DOI: https://doi.org/10.55373/mjchem.v28i4.100

Keywords: Amide-thiourea receptors, anion recognition, binding constant, oxoanions, supramolecular sensing, UV-Vis titration

Abstract

The rising levels of toxic oxoanions such as chromate, permanganate, phosphate, and arsenate in aquatic environments pose significant environmental and health concerns, necessitating the development of efficient detection systems. In this study, three tripodal amide–thiourea receptors (AT1- AT3) were synthesized and evaluated as supramolecular hosts for tetrahedral oxoanions. Structural characterization using UV-Vis, FTIR, and NMR spectroscopy confirmed successful formation of the receptors with multiple hydrogen-bonding donor sites. UV–Vis titration studies revealed distinct bathochromic and hypochromic spectral responses upon anion binding, indicating receptor–anion interactions via N–H·O hydrogen bonding. Job plot analysis suggested a predominant 1:1 binding stoichiometry, while binding constants (Kₐ) derived from BindFit ranged from 8.55 × 10² to 1.88 × 10⁴ M⁻¹. AT3 exhibited the highest affinity toward CrO₄²⁻, attributed to enhanced electronic polarization and favourable structural preorganization induced by the –CN substituent. In contrast, the strongly electron-withdrawing –NO₂ group in AT2 significantly decreased electron density at the thiourea binding sites through its negative inductive (–I) and negative mesomeric (–M) effects. This disrupts effective charge delocalization across the receptor framework and reduces the hydrogen bond-donating ability of the –NH moieties, ultimately resulting in weaker anion binding. These findings highlight the critical role of substituent effects in modulating anion recognition and demonstrate that tripodal amide–thiourea systems are promising candidates for selective optical sensing of environmentally relevant oxoanions.