades, there have been increasing reports on POMs integrating different types of metal-metal bonding. This article conceptualises and reviews the area of metal-metal bonded POMs, and their preparation and physicochemical properties. Attention is given to the changes in the electronic structure of POMs, the emergence of covalent dynamics and its impact on the development of applications in catalysis, nanoswitches, donor-acceptor systems, electron storage materials and nanoelectronics (i.e., "POMtronics").

Controlled Synthesis and Properties of 3d-4f Metals Co-doped Polyoxometalates-Based Materials

It is challenging to explore and prepare polyoxometalates-based nanomaterials (PNMs) with controllable morphologies and diversiform components. Herein, 3d-4f metals are introduced into isopolyoxometalates and Anderson-type polyoxometalates, CeCdW₁₂ nanoflower and EuCrMo₆ microflaky have been fabricated respectively. A series of control experiments are carried out to identify the impact factors on the rare morphologies in PNMs. Furthermore, upon excitation at 396 nm, the emission spectrum of EuCrMo₆ displays five prominent f - f emitting peaks at 674, 685, 690, 707, and 734 nm that are assigned to Eu^{3+ 5}D₀ → ⁷F_J (J = 0, 1, 2, 3, 4) transitions. Meanwhile, the VSM results show that the Cr⁺³ ions in EuCrMo₆ display anti-ferromagnetic interactions when the temperature is lower than - 17.54 K. After rising temperature, this material exhibits paramagnetic property. This work opens up strategies toward the brand new morphologies and components of PNMs, endowing this kind of material with new functions.

Preparation, characterization and electrocatalysis performance of a trimeric ruthenium-substituted isopolytungstate

The ruthenium-containing isopolytungstate Rb₁₀K₃H₆[SeO₃(H₉Ru_5.5W_30.5O₁₁₄)]Cl₃·48H₂O was isolated and then served as a catalyst, showing electrochemical catalytic activity towards the oxidation reaction of nitrite.