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

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.