Synthesis,characterization,luminescence and biological activities of lanthanide complexes with a hydrazone ligand

2.1 Materials

Lanthanide salts, {[LnCl₃.6H₂O]; Ln=Sm, Eu, Dy, Tb and Yb}, 2-hydroxy-1-napthaladehyde and isonicotinyl hydrozide were purchased from Sigma Aldrich. All other solvents and reagents used were of analytical grade received from Sd fine chemicals, Mumbai, India. Throughout the experiment double distilled (DD) water was used.

2.2 Synthesis of schiff base ligand (H₂L)

The hydrazone Schiff base ligand H₂L was prepared from 2-hydroxy-1-napthaladehyde (1.72 g; 10 mmol) and isonicotinylhydrazide (10 mmol) in presence of methanol. To the 100 ml round bottom flask, 20 ml methanol and reactants were added and the resulting reaction mixture was refluxed on water bath for 3 h. On cooling the reaction mixture, a crude yellow product was obtained. Then, the product was collected by filtration, washed with cold ethanol and dried. Finally, the product was recrystallized from hot ethanol. Yield: 60%; m.p. 230°C.

2.3 Synthesis of Ln(III) complexes

A 0.9 mmol (0.18 g) of ligand (H₂L) was added in 60 ml of DD water, the pH of which was adjusted to 7 by the addition of an aqueous solution of NaOH (10 wt%); the solution turned light yellow. Then a solution of LnCl₃.6H₂O (0.3 mmole) was added drop wise to the system. After stirring for 1 hour at 50°C, a yellow precipitate was separated by filtration, washed several times with water, and hot ethanol. The resulting metal complex was dried in a vacuum. Yield; 60%. The other complexes were also synthesized by this method [22].

2.4 Antimicrobial activity

The antimicrobial activity of Lanthanide complexes having a tridentate Schiff base ligand was determined by using an agar diffusion method [33]. Petri-dishes (90 mm) were prepared containing 20 ml of Mueller-Hinton agar. The inoculum density of all bacterial isolates was standardized with 0.5 McFarland turbidity standards. Once the agar is cooled, a bacterial lawn was prepared by spreading 100μl of bacterial culture onto the surface of the dried Mueller-Hinton agar plates using sterile swabs. Wells of 6 mm in diameter were punched into the agar and filled with 100μl of the lanthanides complexes (dissolved in DMF) at a concentration of 1μg/ml [34] and DMF is taken as a control. The plates were incubated at 37°C for 24 h and zones of inhibition were measured using a caliper.

2.5 Characterization

Elemental analysis was carried out on a Perkin-Elmer 2400 CHN elemental analyzer. UV-visible spectra were recorded in DMF solution, concentration of 10^–5 M, at 25°C using a UV-visible spectrophotometer (LAB INDIA, UV-3092). Fourier transform infra-red (FTIR) spectroscopy of the samples was performed using a Perkin Elmer (Spectrum Two, UK) spectrometer. Nuclear magnetic resonance (¹H NMR) spectra of hydrazone Schiff base ligand and Ln(III) complexes were recorded on Mercury 400 NMR spectrometers (Agilent Technologies, Inc. Santa Clara, CA, USA) at 400 MHz, using DMSO-d₆ as a solvent and tetra methyl silane as an internal reference. Molar conductivity was measured in dimethyl formamide (DMF) solution, concentration of 10^–4 M, at 25°C using ELICO conductivity meter equipped with CM162 conductivity cell. Fluorescence spectra (scanned from 200 to 900 nm, with a spectral resolution of 0.2 nm, slit widths ∼2.5 nm) were recorded instrument model Perkin Elmer precisely LS-55 fluorescence spectrophotometer with 1 cm quartz cell at room temperature. The light source and detectors were 450 W xenon lamp and R955 photomultiplier tube, respectively. The thermal analysis was performed on a Perkin-Elmer Pyris STA 6000 thermo balance analyzer operating at a heating rate of 10°C/min in the range of ambient temperature up to 900°C under N₂. X-ray diffractometer (XRD) Philips: PW1830. Electron ionization-mass spectrometer, model: AUTOSPEC-M, Micromass, UK.

Molar conductivity values (2.34–6.52 Ω^–1 cm² mol^–1) in DMF solution at 25°C, suggest for the complexes are non-electrolyte in nature [35, 36].

3.2 UV-visible spectra

The UV-visible spectrum (Fig. 1 & Table 2) recorded for the DMF solution of all the Ln(III) complexes and Schiff base ligand showed maximum absorption bands at 273, 328 and 365 cm^–1 due to the π ⟶ π^* transitions within the aromatic ring and n⟶π^* naphthalene ring and the imine function of hydrazone moiety respectively [37–39]. The band observed at 338, 363 cm^–1 would be due to n⟶π^* transitions of the (C=N) and (C=O) groups of metal-ligand charge transfer coordinate Schiff base lanthanide metal ion [16]. This change is essentially variant for each of the lanthanide complexes of Sm(III), Eu(III), Dy(III), Tb(III) and Yb(III) and is attributed to metal coordination by the ligand is shown in Fig. 1. The higher energy band in the free ligand is observed as a single band upon complexation without much shift in frequency. Based on analytical and spectral data a plausible schematic representation of the lanthanide complex is shown in Scheme 1.

