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Syntheses and characterization of complexes of copper(II) with Schiff-base ligands derived from 2,6-diacetylpyridine: spectroscopic, thermal behavior, magnetic moment and photoluminescent studies

  • P. Dhanakodi
  • M. Jayandran
  • V. Balasubramanian
Article
  • 99 Downloads

Abstract

Transition metal complexes derived from the Schiff base ligands are versatile compounds and playing vital role in organic light emitting diodes, medicinal and pharmaceutical fields due to their luminescent abilities and biological activities. Metal complexes of the two Schiff base ligands with Cu(II) ions were synthesized by reacting each ligand 2, 6-diacetylpyridine bis (benzoylhydrazone) (DAPBH) and 2, 6-diacetylpyridine bis (benzenesulfonylhydrazide) (DAPBSH) with the metal salts in refluxing ethanol. The compound DAPBH and DAPBSH reacted with copper(II) nitrates to provide the corresponding complexes A and B respectively. Both the ligands and complexes are characterized by UV-Visible spectroscopy, thermogravimetry, fourier transform infrared spectroscopy, photoluminescence spectra and scanning electron microscopy. Complexes A and B emit intense luminescence in solution, and a solvent dependent luminescent feature is found. In the solid state, complex B emits weak luminescence under 365 nm excitation from an ultraviolet lamp. Magnetic susceptibility measurements revealed that there are antiferromagnetic exchange interactions between the neighboring spins and IR studies confirm the existence of coordinated water molecules in the complex.

1 Introduction

Compounds consisting an azomethine group (–CH=N–), can be called as Schiff bases, which are formed by the condensation of a primary amine with a carbonyl compound. Schiff bases of aliphatic aldehydes are reasonably unstable and are readily polymerizable. On the other hand, Schiff bases of aromatic aldehydes having an effective conjugation system are more stable. Therefore, schiff bases have number of applications such as preparative use, identification, detection and determination of aldehydes and purification of carbonyl compounds [1]. Besides, Schiff bases form basic units in certain dyes. Commonly, schiff bases are bi-or tri- dentate ligands capable of forming very stable complexes with transition metals. A large number of Schiff bases and their complexes have been studied because of their ability to reversibly bind oxygen [2], act as catalysts [3] and exhibit photochromic properties [4, 5]. The high affinity for the chelation of the Schiff bases towards the transition metal ions, especially copper ions, is utilized in preparing their complexes.

The chemistry of copper complexes is of interest owing to their importance in biological and industrial processes [6, 7]. The copper complexes derived from Schiff bases were found to be extremely efficient catalysts in both homogeneous [8] and heterogeneous [3] conditions. Among all the transition metal complexes, Cu(II) complexes with Schiff bases are a fascinating class owing to their luminescent abilities and potential applications in organic light emitting diodes [9, 10, 11]. In addition, it has gained considerable attention in recent years due to their diversity in magnetic exchange properties and coordination geometries [12, 13].

The present work dealt with the coordination behavior of Schiff bases based on 2,6-diacetylpyridine and hydrazones [14, 15, 16, 17]. The two ligands 2,6-diacetylpyridine bis (benzoylhydrazone) (DAPBH) [18, 19] and 2,6-diacetylpyridine bis (benzenesulfonylhydrazide) (DAPBSH) [20], were prepared and two complexes of Cu(II) with the DAPBH/DAPBSH ligands have been obtained. The structural features of Schiff bases and their metal complexes have been elucidated by various spectral and analytical techniques.

2 Experimental

2.1 Materials and characterization techniques

All chemicals and solvents used in this study were purchased from Sigma-Aldrich, USA and used without further purification. DAPBH and DAPBSH ligands were synthesized as reported in the literature [21]. The UV–Vis spectra were measured using LAMBDA-35 UV–Vis spectrophotometer. The photoluminescence spectra were recorded using a JOBIN YVON Flurolog-3 spectrofluorimeter excited with xenon lamp. Thermo gravimetric analysis was carried out on the Perkin Elmer (Pyris Diamond) instrument at a heating rate of 10 °C min−1 by using alumina powder as reference. Infrared measurements (4000–400 cm−1) were performed on Thermo scientific (USA) Nicolet 6700 spectrometer. Gouy’s Method Apparatus (Model No: HO-ED-EM-08) was used to determine the magnetic susceptibility of prepared samples at room temperature. The morphology of the prepared complexes was observed with scanning electron microscope JEOL F-6400.

