Synthesis, crystal structure and supramolecularity of [Cu(tba)₂] complex (tba = deprotonated of 3-benzoyl-1,1,1-trifluoroacetone)

2.1 General remarks

The respective chemicals were purchased by commercial providers (Fluka Company) and were used as received.

2.2 Physical measurements

Infrared spectra were recorded with a Perkin–Elmer spectrometer (type FT-IR 1000). Melting points were determined using analytically pure samples, sealed off in nitrogen-purged capillaries on a Gallenkamp MFB 595 010 M melting point apparatus. Microanalyses were performed using a thermo FLASHEA 1112 series instrument. The electronic absorption was measured in acetonitrile solution using a Perkin–Elmer Lambda 650 UV–Vis spectrophotometer, working in the wavelength range of 190–900 nm.

2.3 Synthesis of [Cu(tba)₂] (3)

Complex 3 was prepared by the reaction of Cu(OAc)₂·H₂O (OAc = O₂CMe) (39.9 mg, 0.20 mmol) dissolved in 50 mL of warm ethanol with two equivalents of 3-benzoyl-1,1,1-trifluoroacetone (1: Htba) (86.3 mg, 0.40 mmol). After 5 h of stirring at ambient temperatures, 20 mL of distilled water was added until the title complex precipitated. The precipitate solid was then washed three times with petroleum ether and was afterward dried in oil-pump vacuum for 2 days giving a dark green solid in a yield of 86.5%. M.p: 240 °C. IR (KBr), cm⁻¹: 1605 (vs), 1575 (vs) (CO); 1547 (vs), 1505 (s) (C–C); 1460 (s), 1327 (s), 1300 (vs), 1253 (s) (benzoyl ring); 1180 (vs) (ν_C–F); 770 (s), 698 (s) (ν_C–CF3). Anal. Calc. for C₂₀H₁₂O₄F₆Cu (497.82 g/mol): C, 48.6; H, 2.4. Found: C, 49.0; H, 2.6%. λ_max (ε): 233 nm (5.6 × 10¹ L mol⁻¹ cm⁻¹), 339 nm (5.8 × 10² L mol⁻¹ cm⁻¹), 360 nm (4.17 × 10² L mol⁻¹ cm⁻¹), 450–850 nm (broad with low intensity).

2.4 Crystallization of [Cu(tba)₂(3)

A suitable single crystal for X-ray crystallographic analysis was obtained by dissolving 3 in ethanol. After 5 days of storage in ambient room temperature, single dark-green crystals of 3 precipitated from saturated ethanol. One of the crystals was embedded in perfluoropolyether and mounted on a glass needle. The crystal and structure refinement data of 3 are summarized in Table 1. Data were collected on a Oxford Gemini diffractometer at 100 K using Mo K_α radiation (λ = 0.71 Å). The structure was solved by direct methods using SHELXS-97 (Sheldrick, 1990). The structure was refined by full-matrix least-square procedures on F² using SHELXL-97 (Sheldrick, 1997). All non-hydrogen atoms were refined anisotropically and were added on calculated positions.

3.1 Synthesis and characterization

The copper β-diketonate complex [Cu(tba)₂] (3) (tba = deprotonated of 3-benzoyl-1,1,1-trifluoroacetone) was prepared by the reaction of H-tba (1) with Cu(OAc)₂·H₂O (2) in 2:1 M ratio in warm ethanol (Scheme 1 After an appropriate workup, complex 3 could be isolated as dark-green solids dissolving in most of the common organic solvents including tetrahydrofuran, acetonitrile, and ethanol. However, in water and non-polar solvents 3 is not soluble. This complex is stable in both solution and solid state under normal conditions.

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Scheme 1

Synthesis of 3.

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Complex 3 was characterized by elemental analysis and IR spectroscopy. The molecular structure of 3 in the solid state was determined by single X-ray structure analysis.

Complex 3 shows the IR spectrum prominent absorptions in the range of 1600–1410 cm⁻¹ typical for metal β-diketonate complexes. These peaks are assigned for stretching vibrations of diketonate chelating ring ( ${\nu }_{\text{C⚌C}}$ and ${\nu }_{\text{C⚌C}}$ ) (Refat, 2005; Yang et al., 2001; Veeraraj et al., 2000; Bock et al., 1971; Bellamy, 1985; Nakamoto, 1978). The observed downfield shift ( $\mathrm{\Delta }{\nu }_{\text{C⚌O}}$ ), going from free ligand 1 ( ${\nu }_{\text{C⚌C}}=1611{\text{cm}}^{-1}$ ) to 3 indicates for the chelation of 1 with Cu(II) ion in a keto-enol tautomerism (Yang et al., 2001). This result was found inconsistent with the reported one (Veeraraj et al., 2000; Bock et al., 1971) and is supported by X-ray solid state structure (Section 3.2). The progress of the reaction of 1 with 2 could additionally be controlled by IR spectroscopy, since the characteristic absorptions of the acidic proton in the non-coordinated β-diketonate H-tba (3122 cm⁻¹) disappeared during the course of the reaction and new bands characteristic of metal β-diketonate species was observed (Section 2). The respective ν_C–F vibration of the tba trifluoromethyl ligands is found at 1180 cm⁻¹ (3) which, compared to the starting material, is shifted to a somewhat lower frequency (for comparison H-tta (1): 1200 cm⁻¹). In addition, the peak at 775 cm⁻¹ for H-tba (1) ligand was shifted to the higher frequency upon coordination of 1 with copper ion forming 3. The absence of stretching frequency of coordinated aqua in the spectrum indicates the availability of vacant sites, which could be used for extending the complex to coordinated polymers via nitrogen-bridging ligand.

