4'-(5-n-Propylthiophen-2-yl)-2,2':6',2-terpyridine

Jérôme Husson, Laurent Guyard

► To cite this version:

Jérôme Husson, Laurent Guyard. 4'-(5-n-Propylthiophen-2-yl)-2,2':6',2-terpyridine. Molbank, 2021, 2021 (1), pp.M1183. 10.3390/M1183.hal-03121528

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Short Note

4'-(5-*n*-Propylthiophen-2-yl)-2,2':6',2''-terpyridine

Jérôme Husson * and Laurent Guyard

Institut Utinam UMR CNRS 6213, UFR Sciences et Techniques, Université de Bourgogne-Franche-Comté,
16 route de Gray, 25030 Besançon, France; laurent.guyard@univ-fcomte.fr

* Correspondence: jerome.husson@univ-fcomte.fr; Tel.: +33-3-81666291

**Abstract:** A new thiophene-substituted terpyridine derivative has been prepared through the reaction between 5-*n*-propylthiophene-2-carboxaldehyde and 2-acetylpyridine. This terpyridine derivative bears an alkyl chain linked via a thiophene heterocycle.

**Keywords:** chalcogenophene; heterocycles; ligands; pyridine derivatives; thiophene derivatives

**1. Introduction**

2,2':6',2''-Terpyridine (terpy) ligands and their metal complexes have been widely studied [1] owing to the broad range of applications for such molecules. Varying the nature of the substituents on the ligands and/or the metallic centre offers the possibility to prepare an enormous number of different substances. In particular, terpyridines that contain the five membered heterocycle thiophene [2] have attracted widespread of attention. In fact, they can be used in the preparation of materials for solar cells [3–5], for the functionalization of nanoparticles [6], as fluorescent probes [7,8], as antimicrobial agents [9], as electrochromic materials [10] or as chromophores [11], just to name a few. The substituents that are present on the thiophene ring have an important impact, especially on properties of thiophene-substituted terpyridine-based materials [12]. Therefore, the preparation of new thiophene-substituted terpyridines is still of interest. This paper presents the synthesis of the novel 4'-(5-*n*-propylthiophen-2-yl)-2,2':6',2''-terpyridine ligand (1) (Figure 1).

<figure><img src="image_2.png" /><figcaption>**Figure 1.** Chemical structure 4'-(5-*n*-propylthiophen-2-yl)-2,2':6',2''-terpyridine (1).</figcaption></figure>

**2. Results and Discussion**

Many synthetic methods are available for the preparation of terpyridine derivatives [13–15]. In order to prepare 4'-(5-*n*-propylthiophen-2-yl)-2,2':6',2''-terpyridine, the method described by Wang and Hanan in 2005 [16] was selected. This procedure allowed the facile preparation of 1 from 2-acetylpyridine and 5-*n*-propylthiophene-2-carboxaldehyde.

**Citation:** Husson, J.; Guyard, L. 4'-(5-*n*-Propylthiophen-2-yl)-2,2':6',2''-Terpyridine. *Molbank* **2021**, *M1183*. https://doi.org/10.3390/M1183

Academic Editors: Dimitrios Matiadis and Eleftherios Halevas
Received: 8 December 2020
Accepted: 20 January 2021
Published: 21 January 2021

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**Copyright:** © 2020 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).
As in many cases with this synthetic protocol, the crude product was sufficiently pure (>98% by quantitative NMR [17,18] and by combustion analysis) to be used (e.g., for the preparation of metal complexes) without purification.

Ligand **1** was characterized by ¹H and ¹³C-NMR as well as by HR-MS. Firstly, the ¹H-NMR spectrum agrees with the chemical structure. NMR spectra of 4'-functionalized terpyridines exhibit a typical singlet for proton 3' and 5'. In the present molecule, this singlet is seen at δ = 8.63 ppm. Furthermore, as expected, hydrogens that belong to the thiophene heterocycle (a and b) appear as doublets centered at 7.60 and 6.84 ppm, respectively, with a coupling constant of 3.6 Hz. Finally, signals for the propyl chain can be observed as two triplets (at 2.84 and 1.02 ppm) and a multiplet at 1.76 ppm (Figure 2).

