Dichloro-Bis(1-cinnamyl-benzimidazole)-Cobalt(II)

Abstract

Dichloro-bis(1-cinnamyl-benzimidazole)-cobalt(II) was prepared in one step using a cobalt precursor CoCl₂ and corresponding substituted benzimidazole. The complex was fully characterized using IR, elemental analysis, and mass- and NMR spectroscopy. In the solid state, the cobalt atom displays a typical tetrahedral geometry and is coordinated to two chlorine atoms and two benzimidazole moieties.

1. Introduction

Benzimidazole derivatives are aromatic heterocyclic compounds [1] with a range of biological functions [2,3,4,5] and wide coordination properties [6,7,8,9,10], especially with cobalt precursors [11]. The latter complexes are of particular interest due to their magnetic [12,13], catalytic [14,15], and medicinal properties, including their antibacterial [16,17] and anticancer effects [18,19].

Based on the properties of the cobalt complexes discussed above, and based on our experience in the synthesis of organometallic complexes incorporating N-alkyl benzimidazole derivatives [20,21], we present here the synthesis of a cobalt(II) complex (2) comprising two cinnamyl-benzimidazole moieties (Figure 1).

2. Results and Discussion

The desired dichloro-bis(1-cinnamyl-benzimidazole)-cobalt(II) complex (2) was obtained by reacting one equivalent of the CoCl₂ precursor and two equivalents of 1-cinnamyl-benzimidazole 1 in ethanol at room temperature. After 4 h, the blue precipitate was filtered, washed with diethylether, and dried to afford, in a 96% isolated yield, the blue cobalt(II) complex 2 (Scheme 1).

The complex, which is stable against air and moisture in both the solid state and in solution, was characterized using NMR ¹H spectroscopy, elemental analysis, infrared spectroscopy, and mass spectroscopy (see Figures S1–S4 in Supplementary Materials).

The 3d configuration of the cobalt(II) cation unequivocally determines the paramagnetic properties of the formed blue complex. Its ¹H NMR spectrum reveals a signal within the range of −3 to 28 ppm.

The coordination of the cinnamyl-benzimidazole (1) to the metal center is clearly evident from the FT-IR spectrum of complex 2 [20]. There is a distinct redshift of the C=N vibration band. In fact, the ν(CN) band changes from 1485 cm⁻¹ for the free ligand to 1511 cm⁻¹ after coordination, as can be seen in the IR spectra.

The formation of the [CoCl₂(1)₂] complex was unambiguously deduced from the mass spectrum, which shows intense peaks at m/z = 562.14 and 620.10, corresponding to the [M − Cl]⁺ and [M + Na]⁺ cations, respectively, with the expected isotopic profiles. Furthermore, the result of an elemental analysis, carried out on dried crystals, is in accordance with the presence of two cinnamyl-benzimidazole moieties per cobalt atom.

An X-ray diffraction study on a single crystal of the complex dichloro-bis(1-cinnamyl-benzimidazole)-cobalt(II) (2) definitively confirmed the presence of two benzimidazole moieties coordinated to the metal center. Suitable crystals for the solid-state analysis were obtained through the slow diffusion of diethylether in a dichloromethane solution of the cobalt(II) complex. This complex crystallizes in the monoclinic space group P2₁/n with two molecules of dichloromethane, one of which is disordered over two positions with a ratio of 0.9/0.1 (Figure 2).

As anticipated, the coordination sphere of the metal center comprises two chlorine atoms (Co1-Cl1 2.2521(11) and Co1-Cl2 2.2559(12) Å) and two benzimidazole moieties (Co1-N1 2.006(3) and Co1-N3 1.998(3) Å). The cobalt atom has a slightly distorted tetrahedral geometry with N1-Co1-N3 and Cl1-Co1-Cl2 angles of 116.07(13) and 116.37(5)°, respectively. These values are close to those previously reported for [CoCl₂(L)₂] (L = substituted benzimidazole), as, for example, in dichloro-bis [1-(but-2-enyl)-5-nitro-benzimidazole-κN³]-cobalt(II) [22], dichloro-{N,N-bis[(1-allyl- benzimidazol-2-yl)methyl)aniline}-cobalt(II) [23], dichloro-bis[1-(prop-2-en-1-yl)-benz- imidazole]-cobalt(II) [24], dichloro-bis{1-[(2-furyl)methylene]-2-(2-furyl)benzimidazole}- cobalt(II) [11], or dichloro-bis[1-(thiophen-2-yl) methyl-2-(thiophen-2-yl)-benzimida- zole]-cobalt(II) [25]. The two benzimidazole rings are inclined at a dihedral angle of 46.48°. One of the cinnamyl-benzimidazole ligands has a phenyl group perpendicular to the benzimidazole ring, with a dihedral angle of 87.67°, while in the other, the phenyl group is oblique to the ring, with a dihedral angle of 60.81°.

