Self-Assembly of a Family of Isopolytungstates Induced by the Synergistic Effect of the Nature of Lanthanoids and the pH Variation in the Reaction Process: Syntheses, Structures, and Properties

Three types of new lanthanoid­(Ln)-containing isopolyoxotungstates [H₂N­(CH₃)₂]₆Na₆­[Ln₄(H₂O)₂₂­W₂₈O₉₄H₂]₂·­113H₂O [Ln = Pr³⁺ (1), Nd³⁺ (2), Sm³⁺ (3)], Na₂[Eu­(H₂O)₇]₂­[Eu­(H₂O)₅]₂­[W₂₂O₇₄H₂]­·20H₂O (4), and Na₃H₂[Ln­(H₂O)₄]­[Ln (H₂O)₅]₂­[W₂₂O₇₄H₂]­·36H₂O [Ln = Gd³⁺ (5), Tb³⁺ (6), Er³⁺ (7), Tm³⁺ (8), Yb³⁺ (9), Lu³⁺ (10)] have been obtained by reacting Na₂WO₄­·2H₂O with Ln­(NO₃)₃­·6H₂O in the presence of dimethylamine hydrochloride in the acidic aqueous solution and structurally characterized by elemental analyses, IR spectroscopy, UV spectroscopy, electrospray ionization mass spectrometry (ESI–MS), thermogravimetric (TG) analyses, and single-crystal X-ray diffraction. 1, 2, and 3 are isostructural and display a one-dimensional (1-D) chain-like alignment built by hexameric Ln₈-comprising [Ln₄(H₂O)₂₂­W₂₈O₉₄H₂]₂^12– entities via [Ln­(H₂O)₅]³⁺ connectors. The [Ln₄(H₂O)₂₂­W₂₈O₉₄H₂]₂^12– entity consists of two [Ln₄(H₂O)₂₂­W₂₈O₉₄H₂]^6– subunits connected by two W–O–Ln–O–W linkers. Intriguingly, the rare λ-shaped octacosatungstate [W₂₈O₉₄H₂]^18– moiety observed in the [Ln₄(H₂O)₂₂­W₂₈O₉₄H₂]^6– subunit is composed of two undecatungstate [W₁₁O₃₈H]^9– fragments joined through a hexatungstate [W₆O₂₂]^8– fragment by sharing four μ₂-O atoms. In 4, 22-isopolytungstate [W₂₂O₇₄H₂]^14– anions are interlinked together by four W–O–Eu1–O–W linkers giving rise to the 1-D chain motif, and then adjacent 1-D chains are further bridged through multiple W–O–Eu2–O–W connectors to engender the two-dimensional extended sheet structure with the 4-connected topology. The isomorphic 5–10 demonstrate the discrete structure consisting of a [Ln­(H₂O)₄]­[Ln (H₂O)₅]₂­[W₂₂O₇₄H₂]^8– unit. The pH ranges in which 3, 4, and 9 are stable in aqueous solution have been examined by virtue of UV and ESI–MS spectra. The solid-state luminescent properties of 3, 4, and 6 have been probed at room temperature. 3 displays the pink emission derived from characteristic emission bands of the Sm³⁺ cations that correspond to transitions from the ⁴G_5/2 excited-state to lower ⁶H_J (J = 5/2, 7/2, 9/2, 11/2) levels, 4 emits the red light that mainly results from the ⁵D₀ → ⁷F₂ transition of the Eu³⁺ cations, and 6 manifests the green luminescence mainly originating from the ⁵D₄ → ⁷F₅ transition of the Tb³⁺ cations. Their lifetime decay curves all conform to the single exponential function, affording their lifetimes of 8094.19 ns, 149.00 μs and 384.89 μs, respectively.