Recent Advances of Anderson-Type Polyoxometalates as Catalysts Largely for Oxidative Transformations of Organic Molecules

Anderson-type ([XM₆O₂₄]ⁿ⁻) polyoxometalates (POMs) are a class of polymetallic-oxygen cluster inorganic compounds with special structures and properties. They have been paid extensive attention by researchers now, due to their chemical modification and designability, which have been widely applied in the fields of materials, catalysis and medicine. In contemporary years, the application of Anderson-type POMs in catalytic organic oxidation reaction has gradually shown great significance for the research of green catalytic process. In this paper, we investigate the application of Anderson-type POMs in organic synthesis reaction, and these works are summarized according to the different structure of POMs. This will provide a new strategy for further investigation of the catalytic application of Anderson-type POMs and the study of green catalysis.

Cations Modulated Assembly of Triol-Ligand Modified Cu-Centered Anderson-Evans Polyanions

Counter-cations are essential components of polyoxometalates (POMs), which have a distinct influence on the solubility, stabilization, self-assembly, and functionality of POMs. To investigate the roles of cations in the packing of POMs, as a systematic investigation, herein, a series of triol-ligand covalently modified Cu-centered Anderson-Evans POMs with different counter ions were prepared in an aqueous solution and characterized by various techniques including single-crystal X-ray diffraction. Using the strategy of controlling Mo sources, in the presence of triol ligand, NH₄⁺, Cu²⁺ and Na⁺ were introduced successfully into POMs. When (NH₄)₆Mo₇O₂₄ was selected, the counter cations of the produced POMs were ammonium ions, which resulted in the existence of clusters in the discrete state. Additionally, with the modulation of the pH of the solutions, the modified sites of triol ligands on the cluster can be controlled to form δ- or χ-isomers. By applying MoO₃ in the same reaction, Cu²⁺ ions served as linkers to connect triol-ligand modified polyanions into chains. When Na₄Mo₈O₂₆ was employed as the Mo source to react with triol ligands in the presence of CuCl₂, two 2-D networks were obtained with {Na₄(H₂O)₁₄} or {{Na₂(H₂O)₄} sub-clusters as linkers, where the building blocks were δ/δ- and χ/χ-isomers, respectively. The present investigation reveals that the charges, sizes and coordination manners of the counter cations have an obvious influence on the assembled structure of polyanions.

Versatile post-functionalisation strategy for the formation of modular organic-inorganic polyoxometalate hybrids

Hybrid structures incorporating different organic and inorganic constituents are emerging as a very promising class of materials since they synergistically combine the complementary and diverse properties of the individual components. Hybrid materials based on polyoxometalate clusters (POMs) are particularly interesting due to their versatile catalytic, redox, electronic, and magnetic properties, yet the controlled incorporation of different clusters into a hybrid structure is challenging and has been scarcely reported. Herein we propose a novel and general strategy for combining multiple types of metal-oxo clusters in a single hybrid molecule. Two novel hybrid POM structures (HPOMs) bis-functionalised with dipentaerythritol (R-POM1-R; R = (OCH2)3CCH2OCH2C(CH2OH)) were synthesised as building-blocks for the formation of heterometallic hybrid triads (POM2-R-POM1-R-POM2). Such a modular approach resulted in the formation of four novel heterometallic hybrids combing the Lindqvist {V6}, Anderson-Evans {XMo6} (X = Cr or Al) and trisubstituted Wells-Dawson {P2V3W15} POM structures. Their formation was confirmed by multinuclear Nuclear Magnetic Resonance (NMR), infrared (IR) and UV-Vis spectroscopy, as well as Mass Spectrometry, Diffusion Ordered Spectroscopy (DOSY) and elemental analysis. The thermal stability of the hybrids was also examined by Thermogravimetric Analysis (TGA), which showed that the HPOM triads exhibit higher thermal stability than comparable hybrid structures containing only one type of POM. The one-pot synthesis of these novel compounds was achieved in high yields in aqueous and organic media under simple reflux conditions, without the need of any additives, and could be translated to create other hybrid materials based on a variety of metal-oxo cluster building-blocks.

Anderson-type polyoxometalates: from structures to functions

Anderson-type polyoxometalates (POMs) are one of the most important groups of the POM family. In the past decade, the functionalization of Anderson-type POMs has achieved significant progress and these materials have already shown unique charm in catalysis, molecular devices, energy materials, and inorganic biochemical drugs. In particular, their highly flexible topological structure and diverse functionalization methods make them the most convenient and universal platforms for rational design and controllable synthesis. This review provides a deep discussion on the recent progress in the synthetic methodology, structural exploration, and promising applications of Anderson-type POMs. It also summarizes the latest research directions and provides future prospects.

Microwave-Assisted Synthesis of Tris-Anderson Polyoxometalates for Facile CO2 Cycloaddition

Four new tris-Anderson polyoxometalates (POMs), (NH₄)₄[ZnMo₆O₁₈(C₄H₈NO₃)(OH)₃]·4H₂O (1), (NH₄)₄[CuMo₆O₁₈(C₄H₈NO₃)(OH)₃]·4H₂O (2), (TBA)₃(NH₄)[ZnMo₆O₁₇(C₅H₉O₃)₂(OH)]·10H₂O (3) (TBA = n-C₁₆H₃₆N), and (NH₄)₄[CuMo₆O₁₈(C₅H₉O₃)₂]·16H₂O (4), were synthesized by a microwave-assisted method. Single-crystal X-ray diffraction revealed that 1 and 2 contained a tris (trihydroxyl organic compounds) ligand grafted on one side, while two tris ligands were grafted on two sides to form χ/δ and δ/δ isomers in 3 and 4, respectively. ¹H and ¹³C NMR spectra of the χ/δ isomer 3 were obtained for the first time, with six methylenes showing six peaks in the ¹H NMR spectrum and only four peaks in the ¹³C NMR spectrum. Mass spectrometry monitoring revealed that during the microwave-assistant process the tris ligand can graft onto POMs to form 1, while tris directly coordinates with metallic heteroatoms to form isopolymolybdates during the conventional reflux synthesis process. In addition, 1-4 can catalyze CO₂ with epoxides into cyclic carbonates with high selectivity and yields at an atmospheric pressure of CO₂, which is lower than the pressure of CO₂ in other catalysis using POMs as catalysts. Furthermore, 1-4 showed good catalytic stability and cycling properties. Mechanism studies substantiated POMs cocatalyzed with Br^- to improve the catalytic yields.

Direct Syntheses of Nanocages and Frameworks Based on Anderson-Type Polyoxometalates via One-Pot Reactions

A new one-step synthetic protocol of tris-functionalized Anderson polyoxomolybdates directly from heptamolybdate salts was presented in this Communication. Through this new method, we obtained the first example of Anderson-type polyoxomolybdates with vanadium as the heteroatom. Moreover, the crystals of the products exhibited interesting nanocage or framework extended structures, which were greatly affected by the trialkoxyl ligands as well as the counterions.

A novel 3D POMOF based on Wells–Dawson arsenomolybdates with excellent photocatalytic and lithium-ion battery performance

A novel 3D POMOF based on Wells–Dawson arsenomolybdates exhibits fluorescence property and efficient and stable photocatalytic activity for MB and RhB under UV irradiation and has also been evaluated as the anode material for LIBs.