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Communication

Hybrid Pyridine Bis-Anthracene-Imidazolium Salt: NMR Studies on Zn-Acetate Complexation

1
Faculty of Chemistry, Alexandru Ioan Cuza University of Iasi, 11 Carol 1st Blvd, 700506 Iasi, Romania
2
Institute of Interdisciplinary Research-CERNESIM Center, Alexandru Ioan Cuza University of Iasi, 11 Carol I, 700506 Iasi, Romania
*
Author to whom correspondence should be addressed.
Academic Editor: Oleg A. Rakitin
Molbank 2021, 2021(3), M1280; https://doi.org/10.3390/M1280
Received: 3 August 2021 / Revised: 9 September 2021 / Accepted: 10 September 2021 / Published: 14 September 2021
(This article belongs to the Section Organic Synthesis)

Abstract

We report here the design and synthesis of a new hybrid bis-anthracene-imidazolium salt, having a pyridine scaffold. NMR studies of dimer generation, as well as complexation with zinc acetate were performed.
Keywords: hybrid salt; dimer generation; Zn complexation; NMR studies hybrid salt; dimer generation; Zn complexation; NMR studies

1. Introduction

In the field of supramolecular chemistry, the design and synthesis of chemosensors for the detection of metal ions have been widely exploited, due to their biological and environmental significance [1,2,3]. Most of these receptors can be considered hybrid compounds based on a moiety of imidazole, benzimidazole and pyridine, attached to the fluorophore, which is the anthracene unit [1,2,3,4,5].
Considering our experience in the field of the synthesis of hybrid compounds with imidazole/benzimidazole, pyridine and anthracene units [2,6,7,8,9,10,11], and our previous experience in the area of cycloimonium ylides [12,13,14,15,16,17,18,19,20,21,22], we decided to synthesize a novel hybrid pyridine bis-anthracene-imidazolium salt, and also to study its complexation to Zn acetate by NMR.

2. Results and Discussion

The reaction pathway to obtain the new hybrid pyridine bis-anthracene-imidazolium salt 3 involve a quaternization reaction of 2,6-bis((1H-imidazol-1-yl)methyl)pyridine 1, previous reported [7], with and 9-(chloromethyl)anthracene 2, Scheme 1. The structure of new hybrid salt 3 was proved by NMR experiments (1H-, 13C-NMR, 2D: COSY, HMQC, HMBC).
In the next stage, we studied the complexation process of ylide 4 with Zn2+ cation [Zn2+ cation was generated from aqueous deuterated solution of zinc acetate (1.25 × 10−2 M)]. The ylide 4, was generated in situ from the corresponding bis-anthracene-imidazolium salt 3 [previous dissolved in deuterated DMSO (2.5 × 10−3 M)] using aqueous deuterated solutions of potassium carbonate (2 × 10−3 M and 2.5 × 10−1 M).
Our expectation was to obtain a complex of ylide 4 with Zn2+ of type 5, Scheme 2 as in related cases [5]. Instead, because of high reactivity of ylide 4, a dimerisation process took place (via a 3 + 3 dipolar cycloaddition of an ylide molecule to another) when the dimeric structure type 6 was obtained. In the next step, the dimer 6 complexes with Zn2+, leading to the final product, the dimer complex with Zn2+, type 7. The structure of Zn complex, type 7, is a proposed structure but different coordination of Zn2+ ion cannot be excluded.
In Figure 1 are presented the overlapped 1H-NMR spectra of salt 3, dimeric structure 6 and dimeric complex with Zn2+ type 7. Here are described the quantities of reactants used in the experiments and the exchange of the color of solutions.
In the 1H-NMR spectrum of dimeric structure type 6 it can be observed the disappearance of protons (–CH2–)6′, which in salt 3 appears as a singlet at 6.48 ppm. Also, the signal around 9.06 ppm of H2′ from imidazole nucleus of salt 3 does not appear in the NMR spectrum of dimer 6.
The dimer complexation with Zn2+ induces a visible shielding effect on the chemical shifts of the protons from aliphatic and aromatic zone.

