1 Introduction

Schiff bases played a critical part in the development of coordination chemistry. Schiff base metal complexes have been researched widely because of their attractive chemical and physical properties in addition to their wide range of uses in numerous scientific areas [1]. Schiff bases have biological activity as anticancer and antibacterial where they characterized by an imine group –N=CH–, which helps to explain the mechanism of transamination and racemization reaction in biological system [2]. Imine complexes are immersed in the treatment of cancer drug resistance, and frequently tested as antimalarials. It also could be used for the immobilization of enzymes [3]. Additionally, Schiff base coordination complexes are still of great importance because of their simple preparation, diverse chemical structures, wide applications such as biological modeling, catalysis, molecular magnets and material chemistry [4]. The published results have shown that these compounds possess effective role against bacteria and fungi which may be due to hetero atoms and/or azomethine linkage present in these compounds [5].

Transition metal complexes derived from Schiff bases had occupied an essential role in the development of coordination chemistry. Schiff base ligands are able to stabilize them in different oxidation states and to coordinate with many various metals, enabling the use of Schiff base metal complexes for a large range of useful catalytic transformations [6]. The bifunctional carbonyl compound; 4,6-diacetylresorcinol (DAR) serves as precursor for the building of tridentate ligands. Over the past few years, 4,6-diacetyl-resorcinol had also received much attention towards the biological applications as well as the coordination chemistry [7, 8]. 1,8-DAN is primarily used as intermediates in the production of solvent dyes, such as C.I. solvent red 135, C.I. solvent orange 60 [https://issuu.com/nehapatilozone/docs/global_2.docx].

This work intentions to synthesize novel Schiff base ligand (H2L) through the condensation of 4,6-diacetylresorcinol and 1,8-naphthalenediamine (H2L). The metal complexes were prepared using 1:1 (M: L) molar ratio with metal ions like Cr(III), Mn(II), Fe(III), Co(II), Ni(II), Cu(II), Zn(II) and Cd(II). The structures of the ligand and its metal complexes were characterized by elemental analysis, FT-IR, 1H NMR, molar conductance measurements, electronic spectral study and thermal analysis. Docking studies were performed using MOE 2008 and its severe molecular docking software. The Schiff base ligand and its Zn(II) complex were completely optimized employing DFT based B3LYP method along with the LANL2DZ basis set. In order to incorporate the effect of the solvent around the molecule, the TD-DFT method (along with LANL2DZ: basic set) was employed. Anticancer and antibacterial abilities have evaluated [8].

2 Experimental

2.1 Materials and Reagents

All chemicals used were of the analytical reagent grade (AR) and of the highest purity accessible. The chemicals used involved 1,8-naphthalenediamine which was provided from Strem Chemicals Inc., 4,6-diacetyl resorcinol, CrCl3·6H2O, MnCl2·2H2O and FeCl3·6H2O (Sigma-Aldrich), NiCl2·6H2O, CoCl2·6H2O, CuCl2·2H2O and ZnCl2 (BDH) and CdCl2 (Merck). Organic solvents consumed were ethyl alcohol (95%), methyl alcohol and N,N-dimethylformamide (DMF). Bidistilled water was usually getting through all preparations.

2.2 Solutions

1 × 10–3 M Stock solutions of the Schiff base ligand (H2L) and also its metal complexes were prepared by dissolving an exactly weighed amount in DMF solvent. The conductivity then measured for the metal complexes solutions. Dilute solutions of the Schiff base ligand and its metal complexes (1 × 10–4 and 1 × 10–5 M) were prepared by accurate dilution from the previous prepared stock solutions for measuring their UV–vis spectra.

