Synthesis and photoluminescent properties of a Schiff-base ligand and its mononuclear Zn(II), Cd(II), Cu(II), Ni(II) and Pd(II) metal complexes

2.1 Physical measurements

Infrared spectra of solids were recorded in the region 4000–650 cm⁻¹ on a Perkin Elmer Spectrum 100 FT-IR spectrometer. Electronic absorption spectra were recorded in the 200–900 nm region in a JASCO V-530 UV/vis spectrophotometer. Fluorescence spectra were recorded on a Perkin Elmer LS-55 spectrometer at room temperature (298 K) in DMSO solution with a 1 cm path length quartz cell. The molar conductance measurements of 10⁻³ M metal(II) complexes in DMSO were carried out using Fisher Conductometer model AP75 with Cell constant equal to 1 cm⁻¹. Elemental analysis of carbon, hydrogen and nitrogen was determined in the Micro Analytical Unit using Perkin Elmer 2400. The metal ion content was determined using ICP (Inductively Coupled Plasma) – Optical Emission Spectrometer; model Optima 4100 DV, Perkin Elmer. Melting points were carried out on a melting point apparatus, Gallenkamp, England. Perkin Elmer TGA7. Thermogravimetric analyzer was used to record simultaneously TG and DTG curves. Solution NMR spectra were recorded using an AM-500 Bruker spectrometer with TMS or DMSO-d₆ was used as an internal standard and the chemical shifts are given in parts per million (ppm). EPR spectra were recorded on Bruker ESP-300 and JEOL JESRE – IX with variable temperature unit.

2.2 Synthesis of 8-(1-(4-aminophenylamino)ethylidene)-2H-chromene-2,7(8H)-dione, HL₁

A clear solution of P-phenylenediamine (0.13 g, 1.22 mmol) in 10 ml of ethanol was added to a warm solution of 8-acetyl-7-hydroxycoumarin (0.50 g, 2.44 mmol) in the same solvent (30 ml) (Scheme 1). The resulting mixture was refluxed for 2–3 h. The yellow product was precipitated, filtered off and washed with ethanol followed by diethyl ether, dried in a vacuum desiccator and crystallized from chloroform/ethanol (2:1). Yield (80%), m.p. (217) °C. Purity of the ligand was checked using TLC; (methanol:benzene, 1:4).

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Scheme 1

Synthesis of HL and HL₁.

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2.3 Preparation of metal complexes

A solution of HL₁ (0.10 g, 0.34 mmol) in ethanol was adjusted to about pH 8 with 0.1 M of potassium hydroxide and then (0.07 g, 0.34 mmol) zinc(II) acetate, (0.09 g, 0.34 mmol) cadmium(II) acetate, (0.07 g, 0.34 mmol) copper(II) acetate, (0.08 g, 0.34 mmol) nickel(II) chloride, (0.13 g, 0.34 mmol) bis(benzonitrile)-dichloro-palladium(II) in ethanol (15 ml) were added; the resulting mixture was refluxed for 10 min, followed by filtration and washing of the instantly formed precipitate thoroughly with methanol. The product was then dried in a vacuum desiccator over CaCl₂. The elemental analysis of the organic ligand and its metal complexes is tabulated in Table 1. The solid complexes were stable in air, insoluble in water, slightly soluble in alcohols and in all common organic solvent and freely soluble in warm DMSO. The observed lower molar conductivity value for metal complexes revealed their non-electrolytic nature (Refat et al., 2008) (Table 1).

