Time-Resolved Polymorphism in a Cd-MOF with Dual Applications in Iodine Adsorption and One-Pot Tandem Knoevenagel-Michael Cyclocondensation Catalysis.

Metal-organic frameworks (MOFs) are highly advanced porous materials with tunable structures, high surface areas, and multifunctional properties, making them attractive for applications in environmental remediation, gas storage, sensing, and catalysis. In this work, two polymorphic Cd-based mixed-ligand MOFs, MOF-J and MOF-A, were synthesized solvothermally using N'1,N'4-bis((E)-pyridin-4-ylmethylene)terephthalohydrazide (L1) and 4,4'-oxybis(benzene dicarboxylic acid) (H2oba). Their formation was governed by crystallization time, with monoclinic MOF-J representing the kinetically favored phase and triclinic MOF-A the thermodynamically stable phase. Structural analyses by single-crystal and powder X-ray diffraction confirmed their distinct framework architectures. Among the polymorphs, MOF-J exhibited superior multifunctional performance, achieving an iodine vapor adsorption capacity of 606 wt % at 75 °C within 7 h, along with excellent recyclability and structural stability. Iodine adsorption enhanced its electrical conductivity from 6.84 × 10-9 to 2.46 × 10-4 S m-1, attributed to iodine-induced charge transport and band gap reduction. Density functional theory (DFT) calculations revealed strong host-guest interactions responsible for effective iodine confinement. In addition, MOF-J acted as an effective recyclable heterogeneous catalyst for one-pot Knoevenagel-Michael cyclocondensation, affording 2-amino-4H-benzo[b]pyran derivatives in excellent yields under mild conditions. This work highlights time-controlled polymorphism as an effective strategy for designing multifunctional MOFs for iodine capture and heterogeneous catalysis.