Azocalix[4]arene with three distal ethyl ester residues as a highly selective chromogenic sensor for Ca2+ ions

Jie Wang; Hongyu Guan; Chunhua Ge; Ping Fan; Xijuan Xing; Yunshan Shang

doi:10.1515/hc-2017-0239

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Azocalix[4]arene with three distal ethyl ester residues as a highly selective chromogenic sensor for Ca²⁺ ions

Jie Wang , Hongyu Guan , Chunhua Ge , Ping Fan , Xijuan Xing and Yunshan Shang

Published/Copyright: April 27, 2018

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From the journal Heterocyclic Communications Volume 24 Issue 3

Abstract

Three azocalix[4]arenes with distal ethyl ester residues, 5-phenylazo-25,26,27-tris[(ethoxycarbonyl)methoxy]-28-hydroxycalix[4]arene (2), 5-(o-methylphenyl)azo-25,26,27-tris[(ethoxycarbonyl)methoxy]-28-hydroxycalix[4]arene (3), 5-(p-Methylphenyl)azo-25,26,27-tris[(ethoxycarbonyl)methoxy]-28-hydroxycalix[4]arene (4), were synthesized and their binding properties with metal ions were investigated by ultraviolet (UV)/visible spectroscopy. The chromogenic behavior of these compounds upon metal ion complexation indicates a specific selectivity toward Ca²⁺ ion in the presence of other cations tested. The stoichiometry of 3 to Ca²⁺ ion in the complex is 1:1 and the stability constant of the complex is 1.28×10⁴m⁻¹.

Keywords: azocalix[4]arene; calcium ion; chromogenic sensor; ion recognition

Introduction

Over the past two decades, significant efforts have been focused on the design and synthesis of chemosensors that are able to selectively recognize target molecules or ions, particularly those with color or fluorescent response functions [1], [2], [3], [4], [5], [6]. Converting this recognition into an optical signal, a reaction process or a chemical species that could be observed and detected by the naked eye or with inexpensive equipment, is of interest [7], [8], [9]. Azocalixarenes have been reported to change color upon complexation with specific metal ions [10], [11], [12], [13]. In many cases, the azo moiety is introduced into calixarenes at the upper rim to facilitate the monitoring of the binding process upon complexation. Many studies on azocalixarene derivatives also involve the functionalization of the lower rim with metal-chelating groups, such as carboxylic acid, ester and other groups [14], [15], [16], [17], [18]. Other studies focus on azocalix[4]arenes functionalized with distal esters as chromogenic receptors [19], [20], [21], [22], [23], [24].

An antracenylazo derivative was synthesized [19] and shown to undergo complexation with Eu³⁺, Ag⁺ and Cu²⁺ ions [20]. Subsequently, it was reported that this calixarene forms complexes with Ca²⁺, Pb²⁺, Sr²⁺ and Ba²⁺ ions as well, with a moderate selectivity toward Ca²⁺ ions [21]. On the other hand, it was reported that an azocalix[4]arene with four distal ester groups undergoes complexation with Ag⁺, Hg⁺ and Hg²⁺ cations [22]. Similar azocalixarene species bearing diester groups on the lower rim were used for the selective extraction of Pd²⁺ ion in the presence of some other cations in hydrochloric acid solution [23]. Using the azocalixarenes as extractants made the extraction process visible. Additional applications have been reported [24].

Results and discussion

In an effort to synthesize azocalixarene species that exhibit unique chromogenic responses to specific metal ions, three new azocalix[4]arenes with distal ethyl ester groups on the lower rim, 5-phenylazo-25,26,27-tris[(ethoxycarbonyl)methoxy]-28-hydroxycalix[4]arene (2), 5-(o-methylphenyl)azo-25,26,27-tris[(ethoxycarbonyl)methoxy]-28-hydroxy- calix[4]arene (3), 5-(p-methylphenyl)azo-25,26,27-tris[(ethoxycarbonyl)methoxy]-28-hydroxycalix[4]arene (4), were synthesized using methods inspired by Shinkai et al. [25] and Tóth et al. [26]. The synthesis of compounds 2–4 is illustrated in Scheme 1.

Scheme 1

Synthesis of calix[4]arenes 2–4. Reagents and conditions: NaNO₂/4N HCl, pyridine, 0°C, 5 h, aniline in acetone for 2, o-toluidine in acetone for 3, p-toluidine in acetone for 4.

In acetonitrile, compounds 2–4 show a strong ultraviolet/visible (UV/Vis) absorption peak near 362 nm (Figure 1), which can be attributed to the π-π* transition of the -N=N- bond [21]. The binding properties of the three compounds with metal ions were investigated by monitoring UV/Vis spectral changes upon the addition of Na⁺, K⁺, Li⁺, Mg²⁺, Ca²⁺, Ba²⁺, Fe³⁺, Co²⁺, Ni²⁺, Zn²⁺, Cd²⁺, Hg²⁺, Pb²⁺, Mn²⁺, Ag⁺, La³⁺, Nd³⁺, Eu³⁺, Sm³⁺ and Cr³⁺ ions. With the exception of Ca²⁺, addition of these ions does not significantly alter the absorption peaks of compounds 2–4. Upon addition of Ca²⁺ to 2, the intensity of absorption band at 362 nm decreases and a new band appears at 429 nm. The UV/Vis spectral changes for compounds 3 and 4 are similar to those observed for compound 2 (Figure 1).

