5-((4-(-Phenyldiazenyl)phenyl)diazenyl)quinolin-8-ol

Abstract

A new azo compound was synthesized via an azo coupling reaction between 4-(phenyldiazenyl)benzenediazonium chloride and 8-hydroxyquinoline (8-Hq). The new diazene compound can be used to synthesize metal complexes as a derivative of 8-Hq. The structure of the new compound was characterized using UV–Vis, FT-IR, and 2D NMR spectroscopic methods.

1. Introduction

Azo compounds contain at least one azo (diazene) functional group within their molecular structure, which exhibits chromophoric properties. When aromatic or heterocyclic substituents are attached to this functional group, the resulting compound becomes capable of absorbing electromagnetic radiation across various spectra—particularly in the visible range—and, subsequently, emitting a portion of the absorbed radiation. This characteristic renders azo dyes highly suitable for dyeing a wide range of materials. Azo dyes form the largest and most recognized category of colorants, encompassing approximately 60–70% of all colorants [1]. One of the first azo dyes ever produced was 4-(phenyldiazenyl)aniline or Aniline Yellow (synthesized in 1861 by Mene) [2].

The azobenzene moiety can be found in some antibacterial compounds [3], in some compounds with interesting photochemical properties [4,5], and in dyes used in sensitizing solar cells [6].

An 8-Hq fragment is present in many biologically active molecules, including teacleabine, tecleoxine, toddaquinoline, ciprofloxacin, quinine, and chloroquine [7]. 8-hydroxyquinoline-based dyes have many and diverse applications. The 8-Hq moiety can be found in dyes that can be used as chemosensors for metal cation detection [8,9,10]. 8-Hq dyes can be used in human fingerprint detection [11], and an 8-Hq fragment can be found in several food dyes like Quinoline Yellow WS (E104) and Brilliant Black BN (E151). Also, in many other studies, the complexation potential of 8-Hq dyes was investigated. Such complexes can also have biological activity, e.g., antibacterial activity [12,13].

Given the above, we decided to instigate the azo coupling reaction between a compound (4-phenyldiazenylaniline) already containing an azo group in its structure and 8-Hq. The resulting azo dye contains two azo groups and a quinoline moiety, the latter of which is known for its good complexation capability.

2. Results and Discussion

5-(-(4-(-phenyldiazenyl)phenyl)diazenyl)quinolin-8-ol was synthesized via an azo coupling reaction between 4-(phenyldiazenyl)benzenediazonium chloride and 8-hydroxyquinoline in a basic medium. Diazotization and coupling were performed according to a modified procedure from literature [14]. The synthesis followed Scheme 1.

Due to its phenolic hydroxyl group, 8-Hq has an acidic character, which means it can react with bases. The 8-quinolinate anion, formed in a basic medium, has an increased electron density at positions 2, 4, 5, and 7 due to the electron donor effect of the negatively charged oxygen atom that formed as a result of proton loss, as shown in Scheme 2.

The so-called pyridine ring (highlighted in red) of the 8-quinolinate anion is not very active with regard to electrophilic substitution reactions, so reactions such as the azo coupling reaction can occur only at positions C-5 (structure III) and C-7 (structure II) because of the increased electron density in these positions in a basic medium. In our previous work [15], we showed that under similar conditions (i.e., regarding the diazonium salt/8-Hq quantity ratio), coupling takes place exclusively in the C-5 position, which was also confirmed in the experimental NMR data. Data from the literature [16] suggest that at a higher diazonium salt/8-Hq quantity ratio, C-5 and C-7 disubstituted compounds can be obtained. In case the C-5 position is occupied, for example, as in the case of 5-chloro-8-hydroxyquinoline, azo coupling takes place at position C-7, the reaction rate is lower, and different reaction conditions are needed [17].

The obtained azo dye contains the 8-Hq moiety, which is a well-known ligand in coordination chemistry [18,19,20], and two diazene groups, which facilitate extended conjugation along the molecule. The compound can form complexes with metal cations under the appropriate reaction conditions.

Through the acidulation of compound (3)’s methanolic solution using trifluoroacetic acid (TFA), protonation occurs at the quinolinic nitrogen atom’s lone pair, which does not participate in aromatic conjugation. As a result of protonation, a hypsochromic shift can be observed, for which the maximum absorption wavelength changes as follows: λ_max = 409 nm → λ_max = 397 nm. In addition, a hyperchromic effect can be observed as a consequence of the solution’s acidulation, which is manifested by an increasing molar absorption coefficient value (ε = 15,487 M⁻¹ cm⁻¹ → ε = 24,000 M⁻¹ cm⁻¹). The hyperchromic effect was probably caused by the increased HOMO–LUMO gap of the protonated compound (3) compared to the unprotonated compound (3).

3. Materials and Methods

The reagents used were purchased from commercial sources and used as received. Synthesis followed a modified protocol used in our previous work [14].

The ¹H NMR, ¹³C NMR, and ¹⁵N NMR spectra were recorded using a Bruker Avance III 500 MHz spectrometer. Chemical shifts (δ) were measured in ppm, and coupling constants (J) were measured in Hz. Sample was dissolved in non-deuterated TFA (trifluoroacetic acid). DMSO-d6 (a small amount in a sealed tube along with a dissolved sample) was used as a locking solvent. TMS was used as an internal standard.

