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

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.

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).

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).

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 (³J_HH of 9.0 Hz).

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. Important bond lengths and angles.

Bond Lengths (Å)
Zn1-N1 2.011(3)	Zn1-N3 2.048(3)	Zn1-Cl1 2.2266(16)
Zn1-Cl2 2.224(2)	N1-C1 1.310(3)	C1-N2 1.353(4)
N2-C2 1.385(4)	C2-C7 1.389(4)	C7-N1 1.397(3)
N3-C16 1.307(3)	C16-N4 1.356(3)	N4-C17 1.398(3)
C17-C22 1.394(4)	C22-N3 1.403(3)
Angles (°)
N1-Zn1-N3 106.68(9)	Cl1-Zn1-Cl2 120.10(6)	Cl1-Zn1-N1 106.62(9)
N1-Zn1-Cl2 112.70(8)	Cl2-Zn1-N3 103.41(9)	N3-Zn1-Cl1 106.40(9)
C1-N1-Zn1 123.54(19)	C7-N1-Zn1 130.52(19)	C22-N3-Zn1 130.28(18)
C16-N3-Zn1 122.61(19)

). 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].

It is interesting to note that, in the structure of the zinc complex 2, the point group symmetry is C₂ 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, ³J_HH = 9.0 Hz), 6.81 (d, 2H, NCH=CHPh, ³J_HH = 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 R₁ was 0.0354 (I > 2.0 δ(I)) and wR2 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.