ABSTRACT

N,N,N’,N’-tetrakis-(2-benzimidazolylmethyl)-1,2-ethanediamine (TBIMEDA), was prepared by reaction of ethylenediamine tetra-acetic acid disodium salt (EDTA) with 1,2-diaminobenzene in a refluxed glycol solution, and furthermore, three allomeric complexes[(M^II TBIMEDA) SO₄·5H₂O, M = Cd, Co, Ni] were selfassembled by solvothermal method based on reaction of this ligand with the relative sulfates respectively. These allomeric complexes were characterized by elemental analysis and IR spectroscopy and their crystal structures were determined by single crystal X-ray structural analysis. In the crystal architecture of these complexes, every metal(II) ion is chelated by one neutral TBIMEDA ligand to form an octahedral core with configuration of five heterocyclic rings (fivemember ring). These cores then were linked together by multi hydrogen bond interactions with sulfate ions and water molecules to construct their 3D crystal architectures.

1. INTRODUCTION

Azaheterocyclic compounds, with strong coordination ability during complex preparations and as better acceptors in hydrogen bonding formation, were more frequently selected in coordination research [1-4]. After selfassemblies, the conjugated p bonds in the heterocyclic ligands provided abounding information on electromagnetism and photoelectrochemistry, and these special complex’s aggregation was well known as functionalized material in future applications [5-9]. Meanwhile EDTA (ethylenediamine tetra-acetic acid) was regard as the most useful multidentate ligand using in analytic chemistry. In this work, based on reaction of EDTA with benzene-1,2-diamine, a multidentate ligand with four benzoimidazole groups, N,N,N’,N’-tetrakis-(2-benzim-idazolyl methyl)-1,2-ethanediamine(TBIMEDA), was prepared according to the reported method (Scheme 1) [10-13]. Furthermore, by self-assembly of TBIMEDA reacting with different sulfates, three single-nuclear complexes with same crystal configuration were constructed by sovothermal methods, and their crystal structures were well defined by X-ray analysis.

2. EXPERIMENTAL

2.1. Materials and Physical Measurements

All chemicals and solvents were of analytical reagent grade and used as received. Elemental analysis was performed in a Perkin-Elmer 240 elemental analyzer. IR spectrum was obtained using a Nicolet IR200 infrared spectrometer. The fluorescence spectra were taken on an Edinburgh Instruments FLS920 fluorescence spectrumeter respecttively.

2.2. Synthesis of Ligand

Preparation of the ligand, N,N,N’,N’-tetrakis-(2-benzimidazolylmethyl)-1,2-ethanediamine (TBIMEDA) was described as below [10-13]: A solution of EDTA (2.92 g, 0.01 mol) and 1, 2-diaminobenzene (4.32 g, 0.04 mol) in 100 mL of glycol was heated to boiling and kept in reflux for 16h. After cooling down to room temperature, the mixture was added to water (ac. 400 mL) for precipitation over night. The crude product was separated by filtration, purified by recrystallization with small volume of ethanol for three times and dried in air. A white powder product in yield of 70% was obtained with m. p. = 156˚C - 158˚C. IR data (KBr, cm⁻¹): 3186 vs (n_N-H), 2966 w, 2823 w, 1617 s, 1535 s, 1486 w(s_N-H), 1454 s, 1433 vs, 1348 s, 1311 m, 1274 vs, 1245 m, 1217 w, 1094 s, 1049 m, 1021 m, 996 m, 963 w, 845 m, 747 vs, 616 w, 485 m. Calc. (found) for C₃₄H₃₂N₁₀:C 72.88 (72.90), H 3.84 (3.82), N 7.08 (7.12). This IR data were similar to the reported values. [10] The preparation of TBIMEDA was described as Scheme 1.

2.3. Synthesis of Complexes

2.3.1. Co(TBIMEDA)·SO₄·5H₂O(1)

Keeping under stirring, TBIMEDA (0.0706 g, 0.1 mmol), CoSO₄·7H₂O (0.0422 g, 0.15 mmol) are mixed with water (8 mL). The mixture was sealed in an autoclave and the autoclave was placed in an oven at 120˚C for 60 h. After cooling down to room temperature at rate of 5˚C/h, filtrating and washing, several large and transparent orange crystals were collected (in yield of 34%). Found (Calc.) for CoC₃₄H₄₂N₁₀O₉S: C, 49.44(49.45); H, 5.11(5.13); N, 16.98(16.96)%. IR data(KBr, cm⁻¹): 3415 s, 3101 w, 3056 w, 2917 w, 2774 w, 2643 w, 1621 m, 1540 m, 1470 m, 1450 vs,1392 m, 1274 vs, 1119 vs, 1029 m, 939 m, 910 w, 910 w, 857 w, 743 vs, 620 vs, 555 w, 514 w.

2.3.2. Ni(TBIMEDA)·SO₄·5H₂O(2)

Keeping under stirring, TBIMEDA (0.0706 g, 0.1 mmol), NiSO₄·6H₂O (0.0262 g, 0.1 mmol) were mixed with water (8 mL). The mixture was sealed in an autoclave and the autoclave was placed in an oven at 140˚C for 60 h. After cooling down to room temperature at rate of 5˚C/h, filtrating and washing, several large and transparent blue crystals were collected (in yield of 45%). Found (Calc.) for NiC₃₄H₄₂N₁₀O₉S:C, 46.48(46.47); H, 5.13(5.11); N, 16.97(16.99) %. IR data (KBr, cm⁻¹): 3415 s, 3105 m, 3064 w, 2913 w, 2765 w, 2639 w, 1544 m, 1405 vs, 1392 s, 1331 vs, 1278 vs, 1221 w, 1115 vs, 1029 s, 988 w, 939 s, 906 m, 849 m, 743 vs, 616 vs, 545 w, 518 w.