Figure 1

Absorption spectra of 2 (A), 3 (B) and 4 (C) in the absence and the presence of various metal ions in acetonitrile.

These results indicate that compounds 2–4 possess a specific selectivity for Ca²⁺ ion. Compounds 2–4 may exist in two tautomeric forms containing azophenol and quinone-hydrazone moieties [12], [13], [27]. The metal complexation may induce a release of proton from the azophenol to the quinone-hydrazone function. When compounds 2–4 undergo complexation with Ca²⁺ ion, the formation of quinone-hydrazone tautomer results in the weakening of the double bond character of the azo moiety. As a result, the UV/Vis spectrum is altered, with the new band appearing at 429 nm.

The complexation ratio between azocalixarene 3 and Ca²⁺ was determined using a Job’s plot (Figure 2). The experiment was conducted using a 429 nm wavelength. As can be seen, the maximum point is at a molar fraction of [azocalixarene 3]/([azocalixarene 3]+[Ca²⁺]) of approximately 0.5, indicating a host-metal ion complex ratio of 1:1. The association constant for the 3-Ca²⁺ complex in acetonitrile is 1.28×10⁴m⁻¹ as determined by a Benesi-Hilderbrand plot [12], [28].

Figure 2

Job’s continuous variation plot for 3-Ca²⁺ at 429 nm.

([3]+[Ca²⁺]=1×10⁻³).

A color change is observed upon mixing the ligand 2, 3 or 4 and Ca²⁺ cation. For example, when a dilute colorless solution of compound 4 is treated with Ca²⁺ (Figure 3), the solution becomes yellow. No color change is observed in the presence of other metal cations.

Figure 3

Color of the solution of compound 4 (1.0×10⁻⁵m) in the presence of various metal cations (4.0×10⁻⁵m).

Conclusions

Azocalix[4]arenes 2–4 exhibit a remarkable selectivity toward complexation of Ca²⁺ ion, and the interaction is associated with a color change, suggesting possible applications as new chromogenic sensors for Ca²⁺ ion-sensing materials. The hydroxyl and distal ethyl ester units of azocalixarenes 2–4 take part in binding with Ca²⁺ ion, and the azo moiety is necessary for the color recognition of Ca²⁺ ion.

Experimental

Synthesis of compounds 2–4

An ice-cold solution of NaNO₂ (0.90 g) in hydrochloric acid (4 m, 6 mL) was added to a solution of aniline, o-toluidine or p-toluidine (0.86 mmol) in acetone (5 mL). The mixture was stirred for 20 min and then added to an ice-cold solution of calix[4]arene 1 (0.13 g, 0.19 mmol) in pyridine (8 mL) [25]. The colored mixture was stirred for 5 h at 0°C and then treated with hydrochloric acid (4 m, 50 mL). The resultant colored precipitate was subjected to chromatography on silica gel eluting with ethyl acetate/petroleum ether (1:5).

5-Phenylazo-25,26,27-tris[(ethoxycarbonyl)methoxy]-28-hydroxycalix[4]arene (2)

Yield 20%; mp 199–201°C; IR (KBr): 3392, 2927, 1765 cm⁻¹; ¹H NMR (300 MHz, DMSO-d₆): δ 1.26 (t, J=7 Hz, 3H, OCH₂CH₃), 1.34 (t, J=7 Hz, 6H, OCH₂CH₃), 3.32 (d, J=13 Hz, 2H, ArCH₂Ar), 3.47 (d, J=13 Hz, 2H, ArCH₂Ar), 4.14 (q, J=7 Hz, 2H, OCH₂CH₃), 4.29 (q, J=7 Hz, 4H, OCH₂CH₃), 4.41 (d, J=13 Hz, 2H, OCH₂CH₃), 4.51 (d, J=15 Hz, 2H, OCH₂COO), 4.66 (d, J=15 Hz, 2H, OCH₂COO), 4.97 (d, J=13 Hz, 2H, OCH₂COO), 5.13 (s, 2H, OCH₂COO), 6.56 (m, 6H Ar), 6.93 (t, J=7 Hz, 1H, Ar), 7.13 (d, J=7 Hz, 2H, Ar), 7.18 (s, 1H), 7.46 (m, 3H, Ar), 7.74 (s, 2H, Ar), 7.88 (d, J=8 Hz, 2H, Ar); ¹³C NMR (75 MHz, DMSO-d₆): δ 171.0 (CO), 169.2 (CO), 156.9, 155.9, 153.7, 153.0, 145.3, 135.8, 133.2, 132.0, 129.9, 129.5, 129.3, 129.0, 128.7, 128.4, 124.2, 123.8, 123.4, 122.3, 72.1 (OCH₂CO), 70.3 (OCH₂CO), 61.1 (s, OCH₂), 60.3 (OCH₂), 31.8 (CH₂), 31.0 (CH₂), 14.2 (CH₃). HR-MS. Calcd for M⁺: m/z 702.7054. Found: m/z 702.7060.