In the solid state, cobalt complexes interact intermolecularly via CH•••Cl interactions between the hydrogen and chlorine atoms of the complexes and also via the hydrogen and chlorine atoms of the dichloromethane molecules present in the crystal lattice (Figure 3). These latter molecules of the solvent also interact with aromatic benzimidazole and phenyl groups through Cl•••CH intermolecular bonds (with distances in the range of 3.417 to 3.636 Å).

3. Materials and Methods

3.1. General

The manipulations were carried out under a dry argon atmosphere with dried solvents. Routine ¹H spectrum was recorded with AC 300 Bruker FT instruments (Bruker BioSpin, Rheinstetten, Germany) in CDCl₃ and referenced to residual protonated solvent (δ = 7.26 ppm). Chemical shifts are reported in ppm. Mass spectra were recorded using a Bruker MicroTOF spectrometer (ESI-TOF). Infrared spectra were recorded using a Bruker ATR FT-IR Alpha-P spectrometer. Elemental analyses were carried out by the Service de Microanalyse, Institut de Chimie, Université de Strasbourg. 1-cinnamyl-benzimidazole (1) was prepared according to an adapted published procedure [26].

3.2. Procedure for the Preparation Dichloro-Bis(1-cinnamyl-benzimidazole)-Cobalt(II) (2)

In a Schlenk tube in an inert atmosphere of argon, a solution of CoCl₂ (0.041 g, 0.32 mmol) and (1-cinnamyl-benzimidazole (0.150 g, 0.64 mmol) in ethanol (10 mL) was stirred at room temperature. After 4 h, the formed blue precipitate was filtered, washed with diethylether (3 × 10 mL), and dried under vacuum to give complex 2 as blue solid in 96% yield (0.184 g). FT-IR: ν(CN) 1511 cm⁻¹; ¹H NMR (300 MHz, CDCl₃): δ = 27.28 (s, 2H), 14.99 (s, 4H), 8.93 (s, 2H), 7.81–7.39 (m, 12H), 5.37 (s, 2H), 3.16 (s, 2H), 1.42 (s, 2H), −2.67 (s br, 2H) ppm. MS (ESI-TOF): m/z = 562.14 [M − Cl]⁺ and 620.10 [M + Na]⁺ (expected isotopic profiles). Elemental analysis (%): calcd for C₃₂H₂₈N₄CoCl₂ (598.43): C: 64.23; H: 4.72; N: 9.36; found C: 64.03; H: 4.66 N: 9.23.

3.3. X-Ray Crystal Structure Analysis of Complex 2

Single crystals of cobalt(II) complex 2 suitable for X-ray analysis were obtained via slow diffusion of Et₂O into a CH₂Cl₂ solution of the complex. The samples were studied using a Bruker APEX-II CCD using Mo-Kα radiation (λ = 0.71073 Å). The structures were solved with SHELXT-2018/2 [27] and refined with SHELXL-2019/3 [28]. Crystal Data for C₃₂H₂₈Cl₂N₄Co•(CH₂Cl₂)₂ (M = 768.26 g mol⁻¹) are as follows: monoclinic, space group P2₁/n (no. 11), a = 13.9953(16) Å, b = 16.321(2) Å, c = 15.8898(19) Å, β = 100.662(4)°, V = 3566.8(7) Å3, Z = 4, T = 173(2) K, μ(Mo-Kα) = 0.961 mm⁻¹, D_calc = 1.431 g cm⁻³, 69,308 reflections collected (1.783 ≤ θ ≤ 27.983°), and 8563 unique (R_int = 0.1311, R_sigma = 0.0927). All these data were used in all calculations. The final R₁ was 0.0624 (I > 2.0 σ(I)), and wR2 was 0.1658 (all data). CCDC 2385973 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 (accessed on 15 September 2024).

4. Conclusions

We successfully synthesized and characterized the new dichloro-bis(1-cinnamyl-benzimidazole)-cobalt(II) complex. Although paramagnetic, this complex could be characterized via IR, elemental analysis, mass spectroscopy, and NMR spectroscopy, and its structure was determined via an X-ray study. The cobalt atom adopts a slightly distorted tetrahedral conformation. In the solid state, two molecules of dichloromethane are present in the crystal lattice.