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

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)]center dot 23H(2)O (1), three tetramers Cs8K9[(Ge4W36Nb12O156)Eu(H2O)(3)]center dot 25H(2)O (2), Cs(12)K2([)Cs(GeW9Nb3O40) (4)(SO4)Eu-5(H2O)(36)]center dot 61H(2)O (3), Cs12.5K1.5[Cs-2(GeW9Nb3O40)(4)Eu-4(H2O)(22)]center dot 28H(2)O (4), a one-dimensional compound Cs-11.5[(GeW9Nb3O40)(4)Eu-5.5(H2O)(26)]center dot 24H(2)O (5), and a two-dimensional compound CsK2.25[GeW9Nb3O40Eu1.25(H2O)(12)]center dot 8H(2)O (6). In the polyanions of 1-6 (1a-6a), all the Eu-III atoms are selectively bonded to Ot(Nb) of {GeW9Nb3O40} fragments, indicating the high nucleophilicity of Ot(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.