Reactivity of the Asymmetric Wells-Dawson Ion: Lanthanide-Containing 34-Tungsto-2-Phosphates [Ln(P(H ₄ )W ₁₇ O ₆₁ ) ₂ ] ^19– (Ln = La ³⁺ , Ce ³⁺ , Eu ³⁺ , Gd <sup>

We report on the synthesis of the lanthanide-containing 17-tungsto-1-phosphates [Ln(P(H₄)W₁₇O₆₁)₂]^19- (Ln = La³⁺, Ce³⁺, Eu³⁺, Gd³⁺, Yb³⁺, Lu³⁺, Y³⁺), comprising a lanthanide ion connecting two [P(H₄)W₁₇O₆₁]^11- units in a <i>syn</i>-configuration. The compounds were characterized in the solid state by IR, powder XRD, and TGA, and in solution by ³¹P and ¹⁸³W NMR. Alternating current magnetic susceptibility investigations revealed the Ce³⁺ and Yb³⁺ analogues to be single-molecule magnets (SMM), which was further corroborated by sub-kelvin temperature μSQUID studies. For the Eu³⁺ analogue, we have observed a strong ⁵D₀ → ⁷F_<i>J</i> (<i>J</i> = 0-4) emission along with a weaker ⁵D_<i>J</i> (<i>J</i> = 1, 2, or 3) → ⁷F_<i>J</i> emission arising from higher excited state manifolds, upon excitation via the ⁷F₀ → ⁵L₆ transition at 395 nm. Analysis of the steady-state and time-resolved data suggests a distorted square-antiprismatic coordination geometry around the Eu³⁺ center. The presence of water molecules residing in the outer coordination sphere appears to decrease the intrinsic quantum yield (φ_Eu) by providing O-H oscillators as a nonradiative relaxation channel. The observed branching ratio of about 41% for the ⁵D₀ → ⁷F₄ transition highlights that [Eu(P(H₄)W₁₇O₆₁)₂]^19- exhibits a pronounced ⁵D₀ → ⁷F₄ emission.