One-Dimensional Semi-Rigid Bis(1H-Benzimidazole)-Functionalized Reduced Phosphomolybdate Complexes with Distinct Bridging Metal Assemblies for Electrochemical Sensing of Trace Cr(VI)

Rational development of polyoxometalate-based metal–organic complexes (MOCs) for enhanced electrochemical sensing performance in wastewater remains a significant challenge in environmental remediation. Two reduced phosphomolybdate-based MOCs functionalized with a semirigid bis(1H-benzimidazole) ligand, (H2bbix)2[Mn(H2O)3]2{Mn[P4Mo6O31H7]2}·6H2O (1) and (H2bbix)2[Co(H2O)2][Na(H2O)]2{Co[P4Mo6O31H7]2}·6H2O (2) (bbix = 1,4-bis(benzimidazol-1-ylmethyl)benzene), were hydrothermally synthesized. Single-crystal X-ray diffraction reveals adjacent {Mn[P4Mo6O31H7]2}8– polyoxoanions are linked by homometallic {[Mn(H2O)3]2}4+ ions to form 1D inorganic chains in 1. Meanwhile, 2 features 1D chains assembled from {Co[P4Mo6O31H7]2}8– clusters and heterometallic {[Co(H2O)2][Na(H2O)]2}4+ units. The 3D supramolecular frameworks of 1 and 2 are constructed from these 1D chains and protonated [H2bbix]2+ cations via intermolecular hydrogen bonds. Both compounds display excellent reversible redox properties and favorable electrochemical sensing toward trace Cr(VI). 2 exhibits significantly superior performance with a lower limit of detection (LOD) of 38.9 nM (2.01 ppb) and a higher sensitivity of 0.445 mA·μM–1 than that of 1 (LOD: 52.8 nM (2.74 ppb), sensitivity: 0.327 mA·μM–1). Mechanistic studies indicate that the synergistic effects of reduced {Co[P4Mo6O31H7]2}8– clusters, the heterometallic Co–O–Na bridge, and the bbix ligand in 2 collectively accelerate electron-transfer kinetics and mass transport. This study establishes a design strategy for high-performance electrochemical Cr(VI) sensing materials by modulating reduced phosphomolybdate structures, providing new insights for aqueous trace Cr(VI) detection.