Various types of isopolymolybdate-based metal–organic complexes formed in different conditions: synthesis, structures, luminescence, electrochemical, and photocatalytic performances

Abstract

Under the regulation of different metal ions and organic ligands, five isopolymolybdate-based complexes, namely, [CoL¹_0.5(Mo₃O₁₀)(H₂O)₂]·2H₂O (1), [NiL¹_0.5(Mo₃O₁₀)(H₂O)₂]·2H₂O (2), [ZnL¹(H₂O)₃][γ-Mo₈O₂₆]_0.5 (3), [CdL¹_1.5(θ-Mo₈O₂₆)_0.5]·2H₂O (4), and [ZnL²(Mo₃O₁₀)]·H₂O (5) (L¹ = 1,4-bis(1H-pyrazole-4-carboxamide)p-phenylene, L² = 1,2-bis(1H-imidazole-4-carboxamido)ethyl) were prepared under hydrothermal/solvothermal conditions. Complexes 1, 2, and 4 were obtained under solvothermal conditions. 1 and 2 were isostructural, composed of [M₂L¹(H₂O)₄]⁴⁺ (M = Co^II/Ni^II) cationic units and [Mo₃O₁₀]_n²ⁿ⁻ polymolybdate chains. In the presence of Cd ions, the 2D network structure of 4 was constructed from the [θ-Mo₈O₂₆]⁴⁻ anions bridged by metal–organic belts. Both 3 and 5 were isopolymolybdate-based Zn^II-complexes synthesized under hydrothermal conditions except for the difference of organic ligands and pH, namely, L¹ acted as the ligand and the pH was 3.5 for 3, but they were L² and 3.4 for 5, respectively. Complex 3 had a 2D supramolecular structure containing cationic units ZnL¹(H₂O)₃²⁺ and 1D infinite inorganic chains [γ-Mo₈O₂₈]_n. In 5, the 1D convergent chains [Mo₃O₁₀]_n were connected by binuclear metal–organic cationic rings [Zn₂L²₂]⁴⁺ to form a 2D structure. The solvent ratio and reaction pH had a great influence on the synthesis of the title complexes, and metal ions and ligands played important roles in regulating the diversity of the polymolybdates and the structures of the complexes. The electrochemical, luminescence, and photocatalytic degradation properties for MB of the title complexes were investigated.