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

Four Keggin-based coordination polymers, namely, {[Cu-2(4,4'-bpy)(4,4'-Hbpy)(4)(H2O)(4)](-) (SiW12O40)(2)(H2O)(4)}(n) (1), {[Cu-2(4,4'-bpy)(4,4'-Hbpy)(6)(SiW12O40)(3)](4,4'-Hbpy)(2)(H2O)(7)}(n) (2), {[Cu-2(mu(2)-H2O)(2)(4,4'-bpy)(3)(SiW12O40)](H2O)(6)}(n) (3) and {[Cu-2(mu(2)-OH)(4,4'-bpy)(3)(SiW12O40)(H2O)] [Cu-2(mu(2)-O)(4,4'-bpy)(4)(H2O)(2)](0.5)center dot(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.