Europium(III) Reduction and Speciation within a Wells−Dawson Heteropolytungstate

Abstract

The redox speciation of Eu(III) in the 1:1 stoichiometric complex with the alpha-1 isomer of the Wells-Dawson anion, [alpha-1-P 2W 17O 61] (10-), was studied by electrochemical techniques (cyclic voltammetry and bulk electrolysis), in situ XAFS (X-ray absorption fine structure) spectroelectrochemistry, NMR spectroscopy ( (31)P), and optical luminescence. Solutions of K 7[(H 2O) 4Eu(alpha-1-P 2W 17O 61)] in a 0.2 M Li 2SO 4 aqueous electrolyte (pH 3.0) show a pronounced concentration dependence to the voltammetric response. The fully oxidized anion and its reduced forms were probed by Eu L 3-edge XANES (X-ray absorption near edge structure) measurements in simultaneous combination with controlled potential electrolysis, demonstrating that Eu(III) in the original complex is reduced to Eu(II) in conjunction with the reduction of polyoxometalate (POM) ligand. After exhaustive reduction, the heteropoly blue species with Eu(II) is unstable with respect to cluster isomerization, fragmentation, and recombination to form three other Eu-POMs as well as the parent Wells-Dawson anion, alpha-[P 2W 18O 62] (6-). EXAFS data obtained for the reduced, metastable Eu(II)-POM before the onset of Eu(II) autoxidation provides an average Eu-O bond length of 2.55(4) A, which is 0.17 A longer than that for the oxidized anion, and consistent with the 0.184 A difference between the Eu(II) and Eu(III) ionic radii. The reduction of Eu(III) is unusual among POM complexes with Lindqvist and alpha-2 isomers of Wells-Dawson anions, that is, [Eu(W 5O 18) 2] (9-) and [Eu(alpha-2-As 2W 17O 61) 2] (17-), but not to the Preyssler complex anion, [EuP 5W 30O 110] (12-), and fundamental studies of materials based on coupling Eu and POM redox properties are still needed to address new avenues of research in europium hydrometallurgy, separations, and catalysis sciences.

Series behavior of lanthanoid(iii) complexes with the α-1-Wells-Dawson heteropolyoxoanion in acetonitrile: electrochemistry and Ln coordination

“…37 Concerning the O CNs for the TBA + solid and MeCN solution, the sums of O a-1 (4) and O H2O obtained from the 2 shell fits (7.8(11) and 7.6 (9), respectively) are consistent with the values obtained for the K + -Eu-a-1 salt and its aqueous solution. 13 These metrical results for the TBA + -Eu-a-1 system when combined with those for other TBA + -Ln(III)-a-1 systems across the 4f period provide a practical perspective on the variations of Ln(III) coordination in MeCN solutions, including solvent-exchange behaviors. The effects of the lanthanoid contraction have direct impact on the metrics of Ln-O bonding that, in turn, serve to complicate the MeCN solution electrochemistry as well.…”

“…Analysis of the CV response for the initial reduction reveals a direct, linear correlation between the peak currents and n 54 and stands in obvious contrast with the simple voltammogram of K + -Eu-a-1 in an aqueous electrolyte. 13 Moreover, as with the aqueous voltammetry, there is a substantial concentration dependence to the peak shapes and positions. As such, it is impossible to attribute any single wave to a oneelectron Eu(III)/Eu(II) couple, which may be expected to occur at an electrode potential of ≤ -0.12 V, which is the value for the acetonitrile solvate of Eu(ClO 4 ) 3 in purified, anhydrous MeCN.…”

Series behavior of lanthanoid(iii) complexes with the α-1-Wells-Dawson heteropolyoxoanion in acetonitrile: electrochemistry and Ln coordination

“…37 Concerning the O CNs for the TBA + solid and MeCN solution, the sums of O a-1 (4) and O H2O obtained from the 2 shell fits (7.8(11) and 7.6 (9), respectively) are consistent with the values obtained for the K + -Eu-a-1 salt and its aqueous solution. 13 These metrical results for the TBA + -Eu-a-1 system when combined with those for other TBA + -Ln(III)-a-1 systems across the 4f period provide a practical perspective on the variations of Ln(III) coordination in MeCN solutions, including solvent-exchange behaviors. The effects of the lanthanoid contraction have direct impact on the metrics of Ln-O bonding that, in turn, serve to complicate the MeCN solution electrochemistry as well.…”

“…Analysis of the CV response for the initial reduction reveals a direct, linear correlation between the peak currents and n 54 and stands in obvious contrast with the simple voltammogram of K + -Eu-a-1 in an aqueous electrolyte. 13 Moreover, as with the aqueous voltammetry, there is a substantial concentration dependence to the peak shapes and positions. As such, it is impossible to attribute any single wave to a oneelectron Eu(III)/Eu(II) couple, which may be expected to occur at an electrode potential of ≤ -0.12 V, which is the value for the acetonitrile solvate of Eu(ClO 4 ) 3 in purified, anhydrous MeCN.…”

Syntheses, Crystal Structure, Electrocatalytic, and Magnetic Properties of the Monolanthanide-Containing Germanotungstates [Ln(H2O) n GeW11O39]5– (Ln = Dy, Er, n = 4,3)

“…It generally takes place at very low potentials beyond the electrolyte reduction (water reduction wave). The reduction of Eu III centers has been demonstrated in several cases because this cation is reduced at the same potential range as the W VI centers, but the other Ln cations are expected to remain electrochemically silent in our experimental conditions. − …”

