2-Azahetaryl-2-(oxoindolin-2-ylidene)acetonitriles as Colorimetric Probes for Zn: Synthesis and Optical Properties

A new one-pot approach for the synthesis of the Zn²⁺-sensitive probes 2-azahetaryl-2-(oxoindolin-2-ylidene)acetonitriles 3a-c and 4 is described. The method includes the in situ formation of imidoylchloride and its further condensation with azahetarylacetonitrile 1. The structure of the obtained compounds is studied using ¹H nuclear magnetic resonance (NMR), ¹³C NMR, infrared (IR), high-resolution mass spectrometry (HRMS), and UV-Vis spectroscopy techniques. Two model ligands both exhibiting the highest extinction coefficient and the best solubility in a Tris buffer pH 7.2/dimethyl sulfoxide (DMSO) solution, namely 5-methyl-benzothiazole derivative 3b and benzoxazole derivative 4, are thoroughly studied as colorimetric probes for Zn²⁺. The probe 3b has the highest sensitivity to Zn²⁺, showing a limit of ion detection (LOD) calculated by the 3S criterion of 0.43 μM and selectivity upon masking Cu²⁺ ions with Na₂S₂O₃. The composition of the complexes in the solution was determined by the limited logarithm method. The stability constant (lg K) values of 3b-Zn of 10.27 ± 0.02 and 4-Zn of 12.5 ± 0.2 indicate the formation of complexes of average stability.