Aqueous Speciation Studies of Europium(III) Phosphotungstate

The incorporation of lanthanide ions into polyoxometalates may be a unique approach to generate new luminescent,
magnetic, and catalytic functional materials. To realize these new applications of lanthanide polyoxometalates, it is
imperative to understand the solution speciation chemistry and its impact on solid-state materials. In this study we
find that the aqueous speciation of europium(III) and the trivacant polyoxometalate, PW9O349-, is a function of
pH, countercation, and stoichiometry. For example, at low pH, the lacunary (PW11O39)7- predominates and the
1:1 Eu(PW11O39)4-, 2, forms. As the pH is increased, the 1:2 complex, Eu(PW11O39)211- species, 3, and
(NH4)22{(Eu2PW10O38)4(W3O8(H2O)2(OH)4}·44H2O, a Eu8 hydroxo/oxo cluster, 1, form. Countercations modulate
this effect; large countercations, such as K+ and Cs+, promote the formation of species 3 and 1. Addition of Al(III)
as a counterion results in low pH and formation of {Eu(H2O)3(α-2-P2W17O61)}2, 4, with Al(III) counterions bound
to terminal W−O bonds. The four species observed in these speciation studies have been isolated, crystallized,
and characterized by X-ray crystallography, solution multinuclear NMR spectroscopy, and other appropriate tech-niques. These species are 1, (NH4)22{(Eu2PW10O38)4(W3O8(H2O)2(OH)4}·44H2O (P1̄; a = 20.2000(0), b = 22.6951(6), c = 25.3200(7) Å; α = 65.6760(10), β = 88.5240(10), γ = 86.0369(10)°; V = 10550.0(5) Å3; Z = 2), 2,
Al(H3O){Eu(H2O)2PW11O34}·20H2O (P1̄, a = 11.4280(23), b = 11.5930(23), c = 19.754(4) Å; α = 103.66(3), β
= 95.29(3), γ = 102.31(3)°; V = 2456.4(9) Å3; Z = 2), 3, Cs11Eu(PW11O34)2·28H2O (P1̄; a = 12.8663(14), b =
19.8235(22), c = 21.7060(23) Å; α = 114.57(0), β = 91.86(0), γ = 102.91(0)°; V = 4858.3(9) Å3; Z = 2),
4, Al2(H3O)8{Eu(H2O)3(α-2-P2W17O61)}2·29H2O (P1̄; a = 12.649(6), b = 16.230(8), c = 21.518(9) Å; α =
111.223(16), β = 94.182(18), γ = 107.581(17)°; V = 3842(3) Å3; Z = 1).

将镧系离子（lanthanide ions）引入多金属氧酸盐（polyoxometalates）或许是制备新型发光、磁学及催化功能材料的独特策略。要实现镧系多金属氧酸盐的此类新应用，阐明其溶液物种形成化学及其对固态材料的影响至关重要。本研究发现，铕(III)与三缺位多金属氧酸盐PW9O34^9-的水溶液物种分布是pH值、抗衡阳离子及化学计量比的函数。例如，在低pH条件下，缺位型物种(PW11O39)^7-占主导，并形成1:1型配合物Eu(PW11O39)^4-（即物种2）。随着pH升高，会形成1:2型配合物Eu(PW11O39)2^11-（物种3）以及Eu8羟基/氧簇合物(NH4)22{(Eu2PW10O38)4(W3O8(H2O)2(OH)4}·44H2O（物种1）。抗衡阳离子可调控这一转化过程：大尺寸抗衡阳离子（如K+和Cs+）会促进物种3和物种1的生成。以Al(III)作为抗衡阳离子时，体系处于低pH环境，会形成{Eu(H2O)3(α-2-P2W17O61)}2（物种4），其中Al(III)抗衡阳离子结合于末端W−O键上。本物种形成研究中观测到的四种物种均已通过X射线晶体学（X-ray crystallography）、溶液多核核磁共振（NMR）波谱学及其他合适技术完成分离、结晶与表征。这四种物种具体信息如下：
1. (NH4)22{(Eu2PW10O38)4(W3O8(H2O)2(OH)4}·44H2O（空间群P1̄；晶胞参数a = 20.2000(0)，b = 22.6951(6)，c = 25.3200(7) Å；α = 65.6760(10)°，β = 88.5240(10)°，γ = 86.0369(10)°；晶胞体积V = 10550.0(5) ų；Z = 2）
2. Al(H3O){Eu(H2O)2PW11O34}·20H2O（空间群P1̄，a = 11.4280(23)，b = 11.5930(23)，c = 19.754(4) Å；α = 103.66(3)°，β = 95.29(3)°，γ = 102.31(3)°；V = 2456.4(9) ų；Z = 2）
3. Cs11Eu(PW11O34)2·28H2O（空间群P1̄；a = 12.8663(14)，b = 19.8235(22)，c = 21.7060(23) Å；α = 114.57(0)°，β = 91.86(0)°，γ = 102.91(0)°；V = 4858.3(9) ų；Z = 2）
4. Al2(H3O)8{Eu(H2O)3(α-2-P2W17O61)}2·29H2O（空间群P1̄；a = 12.649(6)，b = 16.230(8)，c = 21.518(9) Å；α = 111.223(16)°，β = 94.182(18)°，γ = 107.581(17)°；V = 3842(3) ų；Z = 1）