# Dichloro-bis[(Z)-1-styryl-benzimidazole]-zinc(II)

Neslihan Şahin, İsmail Özdemir, David Sémeril

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Neslihan Şahin, İsmail Özdemir, David Sémeril. Dichloro-bis[(Z)-1-styryl-benzimidazole]-zinc(II). Molbank, 2024, 2024 (4), pp.M1913. 10.3390/M1913.05336535

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Short Note

# Dichloro-bis[(Z)-1-styryl-benzimidazole]-zinc(II)

Neslihan Şahin <fnref n="1" />,<fnref n="2" />*, İsmail Özdemir <fnref n="3" /> ID and David Sémeril <fnref n="2" />,* ID

**Abstract:** We have successfully synthesized the dichloro-bis[(Z)-1-styryl-benzimidazole]-zinc(II) complex, which was fully characterized by IR, elemental analysis, and mass and NMR spectroscopy. The solid-state structure definitively shows that two benzimidazole moieties are coordinated to the zinc atom, which adopts a tetrahedral geometry.

**Keywords:** zinc; benzimidazole; complex synthesis; solid-state structure

## 1. Introduction

Azole heterocycles form complexes with transition metals with remarkable ease [1]. Benzimidazole derivatives are particularly noteworthy, as they easily coordinate with the first row of transition metals to form complexes of the type [MX₂L₂], in which L represents a N-substituted benzimidazole, X a halogen, and M a metal like manganese [2], iron [3], cobalt [4], nickel [5], copper [2], or zinc [5].

In this context, zinc complexes of the type [ZnCl₂L₂] are particularly interesting due to their promising optical [6,7], catalytic [8], and biological properties, including antioxidant [9], antidiabetic [10,11], antimicrobial [12], and anticancer [13] capabilities.

Based on the above considerations and our extensive experience in coordinating N-alkyl benzimidazole derivatives to transition metals [12,14], we now report the synthesis of a zinc(II) complex 2 substituted with two styryl-benzimidazole moieties (Figure 1).

<figure><img src="image_3.png" /><figcaption>**Figure 1.** Targeted zinc(II) complex 2.</figcaption></figure>

## 2. Results and Discussion

The targeted dichloro-bis[(Z)-1-styryl-benzimidazole]-zinc(II) complex was obtained by mixing two equivalents of (Z)-1-styryl-benzimidazole ligand 1 with one equivalent of ZnCl₂ precursor in ethanol. After 4 h at room temperature, the reaction mixture was filtered and the zinc(II) complex was isolated in 92% yield (Scheme 1).

<figure><img src="image_4.png" /><figcaption>**Scheme 1.** Synthesis of zinc complex 2.</figcaption></figure>

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**Citation:** Şahin, N.; Özdemir, İ.; Sémeril, D. Dichloro-bis[(Z)-1-styryl-benzimidazole]-zinc(II). *Molbank* **2024**, *2024*, M1913.

https://doi.org/10.3390/M1913

Academic Editor: René T. Boeré

Received: 1 October 2024

Revised: 29 October 2024

Accepted: 31 October 2024

Published: 5 November 2024

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**Copyright:** © 2024 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 (https://creativecommons.org/licenses/by/4.0/).

<footnote n="1">¹ Department of Science Education, Faculty of Education, Cumhuriyet University, 58040 Sivas, Turkey</footnote>

<footnote n="2">² Synthèse Organométallique et Catalyse, UMR-CNRS 7177, Strasbourg University, 67008 Strasbourg, France</footnote>

<footnote n="3">³ Drug Application and Research Center, İnönü University, 44280 Malatya, Turkey</footnote>

<footnote>* Correspondence: neslihan@cumhuriyet.edu.tr (N.Ş.); dsemeril@unistra.fr (D.S.); Tel.: +903464874656 (N.Ş.); +33-(0)3-6885-1550 (D.S.)</footnote>
The complex was fully characterized by infrared spectroscopy (FT-IR), multi-nuclear magnetic resonance spectroscopy (¹H and ¹³C NMR), mass spectroscopy, and elemental analysis (see Figures S1–S4 in Supplementary Materials).

In comparison with the ligand **1** (ν(CN) 1481 cm⁻¹), the FT-IR spectra of the complex **2** (ν(CN) 1500 cm⁻¹) displayed a redshifted band for the C=N vibration of 19 cm⁻¹, which is characteristic of an azole coordinated to a transition metal [12]. The NMR analysis realized on the zinc(II) complex revealed a downfield singlet (δ = 8.43 ppm) for the NCHN protons with regard to the ligand (δ = 7.74 ppm) (Figure 2). The cis-stereochemistry of the external carbon–carbon double bonds was confirmed by the coupling constants between olefinic protons (³JHH of 9.0 Hz).