2.3 Solution of Anticancer Study

A fresh stock solution (1 × 10–3 M) of Schiff base ligand (0.12 × 10–2 g L−1) was prepared in the suitable volume of DMF (90%). DMSO was used in cryopreservation of cells. RPMI-1640 medium was used. The medium was used for culturing and keep of the human tumor cell line. The medium was contributed in a powder form. It was gotten ready as follows: 10.4 g of medium was weighed, mixed with 2 g of sodium bicarbonate, completed to 1 l with distilled water and shaken prudently until complete dissolution. The medium was then uncontaminated by filtration in a Millipore bacterial filter (0.22 mL). The prepared medium was kept in a refrigerator (4 °C) and checked at methodical intervals for contamination. Before use, the medium was warmed at 37 °C in a water bath and supplemented with penicillin–streptomycin and FBS. Sodium bicarbonate was used for the preparation of RPMI-1640 medium. Isotonic trypan blue solution (0.05%) was prepared in normal saline and was used for feasibility counting. FBS (10%, heat inactivated at 56 °C for 30 min), 100 units/mL penicillin and 2 mg/mL streptomycin were used for the supplementation of RPMI-1640 medium prior to use. Trypsin (0.25 × 10–1% w/v) was used for the picking of cells. Acetic acid (1% v/v) was used for dissolving unbound SRB dye. SRB (0.40%) dissolved in 1% acetic acid was used as a protein dye. A stock solution of trichloroacetic acid (50%) was prepared and stored. An amount of 50 μL of the stock was added to 200 μL of RPMI-1640 medium per well to yield a final concentration of 10% used for protein precipitation. Isopropanol (100%) and ethanol (70%) were used. Tris base (10 mM; pH 10.50) was used for SRB dye solubilization. Tris base (121.10 g) was dissolved in 1000 ml of distilled water and the pH was attuned using hydrochloric acid (2 M).

2.4 Instrumentation

Microanalyses of carbon, hydrogen and nitrogen were carried out at the Microanalytical Center, Cairo University, Egypt, using a CHNS-932 (LECO) Vario elemental analyzer. Analyses of the metals were controlled by dissolving the solid complexes in concentrated HNO3, and dissolving the remainder in deionized water. The metal content was carried out using inductively coupled plasma atomic absorption spectrometry (ICP-AES), Egyptian Petroleum Research Institute. Fourier transform infrared (FT-IR) spectra were recorded with a Perkin Elmer 1650 spectrophotometer (400–4000 cm−1) as KBr pellets. 1H NMR spectra, as solutions in DMSO-d6, were chronicled with a 300 MHz Varian-Oxford Mercury at room temperature using tetra-methylsilane as an interior standard. Mass spectra were chronicled using the electron ionization technique at 70 eV with an MS-5988 GS-MS Hewlett-Packard instrument at the Microanalytical Center, National Center for Research, Egypt. UV–vis spectra were acquired with a Shimadzu UVmini-1240 spectrophotometer. Molar conductivities of 10–3 M solutions of the solid complexes in DMF solvent were measured with a Jenway 4010 conductivity meter. Thermogravimetric (TG) and differential thermogravimetric (DTG) analyses of the solid complexes were carried out from room temperature to 1000 °C using a Shimadzu TG-50H thermal analyzer. Antimicrobial measurements were carried out at the Microanalytical Center, Cairo University, Egypt. Anticancer activity experiments were performed at the National Cancer Institute, Cancer Biology Department, Pharmacology Department, Cairo University. The optical density (OD) of each in good form was measured spectrophotometrically at 564 nm with an ELIZA microplate reader (Meter tech. R960, USA) [8, 16].

2.5 Synthesis of Schiff Base Ligand

The Novel Schiff base ligand (H2L) was synthesized via condensation of 4,6-diacetylresorcinol (4,6-DAR) with 1,8-naphthalenediamine. A solution of 1,8-naphth-alenediamine (15.2 mmol, 2.4 g) dissolved in ethanol was added dropwise to 4,6-DAR (15.4 mmol, 3 g) dissolved in DMF. The resulting mixture was stirred in reflux for about 4–5 h, through which brown solid compound was separated. It was filtered, washed, recrystallized from diethyl ether and dried in vacuum (Scheme 1).

