Dioxygen in Polyoxometalate Mediated Reactions

Abstract

In this review article, we consider the use of molecular oxygen in reactions mediated by polyoxometalates. Polyoxometalates are anionic metal oxide clusters of a variety of structures that are soluble in liquid phases and therefore amenable to homogeneous catalytic transformations. Often, they are active for electron transfer oxidations of a myriad of substrates and upon reduction can be reoxidized by molecular oxygen. For example, the phosphovanadomolybdate, HPVMoO, can oxidize Pd(0) thereby enabling aerobic reactions catalyzed by Pd and HPVMoO. In a similar vein, polyoxometalates can stabilize metal nanoparticles, leading to additional transformations. Furthermore, electron transfer oxidation of other substrates such as halides and sulfur-containing compounds is possible. More uniquely, HPVMoO and its analogues can mediate electron transfer-oxygen transfer reactions where oxygen atoms are transferred from the polyoxometalate to the substrate. This unique property has enabled correspondingly unique transformations involving carbon-carbon, carbon-hydrogen, and carbon-metal bond activation. The pathway for the reoxidation of vanadomolybdates with O appears to be an inner-sphere reaction, but the oxidation of one-electron reduced polyoxotungstates has been shown through intensive research to be an outer-sphere reaction. Beyond electron transfer and electron transfer-oxygen transfer aerobic transformations, there a few examples of apparent dioxygenase activity where both oxygen atoms are donated to a substrate.

Poly(ethylene glycol) nanocomposites of sub-nanometer metal oxide clusters for dynamic semi-solid proton conductive electrolytes

“…[27][28][29][30][31][32] As the transition zone between small molecules and nanoparticles (NPs), sub-nm POMs (mainly Lindqvist and Keggin-types) have extremely high specic surface areas, admirable catalytic efficiencies and proton conductivities, and excellent stabilities. [33][34][35][36][37][38][39] Especially, with poly(ethylene glycol) (PEG) molecules encapsulated inside the nano-channels dened by Keggin-type POMs, the obtained PEG@POM hybrid materials show comparatively high proton conductivities at high temperature (up to 443 K) and low humidity (<1% RH). 40 It is reported in our recent work that proton conduction in the PEG@POM hybrids is basically facilitated by the diffusive motion of PEG chains and therefore the introduction of water into this system is not necessary.…”

Physical and Electrochemical Modulation of Polyoxometalate Ionic Liquids via Organic Functionalization

“…Polyoxometalates (POMs) are discrete anionic metal oxide clusters with a variety of attractive properties, such as photosensitivity, redox activity, and high thermal and chemical stability These properties make POMs of potential interest in a range of applications including catalysis, electrical‐energy storage, and sensing, to name but a few. Hybrid organic/inorganic clusters are an increasingly important sub‐class of POMs,, in which a near‐limitless selection of organic groups can be incorporated through either weak electrostatic interactions (class I) or via direct covalent functionalization of the POM structure itself (class II) , .…”

Ti‐Substituted Keggin‐Type Polyoxotungstate as Proton and Electron Reservoir Encaged into Metal–Organic Framework for Carbon Dioxide Photoreduction

“…Keggin‐type polyoxometalates (POMs) are consist of well‐defined metal–oxygen clusters with reversible multiple electron redox activities . Therefore, they generally act as an electron reservoir in composite photocatalysts to promote electron–hole separation . Keggin‐type POMs, being as strong Brønsted acid and having quasispherical structure with abundant exterior oxygen atoms, are incorporated into the pores of MOFs to enhance proton transfer .…”