3. Materials and Methods

3.1. Instrumentation

The solvents and reagents were purchased from commercial sources, being used without further purification. The melting point (uncorrected) of compound 3 was determined using an open capillary tubes introduced in a MEL-TEMP Electrothermal apparatus. The nuclear magnetic resonance experiments have been recorded on a Bruker AVANCE III 500 MHz spectrometer (Iasi, Romania), equipped with a 5 mm PABBO detection probe, operating at 500.19 and 125.7 MHz for 1H and respectively 13C nuclei. In 1H and 13C spectra, chemical shifts are reported in δ units (ppm) relative to the residual peak of solvent (ref: DMSO-d6, 1H: 2.50 ppm; 13C: 39.52 ppm). The coupling constants (J) are given in Hz. In the NMR spectra to appointed the multiplicity of signals, were used the abbreviations: s = singlet, d = doublet, t = triplet. The microanalyses were in satisfactory agreement with the calculated values: C, ±0.15; H, ±0.10; N, ±0.30.

3.2. General Procedure for Synthesis of Hybrid Quaternary Salt 3

To a solution of 2,6-bis((1H-imidazol-1-yl)methyl)pyridine 1 (1 mmol, 1 equiv., 0.24 g, dissolved in 40 mL acetone using the ultrasound bath) was added dropwise a solution of 9-(chloromethyl)anthracene 2 (2.8 mmol, 2.8 equiv., 0.63 g, dissolved in 15 mL acetone using the ultrasound bath). The reaction mixture was refluxed for 12 h, and stirred at room temperature for another 24 h to give the corresponding hybrid quaternary salt 3. The completion of the reaction was carried out using TLC. The obtained salt was filtered off, washed two times with the same solvent (10 mL) and dried in vacuum. No other purification required.
1,1′-(pyridine-2,6-diylbis(methylene))bis(3-(anthracen-9-ylmethyl)-1H-imidazol-3-ium) chloride (3): Light brown powder. mp 223–224 °C. 1H-NMR (500 MHz, DMSO-d6) (ppm): 9.06 (s, 2H, 2×H2′), 8.76 (s, 2H, 2×H12′), 8.43 (d, 4H, J = 9.0 Hz, 2×(H8′,H16′)), 8.15 (d, 4H, J = 8.5 Hz, 2×(H11′,H13′)), 8.85 (t, 1H, J = 8.0 Hz, H4), 7.75 (t, 4H, J = 8.5 Hz, J = 7.0 Hz, 2×(H9′,H15′)), 7.56 (t, 4H, J = 8.0 Hz, J = 7.0 Hz, 2×(H10′,H14′)), 7.41 (d, 4H, J = 9.0 Hz, 2×(H4′,H5′)), 7.27 (d, 2H, J = 8.0 Hz, 2×H3), 6.48 (s, 4H, 2×(–CH2–)6′), 5.29 (s, 4H, 2×(–CH2–)5). 13C-NMR (125 MHz, DMSO-d6) (ppm): 153.5 (2×C2), 138.8 (C4), 136.5 (2×C2′), 131.0 (2×(C11′a,C12′a)), 130.6 (2×(C7′a,C16′a)), 130.1 (2×C12′), 129.3 (2×(C11′,C13′)), 127.7 (2×(C9′,C15′)), 125.5 (2×(C10′,C14′)), 123.6 (2×C7′), 123.5 (2×(C8′,C16′)), 123.1 (2×C4′), 122.1 (2×C5′), 122.0 (2×C3), 52.5 (2×(–CH2–)5), 44.9 (2×(–CH2–)6′). Anal. Calcd. for C43H35Cl2N5 C, 74.56; H, 5.09; N, 10.11. Found C, 74.66; H, 5.19; N, 10.01.

