Synthesis, Crystal Structure, and Luminescent Properties of a Helical Complex [(CuL)2.5(H2O)0.375] (H2L = Benzil Bis[(5-bromo oxidobenzylidene)hydrazone])

A new helical copper (II) complex [(CuL)_2.5(H₂O)_0.375] 1 (H₂L = benzil bis[(5-bromo oxidobenzylidene)hydrazone]) was synthesized and characterized by elemental analysis, IR spectra, UV-vis spectra, and single-crystal X-ray diffraction. Complex 1 exhibits a new single-stranded helix with the copper (II) atom having a four-coordination seesaw geometry. The cooperativity of intermolecular C−H···π interactions and C−H···O hydrogen bonds links the helices together to form a three-dimensional network structure. The thermogravimetric and luminescent properties in the solid state of complex 1 have been investigated.

Keywords: copper(II); crystal structure; helical complex; luminescent properties

The construction of coordination complexes via self-assembly of predesigned ligands with appropriate metal ions is a subject of current interest because of their unusual structures and special physical properties such as luminescence, redox activity, and magnetism.[[1]–[4]] Polydentate diazine Schiff-base ligands with two symmetrically coordination portions derived from benzil dihydrazone are excellent ligands. The simplicity of the syntheses and the high yield in a single-step reaction from inexpensive commercially available reagents allow us the possibility to systematically probe the effects of modifying the ligand backbone and through which we are attempting to control the precise topography, or microarchitecture, of the arrays. The most intriguing structural feature of this type of ligands in the solid state is that they should be able to show a monohelix due to the constraint of the rotation around the C−C bond. Their coordination to d group metals leads to a variety of metal helices.[[5]–[21]] As a continuation of our research working on metal helicates derived from multidentate diazine Schiff-base ligands via self-assembly,[[12]]−[[21]] herein we report the synthesis and characterization of a new neutral helical coordination complex [(CuL)2.5(H2O)0.375] (1) displaying multiple fluorescent emission bands at room temperature based on the ligand H2L, benzil bis[(5-bromo oxido benzylidene)hydrazone].

All the chemicals were of reagent-grade quality obtained from commercial sources. Elemental analyses (C, H, and N) were carried out on a Perkin-Elmer 240C analytical instrument. IR spectra were recorded in KBr pellets with a Nicolet 170 SXFT-IR spectrophotometer in the 4000 ∼ 400 cm−1 region. UV-vis spectrum were obtained on a Shimazu UV-250 spectrometer in the range of 600 ∼ 200 nm in MeOH solution. The thermogravimetric analysis was carried out on a Perkin-Elmer-7 thermal analyzer at a heating rate of 10°C/min from 25 to 1000°C, and the luminescent spectra were performed on a Hitachi F-7000 fluorescence spectrophotometer.

Benzil dihydrazone (0.96 g, 4 mmol) and 5-bromosalicy laldehyde (1.60 g, 8 mmol) were mixed in methanol (30 mL). The solution was refluxed for 6 h, and then evaporated to a small volume (10 mL). The yellow precipitation formed was isolated and dried under vacuum. Yield: 63%. Anal. Calc. for C28H18N4O2Br2 (%): C, 55.84; H, 3.01; N, 9.30: Found (%): C, 55.82; H, 3.05; N, 9.35. IR (cm−1, KBr pellet): 3453(w), 3056(m), 1891(w), 1759(w), 1620(vs), 1603(vs), 1565(m), 1473(vs), 1444(w), 1366(w), 1343(w), 1319(w), 1269(vs), 1236(w), 1182(s), 1131(w), 1078(m), 1042(w), 1005(w), 980(w), 906(w), 879(w), 823(m), 784(w), 771(w), 745(w), 729(w), 706(w), 688(m), 667(m), 629(m), 609(m), 582(w), 557(w), 465(w), 431(w).

A solution of NaOH (0.02 g, 0.6 mmol) in MeOH (2 mL) was added slowly to a stirring solution of ligand H2L (0.18 g, 0.3 mmol) in 25 mL MeOH solution, and then Cu(NO3)2·6H2O (0.35 g, 1.5 mmol) methanol solution (15 mL) was added in the previous solution. After refluxing for 4 h, the resulting dark-brown solution was filtered and left for slowly evaporating at room temperature to obtain dark-brown block crystals suitable for single crystal X-ray structure determination. Yield: 67%. Anal. Calc. for C70H45.75Br5Cu2.5N10O5.37 (%): C, 50.31; H, 2.76; N, 8.38. Found (%): C, 50.27; H, 2.81; N, 8.34. IR (cm−1, KBr pellet): 3432(m), 3041(w), 1893(w), 1606(vs), 1578(s), 1513(vs), 1513(vs), 1449(vs), 1407(w), 1384(m), 1369(m), 1306(s), 1242(w), 1176(s), 1136(w), 1074(w), 1001(w), 966(w), 822(m), 778(w), 768(w), 743(w), 710(m), 644(w), 588(w), 510(w), 479(w).

