3.1 Synthesis
The scheme of synthesis of Schiff base ligand and metal complex are shown in Fig. 1. In the first step Schiff base ligand 1 was obtained by condensation of salicylaldehyde with L-histidine. The needed metal salts are added to the Schiff base ligand solution. Finally, ligand 1, 10 phenanthroline was added. The resultant product 2 was purified by recrystallization. Different spectroscopic techniques were used to confirm the structure of the synthesized compounds. All the complexes were soluble in DMF and DMSO and were non-hygroscopic. The CHNS analysis data were in good agreement with calculated values which revealed that the formula of the complexes are as expected. The molar conductivity of the complexes was determined using DMSO solvent (10− 3 M) and summarized in Table 1. The non-electrolytic nature of the complexes were indicated by very low values.
Table 1
The Colour, yield, melting point and molar conductance of the Schiff base transition metal complexes
Complex | Molecular Formula | Yield (%) | Colour | Melting point (oC) | Molar conductance (Ohm− 1 cm− 2 mol− 1) | Elemental analysis (%) Found/(Cal.) |
| C | H | N |
CoL1L2 | C25H19O3N5Co | 72.98 | Dark brown | 265.62 | 2.0 | 60.83 (60.49) | 4.01 (3.86) | 13.75 (14.11) |
CuL1L2 | C25H19O3N5Cu | 76.08 | Dark green | 239.93 | 4.4 | 60.05 (59.93) | 3.31 (3.82) | 13.90 (13.98) |
ZnL1L2 | C25H19O3N5Zn | 77.04 | Pale yellow | 257.35 | 2.0 | 59.49 (59.72) | 3.67 (3.81) | 13.34 (13.93) |
CdL1L2 | C25H19O3N5Cd | 76.22 | Pale brown | 270.00 | 4.3 | 53.98 (54.61) | 5.33 (4.80) | 13.05 (12.74) |
3.2 FT-IR
FT-IR spectra of the synthesized complexes are shown in figure S1, and their corresponding data displayed in Table 2. An intense band corresponding to imine group coordinate with metal ions appeared in the range of 1597 − 1586 cm− 1 [25]. Bands appeared in the range of 1519–1592 cm− 1, [26] assigned to symmetric stretching and bands in the range of 1321–1342 cm− 1 assigned to asymmetric stretching of carboxylate anion which are coordinated to the central metal ion. The disparity between the asymmetric and symmetric stretching frequencies Δυ = [υasCOO− − υsCOO−] was found the higher than the free COO− anion (185 cm− 1). From this the monodentate coordination of the COO− anion to the metal ion was established [27]. IR data of the complexes revealed that the Schiff base ligand coordinated to the metal ion through the imine nitrogen, phenolic oxygen and oxygen atom exist in the carboxylate anion and acted as a tridentate ligand.
Table 2
FTIR data of the synthesized transition metal complexes
Complexes | C = N (cm− 1) | COO− (cm− 1) | Δυ = [υas −υs] (cm− 1) | M-N (cm− 1) | M-O (cm− 1) |
(υas) | (υs) |
CoL1L2 | 1593 | 1416 | 1338 | 220 | 554 | 463 |
CuL1L2 | 1597 | 1406 | 1341 | 210 | 540 | 430 |
ZnL1L2 | 1586 | 1401 | 1343 | 177 | 534 | 498 |
CdL1L2 | 1586 | 1402 | 1342 | 215 | 553 | 418 |
3.3 1H NMR and 13C NMR Spectra
The 1H NMR spectrum of the Cd(II) complex (figure S2) recorded in DMSO-d6 showed characteristic peaks appeared in the aliphatic region at 2.509 ppm (s, H) for the > CH- proton and signals appeared at 3.134 ppm and 3.202 ppm (dd, 2H) corresponds to -CH2- proton. Azomethine (CH = N-) proton peak detected at 4.023 ppm (s, H). The signals of aromatic protons appeared as multiplet in the range of 6.701 ppm to 7.751 ppm. Singlet at 11.062 ppm corresponds to -NH- proton of histidine moiety.
The 13C NMR spectrum of the Cd(II) complex portrayed in figure S3, the distinct peaks appeared in the downfield at 172.51 ppm assigned to carboxylate anion carbon and peak at 171.51 ppm corresponds to azomethine carbon. The signal of phenolic carbon appeared at 162.61 ppm and the signals allocated in the region from 107.94 to 136.76 ppm caused by aromatic carbons. The peaks appeared at 53.91 ppm and 26.91 ppm due to > CH- and > CH2 groups present in histidine.
3.4 Mass spectra
Molecular weight of the synthesized complexes was calculated by HR Mass Spectrometer, EI mass 70 eV. The spectrum of all the complexes is presented in figure S4-S7. The molecular ion peak [M+] of the complexes was found at m/z 496 (8%), 500 (5%), 501 (6%), 551 (8%) corresponds to the [CoL1L2], [CuL1L2], [ZnL1L2] and [CdL1L2] respectively in the mass spectrum.
3.5 Electronic spectral studies
The optical properties of the Schiff base metal complexes were investigated by UV-Visible absorption spectroscopy in 10− 3 M DMSO solution (figure S8). All the prepared Schiff base metal complexes possess two discrete absorption bands. The high intensity peak appear in the region of 285–320 nm were allocated to π-π* and n- π* transitions due to azomethine group and aromatic chromophores [28]. The low intensity/absorption minima of the complex observed in the region of 340–420 nm corresponds to Metal-to-Ligand Charge Transfer (MLCT) transitions [29]. in the complex. There was no d-d absorption in the diamagnetic Zinc (II) complex [15]. Electron transfer from metal d-orbital to filled π-orbital of the ligand has been designated as M(dπ)-to-L(π*). These important strong absorption bands of both charge transfer and M-L transition support the pioneering idea of a square pyramidal habitat for the ligand and five coordination with the metal.
