1 Introduction

The rational design and construction of transition metal complexes containing nitrogen heterocycles has been an increasing area of research field in view of their potential catalytic activity [1], magnetic properties [2–5], optical properties [6–9], and use as membranes or biosensors [10–12], and sorption devices [13–17]. Benzimidazole, as a typical nitrogen heterocyclic ligand, is also an important pharmacophore in drug discovery [18]. Its derivatives have a broad spectrum of biological and pharmaceutical properties; e.g., bactericide, antiparasite and anticarcinogen. In addition, benzimidazole is important in metallo-supramolecular chemistry [19]. Benzimidazole and its derivatives are also appealing for the design of medicinal organic–inorganic hybrid materials [20–24]. Co(II) complexes incorporating benzimidazoles have attracted considerable attention because of their magnetic properties and biological activity [18, 19, 25, 26]. For instance, Xia et al. [25] studied the magnetic properties of Co(II) complexes based on 2-(n-pyridyl)benzimidazole. A DNA binding study of a dinuclear macrocyclic Co(II) complex based on 2,2,2′,2′-S,S[bis(bis-N,N-2-thiobenzimidazolyloxalato-1,2-ethane)] was carried out by Arjmand and Aziz [18]. In addition, Sandoval et al. [26] reported the biological activities of Co(II) complexes with 2-benzimidazole derivatives.

Herein, we report the synthesis of two new Co(II) complexes based on the 1,4-di(benzimidazole-1-yl)benzene (L) ligand (Chart 1), [(CoLCl2)(CHCl3)(DMF)]∞ (1) and [CoL(NO3)2]∞ (2). The analysis of their crystal structures was carried out to give valuable insight into the construction of organic–inorganic hybrid materials. Complexes 1 and 2 were also characterized by IR, elemental analysis and single crystal X-ray diffraction. Both materials possess a 1D chain structure.

Chart 1
structure 1

The ligand L used in this work

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2 Experimental

2.1 Materials and General Methods

All reagents and solvents for the syntheses were commercially available (Alfa Aesar Company) and used as supplied. Ligand L was synthesized from the reaction of 1H-benzimidazole with 1,4-dibromobenzene according to the literatures [27–29]. The IR spectra were obtained with a TENSOR 27 OPUS (Bruker) FT-IR spectrometer from 4000–400 cm−1 using dispersed sample powders on KBr disks. Elemental analyses of C, H, and N were performed with a Perkin-Elmer 240C analyzer. The thermogravimetric analysis (TGA) and differential thermal analysis (DTA) were obtained with a Perkin-Elmer Diamond Thermogravimetric Analyzer from room temperature to 800 °C at a heating rate of 10 °C/min under a N2 stream.

2.2 Synthesis of the Complexes

2.2.1 [(CoLCl2)(CHCl3)(DMF)]∞ (1)

A transition layer (5 mL) of methanol/chloroform (v/v 1:1) was carefully layered over a chloroform solution (4 mL + 2-drops N,N-dimethylformamide (DMF)) of L (0.05 mmol). A solution of CoCl2·6H2O (0.1 mmol) in methanol (4 mL) was carefully layered over the transition layer. The tube was covered and left undisturbed. Blue block crystals stable in air were harvested after 4 weeks. Yield: ~40% (based on L). Anal. Calcd. (%) for (C24H22CoON5Cl5): C 45.56, H 3.51, N 11.07. Found (%): C 45.71, H 3.59, N 11.23. IR (cm−1): 1663s, 1587w, 1556w, 1497w, 1440wm, 1415w, 1391s, 1255m, 1158w, 1104s, 1063w, 1025w, 865w, 729m, 689w, 665s, 566w.

2.2.2 [CoL(NO3)2]∞ (2)

Species 2 was prepared in a similar procedure of 1 using Co(NO3)2·6H2O instead of CoCl2·6H2O. Yield: ~50% (based on L). Anal. Calcd. (%) for (C20H14CoO6N6): C 48.70, H 2.86, N 17.04%. Found (%): C 48.55, H 2.77, N 17.15%. IR (cm−1): 1726w, 1612 m, 1529w, 1500s, 1462w, 1396w, 1301w, 1275s, 1240s, 1163w, 1151w, 1119w, 1010m, 993m, 939w, 913m, 874w, 844m, 806m, 778w, 751s, 627m, 594m, 540m, 432w.