2.3.3. Cd(TBIMEDA)·SO₄·5H₂O(3)

Similar to the assembly of 1, when NiSO₄·6H₂O was replaced by CdSO₄·8H₂O (0.0385 g，0.05 mmol), large and transparent blue crystals were obtained in yield of 45% by the same hydrothermal method. Found(Calc.) for CdC₃₄H₄₂N₁₀O₉S:C, 49.44(49.45); H, 5.07(5.09); N, 15.95(15.93)%. IR data (KBr, cm⁻¹): 3395 s, 3096 m, 2761 w, 1621 m, 1540 m, 1446 vs, 1384 s, 1335 m, 1278 s, 1094 vs, 1025 s, 755 s, 620 m.

2.4. Single Crystal X-Ray Diffraction Analysis

The X-ray data collections and structure determinations were performed on a Bruker SMART CCD. The data were collected using graphite-monochromatic Mo-Ka radiation (l = 0.71073 Å). The crystal structure was solved by direct methods and refined by full-matrix least-square calculation on F² with SHELX-97 program package. [14]. All non-hydrogen atoms were treated anisotropically. Hydrogen atoms were placed in calculated positions. The crystallographic data for 1 - 3 are summarized in Tables 1 and 2.

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3. RESULTS AND DISCUSSION

3.1. Crystal Analyses for 1 - 3

Complexes 1 - 3 are alloisomers with each other, 1 is therefore selected as a typical example for discussing their crystal configurations. The view of the single core of 1 is depicted in Figure 1. Some selected bond distances and angles are listed in Table 1, and the hydrogen bonding data are listed in Table 2. 1 adopted orthorhombic system with P2₁2₁2₁ space group (unit cell parameters: a = 11.2807(16) Å, b = 14.980(2) Å, c = 23.081(3) Å, β = 90.00˚, V = 3900.2(10) ų, Z = 4, Dc = 1.497 mg/cm³). Cadmium(II) ion in the core of complex 1 is coordinated by one neutral TBIMEDA ligand, forming five of fivemember rings (Figure 1). The octahedral coordination core around cadmium(II) ion includes six N atoms, two of them originate from the chain of ethanediamine and the others originate from the unsaturated N atoms in imidazole rings. All distances of Cd-N described above are near to or shorter than the sum of Van der Waals radii for Cd and N, and the effective chelations result in distorted octahedron around Cd(II) ions. However, the saturated N atoms in imidazoles, include N3, N5, N11 and N12, are protonated and without any coordination contribution.

3.2. Hydrogen Bond Interactions in 1

In the crystal structure of complex 1, the Cd(II) coordination cores are linked through intermolecular N-H···O and O-H···O hydrogen bonds with sulfate ions and water molecules (Figure 2), forming a complicate 3D architecture. If all water molecules are omitted, the Cd(II)

coordination water molecules are omitted, the Cd(II) coordination cores and sulfate ions will be linked by hydrogen bonding interactions to form a metal organic framework (MOF, as describing in Figure 3), and this MOF can be simplified as a topologic diagram with characters of 3 × 7² around Cd cores and 3² × 7⁵ × 8³ around sulfate cores (Figure 4). Meanwhile, if all Cd cores were ignored, the sulfate ions and water molecules will be linked together to construct a 2D chain running along the a axis (Figure 5).

3.3. Photoluminescence

Excitation under l_Ex = 341 nm, ligand TBIMEDA gave a emission at l_Em = 375 nm, which was contributed from its conjugated configuration with π-π^* electron transition (Figure 6). After crystal assembly using the ligand with the metal ions with d¹⁰ configuration, such as Cd(II) in 1, its crystal state also presented a strong emission peaks at l_Em =417 nm (l_Ex = 339 nm). Obviously, compared with

the ligand’s emission, the fluorescent peaks of 1 emerged with red shift. This photoluminescence mechanism originated from ligand-metal charge transition (LMCT) [15], and its emission peak position was decided by the coordination situation of ligand with metal ion [16].

4. CONCLUSION

Ligands of diamine with imidazole group exhibited potentially application in biodegradation. In this work, after preparation of N,N,N’,N’-tetrakis-(2-benzimidazolylmethyl)-1,2-ethanediamine(TBIMEDA), and using it as ligand reacting with different salts, three crystal architectures were self-assembled under solvothermal conditions. In crystal assembly, beside coordination between metal and ligands, the crystal architectures were also sustained by multi hydrogen bonding interactions from the complex cores with sulfate anions and water molecules. Three crystal architectures of M(TBIMEDA) SO₄·5H₂O(M = Cd^II, Co^II, Ni^II) all adopted orthorhombic crystalline with P2₁2₁2₁ space group. The metal center was chelated by three TBIMEDA to construct an octahedral configuration with five 5-numbered chelating rings, where the coordinating atoms were unsaturated N in imidazole ring and saturated N from ethylenediamine chain. These complexes can be used as a model to study

the effect of stereochemistry on the coordination polyhedron of M(II) ions. Although these M(II) ions were coordinated by six atoms, their coordination bonds were different in length, and therefore they may use different hybrid obits to form inner orbital or outer orbital coordination compounds. Obviously, these conclusions should be supported by magnetic determination and thermal gravimetric analysis. Further study should be intensively investigated.

5. ACKNOWLEDGEMENTS

This work was financially supported by the National Natural Science Foundation of China (No. 20771073); by the “211” Project of Guangdong Province (the third time): The Key and Common Technologies in Fine Chemical Engineering; and by The Key Project of Education Office from Guangdong Province, China.

REFERENCES

NOTES

^*Corresponding author.