A suitable sample of size 0.28 mm × 0.10 mm × 0.03 mm was chosen for the crystallographic study and then mounted on a Bruker SMART APEX CCD diffractometer with ω and ϕ scan mode in the range of 1.70° < θ < 25.00°. All diffraction measurements were performed at room temperature using graphite monochromatized Mo Kα radiation (λ = 0.71073 Å). A total of 12268 (4529 independent, Rint = 0.0996) reflections were measured. The structure was solved by direct methods and refined by full-matrix least-squares on F2 using the SHELXL 97 program.[[22]] The Cu(3) atom is located on the C2 axis special position. The O(1W) and O(2W) atoms are considered as disorder site occupation factor of 0.25 and 0.125 in complex 1, respectively. Anisotropical thermal factors were assigned to most of the non-disordered non-hydrogen atoms except the H2O molecule. The water hydrogen atoms could not be located and were not calculated. CCDC reference number: 752349 (1). Space group, lattice parameters, and other relevant information are listed in Table 1. The relevant bond lengths and bond angles are listed in Table 2.

TABLE 1 Crystallographic data and structure refinement parameters for complex

TABLE 2 Selected bond distances (Å) and bond angles (°) for complex

In the IR spectrum, ν (C˭N) bands are exhibited at 1620 and 1603 cm−1 for H2L and at 1606 and 1578 cm−1 for the title complex, respectively. From 1620 to 1578 cm−1, a 42 cm−1 blue-shifted strongly suggests coordination of the imine nitrogen atoms with copper atoms. The other ν (C˭N) band was almost the same as that of the ligand. Their identity was finally confirmed by X-ray crystallography. The UV absorption spectrum of H2L and the title complex were investigated in MeOH solution (Figure 1). The UV spectra of complex 1 displays three absorption peaks at 233, 287, and 412 nm. Compared with 221, 300, and 363 nm assigned to the π-π* and n-π* transition of H2L, 12 nm blue-shifted, 13 nm red-shifted, and 49 nm blue-shifted are observed.[[11]]

Graph: FIG. 1 The UV-vis spectra of H2L ligand (dash) and complex 1 (solid) in MeOH solution at room temperature (color figure available online).

Graph: FIG. 2 ORTEP drawing of complex 1 with the atom numbering scheme and the geometry of Cu atoms. The atoms are represented by 30% probability thermal ellipsoids. Hydrogen atoms and solvent molecules are omitted for clarity (symmetry code: A: 2 − x, y, 2 − z).

The structure analysis by X-ray diffraction reveals that title complex crystallizes in the monoclinic space group C2. An ORTEP diagram of complex 1 with the atomic numbering scheme and the coordination environment of Cu(II) atom is depicted in Figure 2. The asymmetric unit consists of 2.5 neutral single-stranded helices and each a helix contains one copper center and one two net negative charge ligand, L2−. Each copper center has a four-coordination environment, formed by two imine nitrogen atoms and two phenolate oxygen atoms from a ligand. The values of the topological parameter τ of ca. 0.34, 0.36, and 0.43 indicate seesaw structure for Cu(1), Cu(2), and Cu(3), respectively.[[23]] All the Cu–N and Cu–O bond distances are in the range of 1.927(8)–1.958(8) and 1.868(7)–1.891(7), respectively, and the bond angles around Cu(II) center range from 87.3(3)° to 156.9(3)°. These results agree well with those of related compounds.[[5]–[20]] It should be pointed out that there are weak interactions between copper and bromine atoms with the Cu(1)···Br(2B) distances of 3.26 Å, the Cu(2)···Br(3C) distances of 3.36 Å, Cu(3)···Br(4D), and Cu(3)···Br(4E) distances of 3.94 Å, respectively (symmetric code B: 3/2 – x, 1/2 + y, 2 – z; C: 3/2 – x, – 1/2 + y, 3 – z; D: x, 1 + y, z; E: 2 – x, 1 + y, 2 – z). In addition, two types of C−H···N hydrogen bonds between the C(38) and C(66) atoms and the N(6) and N(10) are found in the helices and also play an important role in stabilizing the helical structure (Table 3).

TABLE 3 Hydrogen bonds for complex

Graph: FIG. 3 Packing diagram for complex 1 along the c-axis (color figure available online).

