A Highly Active Protonated Tetranuclear Peroxotungstate for Oxidation with Hydrogen Peroxide

Peroxide-Selective Reduction of O2 at Redox-Inactive Rare-Earth(III) Triflates Generates an Ambiphilic Peroxide

Metal peroxides are key species involved in a range of critical biological and synthetic processes. Rare-earth (group III and the lanthanides; Sc, Y, La-Lu) peroxides have been implicated as reactive intermediates in catalysis; however, reactivity studies of isolated, structurally characterized rare-earth peroxides have been limited. Herein, we report the peroxide-selective (93-99% O₂^2-) reduction of dioxygen (O₂) at redox-inactive rare-earth triflates in methanol using a mild metallocene reductant, decamethylferrocene (Fc*). The first molecular praseodymium peroxide ([Pr^III₂(O₂^2-)(18C6)₂(EG)₂][OTf]₄; 18C6 = 18-crown-6, EG = ethylene glycol, ^-OTf = ^-O₃SCF₃; 2-Pr) was isolated and characterized by single-crystal X-ray diffraction, Raman spectroscopy, and NMR spectroscopy. 2-Pr displays high thermal stability (120 °C, 50 mTorr), is protonated by mild organic acids [pK_a1(MeOH) = 5.09 ± 0.23], and engages in electrophilic (e.g., oxygen atom transfer) and nucleophilic (e.g., phosphate-ester cleavage) reactivity. Our mechanistic studies reveal that the rate of oxygen reduction is dictated by metal-ion accessibility, rather than Lewis acidity, and suggest new opportunities for differentiated reactivity of redox-inactive metal ions by leveraging weak metal-ligand binding events preceding electron transfer.

Copper-Containing Polyoxometalate-Based Metal-Organic Framework as a Catalyst for the Oxidation of Silanes: Effective Cooperative Catalysis by Metal Sites and POM Precursor

The oxidation of silanes into silanols is a very necessary transformation, and yet the rational fabrication of efficient catalysts for this reaction remains a challenging task. Here, a 3D polyoxometalate-based metal-organic framework (POMOF), [Cu^Ι₃(pz)₃{PMo₁₂O₄₀}]·H₂O (HENU-8, HENU = Henan University; pz = pyrazine) was consciously prepared and first employed in the oxidation of dimethylphenylsilane with tert-butyl hydroperoxide (TBHP) as an oxidant, achieving 89% yield at a production rate of 132 mmol·g^-1·h^-1. Control experiments indicated that polyoxometalates and Cu atoms together affected the ultimate outcome in this catalytic system, and the designed catalyst followed a free radical mechanism.

Supramolecular assemblies of organo-functionalised hybrid polyoxometalates: from functional building blocks to hierarchical nanomaterials

This review provides a comprehensive overview of recent advances in the supramolecular organisation and hierarchical self-assembly of organo-functionalised hybrid polyoxometalates (hereafter referred to as hybrid POMs), and their emerging role as multi-functional building blocks in the construction of new nanomaterials. Polyoxometalates have long been studied as a fascinating outgrowth of traditional metal-oxide chemistry, where the unusual position they occupy between individual metal oxoanions and solid-state bulk oxides imbues them with a range of attractive properties (e.g. solubility, high structural modularity and tuneable properties/reactivity). Specifically, the capacity for POMs to be covalently coupled to an effectively limitless range of organic moieties has opened exciting new avenues in their rational design, while the combination of distinct organic and inorganic components facilitates the formation of complex molecular architectures and the emergence of new, unique functionalities. Here, we present a detailed discussion of the design opportunities afforded by hybrid POMs, where fine control over their size, topology and their covalent and non-covalent interactions with a range of other species and/or substrates makes them ideal building blocks in the assembly of a broad range of supramolecular hybrid nanomaterials. We review both direct self-assembly approaches (encompassing both solution and solid-state approaches) and the non-covalent interactions of hybrid POMs with a range of suitable substrates (including cavitands, carbon nanotubes and biological systems), while giving key consideration to the underlying driving forces in each case. Ultimately, this review aims to demonstrate the enormous potential that the rational assembly of hybrid POM clusters shows for the development of next-generation nanomaterials with applications in areas as diverse as catalysis, energy-storage and molecular biology, while providing our perspective on where the next major developments in the field may emerge.

