Application of Tetrazole-Functionalized Thioethers with Different Spacer Lengths in the Self-Assembly of Polyoxometalate-Based Hybrid Compounds

Three metal-organic networks based on Keggin-type polyoxometalates (POMs) have been hydrothermally synthesized by tuning the spacer lengths of bis(tetrazole)-functionalized thioether ligands and structurally characterized: [Cu-4(bmtm)(4)][SiW12O40]center dot 2H(2)O (1), [Cu-4(bmte)(3.5)][SiW12O40] (2), and [Cu-4(bmtp)(4)][SiW12O40] (3) [bmtm = 1,1'-bis(1-methyl-5-mercapto-1,2,3,4-tetrazole)methane, bmte = 1,2-bis(1-methyl-5-mercapto-1,2,3,4-telrazole)ethane, and bmtp = 1,5-bis(1-methyl-5-mercapto-1,2,3,4-tetrazole)pentane]. The spacer lengths and sulfhydryl of bis(tetrazole)-functionalized thioether ligands play important roles in the final framework formation, as shown by X-ray diffraction analysis. In compound 1, with the connection of a N,S bridge of bmtm, two kinds of binuclear Cu-I units are formed and linked to construct a one-dimensional (1D) chain. The [SiW12O40](4-) (SiW12) cluster provides four terminal O atoms linking four binuclear units to generate a two-dimensional layer with (8(3))(2)(8(5).10) topology. In compound 2, centrosymmetric octameric moieties composed of two equivalent tetrameric Cu-I units are bridged by bmte ligands to form a 1D chain. The SiW12 clusters show an unusual (2,8)-connected mode to connect with the 1D chain and construct a four-connected three-dimensional (3D) network with 5(3).6(2).7 topology. Compound 3 exhibits a rare 3D host framework with a type of large cavity and two types of small windows. The SiW12 clusters as templates are strongly cemented into the large cavities and completely encircled by small windows. Furthermore, the compound 2 bulk-modified carbon-paste electrode (2-CPE) displays good electrocatalytic activity toward the reduction of nitrite.