Scheme 1
scheme 1

Synthesis of Novel Schiff base ligand (H2L)

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Yield 89%; m.p. 96 °C; brown solid. Anal. Calcd for C20H18N2O3 (%): C, 71.72; H, 5.38; N, 8.36. Found (%): C, 71.59; H, 5.26; N, 8.30, FT-IR (cm−1): phenolic ν(OH) 3402, carbonyl ν(C=O) 1637, azomethine ν(C=N) 1590, (NH2) bending 661. 1H NMR (300 MHz, DMSO-d6, δ, ppm): 7.05–6.39 (m, 6H, Ar–H(naph)), 8.4 (s,2H (benzene)), 12.22 (s, H, OHa), 9.83 (s, H, OHb). 5.45 (s,2H, NH2) 2.65 (s, 6 H (methyl)). λmax (nm): 233 and 276 (π–π*) and 324 (n–π*).

2.6 Synthesis of Metal Complexes

Complexes of H2L with Cr(III), Mn(II), Fe(III), Co(II), Ni(II), Cu(II), Zn(II) and Cd(II) metal ions were prepared through reaction of 1:1 molar mixture of warm ethanolic solution (60 °C) of the suitable metal chloride (0.90 mmol) and H2L (0.3 g, 0.90 mmol). The resulting mixture was stirred in reflux for 1 h at which the complexes precipitated. They were accumulated by filtration and purified by washing numerous times with diethyl ether.

2.6.1 [Cr(H2L)(H2O) Cl2]Cl·6H2O

Yield 87%; black solid, m.p. 198 °C. Anal. Calcd for Cr(C20H32Cl3N2O10) (%): C, 38.78; H, 5.17; N, 4.52; Cr, 8.40. Found (%): C, 38.18; H, 5.13; N, 4.42; Cr, 8.34. FT-IR (cm−1): phenolic ν(OH) 3430, carbonyl ν(C=O) 1638, azomethine ν(C=N) 1596, ν(H2O) stretching bands of coordinated water 955 and 893, NH2 bending 695, ν(M–O) 566, ν(M–N) 450. λmax (nm): 227 and 277 (π–π*) and 316 (n–π*). µeff 3.98 B.M., Λm 57.80 Ω−1 mol−1 cm2.

2.6.2 [Mn(H2L)(H2O)2Cl]Cl·4H2O

Yield 80%; dark brown solid, m.p. > 300 °C. Anal. Calcd for Mn (C20H30Cl2N2O9) (%): C, 42.22; H, 5.29; N, 4.93; Mn, 9.68. Found (%): C, 42.16; H, 5.16; N, 4.83; Mn, 9.30. FT-IR (cm−1): phenolic ν(OH) 3498, carbonyl ν(C=O) 1644, azomethine ν(C=N) 1600, ν(H2O) stretching bands of coordinated water 953 and 880 NH2 bending 668, ν(M–O) 567, ν(M–N) 482. λmax (nm): 226 and 273 ( π–π*) and 323 (n–π*). µeff 5.64 B.M., Λm 60.10 Ω−1 mol−1 cm2.

2.6.3 [Fe(H2L)(H2O)3]Cl3·H2O

Yield 83%; brown solid, m.p. 188 °C. Anal. Calcd for Fe(C20H26Cl3N2O7) (%): C, 42.19; H, 4.57; N, 4.92; Fe, 9.84. Found (%): C, 41.89; H, 4.40; N, 4.69; Fe, 9.84. FT-IR (cm−1): phenolic ν(OH) 3394, carbonyl ν(C=O) 1639, azomethine ν(C=N) 1560, ν(H2O) stretching bands of coordinated water 958 and 890, NH2 bending 697, ν(M–O) 506, ν(M–N) 454. λmax (nm): 223 (π–π*). µeff 5.33 B.M., Λm 139.5 Ω−1 mol−1 cm2.

