Construction of (3,4)-Connected Polyoxometalate-Based Metal–Organic Frameworks (POMOFs) from Triangular Carboxylate and Tetrahedral Zn4-ε-Keggin

By using the methodology of extending the reduced transition-metal-grafted ε-Keggin polyoxoanions with two types of terphenyl-based tricarboxylates of H3L1 (3,5′,3″-position substitution) and H3L2 (4,5′,4″-position substitution), we isolated two polyoxometalate-based metal–organic frameworks, [TBA]9{[H3PMoV8MoVI4O40]2[H2PMoV4MoVI8O40]}[Zn12(L1)4] (1, YZU-103), and [TBA]3[H4PMoV8MoVI4O40][Zn4(HL2)2]·0.5H2O (2, YZU-104) (H3L1 = [1,1′;3′,1″-terphenyl]-3,5′,3″-tricarboxylic acid; H3L2 = [1,1′;3′,1″-terphenyl]-4,5′,4″-tricarboxylic acid; TBA = tetrabutylammonium). The combination of tetrahedral inorganic building blocks and rigid triangular organic linkers is expected to produce (3,4)-connected nets. To date, only three types of topologies of ctn, bor, and ofp were identified. Interestingly, in this work, an unprecedented (3,4)-connected (6·84·10)3(62·8)4 topology (labeled as yzu) was identified in compound 1, in which the 4-connected node of {Zn4-ε-Keggin} maintains the tetrahedral extending and the L1 ligand exerts the triangular linking role. However, under the influence of a 4,5′,4″-substitued ligand of H3L2, the 4-connected node of {Zn4-ε-Keggin} exhibits the distorted tetrahedral extending, as a result, a 2D network of 44 topology is formed in compound 2, in which the HL2 ligand acts as an angular linker. Moreover, it is expected that the entrapment of POMs in a 3D configuration could induce huge enhancement of their electrocatalytic activity. Therefore, compounds 1 and 2 were employed as electrocatalysts for the electrocatalytic reduction of bromate.