The IR spectrum was recorded using a Jasco FT/IR-410 spectrophotometer in a KBr pellet.

UV–Vis spectra were recorded using a Jasco V-530 spectrometer. The samples were dissolved in methanol, and TFA was used for acidulation.

Melting point was measured using a Böetius PHMK (Veb Analytik, Dresden, Germany) apparatus and was uncorrected.

4. Experimental Procedure

Synthesis of 5-(-(4-(-Phenyldiazenyl)phenyl)diazenyl)quinolin-8-ol (3)

• Diazonium Salt Synthesis

In a beaker, 4-(phenyldiazenyl)aniline (1.057 g; 5.25 mmol) was dissolved in 20 mL of DMF. After the complete dissolution of the amine, 5 mL of hydrochloric acid solution (4:1 v/v) was added to the amine solution while stirring was applied. A dark red precipitate formed, and the beaker was placed in an ice water bath to cool the obtained suspension. In parallel, a sodium nitrite solution was prepared by dissolving nitrite (0.384 g; 5.51 mmol) in 2 mL of distilled water. Both solutions were cooled in ice water baths to ensure that the temperature was below 5 °C. When the solutions reached the necessary temperature, sodium nitrite solution was added dropwise, while stirring was applied, to an acidic amine solution. An immediate color change was observed, i.e., the solution turned yellow, and the initial precipitate had dissolved. Stirring was continued for 30 min, while the temperature was maintained below 5 °C.

• Azo Coupling Reaction

In a round bottom flask equipped with a magnetic stirrer and a thermometer, 8-hydroxyquinoline (0.734 g; 5 mmol) and sodium hydroxide (0.42 g; 10.5 mmol) along with 15 mL of pyridine and 15 mL of 96% ethanol were added and stirred. Once the solids were fully dissolved, the flask was placed in an ice water bath to maintain the temperature below 5 °C.

After both solutions (diazonium chloride and 8-Hq) were cooled, diazonium chloride solution was added dropwise to the 8-Hq solution, with the two solutions being vigorously stirred, and cooling was maintained. A dark violet coloration of the resulting solution was observed. Another 10 mL of pyridine was added to the suspension obtained to facilitate stirring. Using a 10% sodium hydroxide solution, the pH was adjusted to 7–8. Cooling was maintained for 2 more hours. Then, the reaction mass was stirred for 24 h.

Subsequently, the contents of the flask were acidified using 99.8% acetic acid until reaching a mildly acidic pH of 5.5–6. The dark red suspension was then poured into a beaker that contained 400 mL of distilled water in order to ensure complete precipitation of the azo dye. The precipitate was then separated via vacuum filtration.

The compound obtained was purified via recrystallization with 215 mL of 2-ethoxyethanol. Finally, 1.3 g of a dark red solid was obtained along with the following additional parameters: an isolation yield of 74%; a melting point of 246–248 °C; ¹H NMR (TFA/DMSO-d6, 500 MHz) δ (ppm): 9.71 (dd, 1H, J₁ = 8.5 Hz, J₂ = 1.3 Hz, 4-H), 8.99 (dd, 1H, J₁ = 5.5 Hz, J₂ = 1.3 Hz, 2-H), 8.47 (d, 1H, J = 10.4 Hz, 6-H), 8.32 (dd, 1H, J₁ = 8.5 Hz, J₂ = 5.5 Hz, 3-H) 8.26 (d, 2H, J = 9.3 Hz, 2′-H, 6′-H), 8.00 (d, 2H, J = 7.6 Hz, 2″-H, 6″-H) 7.90 (d, 2H, J = 9.2 Hz, 3′-H, 5′-H), 7.68 (t, 1H, J = 7.6 Hz, 4″-H), and 7.60–7.56 (m, 2H, 3″-H, 5″-H), 7.22 (d, 1H, J = 10.4 Hz, 7-H); ¹³C NMR (TFA/DMSO-d6, 125 MHz) δ (ppm): 175.3 (5-C), 151.2 (1′-C), 143.8 (4-C), 143.1 (2-C), 141.8 (4′-C), 141.1 (2″-C), 136.2 (4″-C), 134.2 (4a-C), 133.8 (8-C), 131.8 (8a-C), 130.4 (3″-C, 5″-C), 129.4 (2′-C, 6′-C), 128.6 (3-C), 127.6 (6-C), 127.5 (7-C), 122.5 (2″-C, 6″-C), and 118.5 (3′-C, 5′-C); ¹⁵N NMR (TFA/DMSO-d6, 50 MHz) δ (ppm): 288.4 (2″-C-N), 198.4 (4′-C-N), 186.9 (1-N), −6.1 (1′-C-N=), and −37.2 (5-C-N=); FT-IR (cm⁻¹):, 3462, 3050, 1572, 1505, and 1287; UV–Vis (EtOH 96%): λ _max = 409 nm, ε(λ _max) = 15,487 M⁻¹ cm⁻¹, λ _max = 397 nm (methanol solution acidified with TFA), ε(λ _max) = 24,000 M⁻¹ cm⁻¹ (methanol solution acidified with TFA).

All spectra are reported in supplementary materials.