Two Anderson-type polyoxometalate-based metal–organic complexes with a flexible bis(pyrazine)-bis(amide) ligand for rapid adsorption and selective separation of cationic dyes

Abstract

Two new Anderson-type polyoxometalates (POMs)-based metal–organic complexes, namely, {HCu(L)[CrMo6(OH)6O18]}·4H2O (1) and [Cu3(L)0.5(TeMo6O24)(H2O)9] (2), were synthesized under the hydrothermal conditions by using a flexible bis(pyrazine)-bis(amide) ligand (L = N,N′-bis(2-pyrazinecarboxamide)-1,2-ethane) and two different Anderson-type POM anions. The structures of 1–2 were characterized by single-crystal X-ray diffraction analysis, IR spectroscopy, and powder X-ray diffraction (PXRD). Complex 1 is a 2D network based on the bidentate bridging B-type Anderson anions [CrMo6(OH)6O18]3− and the μ2-bridging L ligands. Complex 2 displays an unusual 3,4,4,5-connected complicated 3D topological network derived from the tetradentate bridging A-type Anderson anions [TeMo6O24]6− and the μ4-bridging L ligands. Complexes 1–2 exhibited excellent and selective adsorption capacities for the cationic dyes methylene blue (MB), gentian violet (GV), toluidine blue dye (TB), and methylene violet (MV). Most strikingly, complex 1 can be used as the adsorbent to rapidly adsorb GV within 2 min with the highest adsorption capacity of 225 mg/g, and it possesses the outstanding selective separation abilities towards some cationic dyes from the binary dye solutions. The adsorbed dyes can be easily desorbed, and there was no significant change after four cycles. In addition, the electrochemical activities of complex 2 were also studied.