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

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(mu2-H2O)2(4,4'-bpy)3(SiW12O40)](H2O)6}n (3) and {[Cu2(mu2-OH)(4,4'-bpy)3(SiW12O40)(H2O)] [Cu2(mu2-O)(4,4'-bpy)4(H2O)2]0.5 x (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.