2.2 Preparation of the complexes

2.2.1 [Cu (DAPBH)(H2O)2](NO3)2(H2O)2 (complex A)

In order to prepare this complex, in a suspension of 0.4010 g (1.0 mmol) DAPBH in 15 mL water, a solution of 0.2974 g (1.0 mmol) Cu (NO3)2·3H2O in 25 ml 90% ethanol was gradually added. The mixture was stirred at 40 °C for 5 h. These were filtered off, washed with water and air dried. As a result, dark green crystals were collected. Yield: ~ 80% based on Cu(NO3)2·3H2O. This prepared complex is named as ‘Complex A’ and it will be used further to denote this complex.

2.2.2 [Cu(DAPBSH)(DMSO)] (complex B)

In order to prepare the complex, in a solution of 0.4710 g (1.0 mmol) DAPBSH in 4 mL dimethylsulfoxide and 10 mL ethanol, a solution of 0.2974 g (1.0 mmol) Cu(NO3)2·3H2O in 15 ml 95% ethanol was added. After the addition of a droplet of triethylamine, the solution was stirred at 40 °C for 5 h, and then filtered by suction. The filtrate from the reaction on further evaporation afforded yellow crystals. Green block crystals of complex B were obtained after 3 days. Yield:~ 75% based on Cu(NO3)2·3H2O. This prepared complex is named as ‘Complex B’ and it will be used further to denote this complex.

3 Results and discussion

3.1 UV–Vis absorption spectra

The UV–Vis spectra of complex A and B (in ethanolconcentration: 1 × 10−5 mol L−1) were recorded at ambient temperature in the range between 100 and 1000 nm. UV–Vis spectra is shown in Fig. 1, all the spectra are same. The UV–Vis spectrum of the ligands and corresponding complexes showed two bands. An intense band centered in the range 270–315 nm is attributed to n–π* transitions related with azomethinechromophore. It is worth noting that a new band occurs at 400–468 nm in the UV–Vis spectra of A and B, and this band probably arises from a Ligand to Metal Charge Transfer (LMCT)transition [22].

Fig. 1

a UV–Vis spectra of the ligand DAPBH and complex A, b UV–Vis spectra of the ligand DAPBSH and complex B

3.2 Luminescence spectra

The photoluminescent properties of complexes A and B, ligands DAPBH and DAPBSH were examined in both the solid state and solution at ambient temperature. Ligand DAPBSH and the corresponding Cu(II) complex B emit luminescence in the solid state under 350 nm excitation from an ultraviolet lamp. As shown in Fig. 2, DAPBSH and complex B display emission bands with maxima at 425 and 527 nm. The significant shift of the emission band in B is due to the coordination of the ligand (DAPBSH) with the Cu2+ ion. This is assigned to a Metal-to-Ligand Charge Transfer (MLCT).

Fig. 2

Luminescence spectra of DAPBSH and complex B in the solid state. Note DAPBH and complex A display no emission of luminescence in the solid state

Complexes A and B were dissolved in the DMF solution with a concentration of ca. 1.0 × 10−5 mol L−1, for the measurement of photoluminescence spectra in solution, and the emission spectra of the two complexes are shown in Fig. 3. The emission spectrum of A shows an emission band with a maximum at 440 nm (Excitation: 375 nm); the emission peak of the ligand DAPBH is in the range 410–465 nm (Excitation: 359 nm). Compared to complex A, complex B emits a more intense emission with a maximum at 440 nm (optimal excitation at 375 nm). The ligand DAPBSH shows an emission band at 415 nm on being excited at 305 nm.

Fig. 3

a Luminescence spectra of DAPBH and complex A in DMF solution. b Luminescence spectra of DAPBSH and complex B in DMF solution

3.3 Magnetic susceptibility measurements

Magnetic moments were estimated by the equation ueff = 2.828 (χAT)1/2, where A is the magnetic susceptibility per copper. Magnetic susceptibility measurements display that complexes A and B have magnetic moments of 1.58 B. M and 1.65 B. M respectively. It lies within the range reported for tetrahedral and distorted octahedral structures [23]. Magnetic moment 1.65 B. M, is anticipated for magnetically non-coupled copper(II) ion [24]. Magnetic values are found to be low, this is either due to antiferromagnetic effect, with spin-orbital coupling in the ground state for spin doublet species [25] or due to the dimeric nature of the complexes. In such cases, antiferromagnetic interaction between the Cu(II) centers might occur.

