When Identification of the Reduction Sites in Mixed Molybdenum/Tungsten Keggin-Type Polyoxometalate Hybrids Turns Out Tricky

The mixed molybdenum/tungsten Keggin-type polyoxometalate (POM) hybrid (TBA)₄[PW₉Mo₂O₃₉{Sn(C₆H₄I)}] (TBA = tert-butylammonium) has been prepared by the reaction between [α-PW₉Mo₂O₃₉]^7– and [Cl₃Sn(C₆H₄I)] in dried acetonitrile, in the presence of tetra-n-butylammonium bromide. A further coupling reaction affords the ferrocenyl derivative (TBA)₄[PW₉Mo₂O₃₉{Sn(C₆H₄)C≡C(C₆H₄)Fc}]. The POM hybrids have been thoroughly characterized by NMR and IR spectroscopies. Electrochemical analysis confirms their ease of reduction compared to the all-W analogue, albeit with a second reduction process occurring at a lower potential than in the all-Mo species. It is noteworthy that the second reduction is accompanied by an unusual red shift of the electronic absorption spectrum. Whereas there is no doubt that the first reduction deals with Mo, the location of the second electron in the bireduced species, on the second Mo or on W, has thus been the subject of a cross-investigation by spectroelectrochemistry, electron spin resonance, and theoretical calculations. Finally, it came out that the second reduction is also Mo-centered with two unpaired and antiferromagnetically coupled extra electrons.

The sites for the successive reduction processes of a mixed molybdenum/tungsten Keggin-type organotin hybrid are debated through a combination of spectroelectrochemical investigations and theoretical calculations.

Unveiling KuQuinone Redox Species: An Electrochemical and Computational Cross Study

The study of the electrochemical properties of variegated quinones is a fascinating topic in chemistry. In fact, redox reactions occurring with quinoid scaffolds are essential for most of their applications in biological systems, in photoelectrochemical devices, and in many other fields. In this paper, a detailed investigation of KuQuinones' redox behavior is presented. The distinctiveness of such molecules is the presence in the structure of two condensed naphthoquinone units, which implies the possibility to undergo multiple one-electron reduction processes. Solvent, supporting electrolyte, and hydrogen bond donor species effects have been elucidated. Changing the experimental parameters provoked significant shift of the redox potential for each reduction process. In particular, additions of 2,2,2-trifluoroethanol as a hydrogen bond donor in solution as well as Lewis acid coordination were crucial to obtain important shifts of the redox potentials toward more favorable values. UV-vis-NIR spectroelectrochemical experiments and DFT calculations are also presented to clarify the nature of the reduced species in solution.

Benefits of Organo-Aqueous Binary Solvents for Redox Supercapacitors Based on Polyoxometalates

A novel redox electrolyte is proposed based on organo-aqueous solvent and a polyoxometalate (POM) redox moiety. The presence of dimethyl sulfoxide (DMSO) plays multiple roles in this system. Firstly, it enhances the cathodic electrochemical stability window by shifting the H2 evolution to lower potentials with respect to pure aqueous systems; secondly, it improves the reversibility of the redox reaction of the PW12O40 3- anion at low potentials. The presence of DMSO suppresses the Al corrosion, thus enabling the use of this metal as the current collector. An activated carbon-based supercapacitor is investigated in 1 M LiNO3/10 mM H3PW12O40 in a mixed DMSO/H2O solvent and compared with a POM-free electrolyte. In the presence of POMs, the device achieves better stability under floating conditions at 1.8 V. At 1 kW kg-1, it delivers a specific energy of 8 Wh kg-1 vs. 4.5 Wh kg-1 delivered from the POM-free device. The H2 evolution is further shifted by the POMs adsorbed on the activated carbon, which is one reason for the improved stability. The POM-containing cell demonstrates a mitigated self-discharge, owing to strong POMs adsorption into the carbon pores.

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.

Molecular signature of polyoxometalates in electron transport of silicon-based molecular junctions

Polyoxometalates (POMs) are unconventional electro-active molecules with a great potential for applications in molecular memories, providing efficient processing steps onto electrodes are available. The synthesis of the organic-inorganic polyoxometalate hybrids [PM11O39{Sn(C6H4)C[triple bond, length as m-dash]C(C6H4)N2}]3- (M = Mo, W) endowed with a remote diazonium function is reported together with their covalent immobilization onto hydrogenated n-Si(100) substrates. Electron transport measurements through the resulting densely-packed monolayers contacted with a mercury drop as a top electrode confirms their homogeneity. Adjustment of the current-voltage curves with the Simmon's equation gives a mean tunnel energy barrier ΦPOM of 1.8 eV and 1.6 eV, for the Silicon-Molecules-Metal (SMM) junctions based on the polyoxotungstates (M = W) and polyoxomolybdates (M = Mo), respectively. This follows the trend observed in the electrochemical properties of POMs in solution, the polyoxomolybdates being easier to reduce than the polyoxotungstates, in agreement with lowest unoccupied molecular orbitals (LUMOs) of lower energy. The molecular signature of the POMs is thus clearly identifiable in the solid-state electrical properties and the unmatched diversity of POM molecular and electronic structures should offer a great modularity.

Recent advances in the field of selective epoxidation of vegetable oils and their derivatives: a review and perspective

The present critical review reports the recent progress of the last 15 years in the selective epoxidation of vegetable oils and their derivatives, in particular unsaturated fatty acids (UFAs) and fatty acid methyl esters (FAMEs).