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

Abstract

Copper(II) metal–organic frameworks (MOFs) of 4,4′,4″-tri-tert-butyl-2,2′:6′,2″-terpyridine(N₃ttb) formulated as [Cu(btc)(N₃ttb)]·(DMF)₂ 1 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-tert-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 R² 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 (η = 0.31 eV, S = 1.61 eV⁻¹), high electrophilicity (ω = 74.56 eV) which enables charge-transfer-driven binding with both favourable ΔG (−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.