The UV–Vis spectrum of 3-benzoyl-1,1,1-trifluoroacetone (htba:1) exhibits one absorption band in the UV-region from 300 to 390 nm (1: λ_max = 334 nm). This absorption is attributed to π–π^∗ in the 1-enol form (Gilli et al., 1993). The spectral shape in complex and free ligands in acetonitrile solution is very similar, but it slightly shifts to the higher wavelength (3: 339 nm (5.8 × 10² L mol⁻¹ cm⁻¹)). This could accordingly suggests that the coordination of Cu(II) ion does have not a significant influence on π–π^∗ state energy. Upon coordination of ligand with Cu(II), the energy gap of π–π^∗ state energy decreases. This is due to the slight electron donation of ligand. Upon coordination, of the ligand, one new shoulder band has appeared in a high intensity at λ_max = 360 nm (ε = 4.17 × 10² L mol⁻¹ cm⁻¹), reflecting the metal to ligand charge transfer (MLCT). This band is an evidence for coordinating and forming the dative covalent bond. The spectrum does not show the d–d transition in low concentration solution, but in concentrated solution as a broad band in the range 450–850 nm (λ_max = 680 nm), which may be attributed to the d–d transition as expected for a square planar geometry (Atre et al., 1982; Warad et al., 2000; Lever, 1990; Meek and Ehrhardt, 1990).

3.2 Crystal structural studies of 3

The molecular structure of 3 in the solid state was determined by single crystal X-ray. The result thereof is shown in Fig. 1 together with the atomic-labeling scheme. Crystal and experimental data are given in Table 1, and the bond distances (Å) and angels (°) are given in Table 2 (Section 2). Complex 3 crystallizes in the P2(1)/n space group and the Cu1 center lies on the (0, 0, 0) inversion center of monoclinic cell. The Cu1 atom is tetra-coordinated by two η²-chelate of tba (deprotonated-1) ligand forming a distorted square-planar. The crystallographic unit does not show any coordinated solvent molecules. The setup of CuO₄ in 3 is precisely planar, wherein the rms deviation from planarity for all C-, O- and the Cu-atoms is 0.035 Å, with the highest deviation observed for O2:0.073 Å. The same setup of CuO₄ was reported for α-Cu(tta)2 and β-Cu(tta)₂ (tta = deprotonated of 1-thenoyl-4,4,4-trifluoroacetone) in the P⁻¹ and P2(1)/c space groups, respectively (Lecomte et al., 1988; Al-Anber et al., submitted for publication). Wherein, the Cu1 atom lies on the (0, 0, 0) inversion center of triclinic cell as a square bipyramid with different metallocycle bent angles (93.3°, and 92.1°, respectively). The four bond angels in the setup of CuO₄ in 3 show significant distortion from the square planar, for example, O(2)–Cu(1)–O(1) and O(2)–Cu(1)–O(1A) angles have values of 92.97(5)° and 87.03(5)°, respectively. These angles are far from the octahedral value of 90°. The chelating bond length of Cu(1)–O(2) is found ca. 1.90 Å (see Table 2). These results are entirely consistent with those found for non solvated and four-coordinate Cu(II) β-diketonate Lecomte et al., 1988; Yang et al., 2001). These Cu–O bond distances in the four coordinate complexes are slightly, but significantly shorter than those of the five-coordinate Cu(II) β-diketonate and related complexes containing a coordinated water or alcohol molecule (range 2.191(4)–2.256(3) Å) (Pinkas et al., 1995; Toscano et al., 1996; Thompson et al., 1992; Vogelson et al., 1998; Doppelt and Baum, 1994; Patnaik et al., 1994).

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Figure 1

ORTEP diagram (50% probability level) of the molecular structure of 3. Disordered F atoms have been omitted. Symmetry transformations are used to generate equivalent atoms labeled with A: -x,-y,-z + 1.

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The bond lengths of C2−O2 (1.270(2) Å) and C4–O1 (1.266(2) Å) indicate the equivalent binding with copper ion. These bond lengths are found longer than C⚌O_keto form and shorter than C–O_enol form in contrast with the free ligand 1. These values indicate a stronger π-backdonation and delocalization of double bond inside the hexa-membered ring of Cu1–O2–C2–C3–C4–O1.

Within the crystal packing of 3, a 1-D supramolecular chain is generated via a regular pattern of π–π interaction between the adjacent [Cu(tba)₂] units. Fig. 2 shows the one selected part of 1D-supramolecular chain along the a-axis. This figure shows three levels of intermolecular π–π interactions. The largest distance of interacting atoms is 3.4 Å, which is found between ortho-C10A in benzyl ring and O2′ in butadiene entity. The shortest distance of interacting atoms is 3.171 Å between the C4′ and O1A in butadiene entities. The intermediate of the intermolecular contacts is 3.363 Å, which is found between the meta-C7B in benzyl ring and O1A in butadiene entity. This π–π interaction was not found in the same setup of the reported structure of Cu(tta)₂ (Al-Anber et al., submitted for publication). Figs. 3 and 4 show the top-view along the crystallographic a- and b-axes on selected parts of 1-D supramolecular chain, illustrating their different orientations to each other.

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Figure 2

π-interactions within one selected part of 1D chain. Dashed open lines refer to the largest (3.400 Å), small dashed lines to the shortest (3.171 Å) and dashed lines to the intermediate (3.363 Å) intermolecular contacts of interacting atoms.

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Figure 3

Top-view along the crystallographic b-axes on selected parts of 1D chains of 3 illustrating their different orientations to each other.

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Figure 4

Top-view along the crystallographic a-axes on selected parts of 1D chains of 3 illustrating their different orientations to each other.

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