Synthesis, characterization and computational studies of copper( <scp>ii</scp> ) terpyridine-based metal–organic frameworks for the removal of emerging herbicide contaminant from aqueous solution

Copper(ii) metal-organic frameworks (MOFs) of 4,4',4″-tri-<i>tert</i>-butyl-2,2':6',2″-terpyridine(N₃ttb) formulated as [Cu(btc)(N₃ttb)]·(DMF)₂1 and [Cu(N₃ttb)(H₂O)₂] 2 (DMF = dimethylformamide) were synthesized and characterized by elemental analyses, and spectroscopic techniques, and compound 1 was further characterized by single crystal X-ray crystallography. The molecular structure of compound 1 revealed a five-coordinate geometry with three meridional nitrogen atoms of 4,4',4″-tri-<i>tert</i>-butyl-2,2':6',2″-terpyridine and two oxygen atoms of 1,3,5-benzenetricarboxylic acid to form a square pyramidal geometry. Compound 2 was functionalized with 1,2-ethanedithiol (TH) to prepared [Cu(N₃ttb)(H₂O)₂]-TH (compound 3). The copper(ii) metal-organic frameworks (MOFs) were used as adsorbents for the removal of 2,4-dichlorophenoxyacetic acid. The adsorption processes followed pseudo-second-order kinetics, and the adsorption equilibrium data best fit the Langmuir isotherm, with <i>R</i> ² values of 0.981, 0.991, and 0.991 for 1, 2 & 3 respectively. The quantity of 2,4-dichlorophenoxyacetic acid removed was 588.24, 333.33 and 833.33 mg g^-1 over 1, 2 & 3 respectively. The results indicate that the functionalized compound 3 has a higher adsorption capacity than 1 & 2 which could be ascribed to electrostatic interactions between the thiol groups of 1,2-ethanedithiol and the carboxylic acid group of 2,4-dichlorophenoxyacetic acid. Computational studies revealed that 3 outperforms 1 & 2 in herbicide adsorption, this can be adjudged to its ultra-soft character (<i>η</i> = 0.31 eV, <i>S</i> = 1.61 eV^-1), high electrophilicity (<i>ω</i> = 74.56 eV) which enables charge-transfer-driven binding with both favourable Δ<i>G</i> (-39.500 kcal mol^-1) and high experimental capacity (833.33 mg g^-1). FT-IR spectroscopic analysis of the MOFs post-adsorption revealed the presence of the herbicide. The study's findings indicated that the prepared MOFs are effective adsorbent for removal of 2,4-dichlorophenoxyacetic acid from wastewater.