Metal-metal bonded pentamolybdate hybrids as electron storage materials

Two electron-rich, metal-metal bonded pentamolybdate hybrids, 2D-[MoIV3MoVI2O10Sr2(H2O)5(C6H4O7)2py3]·3.5H2O (1) and 1D-[MoIV3MoVI2O10Sr(H2O)3(C6H4O7)2py3]·py·2[NH2(CH3)2]·2H2O (2, py = pyridine), were prepared by the partial solvothermal oxidation of [MoIV3O2(O2CCH3)6(H2O)3]ZnCl4·8H2O and citric acid in py/H2O (for 1) or py/H2O/DMF (for 2). Both 1 and 2 feature a triangularly metal-metal bonded incomplete cuboidal [MoIV3O4] unit. Redox-active 6e-[MoIV3O4] units can serve as an "electron sponge" to store/release six electrons reversibly via Δ-bond breakage and re-formation during charging/discharging processes. 1 and 2 further form 3D and 2D supramolecular structures, respectively, through slipped π-π stacking interactions between the pyridine ligands. Both the incorporated 6e-redox active [MoIV3O4] unit and the 3D/2D supramolecular conductive networks in hybrid-POM 1/2 remarkably enhance the electronic conductivity and reversible multi-electron redox ability with the structural integrity retained. Consequently, 1 and 2 exhibited high discharge specific capacities of 236.0 and 277.0 mA h g-1 at 50 mA g-1, respectively, and a good cycling performance at high current density (121.8 mA h g-1, 2 A g-1 for 2), providing a new way for improving POM-based electrode materials.

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").

Deeply reduced empty Keggin clusters [MoIVxMVI12−xO40−xpyx] (x = 3, 6; M = Mo, W; py = pyridine): synthesis, structures, and Lewis field catalysis

First MoIV3-Keggin anions [MoIVxMVI12−xO40−xpyx]8− (M = Mo, W; x = 3, 6) display unprecedented empty Keggin structure caused by the weak u3-O coordinating capability of the [MoIV3O4] triads and LCF-dependent hydrogen transfer catalysis.

Mo[O8Mo4]3 Lewis acid-base cluster pairs: highly efficient and stable Lewis catalysis fields frustrated in crystalline nanoclusters

By using metal compounds or oxide/organic acid and enhanced reaction temperatures in the controlled solvothermal oxidation of [Mo3O2(MeCO2)6(H2O)3]2+, more interstitial metal atoms were introduced to produce the largest nanoscale MoIV-polyoxomolybdates, [M2@(MoIV3py3)4Mo18Ox]q- (M = Al, V, Mo). Each [H4V2@(MoIV3py3)4Mo18O84]12- (2a) nanocluster is surrounded by 12 [V3Mo12O42] to build a Lewis catalysis field (LCF) composed of MoIV3[O8Mo4]3 Lewis acid-base cluster pairs in the crystalline 2, accounting for the excellent and stable catalysis performance in the hydrazine reduction of nitroarenes to arylamines in varied solvents. The proposed new concept LCF provides a new way of thinking for designed synthesis and real applications of highly efficient LCF catalysts.

MoIV3-Polyoxomolybdates with frustrated Lewis pairs for high-performance hydrogenation catalysis

Highly efficient catalytic hydrogenation of the MOIV6-γ-Keggin hybrid with Mo^IV–O–Mo^VIO multiple Lewis pairs has been established.

POM-FLPs: [MoIV3]n-polyoxometalate bifunctional catalysis by [MoIV3]n–Om Lewis pairs frustrated by triangular MoIV–MoIV bonds

The largest [Mo@Mo₂₀]-1 and the first [Ge@Mo₁₆Zn_0.5Ge_1.5]-2 incorporating group IV metals are added to the [MoIV3]_n-POMs hybrid cluster family, which allows for the first systematic investigation into their reasonable syntheses, diversified molecular and electronic structures, and their influence on the catalytic hydrogenation performance of such POM-FLPs system.

Highly efficient MoIV3⋯SbIII cluster frustrated Lewis pair hydrogenation

A new concept of a cluster frustrated Lewis pair (CFLP) featuring extensive cooperative effects both within Lewis acidic/basic cluster components and between Lewis cluster partners has been proposed.

An unprecedented nanocage-like and heterometallic [MoO4]-polyoxomolybdate hybrid

The solvothermal oxidation of [Mo₃O₂(O₂CCH₃)₆(H₂O)₃]·ZnCl₄ has been established as a general route toward [Mo⁴⁺₃O₄]-incorporated polyoxomolybdates (Mo^IV-POMs). Two unprecedented family members: the first Mo⁴⁺-Mo⁵⁺-Mo⁶⁺ nanocage cluster, Na[MoMoMoO₄₃(OH)Py₁₂]·11H₂O (1) and the first heterometallic hybrid, [MoMoZn(PO₄)₄(OH)₂O₃₁py₃]·2(C₅H₆N)·3(C₅H₅N)·2H₂O (2) have been prepared and characterized by X-ray crystallography, elemental and DFT theoretical analyses, XPS, mass spectroscopy, cyclic voltammetry, and IR spectroscopy.