2-(((4-([2,2′:6′,2″-Terpyridin]-4′-yl)phenyl)imino)methyl)-6-methoxyphenol

Abstract

In this short note, we elaborate on a synthetic method for a new terpyridine Schiff base-bearing vanillin motif. The structure of the product was confirmed via ¹H and ¹³C-NMR spectroscopy and FT-IR spectroscopy. Due to the peculiar coordination nature and optical property of the synthesized compound, this work presents a promising colorimetric sensing molecular compound.

1. Introduction

The functionalization of 2,2′:6′,2″-terpyridine (tpy) has attracted increasing attention in the past few decades on account of the distinguished electrochemical and photophysical properties [1,2,3,4]. Terpyridine derivatives have been broadly applied to ligands, organic optoelectronic materials, and supermolecular materials due to their superior metal-binding ability, π-stacking ability, and strong hydrogen bond interaction [5,6,7,8,9,10]. Particularly, terpyridine-containing imine structure gaining researchers’ insight into designing fluorescent and colorimetric chemosensors because of the peculiar coordination properties encompassed by the tridentate ligand feature of tpy and the strong binding affinity of C=N bond in the imine group [11,12,13]. To the best of our knowledge, the wealth of literature where terpyridine Schiff base with an o-vanillin motif is not observed. This paper described the synthesis of 2-(((4-([2,2′:6′,2″-terpyridin]-4′-yl)phenyl)imino)methyl)-6-methoxyphenol 1 (Scheme 1). The synthesis effort has been complemented by detailed FT-IR, ¹H, and ¹³C NMR spectroscopic analysis.

2. Results and Discussion

We designed the synthetic method of title Compound 1 shown as Scheme 2. The starting material 4-([2,2′:6′,2″-terpyridin]-4′-yl)aniline was synthesized from the commercially available 2-acetylpyridine and 4-nitrobenzaldehyde in a four-step procedure according to the literature [14,15,16,17]. A tentative synthesis of Compound 1 was first performed by condensation of 4-([2,2′:6′,2″-terpyridin]-4′-yl)aniline and o-vanillin in refluxing ethanol using protonic acid catalysts such as p-toluenesulfonic acid. The conventional reaction condition for Schiff base synthesis, however, failed to provide the target product 1 even given a relatively long reaction time. Gratifyingly, using Lewis acid as a catalyst and n-butanol as a solvent, which allows the reaction temperature to override 110 °C, facilitates the reaction, and affords Compound 1 a yield of 80%.

Compound 1 was characterized by ¹H-NMR, ¹³C-NMR, and FT-IR spectroscopy. Firstly, the ¹H-NMR spectrum exhibits the typical signals for the methoxy group as a single peak at 3.92 ppm with the integration of 3H (Figure S1). Due to the intramolecular hydrogen bond in Compound 1, the chemical shifts of CH=N proton and phenol proton shifted downfield at 8.71 ppm and 13.62 ppm, respectively. Chemical shifts on the substituted terpyridine ring are concentrated in the range of 6.95 to 8.86 ppm, as reported for other terpyridine derivatives [15]. The ¹³C-NMR spectrum exhibits 20 peaks, which fully agree with the proposed structure for 1 (Figure S2).

The FT-IR spectra (Figure S3) of Compound 1 present the characteristic absorption of (C=N) vibration at 1586 cm⁻¹, and the signals of 1466 and 1257 cm⁻¹ in the IR spectrum represent skeleton vibration of a benzene ring and stretching vibration of C-O bond, respectively. An O-H stretching band for 1 appears as a broad band centered at 3445 cm⁻¹.

Due to the terpyridine skeleton and imine group, Compound 1 exhibits excellent optical properties and coordination ability and has the potential application in the selective recognition of metal ions. UV-Vis spectra of Compound 1 (100 μM) mixed with 12 common metal ions (100 μM) were tested in MeOH solution, respectively (Figure S4). The absorbance signal profile curve displayed significant enhancement in the presence of Co³⁺, which means Compound 1 can be acted as the colorimetric probe for specific detection of Co²⁺.

3. Materials and Methods

All reagents were purchased from Shanghai Aladdin Bio-Chem Technology Co., LTD (Shanghai, China) and direct used as received; the reagent grades were A. R. The starting 4-([2,2′:6′,2″-terpyridin]-4′-yl)aniline was synthesis in 73% yield according to the literature [14,15,16,17]. Infrared spectra were recorded on an Alpha II spectrometer (Bruker, Wissembourg, France) as KBr discs. Melting points were recorded with a Stuart SMP 10 melting point apparatus (Bibby Sterilin, Stone, UK) and were uncorrected. NMR spectra were recorded on a Bruker Avance AV400 (400/100 MHz ¹H/¹³C)spectrometer (Bruker, Billerica, MA, USA), and chemical shifts (δ, ppm) were downfield from TMS. Absorption spectra were recorded on a UV-6300 spectrophotometer, with a scanning voltage of 250 V, scanning accuracy of 0.5 nm, and scanning speed of 10 nm/s.

2-(((4-([2,2′:6′,2″-terpyridin]-4′-yl)phenyl)imino)methyl)-6-methoxyphenol (1): In a 100 mL three-neck flask, 4-([2,2′:6′,2″-terpyridin]-4′-yl)aniline (0.32 g, 1 mmol), o-vanillin (0.18 g, 1.2 mmol) and Ti(OEt)₄ (0.1 mL, 0.48 mmol) were added into n-butanol (7 mL). The reaction mixture was stirred under reflux for 1 h and then cooled to ambient temperature. The precipitate was collected via filtration and recrystallized in n-butanol to achieve orange powder (0.37 g, 80%). m.p. = 159–161 °C. ¹H NMR (400 MHz, CDCl₃) δ 13.62 (s, 1H), 8.71 (s, 4H), 8.64 (dd, J = 10.1, 5.0 Hz, 3H), 7.93 (d, J = 8.4 Hz, 2H), 7.85 (td, J = 7.8, 1.7 Hz, 2H), 7.39–7.30 (m, 4H), 7.04–6.96 (m, 2H), 6.87 (t, J = 7.9 Hz, 1H), 3.92 (s, 3H); ¹³C NMR (100 MHz, CDCl₃) δ 196.7, 162.9, 156.1, 156.0, 151.5, 149.2, 149.1, 148.7, 148.5, 136.9, 128.4, 124.0, 123.9, 121.8, 121.4, 119.1, 118.7, 118.5, 114.9, 56.2. FTIR (KBr, cm⁻¹): 3445, 1586, 1466, 1257.

4. Conclusions

A novel terpyridine derivative named 2-(((4-([2,2′:6′,2″-terpyridin]-4′-yl) phenyl)imino)methyl)-6-methoxyphenol (1) was handily synthesized from 4-([2,2′:6′,2″-terpyridin]-4′-yl)aniline and characterized. High heating temperature and Lewis acid catalyst were found to be essential to the successful reaction. Future work will emphasize exploring the fluorescence recognition ability of Compound 1 on the heavy metal ions.