Capped Polyoxometalate Pillars between Metal-Organic Layers for Transferring a Supramolecular Structure into a Covalent 3D Framework.

Two robust metal-organic frameworks (MOFs), {H4[Ni(π-H2O)2]2[Ni(rt-H2O)2]8Ni4(Tri)24}[VIVW12O40]2·24H2O (1) and {H[Ni(π-O)2]2[Ni(rt-H2O)2]8Ni4(Tri)24}[VIVW10VV2O40V2][VIVW9VV3O40VIV2]·24H2O (2) (Tri = 1,2,4-triazole), composed of polyoxometalates (POMs) and metal-organic units, were designed and synthesized by a hydrothermal method. Structure analysis indicates that there is a metal-organic crown [{Ni3(Tri)6(H2O)4}4] ({Ni12}) in these two compounds. In 1, the {Ni12} crown embraces four pendant Tri ligands that could capture a cationic [Ni(H2O)2]2+ group, resulting in the Ni13-Tri building unit [Ni(H2O)2{Ni3(Tri)6(H2O)4}4] ({Ni13}). The {Ni13} building unit was fused together by Tri bridges into the 2D metal-organic layers, which are pillared by a typical Keggin-type POM [VW12O40]4- to construct a 3D supramolecular framework via the hydrogen bonds. Interestingly, the 2D metal-organic layer in 1 was successfully transferred into a 3D covalent MOF via extension of the length of the pillars by capping a Keggin-type POM with V-O units. Moreover, electrochemical behaviors and electrocatalytic properties of these two compounds were both studied, which can act as bifunctional electrocatalysts toward the reduction of H2O2 and oxidation of nitrite in neutral aqueous solution.