Constructing nanosized polyanions with diverse structures by the self-assembly of W/Nb mixed-addendum polyoxometalate and lanthanide ion

Abstract

The reaction activity of W/Nb mixed-addendum POMs towards lanthanide has been demonstrated by experimental and computational methods. Utilizing the nucleophilicity of O_t(Nb) (O_t: terminal oxygen), six Nb–O–Eu bridge-based POMs with different structures have been isolated by the reactions of {GeW₉Nb₃O₄₀} and lanthanide ions Eu³⁺ under different conditions, including a dimmer Cs₃K₄[(Ge₂W₁₈Nb₆O₇₈)Eu(H₂O)₄]·23H₂O (1), three tetramers Cs₈K₉[(Ge₄W₃₆Nb₁₂O₁₅₆)Eu(H₂O)₃]·25H₂O (2), Cs₁₂K₂[Cs(GeW₉Nb₃O₄₀)₄(SO₄)Eu₅(H₂O)₃₆]·61H₂O (3), Cs_12.5K_1.5[Cs₂(GeW₉Nb₃O₄₀)₄Eu₄(H₂O)₂₂]·28H₂O (4), a one-dimensional compound Cs_11.5[(GeW₉Nb₃O₄₀)₄Eu_5.5(H₂O)₂₆]·24H₂O (5), and a two-dimensional compound CsK_2.25[GeW₉Nb₃O₄₀Eu_1.25(H₂O)₁₂]·8H₂O (6). In the polyanions of 1–6 (1a–6a), all the Eu^III atoms are selectively bonded to O_t(Nb) of {GeW₉Nb₃O₄₀} fragments, indicating the high nucleophilicity of O_t(Nb). The Density Functional Theory (DFT) calculation on anion [GeW₉Nb₃O₄₀]⁷⁻ further approves this. Comparing with the (Nb)O–Eu bonds, the formation of Nb–O–Nb bridges needs stronger acidity. Thus, weaker acidity (pH>1.5) is favorable to the former. The Cs⁺ cation plays an important templating role in the formation and stability of polyanions 1a–5a. The photoluminescence behaviors of 1 and 2 have been investigated and compound 1 exhibits selective luminescence response to malonic acid.