Cyclic vs. acyclic alkyne towards Hg2+ ion detection: combined experimental and theoretical studies

Abstract

Alkynes constitute a versatile class of functional groups, which can potentially undergo a variety of transformations in organic synthesis. The interaction between the alkyne unit and Hg²⁺ ion has been well-known for several decades. However, with the aim to investigate this interaction in more detail, we have designed and synthesized two thermally stable probes, 3 and 4. Probe 3 is an acyclic compound containing two terminal alkyne units, whereas probe 4 is a cyclic molecule with an internally conjugated C₂-symmetric 1,3-dialkyne unit. Interestingly, both receptors possess alkyne units but reacted differently with Hg²⁺ ions. The acyclic probe 3 with two terminal alkynes interacted with two Hg²⁺ ions, whereas cyclic probe 4, with a cage-like structure, interacted with only one Hg²⁺ ion. The Hg²⁺ ion could only interact with one alkyne unit in probe 4 to avoid steric repulsion between two Hg²⁺ ions. The differences in the selective responses of the cyclic and acyclic structures towards Hg²⁺ ions were thoroughly established via photophysical (UV-vis and emission spectroscopy) and electrochemical analysis together with theoretical (DFT) studies. The probable binding sites of the Hg²⁺ ion with synthesized probes were determined by ¹H NMR and IR titrations, which indicated that the terminal and conjugated di-alkyne units interact with the Hg²⁺ ion via a favorable soft-soft interaction. Furthermore, the sweeping motion of the Hg²⁺ ion between the two alkyne units of the 1,3-dialkyne moiety in 4 was also confirmed by DFT calculations. Unlike the Hg²⁺ ion, Cu²⁺ and Fe³⁺ ions did not interact with the probes, rather they induced oxidation of the ferrocene centre. Both receptors 3 and 4 and their corresponding metal complexes, [3·2Hg²⁺] and [4·Hg²⁺], respectively, were stable in the physiological pH range (pH around 7) and thermally stable up to 60 °C. For the first time, the present study focused on the comparative responses of acyclic and cyclic architectures of the same molecular unit towards metal ion recognition, supporting the fact that the alkyne group in different environments behaves differently with the same soft metal.