Various polyoxomolybdate-based hybrids induced by pH and solvents: structures, adsorption activities for dyes and...

By changing the pH values and solvents of reaction systems, four polyoxomolybdate-based compounds, [CoL(H₂O)₂][β-Mo₈O₂₆]_0.5 (1), {[Co₂L₂(H₂O)₃][Mo₇O₂₃]}·3H₂O (2), [Co₂L₃][δ-Mo₈O₂₆]_0.5[β-Mo₈O₂₆]_0.5·10H₂O (3), [Co₂L₃][H₄Mo₈O₂₈]·4DMA·2H₂O (4), (L = 1,3-bis(1H-imidazole-4-carboxamido)metaphenylene, DMA = N,N'-dimethylacetamide), have been synthesized in the presence of the Co²⁺ ion, L ligand and Mo₇O₂₄^6-. Compounds 1 and 2 were obtained under hydrothermal conditions; the different pH values result in two kinds of polyoxomolybdate anions, [β-Mo₈O₂₆]^4- in 1 and [Mo₁₄O₄₆]^8- in 2, and the similar [CoL(H₂O)₂]²⁺ or [Co₂L₂(H₂O)₃]⁴⁺ metal-organic chains. Compounds 3 and 4 were synthesized under mixed-solvent conditions. There are two kinds of octamolybdate isomers in 3, [β-Mo₈O₂₆]^4- and [δ-Mo₈O₂₆]^4- anions, but only one kind of [Mo₈O₂₈]^8- anion in 4, but the two compounds have a similar metal-organic cage [Co₂L₃]⁴⁺. The results reveal that the pH values and different solvents play key roles in tuning the diverse architectures of polyoxomolybdate anions and metal-organic motifs. The adsorption activities for organic dyes, crystal violet (CV), methylene blue (MB) and Congo red (CR), and the electrocatalytic properties of hydrogen peroxide (H₂O₂) and ascorbic acid (AA) of compounds 1-4 are reported here.