Inorganic Chemistry, Vol.60, No.6, 3980-3987, 2021
Microwave-Assisted Synthesis of Tris-Anderson Polyoxometalates for Facile CO2 Cycloaddition
Four new tris-Anderson polyoxometalates (POMs), (NH4)(4)[ZnMo6O1 (8)(C4H8NO3)(OH) (3)]center dot 4H2O (1), (NH4)(4)[CuMo6O1 (8)(C4H8NO3)(OH) (3)]center dot 4H2O (2), (TBA)(3)(NH4)[ZnMo6O17(C5H9O3)(2)(OH)]center dot 10H2O (3) (TBA = n-C16H36N), and (NH4)(4)[CuMo6O18(C5H9O3)(2)]center dot 16H2O (4), were synthesized by a microwave-assisted method. Single-crystal X-ray diffraction revealed that 1 and 2 contained a tris (trihydroxyl organic compounds) ligand grafted on one side, while two tris ligands were grafted on two sides to form chi/delta and delta/delta isomers in 3 and 4, respectively. H-1 and C-13 NMR spectra of the chi/delta isomer 3 were obtained for the first time, with six methylenes showing six peaks in the 1H NMR spectrum and only four peaks in the C-13 NMR spectrum. Mass spectrometry monitoring revealed that during the microwave-assistant process the tris ligand can graft onto POMs to form 1, while tris directly coordinates with metallic heteroatoms to form isopolymolybdates during the conventional reflux synthesis process. In addition, 1-4 can catalyze CO2 with epoxides into cyclic carbonates with high selectivity and yields at an atmospheric pressure of CO2, which is lower than the pressure of CO2 in other catalysis using POMs as catalysts. Furthermore, 1- 4 showed good catalytic stability and cycling properties. Mechanism studies substantiated POMs cocatalyzed with Br- to improve the catalytic yields.