pH-Dependent Assembly of 1D to 3D Octamolybdate Hybrid Materials Based on a New Flexible Bis-[(pyridyl)-benzimidazole] Ligand

Four octamolybdate hybrid materials based on a new flexible multidentate ligand, namely, H4L(γ-Mo8O26)·2H2O (1), [Cu(H2L)(β-Mo8O26)(H2O)2]·3H2O (2), [Cu(H2L)(γ-Mo8O26)]·3H2O (3), and Cu(HL)2(β-Mo8O26) (4), where L is 1,1′-(1,4-butanediyl)bis[2-(4-pyridyl)benzimidazole], have been synthesized at different pH values under hydrothermal conditions. Compound 1, which is hydrothermally prepared at pH ≈ 1, exhibits a rare one-dimensional (1D) molybdenum oxide chain. Compounds 2 and 3 are hydrothermally obtained at pH ≈ 2.5. In 2, CuII cations are bridged by H2L2+ ligands and (β-Mo8O26)4– anions to form an interesting two-dimensional (2D) layer. In 3, the H2L2+ ligands and (γ-Mo8O26)4– clusters bridge adjacent CuII cations to form a 3D framwork. When the pH value is adjusted to 3.5, compound 4 is obtained. Because the HL+ cation acts as a monodentate ligand, compound 4 only exhibits a 1D chain structure. The structural diversities of 1–4 reveal that the pH value of the reaction system plays a crucial role in the assembly of POM-based metal–organic frameworks (MOFs). In addition, the electrochemical property of compound 4 has also been investigated in 1 M H2SO4 aqueous solution.