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 e-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. [ABSTRACT FROM AUTHOR]