Synthesis and structural characterization of two copper(II) complexes with 3-(4-(1H-benzo[d]imidazol-1-yl)-4-methoxyphenyl)-1-phenylprop- 2-en-1-one ligands

2.1 Preparation and characterization of the Cu(II) complexes

The two complexes, Cu(tta)₂(L¹)₂ (1) and Cu(tta)₂(L¹) (2), were synthesized in straightforward manner in reasonable yield (70∼80 %). Drop-wise addition of a CH₂Cl₂ solution of L¹ to the Cu(tta)₂ complex in a molar ratio of 1:1 afforded the complex 2. In complex 2, one L¹ ligand coordinates to Cu^II, leaving one coordination site on the metal ion available for bonding to other incoming ligands. When a CH₂Cl₂ solution of L¹ was treated with Cu(tta)₂ in a molar ratio of 2:1, the well-characterized complex 1 was obtained. Similar results were also obtained when reactions were performed between L¹ and complex 2 in a 1:1 molar ratio.

The reaction of 3-(4-(1H-benzo[d]imidazol-1-yl)-4-methoxyphenyl)-1-phenylprop-2-en-1-one (L¹) with Cu(tta)₂ in molar ratios of 2:1 and 1:1, respectively, in dichloromethane at room temperature is complete within ca. 30 min as judged from the absorption spectra with a green solution as a result (Fig. 1). Two products, 1 and 2, were obtained as crystalline products with different color at r. t. (1 is light green and 2 is green). Both 1 and 2 are air-stable and soluble in common organic solvents such as acetonitrile, acetone, alcohol and chloroform. The infrared spectra of 1 and 2 show characteristic bands corresponding to enolized 1,3-dicarbonyl stretching frequencies at ca. 1650 (1655 for 1, 1656 for 2) and sharp C–F deformation frequencies at 1455 for 1 and 2.

Fig. 1:

Electronic spectra solution with the Cu(tta)₂: L¹ (in ratio of 1:1) reaction mixture in dichloromethane. The spectra ( –) were recorded 1, 3, 6, 10, 14, 18, 22, 26, 30, 33 and 45 min after dissolution of Cu(tta)₂+L¹.

2.2 Molecular structures of 1 and 2

Numerical details of the crystal structure determinations of 1 and 2 are given in Table 1. A perspective view together with the atom labeling scheme for compound 1 is given in Fig. 2, and selected bond parameters are given in Table 2. The structure of compound 1 consists of two L¹ ligands, two tta ligands and one copper ion. The central Cu(II) ion is hexa-coordinated with a distorted coordination polyhedron. The hexa-coordination around the Cu(II) center is achieved by the four oxygen atoms of two tta ligands and two nitrogen atoms of two L¹ ligands. The equatorial plane is formed by the four tta oxygen atoms O(1), O(2), O(3) and O(4) atom [deviation of Cu(1) by 0.003 Å towards plane defined by O(1), O(2), O(3) and O(4)]. The Cu(1)–O(tta) bond lengths in the equatorial sites are in the 2.090(2)–2.173(2) Å range. The apical sites of the compressed octahedron are occupied by N(1) and N(3), with Cu(1)–N(1) = 2.006(3) and Cu(1)–N(3) = 1.991(3) Å. The N(1)–Cu(1)–N(3) bond angle is 176.33(12)°.

Fig. 2:

Molecular structure of complex 1 showing 30 % probability ellipsoids. The hydrogen atoms are omitted for clarity.

The ORTEP drawing for 2 and the atom labeling scheme are illustrated in Fig. 3, and selected bond parameters are given in Table 2. The copper(II) center is penta-coordinated with a distorted coordination polyhedron close to a square pyramid with s = 0.06 [13]. The penta-coordination around the Cu²⁺ center is achieved by the four oxygen atoms O(1), O(2), O(3) and O(4) [Cu(1)–O = 1.936(2)–1.951(2) Å]. The apical site of the square pyramid is occupied by N(1) of the L¹ ligand [Cu(1)–N(1) = 2.252(3) Å]. The copper atom is shifted by 0.184 Å towards N(1) from the basal O plane, which itself forms an interplanar angle of 75.0° with the benzimidazole ring of L¹.

Fig. 3:

Molecular structure of complex 2 showing 30 % probability ellipsoids. The hydrogen atoms are omitted for clarity.

