Two reduced phosphomolybdate-based metal–organic complexes modified by tunable bis-1<i>H</i>-benzimidazole ligands for enhanced photocatalytic Cr(<scp>vi</scp>) reduction

The strategic development of polyoxometalate-based complexes for enhanced photocatalytic Cr(VI) reduction in wastewater remains a significant challenge in environmental remediation. In this work, two flexible bis-1<i>H</i>-benzimidazole ligand-functionalized reduced phosphomolybdate-based complexes with the formulas (H₂bbbm)₃[Mn(H₂O)₂]{Mn[P₄MoV6O₃₁H₇]₂}·8H₂O (1) and (H₂bbbm)₃{Ni[P₄MoV6O₃₁H₈]₂}·6H₂O (2) (bbbm = 1,1'-(1,4-butanediyl)bis-1<i>H</i>-benzimidazole) were hydrothermally prepared and comprehensively characterized by multiple analytical techniques. Single-crystal analysis reveals that compound 1 exhibits a 3D network structure constructed through hierarchical assembly: Mn²⁺-bridged {Mn[P₄Mo₆O₃₁H₇]₂}^8- dimers first form 1D inorganic chains, which are then interconnected by protonated [H₂bbbm]²⁺ ions <i>via</i> supramolecular interactions to generate the final 3D architecture. However, compound 2 is revealed as a 3D supramolecular network, which is formed by classical 0D {Ni[P₄MoV6O₃₁H₈]₂}^6- dimeric clusters and protonated [H₂bbbm]²⁺ ions. When employed as visible-light photocatalysts, compounds 1 and 2 manifested excellent photocatalytic performance in Cr(VI) reduction with removal rates of 99.25% for 1 and 98.09% for compound 2 after only 8 minutes, respectively. In addition, the reduction process followed pseudo-first-order kinetics with respect to Cr(VI) concentration, exhibiting outstanding <i>k</i> values of 0.623 min^-1 for compound 1 and 0.511 min^-1 for compound 2, respectively. Notably, compound 1 demonstrates superior photocatalytic performance. Meanwhile, both compounds still displayed excellent capability for Cr(VI) reduction in real water samples, underscoring their significant potential for practical application. Mechanistic studies demonstrate that the M{P₄Mo₆}₂ clusters play a critical role in photocatalytic performance. The synergistic effect among polyoxometalate anions, bridging metal centers (M), and the flexible bis(1<i>H</i>-benzimidazole) ligand enhances photocatalytic activity by modulating the band gap of the photocatalysts. This study establishes a design strategy for high-performance visible-light photocatalysts by adjusting the structural composition of reduced phosphomolybdate systems, offering new solutions for environmental pollutant treatment.