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Scheme1

Schematic representation of the Keto-enol toutomerism in Schiff base (H₂L) of the ligand and its lanthanide complexe [Ln = Sm, Eu, Dy, Tb and Yb].

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Fig.1

UV-Visible absorption spectra for the Ln complexes.

The main stretching frequencies of the FTIR spectrum of the ligand (H₂L) and its lanthanide complexes are tabulated in Table 2 data of the free ligand broad absorption band at 3211 cm^–1 are observed due to weak hydrogen bonded and free phenolic ν(OH) stretching vibrations. After the complexation, phenolic -OH peak disappears which due to deprotonation followed by coordination of the ligand to the Ln(III) metal ion (Scheme 1). The FTIR spectrum of the free ligand show strong band at 1680 cm^–1, which is attributable to ν(C=O) stretching vibrations. The irrational bands at 1600 cm^–1 can be assigned to the ν(C=N) of azomethine. In the FTIR spectra of lanthanide complexes, the ν(C=O) and ν(C=N) bands are shifted by 57–59, 28–32 cm^–1. The shifts of the ν(C=O) and ν(C=N) vibrations of the bands towards lower wave numbers on complexation indicate participation of the carbonyl oxygen and azomethine nitrogen in coordination to the rare earth metal ions [40, 41]. These data were further supported by the appearance of a medium intensity band in range 610–615 and 460–465 cm^–1, which could be assigned to ν(Ln-O), ν(Ln-N) respectively. The vibrational band at 3198 cm^–1 can be assigned to the ν(N-H) for the free ligand. The ν(N-H) band is observed in the range 3202–3220 cm^–1 for the complexes. Observation of the ν(C=O) and ν(N-H) bands observed in the, infrared spectra of the complexes indicate that the H₂L ligand acts as monobasic to tridentate ligand in the complex formation. The presence of ν(OH) stretching vibrations at higher frequency in the spectra of complexes suggest that the naphtholic oxygen binds metal ion with deprotonation. The ν(C-O) of the phenolic group which occurs at 1285 cm^–1 in the free ligand is shifted to lower wave numbers by 93–97 cm^–1 in the spectra of lanthanide complexes. These observations suggest that the ligand binds metal ions through naptholic oxygen atom [41]. Thus the FTIR data (Figure S1) corroborate the conductivity values suggesting that the complexes exhibit non-electrolytic behaviour.

3.4 ¹H-NMR spectra

¹H-NMR spectra of H₂L (Fig. 2) and its lanthanide complexes with Sm(III), Eu(III), Dy(III), Tb(III) and Yb(III) were recorded in (DMSO-d₆) (Figure S2-S7). The ligand spectrum of the signals observed in the δ (ppm): 1.53 (3H, singlet, CH₃); 7.12-7.86, 8.31 (11H, multiplet, naphthalene, -NH-); 7.96 (2H, doublet, J = 6 Hz, H³, H⁵ of pyridine ring); 8.31 (2H, broadband, H², H⁶ of pyridine ring); 12.46 (naphthoic OH). It was observed that the proton signal at 12.46 ppm in the ligand assigned to phenolic OH has disappeared in the complex suggesting coordination of the phenolic OH via deprotonation. The proton of -NH group was not observed. This may be due to the formation of intra molecular hydrogen bonding or exchangeable with solvent.

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Fig.2

¹H-NMR spectra of ligand (N,N-bis (2-hydroxy-1-naphthylidene) isonicotinylhydrazone).

Thermogravimetric (TG) and differential thermal analysis (DTA) of lanthanide complexes of H₂L ligand are carried out within the temperature range from ambient temperature up to 800°C under nitrogen (inert) gas flow with a heating rate 10°C/min. All the complexes showed similar thermal decomposition. TGA curve of the complexes undergoes two-stage changes. All lanthanide complexes are thermal by stable up to 290°C indicating the absence of lattice or coordinated water and solvent molecules. The first degradation of H₂L ligand was observed in the range of 291–663°C and is characterized by the obvious endothermic peak in the DTA-curve, corresponding to the weight loss of 77.67% (calculated 76.12%). The second weight loss 17.52% ligand molecule and formation of stable Ln₂O₃ (calculated 17.39%) oxide residue which occured between 664–800°C. The TGA profile of the complex is as shown in Fig. 3.

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Fig.3

DTGA and TGA spectra for the Sm complex.

X-ray diffraction studies of the powder sample were carried out as it could not be possible to grow suitable crystals for complete X-ray analysis. The observed interplanar spacing values (’d’ in A°), have been measured from the diffractogram of the [Sm(H₂L)₃] complex as shown in Fig. 4 with respect to major peaks having relative intensity at wavelength = 1.54059 and the Millar indices, h, k, l have been assigned to each d value and 2θ angles. The system belongs to ‘Orthorhombic’ form. The direct unit cell dimensions determined are.