Self‐Assembled Tetrameric Lanthanide‐Containing Germanotungstates [(Ln 2 GeW 10 O 38 ) 4 (W 3 O 8 )(OH) 4 (H 2

“…It has a pre‐wave which clearly forms before the main wave. In fact, several previous studies indicated that the Eu III centres in tungstic POMs are reduced in the same potential range as the W VI centres . This pre‐wave is therefore attributed to the reduction of the Eu III centres.…”

Series behavior of lanthanoid(iii) complexes with the α-1-Wells-Dawson heteropolyoxoanion in acetonitrile: electrochemistry and Ln coordination

“…37 Concerning the O CNs for the TBA + solid and MeCN solution, the sums of O a-1 (4) and O H2O obtained from the 2 shell fits (7.8(11) and 7.6 (9), respectively) are consistent with the values obtained for the K + -Eu-a-1 salt and its aqueous solution. 13 These metrical results for the TBA + -Eu-a-1 system when combined with those for other TBA + -Ln(III)-a-1 systems across the 4f period provide a practical perspective on the variations of Ln(III) coordination in MeCN solutions, including solvent-exchange behaviors. The effects of the lanthanoid contraction have direct impact on the metrics of Ln-O bonding that, in turn, serve to complicate the MeCN solution electrochemistry as well.…”

“…Analysis of the CV response for the initial reduction reveals a direct, linear correlation between the peak currents and n 54 and stands in obvious contrast with the simple voltammogram of K + -Eu-a-1 in an aqueous electrolyte. 13 Moreover, as with the aqueous voltammetry, there is a substantial concentration dependence to the peak shapes and positions. As such, it is impossible to attribute any single wave to a oneelectron Eu(III)/Eu(II) couple, which may be expected to occur at an electrode potential of ≤ -0.12 V, which is the value for the acetonitrile solvate of Eu(ClO 4 ) 3 in purified, anhydrous MeCN.…”

Syntheses, Crystal Structure, Electrocatalytic, and Magnetic Properties of the Monolanthanide-Containing Germanotungstates [Ln(H2O) n GeW11O39]5– (Ln = Dy, Er, n = 4,3)

“…It generally takes place at very low potentials beyond the electrolyte reduction (water reduction wave). The reduction of Eu III centers has been demonstrated in several cases because this cation is reduced at the same potential range as the W VI centers, but the other Ln cations are expected to remain electrochemically silent in our experimental conditions. − …”

Self‐Assembled Tetrameric Lanthanide‐Containing Germanotungstates [(Ln 2 GeW 10 O 38 ) 4 (W 3 O 8 )(OH) 4 (H 2

“…It has a pre‐wave which clearly forms before the main wave. In fact, several previous studies indicated that the Eu III centres in tungstic POMs are reduced in the same potential range as the W VI centres . This pre‐wave is therefore attributed to the reduction of the Eu III centres.…”

Series behavior of lanthanoid(iii) complexes with the α-1-Wells-Dawson heteropolyoxoanion in acetonitrile: electrochemistry and Ln coordination

“…37 Concerning the O CNs for the TBA + solid and MeCN solution, the sums of O a-1 (4) and O H2O obtained from the 2 shell fits (7.8(11) and 7.6 (9), respectively) are consistent with the values obtained for the K + -Eu-a-1 salt and its aqueous solution. 13 These metrical results for the TBA + -Eu-a-1 system when combined with those for other TBA + -Ln(III)-a-1 systems across the 4f period provide a practical perspective on the variations of Ln(III) coordination in MeCN solutions, including solvent-exchange behaviors. The effects of the lanthanoid contraction have direct impact on the metrics of Ln-O bonding that, in turn, serve to complicate the MeCN solution electrochemistry as well.…”

“…Analysis of the CV response for the initial reduction reveals a direct, linear correlation between the peak currents and n 54 and stands in obvious contrast with the simple voltammogram of K + -Eu-a-1 in an aqueous electrolyte. 13 Moreover, as with the aqueous voltammetry, there is a substantial concentration dependence to the peak shapes and positions. As such, it is impossible to attribute any single wave to a oneelectron Eu(III)/Eu(II) couple, which may be expected to occur at an electrode potential of ≤ -0.12 V, which is the value for the acetonitrile solvate of Eu(ClO 4 ) 3 in purified, anhydrous MeCN.…”

Syntheses, Crystal Structure, Electrocatalytic, and Magnetic Properties of the Monolanthanide-Containing Germanotungstates [Ln(H2O) n GeW11O39]5– (Ln = Dy, Er, n = 4,3)

“…It generally takes place at very low potentials beyond the electrolyte reduction (water reduction wave). The reduction of Eu III centers has been demonstrated in several cases because this cation is reduced at the same potential range as the W VI centers, but the other Ln cations are expected to remain electrochemically silent in our experimental conditions. − …”

Self‐Assembled Tetrameric Lanthanide‐Containing Germanotungstates [(Ln 2 GeW 10 O 38 ) 4 (W 3 O 8 )(OH) 4 (H 2

“…It has a pre‐wave which clearly forms before the main wave. In fact, several previous studies indicated that the Eu III centres in tungstic POMs are reduced in the same potential range as the W VI centres . This pre‐wave is therefore attributed to the reduction of the Eu III centres.…”