pH-dependent assembly of 0D to 3D Keggin-based coordination polymers: Structures and catalytic properties

Abstract

Four Keggin-based coordination polymers, namely, {[Cu₂(4,4′-bpy)(4,4′-Hbpy)₄(H₂O)₄](SiW₁₂O₄₀)₂(H₂O)₄}_n (1), {[Cu₂(4,4′-bpy)(4,4′-Hbpy)₆(SiW₁₂O₄₀)₃](4,4′-Hbpy)₂(H₂O)₇}_n (2), {[Cu₂(μ₂-H₂O)₂(4,4′-bpy)₃(SiW₁₂O₄₀)](H₂O)₆}_n (3) and {[Cu₂(μ₂-OH)(4,4′-bpy)₃(SiW₁₂O₄₀)(H₂O)] [Cu₂(μ₂-O)(4,4′-bpy)₄(H₂O)₂]_0.5·(H₂O)₃}_n (4) (4,4′-bpy = 4,4′-bipyridine) were prepared through the hydrothermal reaction of silicotungstic acid, copper(II) nitrate and 4,4′-bipyridine under different pH conditions. Coordination polymers 1 and 2, which exhibit 0D and 1D structures respectively, were prepared at pH = 3.5. At pH = 5.5, a 2D coordination polymer 3 was obtained. Increasing the pH of the reaction to 8.5 led to a 3D coordination polymer 4. The structural diversities of 1–4 reveal that the pH value of the reaction plays a key role in the assembly of POM-based coordination polymers. Investigation of the catalytic properties of 1–4 for the oxidation of ethylbenzene indicates that the catalytic activity of the coordination polymers is closely related to the protonated extent of 4,4′-bpy in the coordination polymers.