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

Abstract

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 Mo₇O₂₄⁶⁻. 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.