3.4 Thermal analysis

Thermo gravimetric analysis (TGA) and differential thermal analysis (DTA) techniquesare used to determine the decomposition nature of the complex. Thermogram of Cu(II) complexes A and B is shown in Fig. 4 and thermo gravimetric data is listed in Table 1. The heating rates were suitably controlled at 10 °C min−1 under nitrogen atmosphere and the weight loss was calibrated from the 30 to 1000 °C. Thermogramof Cu(II) complex revealed a total weight loss of 65% up to 1000 °C. This weight loss occurred in three stages, (i) an insignificant weight loss was observed at 102 °C due to loss of lattice water, (ii) maximum weight loss in the range of 356–380 °C is corresponded to the loss of coordinated water (iii) and gradual weight loss in the range of 600–623 °C can be associated to complete decomposition of ligand around the metal ion [26] respectively. Finally the complex is transformed into its metal oxide. The existence of water molecules is further confirmed by the endothermic curve observed in respective DTA curve in the temperature region where TGA curves indicate.

Fig. 4

a TG-DTG curve of complex A, b TG-DTG curve of complex B

Table 1

Thermo gravimetric analysis data of copper(II) complexes

Complex

Temperature

% Loss

Fragment lost

Nature of water molecules

Complex A

102

5.2

9H2O

Lattice water

356

14.4

H2O

Coordinated water

600

12.3

H2O

Decomposition of ligand

Complex B

102

5.8

9H2O

Lattice water

380

12.4

H2O

Coordinated water

623

10.3

H2O

Decomposition of ligand

3.5 Infrared spectra

The IR spectra give information about the nature of functional group attached to the metal ions. In order to investigate the bonding mode of Schiff base to the metal complexes, the main IR spectra data and their assignments are listed in Table 2. The IR spectra of the Schiff base ligands display various bands in the hydroxyl stretching frequency region. On coordination of the Schiff base ligand, the C=N stretching frequency are slightly shifted to lower frequencies denoting a reduction in the C=N bond order. This is mainly due to the coordinate bond of copper with the azomethine nitrogen pair [27]. The (C=N) band of the quinoxaline ring appears in the region 1544–1562 cm−1 in the ligand, it undergoes a minor shift in the spectra of complexes A and B denoting the involvement of ring nitrogen atom in coordination to the copper [28]. The presence of broad band is seen in the spectra of complexes A and B in the 3385–3400 cm−1 region, which is corresponded with the solvent water molecules [27]. Weak bands at ~ 3000 cm−1 in both complexes could be assigned to aromatic C–H stretching of quinoxaline ring. The bands due to Cu–N and Cu–O stretching occur in the region ~ 450 and 550 cm−1 respectively.

Table 2

FTIR spectral data for the copper(II) complexes. (ν in cm−1)

Complex

ν (C=N)a

ν (C=19)q

ν (OH)

ν (Cu–N)

ν (Cu–O)

Complex A

1635

1565

3400

456

550

Complex B

1618

1550

3385

444

524

3.6 SEM micrographs

The metal coordination to ligand considerably modifies the surface morphology of the complexes and this was studied by SEM analysis. The SEM micrograph of metal complexes are shown in Fig. 5. There is no significant differences are seen in surface morphology of the metal complexes. The SEM micrograph of complex A displays agglomerated morphology with small sized grains dispersed in homogenous matrix and complex B exhibits that small sized particles powdered together to give rock-like structure.

Fig. 5

a The SEM micrograph of complex A, b The SEM micrograph of complex B

4 Conclusion

Complexes of Cu(II) ions comprising two Schiff-base ligands were prepared and characterized. The surface morphologies of complexs were examined. The complexes have a molar ratio of ligand: transition metal of 1:1. Complex B emits slight luminescence in the solid state under 365 nm excitation from an ultraviolet lamp. Complexes A and B show strong luminescence intensity in organic solvents. Therefore, Cu(II) complexes with Schiff bases could be a potential candidate in organic light emitting diodes. The bonding of ligand to metal ion is confirmed by spectral studies (UV–Vis, FT-IR) and TGA/DTA measurements. Magnetic susceptibility measurements confirm that an antiferromagnetic exchange occurs in complex A and B.

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Copyright information

© Springer Science+Business Media, LLC, part of Springer Nature 2018

Authors and Affiliations

  • P. Dhanakodi
    • 1
  • M. Jayandran
    • 2
  • V. Balasubramanian
    • 1
  1. 1.Department of ChemistryAMET UniversityChennaiIndia
  2. 2.Department of ChemistryGovernment Arts CollegeUdumalpetIndia

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