<figure><img src="image_7.png" /><figcaption>**Figure 2.** ¹H NMR spectra of 1-styryl-benzimidazole (1) (top) and its corresponding dichloro-bis[(Z)-1-styryl-benzimidazole]-zinc(II) (2) complex (bottom) in CDCl₃.</figcaption></figure>

The formation of the [ZnCl₂(1)₂] complex is unambiguously deduced from the mass spectrum, which shows intense peaks at *m*/*z* = 539.10 corresponding to the [M − Cl]⁺ cation with the expected isotopic profiles. Furthermore, the elemental analysis, carried out on dried crystals, is in accordance with the presence of two styryl-benzimidazole moieties per zinc atom.

The formation of the dichloro-bis[(Z)-1-styryl-benzimidazole]-zinc(II) complex (2) was confirmed by a single-crystal X-ray diffraction study. Suitable crystals were obtained by the slow diffusion of diethyl ether into a dichloromethane solution of zinc(II) complex. The complex crystallized in the triclinic space group *P*-1.

Figure 3 shows that the zinc atom in complex **2** adopts a slightly distorted tetrahedral geometry with L-Zn-L' angles in the range 103.41(9) to 120.10(6)° (with L and L' = N or Cl). Two nitrogen atoms of the two benzimidazole moieties (Zn1-N1 2.011(3) and Zn1-N3 2.048(3) Å) and two chloride atoms (Zn1-Cl1 2.2266(16) and Zn1-Cl2 2.224(2) Å) are clearly coordinated to the metal center (Table 1). These distances and angles are in agreement with those reported for the dichloro-{N,N-bis[(1-allyl-1H-benzimidazol-2-yl)methyl]aniline}-zinc(II) or the dichloro-bis{2-(thiophen-2-yl)-1-[(thiophen-2-yl)methyl]-1H-benzimi-dazole}-zinc(II) complexes [15,16].
<figure><img src="image_8.png" /><figcaption>**Figure 3.** ORTEP drawing of the zinc(II) complex 2 (50% probability thermal ellipsoids).</figcaption></figure>

<table><caption>**Table 1.** Important bond lengths and angles.</caption><thead><tr><th colspan="3">Bond Lengths (Å)</th></tr></thead><tbody><tr><td>Zn1-N1 2.011(3)</td><td>Zn1-N3 2.048(3)</td><td>Zn1-Cl1 2.2266(16)</td></tr><tr><td>Zn1-Cl2 2.224(2)</td><td>N1-C1 1.310(3)</td><td>C1-N2 1.353(4)</td></tr><tr><td>N2-C2 1.385(4)</td><td>C2-C7 1.389(4)</td><td>C7-N1 1.397(3)</td></tr><tr><td>N3-C16 1.307(3)</td><td>C16-N4 1.356(3)</td><td>N4-C17 1.398(3)</td></tr><tr><td>C17-C22 1.394(4)</td><td>C22-N3 1.403(3)</td><td></td></tr></tbody><thead><tr><th colspan="3">Angles (°)</th></tr></thead><tbody><tr><td>N1-Zn1-N3 106.68(9)</td><td>Cl1-Zn1-Cl2 120.10(6)</td><td>Cl1-Zn1-N1 106.62(9)</td></tr><tr><td>N1-Zn1-Cl2 112.70(8)</td><td>Cl2-Zn1-N3 103.41(9)</td><td>N3-Zn1-Cl1 106.40(9)</td></tr><tr><td>C1-N1-Zn1 123.54(19)</td><td>C7-N1-Zn1 130.52(19)</td><td>C22-N3-Zn1 130.28(18)</td></tr><tr><td>C16-N3-Zn1 122.61(19)</td><td></td><td></td></tr></tbody></table>

It is interesting to note that, in the structure of the zinc complex 2, the point group symmetry is $C_2$ with the two phenyl rings pointing to opposite directions, whereas the point group symmetry of a closely related cobalt complex, the dichloro-bis(1-cinnamyl-benzimidazole)-cobalt(II), recently reported by our group is $C_s$ with a pseudo mirror plane and the two phenyl rings pointing in the same direction [17]. The two benzimidazole rings are inclined at a dihedral angle of 65.61° due to intermolecular interactions with a second molecule of the complex. For ligand 1, the phenyl groups are oblique to the benzimidazoles with dihedral angles of 56.89° and 60.99°, respectively.