2.3 X-Ray Crystallographic Studies

X-ray single-crystal diffraction data for 1 was collected on a Rigaku Raxis Rapid IP diffractometer at 293(2) K. The data for 2 was collected on a Bruker Smart 1000 CCD diffractometer at 293(2) K. The program SAINT [30] was used for integration of the diffraction profiles. All the structures were solved by direct methods using the SHELXS program of the SHELXTL package and refined by full-matrix least-squares methods with SHELXL [31]. Metal atoms in each complex were located from the E-maps. The other non-hydrogen atoms were located in successive difference Fourier syntheses and refined with anisotropic thermal parameters on F 2. All hydrogen atoms were placed geometrically. Crystallographic data and experimental details for structural analyses are summarized in Table 1.

Table 1 Crystal data and structure refinement summary for complexes 1 and 2
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3 Results and Discussion

3.1 Synthesis and General Characterizations of Complexes 1 and 2

Two different reactions of 1,4-di(benzimidazole-1-yl)benzene with cobalt(II) salts led to different crystalline materials. The reaction of 1,4-di(benzimidazole-1-yl)benzene with CoCl2·6H2O in methanol/chloroform produced crystallographically characterizable [(CoLCl2)(CHCl3)(DMF)]∞ (Fig. 1), whereas Co(NO3)2·6H2O yielded [CoL(NO3)2]∞ (Fig. 2). The results of elemental analyses for the two complexes were in good agreement with the theoretical compositions.

Fig. 1
figure 1

a Coordination environment of Co(II) in 1; b the 1D zigzag structure of 1 (all solvent molecules omitted for clarity); c 2D supramolecular framework linked by π···π interactions

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Fig. 2
figure 2

Perspective view of a the coordination environment around the Co(II) ions in 2; b 1D chain of 2; c 2D supramolecular framework linked by π···π interactions in 2

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3.2 Crystal Structures of 1 and 2

Complex 1 crystallizes in a monoclinic system with the P2(1)/n space group (Table 1). The basic structural unit consists of one Co(II) ion, one L ligand, two chlorine anions, one chloroform and one DMF molecule (Fig. 1a). The Co(II) ion adopts a tetrahedral geometry involving two chlorine anions and two N-atoms from two L ligands. The Co–Cl bond [2.2397(2) and 2.2527(2) Å] and Co–N bond [2.024(4) and 2.010(4) Å] distances are consistent with the similar structures reported earlier [32]. The L ligands have two types of configuration in 1; i.e., configuration 1 and configuration 2, which bridge two Co(II) ions by turns to a linear zigzag chain (Fig. 1b). The two benzimidazole rings of each L configuration are in the same plane. The dihedral angle between the benzene ring (C19–C18–C20–C19′–C18′–C20′) and the benzimidazole ring (N3–C11–N4–C12–C13–C14–C15–C16–C17) in configuration 1 is 47.5°, and the dihedral angle between the benzene ring (C9–C8–C10–C9′–C8′–C10′) and the benzimidazole ring (N1–C1–N2−C2–C3–C4–C5–C6–C7) in configuration 2 is 59.7°. The benzimidazole rings of configuration 1 in each chain are parallel to the benzene rings of configuration 2 of the adjacent chains with a centroid-to-centroid distance of 3.777(8) Å and interplanar distance of 3.719 Å, which indicates that there are π···π interactions between the adjacent chains. Such π···π interactions link the 1D chains into 2D framework structures (Fig. 1c). Chloroform and DMF act as solvent molecules filling in the interspaces of the 2D sheets.

