Microwave-Assisted Neat Synthesis of a Ferrocene Appended Phenolphthalein Diyne: A Designed Synthetic Scaffold for Hg2+ Ion.

A C2-symmetric internally conjugated 1,3-dialkyne system 5, containing phenolphthalein as a fluorophore and ferrocene as a redox moiety, has been synthesized via a microwave-assisted synthetic procedure. Compound 5 was synthesized by Cu-catalyzed Glaser-Hay coupling using a microwave reactor in neat condition for the first time. Compound 5 was found to be highly selective toward Fe3+, Cu2+, and Hg2+ ions via multichannels. Interestingly, Fe3+ and Cu2+ ions simply promote the oxidation of ferrocene unit to ferrocenium ion without binding to the receptor, whereas Hg2+ binds with the receptor 5 (ΔE1/2 = 71 mV). The oxidation and binding phenomena were investigated by optical and electrochemical analyses. Furthermore, the binding site of Hg2+ ion with our designed probe was confirmed by 1H, 13C NMR and IR titrations, which indicated that conjugated dialkyne unit interacts with Hg2+ ion by a favorable soft-soft interaction. Both receptor 5 and its metal complex, [5·2Hg2+], are stable in the physiological pH range (pH = 6-7) and thermally stable up to 78 °C. The experimental results of metal binding have been further supported by quantum chemical calculations (DFT), which explore the favorable geometry of the free ligand as well as its Hg2+ complex.