Upon coordination, the rotating freedom of the ligand is restrained and the ligand can be locked in a twist helical conformation. The torsion angle of carbon− carbon bond is 65.6° for N(2)˭C(8)−C(15)˭N(3), 71.7° for N(6)˭C(36)−C(43)˭N(7), and 76.8° for N(10)˭C(64)−C(64A)˭N(10A), respectively. The torsion angle is 130.7°, 127.7°, 128.0°, 138.6°, and 137.3° around C(7)˭N(1)−N(2)˭C(8), C(15)˭N(3)−N(4)˭C(23), C(35)˭N(5)−N(6)˭C(36), C(43)˭N(7)−N(8)˭C(50), and C(63)˭N(9)−N(10)˭C(64), respectively. The dihedral angle between two benzene rings of a ligand is 94.4° of C(9)−C(14) and C(16)−C(21), 92.2° of C(37)−C(42) and C(44)−C(49), and 98.4° of C(65)−C(70) and C(65A)−C(70A), respectively. These results are similar to the values in related compounds.[[5]–[21]]

Detailed structural analyses revealed that copper(II) helices were further interacted each other feathering three-dimensional network through five kinds of C−H···O hydrogen bonds (Table 3) and eight kinds of C−H−π interactions (Figure 3). The C···M distances and the C−H···M angels are 3.88 Å and 124.9° for C(67E)–H(67E)···MI, 3.89 Å and 127.2° for C(66E)–H(66E)···MI, 3.96 Å and 136.8° for C(26B)– H(26B)···MII, 3.72 Å and 167.6° for C(3F)–H(3F)···MIII, 3.53 Å and 118.2° for C(13)–H(13A)···MIV, 3.57 Å and 115.9° for C(46G)–H(46G)···MIV, 3.78 Å and 170.2° for C(54F)–H(54F)···MV, and 3.87 Å and 136.4° for C(31C)–H(31C)···MIV, respectively (M: midpoint of the aromatic ring, symmetric code E: 2 – x, y, 2 – z; F: x, y, 1 + z; G: x, y, z – 1).[[24]] Although these C−H···π interactions and hydrogen bonds are weak compared with the metal-nitrogen and metal-oxygen coordination bonds, they are important in the formation of the 3D supramolecular network.

In order to study the thermal stability of the title complex, the thermal gravimetric analysis (TGA) was carried out in the temperature range of 25–1000 °C (Figure 4). The title complex keeps the weight until 212°C. On raising the temperature further, complex 1 loses 2.5 ligands and makes the weight lose 86.02% till 700°C, which is in good accordance with the calculated value 87.69%. The final residue after 700°C should be CuO (found: 12.67%, calcd: 11.90%).

Graph: FIG. 4 The TGA curve for complex 1.

Graph: FIG. 5 Emission spectrum of H2L (dash) and complex 1 (solid) in the solid state at room temperature. (a) λex = 220 nm for H2L and λex = 210 nm for 1; (b) λex = 360 nm for H2L and λex = 350 nm for 1 (color figure available online).

The solid-state luminescent spectra of the ligand and title complex have been investigated in the solid state at room temperature. H2L and complex 1 display multiple emission bands at different excitations. As shown in Figure 5, the luminescent spectra of H2L and complex 1 display emission maximum (λem, max) at 314 and 316 nm with using excitation wavelengths at 220 and 210 nm, respectively. Excitated at 360 nm, the H2L shows weak emission band at 463 and strong emission band 559 nm (Figure 5). Excitated at 350 nm, complex 1 exhibits emissions at 468 and 561 nm. Therefore, the emission bands of the title complex should be ascribed to neither ligand-to-metal nor metal-to-ligand charge transfer but the intraligand π*−π charge transfer.

In summary, we synthesized a neutral single-stranded helical coordination complex [(CuL)2.5(H2O)0.375] in a MeOH environment with a new ligand H2L, benzil bis[(5-bromo oxidobenzylidene)hydrazone]. The structure of the complex has been established by X-ray single-crystal diffraction analysis and also characterized by IR, UV, elemental analysis, TG technique, and luminescent properties. Each copper(II) center is bound to two imine nitrogen atoms and phenolate oxygen atoms of one ligand to form a neutral single-stranded helix. The copper helices were further interacted each other feathering three-dimensional network through extensive C−H···O hydrogen bonds and C−H−π interactions. The fluorescence measurement of 1 showed the multiple emission bands at 316, 468, and 561 nm.

This work was supported by the National Natural Science Foundation of China (No. 20701012), the Foundation of Henan Educational Committee of China (No. 2007150003 and 2008B150002), the Foundation for University Youth Key Teacher of Henan Province of China and Innovation Experiment Program for University Students of Henan University (No. 09NB021).