Adsorptive Removal of Organic Dyes from Aqueous Solutions Using Cadmium(II)-Based Coordination Polymers Derived from the Thiophene–Isophthalic Acid Hybrid

Three novel cadmium(II)-based coordination polymers, namely [Cd(H₂O)₃L]_n·2nDMF·nH₂O (1), [Cd(DMF)(H₂O)L]_n·nDMF (2) and [Cd₂(4,4′-bpy)(H₂O)₄L₂]_n·2nH₂O (3), were successfully synthesized under solvothermal conditions using the custom-designed organic linker 5-((thiophen-2-ylmethyl)amino)isophthalic acid (H₂L). Structural diversity was achieved by modulating the solvent system (DMF/MeOH for 1, DMF/EtOH for 2 and 3) and, for compound 3, incorporating 4,4′-bpy as an auxiliary N-donor ligand. Single-crystal X-ray diffraction revealed that all three compounds feature mononuclear cadmium(II) centers coordinated by doubly deprotonated L^2– ligands, which function exclusively as bridging linkers through their carboxylate groups. Coordination geometries vary from capped trigonal prismatic CdO₇ in 1 to distorted pentagonal bipyramidal CdO₇ in 2 and CdNO₆ in 3. Compound 1 forms 1D double-chain motifs assembled through O–H···O hydrogen bonds and π···π interactions; compound 2 displays a 2D layered structure; and compound 3 adopts a 1D ladder-like motif reinforced by π···π stacking between thiophene rings. Thermogravimetric analysis demonstrated thermal stability following the trend 3 > 2 ≈ 1. Adsorption performance was systematically evaluated for Methylene Blue (MB), Crystal Violet (CV), and Congo Red (CR). Compounds 1 and 2 exhibited remarkable selectivity toward CV (87–88% and 78–83%, respectively), while compound 3 displayed exceptional efficiency for CR (84–85%, independent of contact time). All three compounds showed consistently poor performance for MB (≤67%). The adsorption behavior of 1–3 was further analyzed, demonstrating that the kinetic data conform to a pseudo-second-order model, while the equilibrium data are best described by the Langmuir isotherm. The distinct adsorption behaviors underscore the critical role of framework dimensionality, coordination geometry, supramolecular architecture, and surface functionality in governing dye uptake, highlighting the potential of these materials for targeted wastewater remediation.