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

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.

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 H_2′ 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×H_2′), 8.76 (s, 2H, 2×H_12′), 8.43 (d, 4H, J = 9.0 Hz, 2×(H_8′,H_16′)), 8.15 (d, 4H, J = 8.5 Hz, 2×(H_11′,H_13′)), 8.85 (t, 1H, J = 8.0 Hz, H₄), 7.75 (t, 4H, J = 8.5 Hz, J = 7.0 Hz, 2×(H_9′,H_15′)), 7.56 (t, 4H, J = 8.0 Hz, J = 7.0 Hz, 2×(H_10′,H_14′)), 7.41 (d, 4H, J = 9.0 Hz, 2×(H_4′,H_5′)), 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×C_2′), 131.0 (2×(C_11′a,C_12′a)), 130.6 (2×(C_7′a,C_16′a)), 130.1 (2×C_12′), 129.3 (2×(C_11′,C_13′)), 127.7 (2×(C_9′,C_15′)), 125.5 (2×(C_10′,C_14′)), 123.6 (2×C_7′), 123.5 (2×(C_8′,C_16′)), 123.1 (2×C_4′), 122.1 (2×C_5′), 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×H_12′), 8.23 (d, 8H, J = 8.5 Hz, 4×(H_8′,H_16′)), 8.00 (d, 8H, J = 8.0 Hz, 4×(H_11′,H_13′)), 7.79 (t, 2H, J = 8.0 Hz, 2×H₄), 7.58 (t, 12H, J = 7.0 Hz, J = 7.5 Hz, 4×(H_9′,H_15′,H_4′)), 7.48 (t, 12H, J = 7.0 Hz, J = 8.0 Hz, 4×(H_10′,H_14′,H_5′)), 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×C_2′), 153.5 (4×C₂), 139.2 (2×C₄), 131.2 (4×(C_11′a,C_12′a)), 130.8 (4×(C_7′a,C_16′a)), 130.5 (4×C_12′), 129.7 (4×(C_11′,C_13′)), 128.2 (4×(C_4′,C_9′,C_15′)), 127.5 (4×C_1”), 125.9 (4×(C_5′,C_10′,C_14′)), 123.4 (4×(C_7′,C_8′,C_16′)), 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×H_12′), 8.02 (d, 8H, J = 8.5 Hz, 4×(H_8′,H_16′)), 7.84 (d, 8H, J = 8.5 Hz, 4×(H_11′,H_13′)), 7.76 (t, 2H, J = 8.0 Hz, 2×H₄), 7.51 (t, 12H, J = 7.0 Hz, J = 8.5 Hz, 4×(H_9′,H_15′,H_4′)), 7.41 (t, 12H, J = 7.5 Hz, 4×(H_10′,H_14′,H_5′)), 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×C_2′), 153.6 (4×C₂), 139.9 (2×C₄), 131.6 (4×(C_11′a,C_12′a)), 131.3 (4×C_1′’), 131.2 (4×C_12′), 130.3 (4×(C_11′,C_13′)), 129.0 (4×(C_4′,C_9′,C_15′)), 126.5 (4×(C_5′,C_10′,C_14′)), 123.6 (4×(C_8′,C_16′)), 123.3 (4×C_7′), 123.1 (4×C₃), 45.6 (4×(–CH₂–)₅).