3.1 Infrared absorption spectra

The infrared spectrum of 8-acetyl-7-hydroxycoumarin showed a weak band in the region 3600–2300 cm⁻¹ attributed to intramolecular hydrogen bonding between the 7-hydroxy and 8-acetyl group (Fig. 1). The IR spectrum of the Schiff base showed characteristic bands for C⚌N, C⚌O and C–O vibrations (Table 2). The signals appearing at v 3364 and 3463 cm⁻¹ in HL₁ spectrum were assigned to v_sym NH₂ and v_asym NH₂, respectively. These bands remained unchanged in all metal complexes, thus excluding coordination of the ligand through the amine group (Fig. 2) (Donia et al., 2003). The v(C⚌N) and v(C–O) for metal complexes exhibited a downward shift (15–45 cm⁻¹) and (9–22 cm⁻¹), respectively, thus supporting the participation of the azomethine group and the formation of M–O bonds via deprotonation (Bagihalli et al., 2008). While v(C⚌O) and v_asym(C–O–C) of the complexes remained as in HL₁, thus excluding their participation in chelate formation (Table 2 and Fig. 2). The bands observed at 1514–1545 cm⁻¹ and 1402–1408 cm⁻¹ in the spectra of [ZnHL₁(OAc)₂(H₂O)₂], [CdHL₁(OAc)₂]·2H₂O and [CuHL₁(OAc)₂] were assigned to v_as(COO⁻) and v_s(COO⁻) of acetate group, respectively. The magnitude of separation between these two vibrations (Δv = 112–137 cm⁻¹) suggests the binding of acetate to the metal in a monodentate fashion (Lever and Ogden, 1967). The IR spectra of v(H₂O) of coordinated water appeared at 825–846 cm⁻¹, indicating the binding of water molecules to the metal ions.

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Figure 1

Infrared spectra of (a) 8-acetyl-7-hydroxycoumarin AND (b) HL₁.

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Figure 2

Infrared spectra of (a) HL₁, (b) [ZnHL₁(OAc)₂(H₂O)₂], (c) [CdHL₁(OAc)₂]·2H₂O, (d) [CuHL₁(OAc)₂], (e) [NiHL₁Cl₂(H₂O)₂] and (f) [PdHL₁Cl₂].

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3.2 Thermal studies

Thermogravimetric (TG) and differential thermogravimetric (DTG) analyses were carried out for [ZnHL₁(OAc)₂(H₂O)₂] and [CdHL₁(OAc)₂]·2H₂O complexes in the 800 °C range under N₂ atmosphere. Typical TG and DTG curves are presented in Figs. 3 and 4. Thermal data of the complexes are given in Table 3. The correlation between the different decomposition steps of the complexes with the corresponding weight losses is discussed in terms of the proposed formula of the complexes.

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Figure 3

TG and DTG curves of [ZnHL₁(OAc)₂)H₂O)₂].

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Figure 4

TG and DTG curves of [CdHL₁(OAc)₂]·2H₂O.

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The TG and DTG curves of [ZnHL₁(OAc)₂(H₂O)₂] complex with molecular formula [C₂₁H₂₄N₂O₉Zn] are shown in Fig. 3 and Table 3. The TG shows three stages of decomposition within temperature range 22–800 °C. The first step of decomposition in the temperature range 22–306 °C shows that a mass loss 12.49% (calculated mass loss = 12.47%) corresponds to loss of 2H₂O + CO. The second step occurs within temperature range 306–385 °C with mass loss 10.20% (calculated mass loss = 10.13%) which could be attributed to the liberation of CH₃OH + NH₂ + 2H₂. DTG curve gives an endothermic peak at 358 °C. The third step is reasonably accounted for the loss of the rest of the ligand molecule C₉H₄ON with estimated mass loss 27.21% (calculated mass loss = 27.66%). The total mass loss up to 486 °C is in agreement with the formation of metal acetate and few carbon atoms as the final residue (TG 49.90%, calculated 50.34%).

The TG curve of [CdHL₁(OAc)₂]·2H₂O with the general formula [C₂₁H₂₃N₂O₉Cd] (Fig. 4), (Table 3) displays an initial mass loss in the temperature range 33–90 °C, corresponding to the decomposition of the complex to anhydrous complex by the loss of two uncoordinated water molecules. This is followed by another mass loss 70.80% (calculated mass loss = 70.66%) in the temperature range 263–480 °C corresponding to the decomposition of the ligand molecules with a final cadmium oxide residue and a total mass loss 76.90% (total calculated mass loss = 77.09%).