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Fig.4

Powder X-ray diffraction spectrum for the Sm complex.

a = 7.3592A°

b = 12.1431 A°

c = 12.2745A°

V = 1036.883(A°)³

α = β = γ = 90° _. 

Based on the experimental evidences, the average crystallite or grain size varies from 20 to 32 nm. The system exhibits a nano crystalline phase.

The emission spectrum of the lanthanide complexes recorded in DMF solution with excitation wavelength was fixed at 366 nm of the free ligand and is shown in Fig. 5 and corresponding data given in Table 3. The ligand exhibits a broad fluorescence band centered 512 nm attributed to π ⟶ π^* transition [36]. Sm(III), Eu(III), Dy(III), Tb(III) and Yb(III) complexes show a broad excitation bands in the range 366–426 nm. The band is assigned to π ⟶ π^* transition (Fig. 5).

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Fig.5

Fluorescence spectrum of excitation (red) emission (blue) for the Ln complexes.

The emission of Eu^3 + displays three luminescence bands corresponding to the π ⟶ π^* transition at 426 nm (23,474 cm^–1), ⁵D₀⟶⁷F₂ transition at 512 nm (19,531 cm^–1) and ⁵D₀⟶⁷F₃ transition at 600 nm (16,666 cm^–1). The intensity sharp emission peak at 512 nm(19,531cm^–1) in Fig. 5 is characteristic of the hypersensitive ⁵D₀⟶⁷F₂ transition of Eu^3 +, which is much more intense than the ⁵D₀⟶⁷F₃ transition at 600 nm (16,666 cm^–1). This is consistent with the observation that the ⁷F₂ transition is the preferred transition for europium-containing luminescence materials [37].

In the emission spectrum of Sm^3 + complex, two luminescence bands are observed. The band observed at 366 nm (27,322 cm^–1), 512 nm (19,531 cm^–1) and 600 nm (16,666 cm^–1) are respectively assigned to π ⟶ π^*, ⁴G_5/2⟶⁶H_7/2 and ⁴G_5/2⟶⁶H_9/2 transitions. The most intense emission sharp peak centered at 512 nm (19,531 cm^–1) corresponding to the more hypersensitive ⁶H_9/2⟶⁶H_7/2 transition [38]. In the emission spectrum of Dy^3 + complex, a strong band is observed at 366 nm corresponding to the π ⟶ π^* transition (Fig. 5). The complex shows three luminescence bands at 483 nm (20,703 cm^–1), 516 nm (19,379 cm^–1) and 600 nm (16,666 cm^–1). These bands are respectively assigned to ⁴F_9/2⟶⁶H_15/2, ⁴F_9/2⟶⁶H_13/2 and ⁴F_9/2⟶⁶H_11/2 transitions. The intensity sequence of the peaks (Fig. 5) is I₆H_13/2 >I₆H_15/2 >I₆H_11/2 for Dy^3 + complex [39]. The emission spectra exhibited characteristic emission of Tb^3 + ions at 426, 512 and 600 nm, which are designed transitions of ⁵D₄ excitation state to ⁷F_j (j = 5,4) ground state of Tb^3 + ion. The strong intensity peak at 512 nm (⁵D₄⟶⁷F₅) transition for terbium-containing luminescence materials [41] _.  The intensity sequence of the peaks (Fig. 5) is I_7F5 >I_7F4 for Tb^3 + complex. Yb^3 + also has exhibited the similar emission spectra as above. The results indicate that the ligand H₂L is a good chelating organic chromosphere and can be used by the efficiency energy transfer from Schiff base ligand H₂L to the Ln(III) ions. The europium, samarium and terbium complexes show stronger luminescence intensities than those of dysprosium and ytterbium complexes.

The antimicrobial activity was evaluated against gram-negative (Eschereshia coli and Klebsiella Pneumoniae) and gram-positive (Staphylococcus aureus and Bacillus subtilis) bacterial strains using an agar disc diffusion method [42–45]. The results are expressed in the diameter of the growth of inhibition area and the data of microbial activity is given in Table 4 (Figure S8-S12). The results showed that the three complexes exhibited highly good activities compared to control (DMF) and free ligand. The metal complexes of samarium, europium and dysprosium showed high inhibition activity against Eschereshia coli and Staphylococcus aureus while the moderate activity is observed for complexes of terbium and dysprosium and the least is for ytterbium complex against Klebsiella Pneumoniae and Bacillu subtilis which indicated that the free ligand has less activity than the complexes. Such extreme activity of metal complexes is due to penetration of chelates with the bacterial cell membrane where in the Schiff base metal ion coordinates to lipopolysaccharide (LPS) of cell membrane through its nitrogen or oxygen donor atoms, which leads to damage the outer cell membrane and consequently inhibits growth of bacteria.