The single-crystal X-ray diffraction analysis of 2 indicates that it crystallizes in the triclinic space group of P-1 (Table 1). The asymmetric unit of 2 contains one Co(II) ion, one L ligand and two NO3 − anions (Fig. 2a). The Co(II) ions possess a distorted octahedral coordination environment composed of O4N2 donor atoms from the two L ligands and two NO3 − anions. All the Co–O bond [2.042(4)–2.348(2) Å] and Co–N bond [2.043(4)–2.045(4) Å] distances are in the normal range [33] (Table 2). Similar to complex 1, L ligands adopt two kinds of trans-configurations to connect Co(II) ions by turns forming a zigzag chain (Fig. 2b). The dihedral angles between the benzene ring and the benzimidazole ring in two configurations are 40.3° and 40.6°, respectively. The NO3 − ions coordinate to the Co(II) ion in a bidentate chelate coordination mode. Furthermore, the two benzimidazole rings from two adjacent chains are parallel to each other with a centroid-to-centroid distance of 3.833 Å. Here too π···π interactions link the 1D chains into 2D sheets (Fig. 1c).

Table 2 Selected bond lengths [Å] and angles [°] for 1 and 2
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The reaction of 1,4-di(benzimidazole-1-yl)benzene with Co(ClO4)2·6H2O produced crystallographically characterizable {[Co(L)2](ClO4)2}∞ (3), which has been recently reported [34]. In contrast to 1 and 2, 3 consists of a 3D three-fold interpenetrating diamondoid network, which crystallizes in the monoclinic space group of C2/c.

3.3 Analysis of Structure

Comparing the structures of 1, 2, and 3, we find that the different coordination abilities, coordination modes and spatial structures of the three anions, Cl−, NO3 − and ClO4 −, determine the final structural features of the three Co(II) complexes. Due to its relatively small volume and strong coordination ability, Cl− is a normal monodentate and terminal ligand [26, 32, 35]. In 1, two Cl− anions occupy two coordination sites of the Co(II) ions and limit the dimension of 1 to a 1D chain. NO3 − often adopts multiple coordination modes; e.g., a monodentate terminal ligand [6, 36] (Chart 2a), a bidentate chelate ligand [6, 37] (Chart 2b) and a bidentate bridging ligand [38] (Chart 2c). Therefore, the monodentate terminal and bidentate chelate coordination modes are able to limit the dimensions of the complexes; and, the bidentate bridging coordination mode does just the opposite. In 2, the NO3 −ion has an intermediate volume and serves as a bidentate chelate ligand. Its strong coordination ability limits the complex to low dimensional topology. The ClO4 − with its large volume but weak coordination ability is a good choice for the design of high dimensional polymers [34]; and, complex 3 is a good example.

Chart 2
structure 2

a Terminal ligand; b Chelate ligand; c Bridging ligand

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3.4 Thermal Analysis

The thermal stabilities of the complexes were studied. The structures of 1 and 2 have a similar 1D chain structure. Therefore, the TGA and DTA of 2 were examined. The result is given in Fig. 3. The TGA curve of 2 exhibits three stages of weight loss. The first weight loss of 6% occurs between 25 and 200 °C and corresponding to the loss of solvent molecules (H2O, DMF, CHCl3) attached to the sample surfaces. The second weight loss of 29% occurs between 200 and 315 °C and results from the loss of two NO3 −. The third weight loss of 55% occurs between 315 and 465 °C and results from the loss of one L ligand. Two obvious endothermal peaks are observed at 308 and 370 °C (DTA curve), and are related to the heat-absorbing process of the loss of NO3 − and L ligand, respectively. Thus, 2 decomposes at ca. 200 °C.

Fig. 3
figure 3

TG–DTA curves of the complex 2

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4 Conclusions

Two new Co(II) complexes of the 1,4-di(benzimidazole-1-yl)benzene ligand were synthesized and characterized by elemental analyses, IR, and X-ray single crystal diffraction. 1 and 2 exhibit different 1D chain structures. These results illustrate that the anions can play an important role in the self-assembly of metal–organic materials.

5 Supplementary Data

Crystallographic data (excluding structure factors) for the structures in this article have been deposited with the Cambridge Crystallographic Data Centre as supplementary publications CCDC 796416 and 796417 (for 1 and 2). Copies of the data can be obtained, free of charge, on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (Fax: +44 1223336033 or E-mail: deposit@ccdc.cam.ac.uk).