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

Abstract

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 (ΔE 1/2 = 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.

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

“…28 Compound 1 was prepared from commercially available phenolphthalein via the propargylation of phenolic alcohol. 29 Chromatography was carried out using 60-120 mesh silica gel in a column with a diameter of 2.5 cm. All the necessary solvents were dried using conventional methods and distilled under an N 2 atmosphere.…”

“…Compound 4 was synthesized via a modified GlaserÀHay coupling reaction under neat conditions using microwaveassisted reaction conditions. 29 Treatment of compound 3 with Cu(OAc) 2 ÁH 2 O as the catalyst and pyridine as the base under microwave conditions afforded the cyclic compound 4. The reaction was performed in a closed reaction vessel in a microwave reactor with a power of 50 watts at a temperature of 60 1C for 30 min to give compound 4 in 30% yield.…”

“…This typical behaviour is consistent with our earlier report. 29 The calculated bond lengths and Wiberg bond indices (WBI) are presented in Table S1 (ESI †).…”

“…In the optimized structure, the Hg-C bond distances are in the range of 3.0-3.4 Å, which is in good agreement with the literature value. 67 Following our earlier findings, 29 we also tried to optimize another possibility of Hg 2+ coordination with only terminal alkyne units. Interestingly, in this optimized geometry, we found that the Hg 2+ ions are far apart from the alkyne units and higher in energy than the aforementioned binding mode.…”

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

“…Such interactions are feasible according to the HSAB principle, but literature reports are rather rare. [10][11][12] The Hg 2+ ion is known to have a high affinity towards the alkyne functionality, and therefore it is anticipated to serve as a potential interactive unit towards any kind of alkyne, but the nature of such interactions in different alkynes with similar surroundings of probes has not been explored yet. The Hg 2+ ion is also well-known to bind with hard donors like N and O atoms; [13][14][15][16][17] however, any insight about the nature of interaction of Hg 2+ in the presence of both hard and soft centres, simultaneously in a probe, is rather very rare.…”