4.1 Materials

All solvents, Cu(CH₃COO)₂·H₂O, Htta, benzimidazole, 4-fluorobenzaldehyde and 4-methoxyacetophenone were commercially available and used as received and purchased from Aladdin Industrial Corporation of Shanghai, China. Cu(tta)₂ and 3-(4-(1H-benzo[d]imidazol-1-yl)-4-methoxy phenyl)-1-phenylprop-2-en-1-one (L¹) were synthesized according to the literature method [14, 15].

4.2 Analyses and physical measurements

C, H and N elemental analysis was performed on a Vario EL-III analyzer. UV/Vis spectra were collected using a Varian Cary-5000 spectrometer. The infrared (IR) spectra were recorded on a Bruker Vector 22 spectrophotometer with KBr pellets in the 4000–400 cm^–1 region.

4.3 Preparation of Cu(tta)₂(L¹)₂ (1)

To a CH₂Cl₂ solution (20 mL) of Cu(tta)₂ (124.5 mg, 0.25 mmol), 3-(4-(1H-benzo[d]imidazol-1-yl)-4-methoxy phenyl)-1-phenylprop-2-en-1-one (177 mg, 0.50 mmol) in CH₂Cl₂ was added slowly. The mixture was stirred for 0.5 h and filtered, and the resulting light-green filtrate was kept at room temperature for several days. Light-green crystals, suitable for X-ray crystallography, formed upon evaporation of the solvent. Yield 272 mg (89 % based on Cu(tta)₂). – Anal. for C₆₂H₄₄CuF₆N₄O₈S₂: calcd. C 61.30, H 3.65, N 4.61; found C 61.28, H 3.62, N, 4.68 %. – IR (KBr, cm^–1): v = 3076, 2932, 2839, 1661, 1655, 1606, 1520, 1455, 1412, 1311, 1236, 1197, 1144, 1063, 1016, 980, 938, 791, 744, 727, 689, 665, 619, 592, 529, 427.

4.4 Preparation of Cu(tta)₂(L¹) (2)

To a CH₂Cl₂ solution (20 mL) of Cu(tta)₂ (124.5 mg, 0.25 mmol), 3-(4-(1H-benzo[d]imidazol-1-yl)-4-methoxy phenyl)-1-phenylprop-2-en-1-one (88.6 mg, 0.25 mmol) in CH₂Cl₂ was added slowly. The mixture was stirred for 0.5 h, the resulting light-green solution was filtrated, and the filtrate was kept at room temperature for several days. Light-green crystals, suitable for X-ray crystallography, formed upon evaporation of the solvent, which proved to be Cu(tta)₂(L¹)₂ (1). Yield 22 mg (7 % based on Cu(tta)₂). Four days later, a large amount of green crystals were collected from the mother liquid which proved to be Cu(tta)₂(L¹) (2). Yield 155 mg, (72 % based on Cu(tta)₂). – Anal for C₃₉H₂₅CuF₆N₂O₆S₂: calcd. C 54.51, H 2.93, N 3.26; found: C 54.50, H 2.89, N 3.27 %. – IR (KBr, cm^–1): v= 3106, 2969, 2844, 1656, 1616, 1593, 1572, 1541, 1517, 1503, 1455, 1412, 1357, 1318, 1301, 1248, 1230, 1141, 1084, 1064, 1030, 1024, 985, 934, 860, 825, 783, 763, 750, 718, 691, 619, 594, 531, 523, 484, 427.

4.5 X-ray crystallography

All measurements were made on a Bruker Smart-1000 CCD diffractometer equipped with a graphite-monochromatized MoK_α radiation (λ = 0.71073 Å) by using a φ-ω scan mode at 296(2) K. The corrections for Lp factors were applied. Absorption corrections were applied using the SADABS program [16]. The structures were solved by Direct Methods [17] with the shelxtl (version 6.10) program [17, 18] and refined by full matrix least-squares techniques on F² with shelxtl [17, 18]. All non-hydrogen atoms were refined anisotropically, and the hydrogen atoms were generated geometrically. In the structure of 2, a disorder was modeled with displacement parameter restraints. The details of the crystal and refinement data for 1 and 2 are given in Table 1. Selected bond length and bond angles are listed in Table 2.

CCDC 1018585 and 1018586 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.