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 Keggi...