The symmetric structure of
1 is comprised of a new tetrameric polyanion [Zr
2(O
2)
2(
β-AsW
10O
38)]
428− (
1a), 8 K
+, 19.5 Na
+, 0.5H
+, and 68 lattice water molecules.
1a is made up of four divacant [
β-AsW
10O
38]
11− {AsW
10} subunits, and four [Zr
2(O
2)
2]
4+ {Zr
2} fragments. The [
β-AsW
10O
38]
11− subunit in
1a is rare compared with the common polyanion [
α-AsW
10O
36]
9− (Fig. S3 in Supporting information). In the structure of
1a, the polyanion exhibits an interesting symmetry, whose whole configuration looks like a square (
Fig. 1a). The structure of
1a can be considered as each {Zr
2} cation encapsuled by two {AsW
10} fragments, and the {Zr
2} group fills the role of the bridge to connect the four {AsW
10} subunits to form a fascinating square configuration (
Fig. 1b). The distances of four As atoms are almost the same, exhibiting the length of 11.1 Å and 11.0 Å, respectively (
Fig. 1c). The {Zr
2} group was constituted by two Zr atoms linked by two peroxyl ions, where the peroxyl bonds are in the vital position on enhancing stability of the structure. The dihedral angles of Zr1−O85−O86−Zr3 and Zr1−O87−O88−Zr3 are
ca. 63.884° and 61.646°, respectively (
Fig. 1d). The transformation of K
14[As
2W
19O
67(H
2O)] → {AsW
10} must have happened during the process of reaction (
Figs. 2a and
b). The structure of
1a can also be seen as each {AsW
10} fragment combined with two Zr atoms to form four [Zr
2(
β-AsW
10O
38)]
3− building units (
Fig. 2c), then every two such building units are bridged by two peroxyl bonds (
Fig. 2d)
via Zr−(O
2)
2−Zr (Zr1−(O
2)
2−Zr3 and Zr2−(O
2)
2−Zr4) to form the square-shaped structure [Zr
2(O
2)
2(
β-AsW
10O
38)]
428−with the length about 2.16 nm (
Fig. 2e). In
1a, all the Zr atoms exhibit the same coordination geometry which show an octa-coordinated, two capped triangular prism configurations (
Fig. 2f and Fig. S4 in Supporting information). Specifically, each Zr atom is surrounded by four O atoms from the {AsW
10} building unit and another four O atoms from two peroxyl ions. Bond valence sum (BVS) calculation verified that the valence states of the W, Zr, As atoms were +6, +4, +5, respectively (Table S2 in Supporting information). It is obvious to find that the valence state of the As atoms in the obtained sample is different from the one that comes from the K
14[As
2W
19O
67(H
2O)] precursor, whose valence state of As atom is +3. The introduction of H
2O
2 in the synthetic process plays an important role to obtain the target product. On the one hand, it can act as an oxidant to oxidize As atom in the precursor from +3 to +5, which is verified by XPS spectra (Fig. S2 in Supporting information); on the other hand, it serves as a bridging ligand to link two Zr atoms. Up to date, there are only several hydrogen peroxide functionalized POMs in Zr-substituted POMs [
44-
46].