2.6.4 [Co(H2L)(H2O)2Cl]Cl·4H2O

Yield 86%; black solid, m.p. > 300 °C. Anal. Calcd for Co(C20H30Cl2N2O9) (%): C, 41.93; H, 5.24; N, 4.89; Co, 10.30. Found (%): C, 41.93; H, 4.96; N, 4.87; Co, 10.23 FT-IR (cm−1): phenolic ν(OH) 3752, carbonyl ν(C=O) 1631, azomethine ν(C=N) 1580, ν(H2O) stretching bands of coordinated water 955 and 829, NH2 bending 620, ν(M–O) 577, ν(M–N) 468. λmax (nm): 226(π–π*). µeff 4.89 B.M., Λm 73.8Ω−1 mol−1 cm2.

2.6.5 [Ni(H2L)(H2O)2Cl]Cl·6H2O

Yield 80%; black solid, m.p. > 300 °C. Anal. Calcd for Ni(C20H34Cl2N2O11) (%): C, 39.45; H, 5.59; N, 4.60; Ni, 9.70. Found (%): C, 38.95; H, 5.37; N, 4.45; Ni, 9.61. FT-IR (cm−1): phenolic ν(OH) 3383, carbonyl ν(C=O) 1646, azomethine ν(C=N) 1598, ν(H2O) stretching bands of coordinated water 955 and 894, NH2 bending 654, ν(M–O) 572, ν(M–N) 459. λmax (nm): 227 (π–π*). µeff 3.11 B.M., Λm 56.60 Ω−1 mol−1 cm2.

2.6.6 [Cu(H2L)(H2O)2Cl]Cl·5H2O

Yield 84%; yellowish brown solid, m.p. 271 °C. Anal. Calcd for Cu (C20H32Cl2N2O10) (%): C, 40.34; H, 5.38; N, 4.71; Cu, 10.68. Found (%): C, 40.04; H, 5.31; N, 4.63; Cu, 10.48. FT-IR (cm−1): phenolic ν(OH) 3433, carbonyl ν(C=O) 1634, azomethine ν(C=N) 1562, ν(H2O) stretching bands of coordinated water 940 and 900, NH2 bending 654, ν(M–O) 593, ν(M–N) 460. λmax (nm): 255 (π–π*). µeff 2.09 B.M., Λm 52.00 Ω−1 mol−1 cm2.

2.6.7 [Zn(H2L)(H2O) Cl2]3H2O

Yield 80%; black solid, m.p. 200 °C. Anal. Calcd for Zn (C20H26Cl2N2O7) (%): C, 44.25; H, 4.80; N, 5.16; Zn, 11.98. Found (%): C, 44.15; H, 4.59; N, 4.97; Zn, 11.83. FT-IR (cm−1): phenolic ν(OH) 3445, carbonyl ν(C=O) 1631, azomethine ν(C=N) 1591, ν(H2O) stretching bands of coordinated water 956 and 894, NH2 bending 658, ν(M–O) 547, ν(M–N) 488. 1H NMR (300 MHz, DMSO-d6, δ, ppm): 7.12–6.23 (m, 6H, Ar–H(naph)) 8.40 (s,1H (benzene)) 7.95 (s,1H (benzene)) 12.23 (s, H, OHa) 10.24 (s, H, OHb) 5.38 (s, 2H, NH2) 2.89 (s, 6 H (methyl(). λmax (nm): 226 (π–π*), 324 (n–π*). µeff diamagnetic, Λm 15.50 Ω−1 mol−1 cm2.