Molybdenum/Tungsten-Based Heteropoly Salts in Oxidations

Oxidation represents one of the most important and practical chemical transformations for both organic synthesis, material science and pharmaceutical area. Among the existing strategies, molybdenum/tungsten-based heteropoly salts involved oxidations with low-cost and environmentally benign terminal oxidant and thus have attracted considerable attention in recent years. In this review, we have summarized the recent development of heteropoly salts utilized in oxidations, mainly the peroxomolybdates and peroxotungstates. We wish to highlight the progress made in the past 20 years of this field. Three categories are classified according to the aggregation state of metal oxides. Special attention is paid to the catalytically active peroxometalate species generated during the oxidation process. It is helpful to shed light on the common features that enable highly efficient and selective oxidations. We aim to inspire fellow chemists to explore more functional metalates for catalytic oxidations, especially asymmetric versions. Meanwhile, we attempt to understand the design principles for the discovery of more efficient, selective and economical catalytic systems.

Unraveling the role of the lacunar Na7PW11O39 catalyst in the oxidation of terpene alcohols with hydrogen peroxide at room temperature

The Na₇PW₁₁O₃₉ lacunar salt was the most active among Keggin lacunar salts (Na₈SiW₁₁O₃₉ and Na₇PMo₁₁O₃₉), a saturated salt (Na₇PW₁₂O₄₀) and heteropolyacid catalysts.

Fragmentation of a Monoclonal Antibody by Peroxotungstate

PURPOSE

Tungsten and tungsten oxide leachates found in glass pre-filled syringes were identified to initiate protein precipitation and aggregation. Here, we tested the possibility of tungsten and tungsten oxide to induce the chemical degradation of proteins via reaction with hydrogen peroxide, a possible impurity present in protein formulations, to yield peroxotungstate.

METHODS

A monoclonal antibody (mAb) was incubated with various concentrations of peroxotungstate and the reaction mixtures analyzed by SDS-PAGE and mass spectrometry.

RESULTS

Exposure of a mAb to 1.07-1070 ppm peroxotungstate (based on tungsten content) at temperatures of 4°C and 22°C (pH 5-7) induced protein fragmentation. The extent of fragmentation increased with higher temperatures, lower pH and higher peroxotungstate concentrations. The mAb fragments were identified to contain different combinations of heavy chains (H) and light chains (L). Analogous mAb fragments were generated when the protein was exposed to H₂O₂ and orthotungstate at levels as low as 5 ppm. In addition, extracts from tungsten pins used to manufacture glass pre-filled syringes, in combination with H₂O₂ caused comparable fragmentation of the mAb. Mass spectrometric identification of the fragments suggests fragment generation by oxidative disulfide bond cleavage between the heavy and light chains, confirmed by mass spectrometry data on product formation. The mechanism of oxidative fragmentation was separately confirmed with insulin.

CONCLUSION

Fragmentation of the mAb by peroxotungstate is proposed to occur through inter-chain disulfide bond oxidation to form thiosulfinate (CyS(═O)SCy) and thiosulfonate [CyS(═O)₂SCy], followed by hydrolysis.

Investigation into the mechanism of polyoxotungstates-catalyzed cyclooctene epoxidation by ESI-MS

The catalytic mechanism of cyclooctene epoxidation was real-time monitored by ESI-MS using H₃PW₁₂O₄₀ and H₂WO₄ as catalysts.

Polyoxometalate-based homochiral metal-organic frameworks for tandem asymmetric transformation of cyclic carbonates from olefins

Currently, great interest is focused on developing auto-tandem catalytic reactions; a substrate is catalytically transferred through mechanistically distinct reactions without altering any reaction conditions. Here by incorporating a pyrrolidine moiety as a chiral organocatalyst and a polyoxometalate as an oxidation catalyst, a powerful approach is devised to achieve a tandem catalyst for the efficient conversion of CO₂ into value-added enantiomerically pure cyclic carbonates. The multi-catalytic sites are orderly distributed and spatially matched in the framework. The captured CO₂ molecules are synergistically fixed and activated by well-positioned pyrrolidine and amine groups, providing further compatibility with the terminal W=O activated epoxidation intermediate and driving the tandem catalytic process in a single workup stage and an asymmetric fashion. The structural simplicity of the building blocks and the use of inexpensive and readily available chemical reagents render this approach highly promising for the development of practical homochiral materials for CO₂ conversion.

Using a single catalyst to promote multiple distinct reactions without alteration in reaction conditions is an attractive synthetic goal. Here, to this end, the authors have developed a polyoxometalate-based metal-organic framework for the synthesis of cyclic carbonates from olefins.

Composites of [γ-H2PV2W10O40]3− and [α-SiW12O40]4− supported on Fe2O3 as heterogeneous catalysts for selective oxidation with aqueous hydrogen peroxide

The titled heterogeneous catalysts showed high performance for selective oxidation with H₂O₂.

Synthesis and oxidation catalysis of a Ti-substituted phosphotungstate, and identification of the active oxygen species

A Ti-substituted phosphotungstate (I) showed high catalytic performance for several oxidation reactions. The truly catalytically active species was successfully isolated, and its anion structure was determined.