Pyridine-functionalized monoamides as selective chromate and phosphate receptors

Abstract Selective recognition of oxyanions such as chromate (CrO 4 2− ) and phosphate (PO 4 3− ) is essential for environmental monitoring; however, it remains challenging because of competitive solvation and diverse anion geometries. Amide N-H donors provide directional hydrogen bonding suitable for anion recognition, and their performance can be tuned through subtle structural variations. Herein, three closely related pyridine-functionalized monoamide receptors, namely methyl 6-((2-(pyridin-2-yl)ethyl)carbamoyl)picolinate ( L1 ), methyl 6-((2-(pyridin-3-yl)ethyl)carbamoyl)picolinate ( L2 ), and methyl 6-((2-(pyridin-4-yl)ethyl)carbamoyl)picolinate ( L3 ) were synthesized to evaluate the effect of pyridyl- N positional isomerism (2-, 3-, and 4-). The ligands were characterized by FTIR, 1 H & 13 C NMR, mass spectrometry, and elemental analysis. Single-crystal X-ray diffraction of L3 confirmed a binding-competent molecular cleft. The high purity of L1 – L3 was verified through elemental analysis, with experimental values closely matching the calculated data (±0.3 %), along with characteristic spectroscopic features. UV–vis titrations in acetonitrile with chloride, nitrate, chromate and phosphate salts showed the largest spectral responses for multiply charged oxyanions (chromate and phosphate) compared with monovalent anions. Quantitative analysis afforded association constants (K a ) ranging from 1.72 × 10 2 to 5.59 × 10 3 M −1 , reveals that chromate exhibited the strongest overall affinity, followed by phosphate, chloride and nitrate. The enhanced binding toward chromate and phosphate reflects the combined influence of higher charge density, stronger electrostatic interactions, and favourable hydrogen-bond complementarity within the amide-based binding cleft, whereas the positional variation of the pyridyl nitrogen modulates receptor preorganization and interaction strength. These results highlight pyridyl- N positional isomerism as a simple handle for tuning oxyanion recognition in monoamide receptors.