3.3 Electronic absorption spectra

The electronic spectra of the Schiff-base ligand HL₁ and its transition M(II) complexes [where M = Zn, Cd, Cu, Ni and Pd] were recorded in 10⁻³ or 10⁻⁴ M. Within the UV spectrum of the ligand (Fig. 5), (Table 4) was observed the existence of two absorption bands assigned to the transition π–π* at 315 and 395 nm, while the longer wavelength band observed at about 450 nm can be assigned to n-π* transition. The copper(II) complex [CuHL₁(OAc)₂] exhibited one broad d–d absorption band at 685 nm, suggesting a square-planar geometry (Vancˇo et al., 2008). The electronic spectrum of the nickel complex [NiHL₁Cl₂(H₂O)₂] (Fig. 6), (Table 4) showed very broad d–d band at 650 nm and well-defined band at 800 nm, these were assigned to the transitions ³A_2g(F) → ³T_1g(P) and ³A_2g(F) → ³T_1g(F). The lowest energy band corresponding to ³A_2g(F) → ³T_2g(F) was not observed possibly because it is outside the range of the spectrometer (at >1000 nm). These transitions are consistent with their well-defined octahedral configuration (Vancˇo et al., 2008). The spectrum of [PdHL₁Cl₂] complex displayed a band at 465 nm, this spin-allowed transition may correspond to ¹A_1g → ¹B_1g transition indicating a square planar geometry of the metal complex. The proposed structures are summarized in Table 5 (Bon et al., 2007).

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Figure 5

Electronic absorption spectra of HL₁ ligand and its Zn(II) and Cd(II) complexes in 10⁻⁴ M DMSO.

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Figure 6

Electronic visible absorption spectra of Cu(II), Ni(II) and Pd(II) complexes of HL₁ in 10⁻³ M DMSO.

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Table 5 Summary of proposed structures for metal complexes.

Complex	Formula	Structure
[ZnHL1(OAc)2(H2O)2]	C21H24N2O9Zn Pale yellow	Octahedral
[CdHL1(OAc)2]·2H2O	C21H24N2O9Cd Green	Tetrahedral
[CuHL1(OAc)2]	C21H20N2O7Cu Brown	Square planar
[NiHL1Cl2(H2O)2]	C17H18Cl2N2O5Ni Green	Octahedral
[PdHL1Cl2]	C17H14Cl2N2O3Pd Green	Square planar

3.4 Fluorescence studies

The fluorescence spectra of the Schiff-base ligand and its M (II) complexes [M = Zn, Cd, Cu, Ni and Pd] were studied in DMSO. The fluorescence quantum yield was determined using 7-amino-4-methylcoumarin (coumarin 120) laser dye as a reference with a known Φ_f of 0.63 in MeCN. The ligand, complexes and the reference dye were excited at 370 nm, maintaining nearly equal absorbance (∼0.1), and the emission spectra were recorded from 350 to 600 nm (Das et al., 2007). The emission intensity was found to increase with the concentration of the ligands up to a concentration of 10⁻⁵ M (Fig. 7). All experiments were carried out at a concentration of 10⁻⁵ M. The fluorescence spectrum of the free coumarin 8-acetyl-7-hydroxycoumarin when excited at 395 nm showed a maxima band at 445 nm (blue region) with fluorescence intensity 73.473, Stokes shift Δλ = 50 nm and Φ_f = 0.47 (Fig. 8 and Table 6). The HL₁ ligand displayed the maximum emission bands at 446 nm (blue region) when excited at 395 nm with fluorescence intensity 193.73, Stokes shift Δλ = 51 nm and Φ_f = 0.48. The high fluorescence quantum yield of HL₁ compared to 8-acetyl-7-hydroxycoumarin may be attributed to the large dipole moment of the fluorescent excited state; other factors such as hydrogen bonds formed between HL₁ and solvent also influence the photophysical properties of the ligand.

