Polyoxometalates in Imidazolim-based Ionic Liquids: Acceptor Number and Polarity Estimated from Their Voltammetric Behaviour

Abstract

The selection of an appropriate solvent is essential for achieving high yields and selectivity in chemical reactions. The chemical and physical parameters of organic solvents have been classified into several groups, and solvents can be compared with each other with respect to these properties. The acceptor number (AN), donor number (DN) and polarity (ET N ) have been widely accepted and used for theoretically and quantitatively evaluating the properties of organic solvents. In a similar manner, the AN, DN and ET N of room temperature ionic liquids (RTILs) have been estimated from spectral changes in solvatochromic compounds. In this paper, the AN and ET N of eight types of imidazolium-based RTILs were estimated from the relationship between the AN and ET N values and the first redox potential obtained from the voltammograms of polyoxometalates (POMs) in various organic solvents. The obtained parameters were compared with those estimated by spectrophotometric methods reported previously by several groups. This new method for estimating the AN and ET N of RTILs using the voltammetric behaviour of POMs with low charge density and high symmetry could provide the other path to obtain more reliable AN and ET N of RTILs.

Cation effect on the electrochemical reduction of polyoxometalates in room temperature ionic liquids

“…Ferrocene was added in situ to the POM/RTIL solution, and the potentials were shifted so that the midpoint of the Fc/Fc + redox couple was at 0 V. Ferrocene is commonly used as a reference potential scale in RTIL solvents. [54,55] The E mid values reported in Table 2 compare very well with other previously reported reduction potentials for similar POMs in RTILs, [17,26] noting that these experiments were performed on different electrodes, so some variation is expected. Interestingly, the E mid values in the 12 RTILs are ~200 mV less negative than previously reported in a conventional solvent system (acetonitrile + 0.5 M [n-Bu 4 N][PF 6 ]), which were −0.233 and −0.606 V vs Fc/Fc + for I/I′ and II/ II′, respectively.…”

“…2). Because POMs are known to have limited solubility in RTILs, [26,28] attempts to dissolve low millimolar concentrations were first made; however, full dissolution was visually observed only in 6 of the 12 RTILs (see supplementary information for details). Therefore, the 'adhered solid' method reported by Zhang and Bond [39] was utilised for the present study in all RTILs for consistency and to enable comparison of the behaviour in all RTILs.…”

“…It can be seen that both electron transfer processes appear to be chemically reversible owing to the presence of an oxidative back-peak after the reduction, with peak shapes relatively symmetric for the forward vs reverse currents, as observed previously for the same POM in RTIL solvents. [17,26] It is important to note that some of the ILs contained intrinsic impurities that showed redox activity in the reductive potential region, making observation of the additional POM reduction processes after the second reduction peak difficult. This was particularly problematic in the ammonium-based RTILs, where large impurity peaks were consistently observed, noting that the additional oxidation peak that can be seen at ~0.2 V vs Pt in the [N 1,8,8,8 ][TFSI] RTIL (Fig.…”

“…[8,[21][22][23][24][25] This relationship was recently applied to the estimation of A N and E T N for a series of eight imidazoliumbased RTILs. [26] Despite several papers reporting POM electrochemistry in RTILs, [17][18][19][26][27][28] their fundamental electrochemical behaviour is still not fully investigated, especially in terms of understanding the solvation effects arising from RTILs with different chemical structures of cations and anions. RTILs have many different chemical structures, and are well known to have charged interfacial layers that are much more ordered and more dense compared with conventional salt/solvent systems.…”