### 3. Materials and Methods

#### 3.1. General

The manipulations were carried out under dry argon with dried solvents. Routine ¹H and ¹³C{¹H} spectra were recorded with AC 300 and 500 Bruker FT instruments. Chemical shifts and coupling constants are reported in ppm and Hz, respectively. ¹H and ¹³C NMR spectra were recorded in CDCl₃ and they referenced residual protonated solvent (δ = 7.26 and 77.16 ppm, respectively). Mass spectra were recorded on a Bruker MicroTOF spectrometer (ESI-TOF). Infrared spectra were recorded on a Bruker ATR FT-IR Alpha-P spectrometer. Elemental analyses were carried out by the Service de Microanalyse, Institut de Chimie, Université de Strasbourg. (Z)-1-Styryl-benzimidazole (1) was prepared according to an adapted published procedure [18].
### 3.2. Procedure for the Preparation Dichloro-bis[(Z)-1-styryl-benzimidazole]-zinc(II) (2)

In a Schlenk tube under an inert atmosphere of argon, a solution of ZnCl₂ (0.093 g, 0.68 mmol) and (Z)-1-styryl-benzimidazole (0.300 g, 1.36 mmol) in ethanol (10 mL) was stirred at room temperature. After 4 h, the formed precipitate was filtered, washed with diethylether (3 × 10 mL), and dried under vacuum to give complex **2** as white solid in 92% yield (0.362 g). FT-IR: *ν*(CN) 1500 cm⁻¹; ¹H NMR (300 MHz, CDCl₃): *δ* = 8.43 (s, 2H, NCHN), 8.09–8.03 (m, 2H, arom CH of C₇H₅N₂), 7.36–7.28 (m, 6H, arom CH of C₇H₅N₂), 7.22–7.17 (m, 2H, arom CH of C₆H₅), 7.12–7.07 (m, 4H, arom CH of C₆H₅), 6.94–6.90 (m, 4H, arom CH of C₆H₅), 6.86 (d, 2H, NCH=CHPh, ³JHH = 9.0 Hz), 6.81 (d, 2H, NCH=CHPh, ³JHH = 9.0 Hz); ¹³C{¹H} NMR (126 MHz, CDCl₃): *δ* = 143.78 (s, NCHN), 139.17, 132.36, 132.33, 129.37, 129.16, 128.51, 125.34, 125.10, 118.97, 111.58 (10 s, arom Cs), 130.22 (s, NCH=CHPh), 119.35 (s, NCH=CHPh) ppm. MS (ESI-TOF): *m/z* = 539.10 [M − Cl]⁺ (expected isotopic profile). Elemental analysis (%): calcd for C₃₀H₂₄N₄ZnCl₂ (576.83): C: 62.47; H: 4.19; N: 9.71; found C: 62.04; H: 4.15 N: 9.64.

### 3.3. X-Ray Crystal Structure Analysis of Complex 2

Single crystals of zinc(II) complex **2**, suitable for X-ray analysis, were obtained by slow diffusion of Et₂O into a CH₂Cl₂ solution of the complex. The samples were studied on a Bruker APEX-II CCD, using Mo-Kα radiation (λ = 0.71073 Å). The structures were solved with SHELXT-2018/2 [19] and the structure was refined with SHELXL-2019/3 [20]. Crystal Data for C₃₀H₂₄Cl₂N₄Zn (M = 576.80 g/mol): triclinic, space group P-1 (no. 2), *a* = 10.386(10) Å, *b* = 10.503(10) Å, *c* = 14.167(14) Å, *α* = 97.98(2)°, *β* = 107.95(2)°, *γ* = 108.65(3)°, *V* = 1344(2) Å³, *Z* = 2, *T* = 173(2) K, μ(Mo-Kα) = 1.139 mm⁻¹, *D*calc = 1.425 g cm⁻³, 44,075 reflections collected (1.566 ≤ *θ* ≤ 25.680°), and 5093 unique (*R*int = 0.0676, *R*sigma = 0.0404) which were used in all calculations. The final *R*₁ was 0.0354 (*I* > 2.0 *δ*(I)) and *wR*2 was 0.0895 (all data). CCDC 2385094 contains the supplementary crystallographic data for this paper. The data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/structures.

## 4. Conclusions

In summary, we successfully synthesized and characterized the new dichloro-bis[(Z)-1-styryl-benzimidazole]-zinc(II) complex. Its composition was confirmed by an X-ray study, which clearly showed the presence of two benzimidazole units coordinated to the zinc atom in a slightly distorted tetrahedral conformation.

**Supplementary Materials:** Characterizing data of dichloro-bis[(Z)-1-styryl-benzimidazole]-zinc(II) (2) with: Figure S1. FT-IR spectrum; Figure S2. Mass spectrum (ESI-TOF); Figure S3. ¹H NMR spectrum (CDCl₃); Figure S4. ¹³C{¹H} NMR spectrum (CDCl₃); Figure S5. Packing pattern in the crystal structure of complex **2**; and Figure S6. Self-organized structure of **2** via CH-π interactions (in red).

**Author Contributions:** Conceptualization, N.Ş. and D.S.; methodology, N.Ş. and D.S.; validation, N.Ş. and D.S.; formal analysis, N.Ş. and D.S.; investigation, N.Ş.; resources, D.S.; data curation, N.Ş. and D.S.; writing—original draft preparation, N.Ş. and D.S.; writing—review and editing, D.S.; supervision, İ.Ö. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Data Availability Statement:** The data are available in a publicly accessible repository.

**Conflicts of Interest:** The authors declare no conflicts of interest.

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