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Figure 7

Fluorescence emission spectra of 10⁻⁴, 10⁻⁵ and 10⁻⁶ M of HL₁ (excitation at 395 nm) in DMSO.

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Figure 8

Fluorescence emission spectra of 10⁻⁵ M 8-acetyl-7-hydroxycoumarin and HL₁(excitation at 395 nm) in DMSO.

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The emission spectral shape of the Zn(II) and Cd(II) complexes closely resembled that of the ligand. The Zn(II) and Cd(II) complexes (Fig. 9 and Table 6) showed fluorescence emission band at 446 nm Stokes shift Δλ = 51 nm with fluorescence intensity 358.97 and 328.52, and Φ_f = 0.50 and 0.49 for Zn(II) and Cd(II) complexes, respectively. The emission intensity of the Zn(II)-complex is stronger than that of the ligand (Yu et al., 2008). These results suggest that HL₁ may be a suitable agent to detect zinc ion. Therefore, this kind of compounds may have potential uses as a Zn²⁺ sensor (Wu et al., 2004). Both the Zn(II) and Cd(II) complexes exhibit strong fluorescence in comparison with HL₁, since the Zn(II) and Cd(II) ions are difficult to oxidize or reduce due to their stable d¹⁰ configurations (Basak et al., 2007). On the other hand, the fluorescent intensity enhancement may be due to the coordination of free ligand to Zn(II) and Cd(II) reducing the loss of energy via radiationless thermal vibrations of the intraligand excited states and due to an increase in the rigidity of the ligand (Ye et al., 2005).

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Figure 9

Fluorescence emission spectra of 10⁻⁵ M of HL₄ and its M(II) complexes (excitation at 395 nm) in DMSO.

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The Cu(II), Ni(II) and Pd(II) complexes (Fig. 9 and Table 6) showed fluorescence intensity 114.42, 141.97 and 43.38, and Φ_f = 0.31, Φ_f = 0.37 and Φ_f = 0.21, respectively. The emission spectra for the complexes were characterized by the emission band around 446 nm and quenching of fluorescence intensity (Wang and Yang, 2008). Quenching of fluorescence of a ligand by transition metal ions during complexation is a rather common phenomenon which is explained by processes such as magnetic perturbation, redox activity, and electronic energy transfer (Bagihalli et al., 2008; Basak et al., 2007).

3.5 ESR studies of [CuHL₁(OAc)₂] complex

The ESR spectrum of [CuHL₁(OAc)₂] in DMSO at 150 K indicates that the value of g_|| (2.057) is less than $g_{\perp }$ (2.1022) and A_ll = 67 G. The spectrum exhibits features similar to the majority of other tetragonally distorted, copper complexes which is consistent with an elongation or a weaker field along the tetragonal axis. The parameter G is found to be greater than 4 (6.462) indicating that the exchange coupling between two copper centers in the solid state is negligible (EL Husseiny et al., in press).

3.6 The nuclear magnetic resonance

The ¹H NMR of the ligand HL₁ showed the presence of two isomers in solution in a ratio 1: 4 (Husseiny et al., in press). The signal resonance at (δ 2.31 and 2.67 ppm) was assigned to azomethine methyl protons (CH₃C⚌N), thus confirming Schiff-base formation. The N–H resonance in HL₁ is broad due to the quadrupole moment of the nitrogen atom. The multiplet (doublet and triplet) resonances at 6.14–8.00 ppm were attributed to the protons of aromatic rings of both the 4-aminophenylimino and coumarin moieties. In the ¹H NMR spectra of [ZnL₁(OAc)₂2H₂O] only one isomer was observed indicting that the slow isomerization about C⚌N bond is stopped due to bonding to the metal ion (Ray and Bharadwaj, 2008). An important signal in the NMR spectrum of [ZnL₁(OAc)₂2H₂O] is that due to the coordination of two acetyl groups occurring at δ 2.38 and 2.56 ppm thus confirming its proposed structure (Table 7).