<figure><img src="image_4.png" /><figcaption>**Figure 2.** ¹H-NMR spectra of compound **1** (inset: structure and atom numbering of **1**).</figcaption></figure>

Additionally, the structure of **1** was further confirmed by ¹³C-NMR as well as by HR-MS (Supplementary Materials). For instance, the ¹³C-NMR spectrum features 15 signals due to the symmetry of the molecule, while mass spectra exhibit the molecular ion peak at 358.13703 (calc. for [C₂₂H₁₉N₃S + H]⁺: 358.13724). The UV-Vis spectrum of compound **1** recorded in acetonitrile exhibits bands at 231, 252, 286 and 314 nm (Figure 3).
<figure><img src="image_5.png" /><figcaption>**Figure 3.** UV-Vis. spectrum of terpyridine **1** (1.21 × 10⁻⁴ M in acetonitrile).</figcaption></figure>

The strong absorption band can be assigned to π-π* transitions of the terpyridine part, and the shape of the spectrum is similar to previously reported ones for such five-membered heterocycle-substituted terpyridines [11,19].

As expected, introduction of an aliphatic chain onto the thiophene ring subsequently lowered the melting point of the product (98–99 °C) when compared to other non-functionalized chalcogenophene-substituted terpyridine molecule [20–23]. This phenomenon is also observed in an hexylthiophene-functionalized 2,2':2',2"-terpyridine [11] (Table 1).

<table><caption>**Table 1.** Comparison of melting points for some chalcogenophene-substituted terpyridines. a Values from literature.</caption><thead><tr><th></th><th>X=</th><th>S</th><th>O</th><th>Se</th><th>Te</th></tr></thead><tbody><tr><th>R=</th><td>nC<sub>6</sub>H<sub>13</sub></td><td>nC<sub>3</sub>H<sub>7</sub></td><td>H</td><td>H</td><td>H</td></tr><tr><th>Mp (°C)</th><td>70–72 <sup>a</sup></td><td>98–99</td><td>197–199 <sup>a</sup></td><td>219 <sup>a</sup></td><td>215–218 <sup>a</sup></td></tr></tbody></table>

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### 3. Materials and Methods

All reagents were purchased from commercial suppliers (ACROS Organics, Geel, Belgium and TCI Chemicals, Zwijndrecht, Belgium) and used as received. Starting material 5-*n*-propylthiophene-2-carboxaldehyde [24] was prepared from thiophene via 2-*n*-propylthiophene [25] and 1-(thiophen-2-yl)-propan-1-one [26] according to literature procedures. ¹H and ¹³C-NMR spectra were recorded on a Brucker AC 400 (Bruker, Wissembourg, France) at 400 and 100 MHz, respectively, using CDCl₃ as a solvent. Melting point was recorded with a Stuart SMP 10 melting point apparatus (Bibby Sterilin, Stone, UK) and was uncorrected. The UV-Vis spectrum was recorded on a Cary 300 (Agilent Technologies, Santa Clara, CA, USA) using acetonitrile (C = 1.21 × 10⁻⁴ M) as solvent. HR-MS was recorded at Sayence SATT, Dijon, France. Elemental analysis was performed at Service d'Analyse Élémentaire, Vandoeuvre-les-Nancy, France.

4'-(5-*n*-propylthiophen-2-yl)-2,2':6',2"-terpyridine (1): to a solution of 2-acetylpyridine (7.43 g; 61 mmol) in ethanol (154 mL), 5-*n*-propylthiophene-2-carboxaldehyde (4.73 g; 31 mmol), 85% potassium hydroxide pellets (4.73 g; 72 mmol) and 25% aqueous ammonia (89 mL) were added. The reaction mixture was stirred at room temperature for 24 h.
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