3.6 PXRD analysis
In order to check the crystalline nature of the synthesized complex, powder XRD was recorded between 2 theta from 10o to 90o. Powder X-ray diffraction analysis of Schiff base metal complexes shown in figure S9, Co(II) complex displayed well-defined sharp peaks while other complexes are not. Co(II) complexes are highly crystalline in nature, while Cu(II) and Zn(II) complexes exhibited broad peak and no identifiable peak is observed for Cd(II) complex. The average crystallite size of CoL1L2, CuL1L2, and ZnL1L2 complexes are135 nm, and 145 nm respectively.
3.7 Morphology Study
The surface morphology and particle size of the synthesized complexes are investigated using a scanning electron microscope (SEM). Figure 2 displays SEM image of complexes. The SEM images of the synthesized complexes unveil homogeneous spongy like structure. Schiff base cobalt metal complex [Figure 2 (a)] appeared as aggregated grains like structure. SEM micrograph of the copper and zinc complex [Figure 2 (b) and 2 (c)] indicates a homogeneous environment with aggregation. Cadmium complex [Figure 2 (d)] shows small sized grains with agglomeration and appeared as coral-rag-like structure. The layers in each complex micrograph demonstrate that the system contains atoms in a well-defined pattern, indicating that the reactants have completely reacted to form a distinct homogenous product. The particles in the complexes are only a few microns in size. Particles smaller than 100 nm were also found, which clumped together to form bigger agglomerates.
3.8 Electrochemical studies
The cyclic voltammetry (CV) measurement is a primary characterization technique to identify the redox behavior of synthesized complexes. Cyclic voltammetry analysis was performed in dimethyl sulfoxide solution (DMSO) using 0.1 M TABPF6 as supporting electrolyte, glassy carbon, Pt and Ag/AgCl as working electrode, counter electrode, and reference electrode respectively with the voltage scan rate of 50 mV/s. The resultant cyclic voltammograms of the complexes 1–4 are displayed in figure S10 and the electrochemical data are tabulated in Table 3.
Table 3
CV analysis of Schiff base metal complexes
Comlpex | EHOMO(eV)a | ELUMO(eV)b | Eg (eV)c | Egoptical (eV)d |
CoL1L2 | -5.56 | -2.38 | 3.18 | 2.75 |
CuL1L2 | -5.32 | -2.61 | 2.73 | 2.69 |
ZnL1L2 | -5.31 | -2.21 | 3.10 | 2.18 |
CdL1L2 | -5.34 | -2.59 | 2.74 | 2.75 |
aValues of HOMO level found from CV using EHOMO = -e (Eox + 4.4).
bValues of LUMO level found from CV using ELUMO = Eoptical + EHOMO.
Eoptical onset values obtained from oxidation peak in cyclic voltagram.
cBand gap calculated using Eg = EHOMO - ELUMO
dOptical band gap calculated from onset/edge of absorption using the equation e (Eoptical) = 1240/λonset.
Eoptical onset values of oxidation peak in CV.
The electrochemical reaction of complexes showed two oxidation and two reduction peak potential. The oxidation potential of the complex 1 (0.732, 1.166 eV), complex 2 (-0.664, 0.904 eV), complex 3 (-0.621, 0.88 eV), and complex 4 (-0.53, 0.94 eV) occur at the anodic half cycle. The reduction potential of the complex 1 (0.615, 1.119 eV), complex 2 (-1.043, 1.087 eV), complex 3 (-1.109, 0.89 eV), and complex 4 (-1.11, 0.916 eV) occur at the cathodic half cycle in the scan rate of 50 mV/s. Each reduction is associated to quasi-reversible one electron transfer process at room temperature [30]. The first redox couple peak separation (Ep) value is higher than the second redox couple of all the complexes. From this higher Ep value of the complexes, it is observed that there is a difference between the original complex and the reduced species [31].
3.9 Thermal properties
Thermal stability of synthesized Cu(II), Co(II) and Zn(II) complexes was carried out by Thermogravimetric (TGA) and Differential Thermo analysis (DTA) in nitrogen atmosphere with heating rate from 25 oC to 800 oC. Figure S11 and Table 4 depicts the results of TGA and DTA. Thermogram of all the metal complexes showed three weight loss, indicating that decomposition of complexes starts from removal of small molecule (H2O, CO2) and then organic moiety (ligand) and finally formation of metal oxide with increase in temperature. The first weight loss occurs in the range of 45 oC − 140 oC indicate the elimination of water molecule from the lattice. Second weight loss occurs in the range of 212 oC – 345 oC attribute to removal of organic (ligand) moiety and the final weight loss in the range of 360–800 oC represent complete decomposition of metal ligand bond to form a corresponding metal oxide [32]. When compared to other transition metal complexes, the thermal stability of Co (II) complex was found to be higher (large proportion of residue).
Table 4
Thermal analysis of Schiff base metal complexes
Complexes | Tg (oC) | Td/m (oC) | % of residue | Endothermic peak |
CoL1L2 | 82.5 | 265.62 | 56.97 | 510.63 |
CuL1L2 | 79.25 | 239.93 | 44.92 | 489.43 |
ZnL1L2 | 80.81 | 257.35 | 54.25 | 558.10 |