Electron Transfer–Oxygen Transfer Reactions and Beyond With Polyoxometalates

Impact of the Lithium Cation on the Voltammetry and Spectroscopy of [XVM11O40]n- (X = P, As (n = 4), S (n = 3); M = Mo, W): Influence of Charge and Addenda and Hetero Atoms

Polyoxometalates (POMs) have been proposed as electromaterials for lithium-based batteries because they provide access to multiple electron transfer reactions coupled to fast lithium ion transport processes and high stability over many redox cycles. Consequently, knowledge of reversible potentials and Li⁺ cation-POM anion interactions provides a strategic basis for their further development. In this study, detailed cyclic voltammetric studies of a series of [XV^VM₁₁O₄₀]^n- (XVM₁₁^n-) POMs (where X (heteroatom) = P (n = 4), As (n = 4), and S (n = 3) and M (addenda atom) = Mo, W) have been undertaken in CH₃CN in the presence of LiClO₄, with n-Bu₄NPF₆ also present when required to keep the ionic strength close to constant value of 0.1 M. An analysis of the data has allowed the impact of the POM charge, and addenda and hetero atoms on the reversible potentials and the interaction between Li⁺ and the oxidized XV^VM₁₁^n- and reduced XV^IVM₁₁^(n+1)- forms of the V^V/IV redox couple to be determined. The SV^V/IVM₁₁^3-/4- process is independent of the Li⁺ concentration, implying the absence of the association of this cation with either SV^VM₁₁^3- or SV^IVM₁₁^4- redox levels. However, lithium-ion association constants for both V^V and V^IV redox levels were obtained from a comparison of simulated and experimental cyclic voltammograms for the reduction of the more negatively charged XV^VM₁₁^4- (X = P, As; M = Mo, W), since the Li⁺ interaction with these more negatively charged POMs is much stronger. The interaction between Li⁺ and the oxidized, XV^VM₁₁^n-, and reduced, XV^IVM₁₁^(n+1)-, forms was also investigated by ⁵¹V NMR and EPR spectroscopy, respectively, and it was confirmed that, due to their lower charge density, SV^VM₁₁^3- and SV^IVM₁₁^4- interact significantly less strongly with the lithium ion than XV^VM₁₁^4- and XV^IVM₁₁^5- (X = P, As). The lithium-POM association constants are substantially smaller than the corresponding proton association constants reported previously, which is attributed to a smaller surface charge density. The much stronger impact of Li⁺ on the W^VI/V- and Mo^VI/V-based reductions that occur at more negative potentials than the V^V/IV process also has been qualitatively evaluated.

Aggregation Patterns in Low- and High-Charge Anions Define Opposite Solubility Trends

Molecular dynamics simulations in aqueous solution reveal the existence of two distinct patterns of aggregation in low and high charge density Lindqvist-type polyoxometalates (POMs). Our results indicate the presence of contact and solvent-shared ion pairs and specific and preferential interactions of alkalis with POMs. Highly charged POMs are capable of breaking apart the Li⁺ and Cs⁺ solvation shell, thus enhancing the formation of long-lived alkali-POM contact ion pairs, where alkalis act as an electrostatic "glue" forming large oligomers. Stronger ion pair interactions for Li⁺ than for Cs⁺ promote lower solubility for Li⁺ than for Cs⁺, evoking anomalous solubility trends. Lower charge density POMs are not capable of disrupting the Li⁺ solvation shell and only solvent-shared ion pairs are formed, whereas for Cs⁺, contact ion pairs exist. The large number of oxygen atoms in the POM surface enhances the hydrogen bonds between POM and water, thus promoting aggregation. In this case, aggregation follows normal solubility trends. Thus, aggregation depends on the strength of ion pair interactions, the capacity of POM to disrupt alkali's solvation shell, and the contact surface area between the solvent and POM.

The kinetics and mechanism of oxidation of reduced phosphovanadomolybdates by molecular oxygen: theory and experiment in concert

The reactivity of the H₅PV₂Mo₁₀O₄₀ polyoxometalate and its analogues as an electron transfer and electron transfer-oxygen transfer oxidant has been extensively studied in the past and has been shown to be useful in many transformations. One of the hallmarks of this oxidant is the possibility of its re-oxidation with molecular oxygen, thus enabling aerobic catalytic cycles. Although the re-oxidation reaction was known, the kinetics and mechanism of this reaction have not been studied in any detail. Experimentally, we show that both the one- and two-electron reduced polyoxometalate are reactive with O₂, the two-electron one more so. The reactions are first-order in the polyoxometalate and O₂. Solvents also have a considerable effect, protic solvents being preferred over aprotic ones. H₅PV₂Mo₁₀O₄₀ was reduced either by an electron transfer reaction (H₂) or an electron transfer-oxygen transfer reaction (Ph₃P). Similar rate constants and activation parameters were observed for both. DFT calculations carried out on the re-oxidation reactions strongly suggest an inner-sphere process. The process involves first the formation of a coordinatively unsaturated site (CUS) and subsequently the binding of O₂ to form superoxo and then peroxo η²-O₂ adducts. Most interestingly, although vanadium is the reactive redox centre as well as a necessary component for the oxidative activity of H₅PV₂Mo₁₀O₄₀, and a CUS can be formed at both Mo and V sites, O₂ coordination occurs mostly at the Mo CUSs, preferably those where the vanadium centers are distal to each other.