3.3. General Procedure for NMR Studies

3.3.1. Dimer Generation 6

To 400 μL (2.5 × 10−3 M) solution in DMSO-d6 of hybrid quaternary salt 3 was added 25 μL (2 × 10−3 M) solution in D2O of K2CO3 and also 5 μL (2.5 × 10−1 M) solution of K2CO3. It was observed that the solution become pale pink when adding the base (K2CO3). After the preparation of the solution, the NMR spectra were registered and the existence of the dimer 6 was highlighted.
Weak pink solution. 1H-NMR (500 MHz, DMSO-d6) (ppm): 8.56 (s, 4H, 4×H12′), 8.23 (d, 8H, J = 8.5 Hz, 4×(H8′,H16′)), 8.00 (d, 8H, J = 8.0 Hz, 4×(H11′,H13′)), 7.79 (t, 2H, J = 8.0 Hz, 2×H4), 7.58 (t, 12H, J = 7.0 Hz, J = 7.5 Hz, 4×(H9′,H15′,H4′)), 7.48 (t, 12H, J = 7.0 Hz, J = 8.0 Hz, 4×(H10′,H14′,H5′)), 7.29 (d, 4H, J = 7.5 Hz, 4×H3), 6.30 (s, 8H, 4×(–CH2–)5). 13C-NMR (125 MHz, DMSO-d6) (ppm): 157.7 (4×C2′), 153.5 (4×C2), 139.2 (2×C4), 131.2 (4×(C11′a,C12′a)), 130.8 (4×(C7′a,C16′a)), 130.5 (4×C12′), 129.7 (4×(C11′,C13′)), 128.2 (4×(C4′,C9′,C15′)), 127.5 (4×C1”), 125.9 (4×(C5′,C10′,C14′)), 123.4 (4×(C7′,C8′,C16′)), 122.3 (4×C3), 45.9 (4×(–CH2–)5).

3.3.2. Dimer Complex with Zn2+ 7

To the solution of generated dimer 6 (400 μL (2.5 × 10−3M) salt 3, 25 μL (2 × 10−3M) K2CO3, 5 μL (2.5 × 10−1 M) K2CO3) was added 100 μL (1.25 × 10−2 M) solution in D2O of Zn(CH3COO)2 × 2H2O, when the solution becomes poorly colored. After the preparation of the solution, the NMR spectra were recorded and the complex formation with zinc ions was evidenced.
Poorly colored solution. 1H-NMR (500 MHz, DMSO-d6) (ppm): 8.31 (s, 4H, 4×H12′), 8.02 (d, 8H, J = 8.5 Hz, 4×(H8′,H16′)), 7.84 (d, 8H, J = 8.5 Hz, 4×(H11′,H13′)), 7.76 (t, 2H, J = 8.0 Hz, 2×H4), 7.51 (t, 12H, J = 7.0 Hz, J = 8.5 Hz, 4×(H9′,H15′,H4′)), 7.41 (t, 12H, J = 7.5 Hz, 4×(H10′,H14′,H5′)), 7.30 (d, 4H, J = 8.0 Hz, 4×H3), 6.11 (s, 8H, 4×(–CH2–)5). 13C-NMR (125 MHz, DMSO-d6) (ppm): 177.5 (4×C2′), 153.6 (4×C2), 139.9 (2×C4), 131.6 (4×(C11′a,C12′a)), 131.3 (4×C1′’), 131.2 (4×C12′), 130.3 (4×(C11′,C13′)), 129.0 (4×(C4′,C9′,C15′)), 126.5 (4×(C5′,C10′,C14′)), 123.6 (4×(C8′,C16′)), 123.3 (4×C7′), 123.1 (4×C3), 45.6 (4×(–CH2–)5).

Author Contributions

Design, conception and writing were performed by V.A. Synthesis, structure elucidation, NMR studies were performed by all authors, which also reviewed and approved the final version. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by a grant of the Romanian Ministry of Education and Research, CNCS-UEFISCDI, project number PN-III-P4-ID-PCE-2020-0371, within PNCDI III.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Acknowledgments

Acknowledgment to the infrastructure support from Operational Program Competitiveness 2014-2020, Axis 1, under POC/448/1/1 Research infrastructure projects for public R&D institutions/Sections F 2018, through the Research Center with Integrated Techniques for Atmospheric Aerosol Investigation in Romania (RECENT AIR) project, under grant agreement MySMIS no. 127324. Authors are also gratefully to CERNESIM center, for NMR experiments.

Conflicts of Interest

The authors declare no conflict of interest.

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Scheme 1. The route of synthesis of hybrid pyridine bis-anthracene-imidazolium salt 3.
Scheme 1. The route of synthesis of hybrid pyridine bis-anthracene-imidazolium salt 3.
Molbank 2021 m1280 sch001
Scheme 2. The complexation process with Zn2+ of ylide 4.
Scheme 2. The complexation process with Zn2+ of ylide 4.
Molbank 2021 m1280 sch002
Figure 1. The 1H-NMR spectra of salt 3, dimeric structure 6 and dimeric complex 7.
Figure 1. The 1H-NMR spectra of salt 3, dimeric structure 6 and dimeric complex 7.
Molbank 2021 m1280 g001
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