A New Polyoxometalate-Based Metal-Organic Complex for Efficient Photocatalytic Degradation of Organic Dyes and Reduction of Cr(VI)

Abstract Photocatalysis represents a promising strategy for degrading highly toxic agents, thereby contributing to environmental protection and public health. The central challenge in this field lies in the design and development of novel, high-efficiency photocatalysts. Herein, a new polyoxometalate (POM)-based metal-organic complex (POMOC), {[Zn(bcbpy) (γ-Mo8O26)0.5(H2O)2](H2bcbpy)(γ-Mo8O26)0.5}·2H2O [H2bcbpyCl2 = 1,1′-bis(4-carboxybenzyl)-4,4′-bipyridinium dichloride], was successfully synthesized via hydrothermal assembly from an electron-deficient viologen ligand H2bcbpyCl2, (NH4)6Mo7O24·4H2O, and ZnCl2. Its structure features dinuclear metallacyclic units [Zn(bcbpy)2(H2O)4]4+ formed by Zn2+ ions bridged by bcbpy ligands. These units are interconnected through H-bonds to generate a 2D supramolecular network. Notably, the structure also contains 1D helical chains composed of [H2bcbpy]2+ cations linked to {γ-Mo8O26} clusters via H-bonds, which further assemble into double-helical chains. The final architecture is a 3D supramolecular framework formed by the interweaving of the 2D network and the double-helical chains. Under visible-light irradiation, the complex exhibits excellent dual-functional photocatalytic performance. It achieves degradation efficiencies of 95% for methylene blue (MB) within 60 min and 98% for gentian violet (GV) within 75 min, following pseudo-first-order kinetics with rate constants and corresponding half-lives of 0.05 min–1/13.86 min and 0.04 min–1/17.33 min, respectively. Moreover, the complex demonstrates high efficiency in the reduction of toxic Cr(VI) to Cr(III), reaching 97% conversion within 40 min with a pseudo-first-order rate constant of 0.11 min–1 and a half-life of 6.30 min. Control experiments confirm the essential roles of sacrificial agent, catalyst, and the synergistic interaction among components. The complex also demonstrates good recyclability, maintaining over 90% of its initial activity after three cycles. The structural integrity of the complex was well preserved, as evidenced by IR and PXRD patterns before and after reaction.