Strategic design of a 2,6-disubstituted pyridine-based probe having hard-soft centers: responsive divergence from one core

Abstract

Alkyne is a versatile functional group in organic chemistry, and is able to undergo a wide variety of reactions and interactions. Featuring a reactive functional group, alkyne participates in many organic reactions and hence a sharp rise in the interest of utilizing the alkyne functionality has made it an inevitable synthon in a wide domain of purposes and one of such purposes is molecular recognition. On account of this, two pyridine-derived scaffolds, 5 and 7, containing identical molecular cores but different appendages, viz., terminal alkyne (5) and internally 1,3-conjugated alkyne units (7), are successfully synthesized. Both the compounds are subjected to metal ion sensing at the molecular level and are found to bind Cu²⁺ and Hg²⁺ ions with different functionalities. Compounds 5 and 7 interact with Cu²⁺ by the pyridine N and the two adjacent amide N's in a tripodal fashion, whereas they interact with Hg²⁺ by their respective open-end and closed-end alkyne units. The terminal alkynes in 5 undergo chemical reaction in the presence of Hg(ClO₄)₂·H₂O and get converted to a ketone functionality, while the internally conjugated 1,3-dialkyne unit in 7 acts as a binding unit for a Hg²⁺ ion. Both experimental studies and theoretical (DFT) calculations have converged on the result that terminal alkynes cannot function as a chemosensor for Hg²⁺ ions, although they can respond by functional group transformation, whereas cyclic internally conjugated alkynes can perform as potential Hg²⁺ sensors. The combination of Cu²⁺ and Hg²⁺ ions has been used to generate a molecular system exhibiting the OR logic operation. The limits of detection (LODs) of Cu²⁺ ions, obtained for 5 and 7, are 5.5 × 10⁻⁷ M and 5.2 × 10⁻⁷ M, respectively, and that of the Hg²⁺ ion for 7 is 4.4 × 10⁻⁷ M. The synthesized probes, as well as their complexes, are stable around neutral pH and the probes retain their sensitivity within a temperature window of 25–80 °C. This creates an avenue for differential recognition of multiple heavy metal ions simultaneously with similar molecular motifs.