Substituent-dependent selective extraction of environmentally relevant metal ions by benzothioates: a combined experimental and DFT study

Abstract

This study presents a systematic investigation into the substituent-dependent extraction behavior of benzothioate derivatives, focusing on chloro-(BzT-Cl), methyl-(BzT-Me), and methoxy-(BzT-MeO) analogues. Competitive liquid-liquid ion-pair extraction experiments were conducted using six environmentally relevant metal ions (Zn2+, Mn2+, Cr3+, Fe3+, Co2+, and Cu2+), establishing a comprehensive structure-activity relationship (SAR) supported by both experimental data and density functional theory (DFT) calculations. The results indicate a pronounced preference for Fe3+ across all ligand systems; notably, BzT-Cl achieved an extraction efficiency of 87.74% and a selectivity factor (S-f) exceeding 1400 relative to Zn2+. DFT analysis (B3LYP/def2-TZVP) suggests that this selectivity arises from the interplay between the high charge density of Fe3+ and the substituent-induced electronic substituent-dependent electronic properties of the ligand framework. The performance of BzT-Cl correlates with its higher chemical hardness (eta = 2.315 eV) and wider HOMO-LUMO energy gap (Delta E = 4.630 eV), whereas the broader extraction behavior of BzT-MeO toward divalent ions such as Mn2+ is associated with its increased nucleophilicity (epsilon > 2.9 eV). Extraction experiments in reservoir, river, and tap water further revealed pronounced matrix-dependent variations in metal removal efficiency, highlighting the influence of sample composition on ligand performance. Overall, the combined experimental and computational results demonstrate that electronic modification of the benzothioate framework can modulate metal-ion extraction and selectivity, providing a basis for the further development of substituent-engineered extraction systems.