5-(o-Methylphenyl)azo-25,26,27-tris[(ethoxycarbonyl)methoxy]-28-hydroxy- calix[4]arene (3)

Yield 30%; mp 192–194°C; IR (KBr): 3422, 2927, 1763 cm⁻¹; ¹H NMR (300 MHz, DMSO-d₆): δ 1.27 (t, J=7 Hz, 3H, OCH₂CH₃), 1.34 (t, J=7 Hz, 6H, OCH₂CH₃), 2.72 (s, 3H, ArCH₃), 3.32 (d, J=13.5 Hz, 2H, ArCH₂Ar), 3.47 (d, J=13.5 Hz, 2H, ArCH₂Ar), 4.14 (q, J=7 Hz, 2H, OCH₂CH₃), 4.29 (q, J=7 Hz, 4H, OCH₂CH₃), 4.42 (d, 2H, ArCH₂Ar), 4.51 (d, J=16 Hz, 2H, OCH₂COO), 4.66 (d, J=16 Hz, 2H, OCH₂COO), 4.97 (d, J=13.5 Hz, 2H, ArCH₂Ar), 5.13 (s, 2H, OCH₂COO), 6.49–6.65 (m, 6H, Ar), 6.93 (t, J=7 Hz, 1H, Ar), 7.12 (d, J=7 Hz, 3H, Ar), 7.28 (m, 3H, Ar), 7.58 (d, J=7 Hz, 1H, Ar), 7.73 (s, 2H, Ar); ¹³C NMR (75 MHz, DMSO-d₆): δ 170.9 (CO), 169.1 (CO), 156.6, 155.7, 153.8, 151.1, 145.7, 137.0, 135.8, 133.2, 132.0, 131.0, 129.4, 129.3, 128.6, 128.3, 126.3, 124.1, 123.8, 123.3, 115.4, 72.2 (OCH₂CO), 70.2 (OCH₂CO), 61.0(OCH₂), 60.2 (OCH₂), 31.7 (CH₂), 31.0 (CH₂), 17.5 (CH₃), 14.1 (CH₃). HR-MS. Calcd for M⁺: m/z 716.7320. Found: m/z 716.7311.

5-(p-Methylphenyl)azo-25,26,27-tris[(ethoxycarbonyl)methoxy]-28-hydroxycalix[4]arene (4)

Yield 25%; mp 151–153°C; IR (KBr): 3410, 2978, 1733 cm⁻¹; ¹H NMR (300 MHz, DMSO-d₆): δ 1.26 (t, J=7 Hz, 3H, OCH₂CH₃), 1.34 (t, J=7 Hz, 6H, OCH₂CH_3,), 2.42 (s, 3H, ArCH₃), 3.31 (d, J=13.5 Hz, 2H, ArCH₂Ar), 3.46 (d, J=13.5 Hz, 2H, ArCH₂Ar), 4.14 (q, J=7 Hz, 2H, OCH₂CH₃), 4.29 (q, J=7 Hz, 4H,OCH₂CH₃), 4.40 (d, J=13.5 Hz, 2H, ArCH₂Ar), 4.50 (d, J=15 Hz, 2H,OCH₂COO), 4.65 (d, J=15 Hz, 2H, OCH₂COO), 4.96 (d, J=13.5 Hz, 2H, ArCH₂Ar), 5.13 (s, 2H, OCH₂COO), 6.47–6.63 (m, 6H, Ar–H), 6.93 (t, J=7 Hz, 1H, Ar–H), 7.03–7.14 (m, 3H, Ar), 7.29 (d, J=8 Hz, 2H, Ar–H), 7.72 (s, 2H, Ar), 7.78 (d, J=7.5 Hz, 2H, Ar); ¹³C NMR (75 MHz, DMSO-d₆): δ 170.9 (s, CO), 169.0 (s, CO), 156.5 (s), 155.6 (s), 153.6 (s), 151.0 (s), 145.2, 140.2, 135.7, 133.1, 131.9, 129.6, 129.4, 129.2, 128.6, 128.3, 124.1, 123.6, 122.2, 70.9 (OCH₂CO), 70.2 (OCH₂CO), 61.0 (OCH₂), 60.1 (OCH₂), 31.6 (CH₂), 30.9 (CH₂), 21.3 (CH₃), 14.1 (CH₃). HR-MS. Calcd for M⁺: m/z 716.7320. Found: m/z 716.7383.

Acknowledgments

This work was supported by the National Natural Science Foundation of China (grant 21171081), the Science Foundation of Education Department of Liaoning Province (grant L2011007) and the Foundation of Project 211 for Innovative Talents Training, Liaoning University. The authors also thank the colleagues and students who participated in this work.

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Received: 2017-12-6

Accepted: 2018-1-26

Published Online: 2018-4-27

Published in Print: 2018-6-27

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https://doi.org/10.1515/hc-2017-0239

Keywords for this article

azocalix[4]arene; calcium ion; chromogenic sensor; ion recognition

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