2.6.8 [Cd(H2L)(H2O) Cl2]

Yield 85%; reddish brown solid, m.p. 186 °C. Anal. Calcd for Cd (C20H20Cl2N2O4) (%): C, 44.71; H, 3.73; N, 5.23; Cd, 20.98. Found (%): C, 44.23; H, 3.51; N, 5.11; Cd, 20.33.FT-IR (cm−1): phenolic ν(OH) 3376, carbonyl ν(C=O) 1639, azomethine ν(C=N) 1595, ν(H2O) stretching bands of coordinated water 955 and 891, NH2 bending 653, ν(M–O) 587, ν(M–N) 480. 1H NMR (300 MHz, DMSO-d6, δ, ppm): 7.15–6.24 (m, 6H, Ar–H(naph)), 8.41 (s,1H (benzene)), 7.80 (s,1H (benzene)), 12.21 (s, 1H, OHa), 9.92 (s, H, OHb), 5.40 (s, 2H, NH2), 2.88 (s, 6 H (methyl)). λmax (nm): 227 and 277 (π–π*), 316 (n–π*). µeff diamagnetic, Λm 1.00 Ω−1 mol−1 cm2.

2.7 Spectrophotometric Studies

The absorption spectra were recorded for 1 × 10–4 and 1 × 10–5 M solutions of the Schiff base ligand and its metal complexes in DMF solvent, except for Cu(II) complex that had a concentration of 10–4 M. The spectra were scanned within the wavelength range from 200 to 700 nm [9].

2.8 Pharmacology

2.8.1 Antibacterial Activities

Antimicrobial activity of the tested samples was determined by a modified Kirby-Bauer disc diffusion method [10]. Briefly, 100 µl of the test bacteria were grown in 10 ml of fresh media until they reached a count of approximately 108 cells/ml for bacteria [11]. 100 µl of microbial suspension was spread onto agar plates corresponding to the broth in which they were maintained. Isolated colonies of each organism that may be playing a pathogenic part should be selected from primary agar plates and tested for susceptibility by disc diffusion method [12]. Plates inoculated Gram (+) bacteria as Staphylococcus aureus and Bacillus subtilis; Gram (−) bacteria as Escherichia coli and Pseudomonas aeruginosa. They were incubated at 35–37 °C for 24–48 h and then the diameters of the inhibition zones were calculated in millimeters [10].

Standard discs of amikacin (Antibacterial agent), served as positive controls for antimicrobial activity but filter discs impregnated with 10 µl of solvent (distilled water, chloroform, DMSO) were used as a negative control. Blank paper disks (Schleicher & Schuell, Spain) with a diameter of 8.0 mm were impregnated 10 µ of tested concentration of the stock solutions (20 mg/ml). When a filter paper disc impregnated with a tested chemical is placed on agar the chemical will diffuse from the disc into the agar. This diffusion will place the chemical in the agar only around the disc. The solubility of the chemical and its molecular size will determine the size of the area of chemical infiltration around the disc. If an organism is placed on the agar it will not grow in the area around the disc if it is susceptible to the chemical. This area of no growth around the disc is known as a “Zone of inhibition” or “Clear zone”. For the disc diffusion, the zone diameters were measured with slipping calipers of the National Committee for Clinical Laboratory Standards. Agar-based methods such as E-test and disk diffusion can be good alternatives because they are easier and more rapidly than broth-based methods [13, 14].

2.8.2 Anticancer Activity

Potential cytotoxicity of the compounds was tested via the method of Skehan and Storeng [15]. Cells were plated in 96‐multiwell plate (104 cells/well) for 24 h before treatment using the compounds to let attachment of cell to the wall of the plate. Different concentrations of the compounds under investigation (0, 5, 12.5, 25, 50 and 100 μg/ml) were added to the cell monolayer and triplicate wells were prepared for each individual dose. The monolayer cells were keep warm with the compounds for 48 h at 37 °C and in 5% CO2 atmosphere. After 48 h, cells were fixed, washed and stained with SRB stain. Excess stain was washed by acetic acid and attached stain was recovered with tris‐EDTA buffer. The optical density (O.D.) of each well was measured spectrophoto-metrically at 564 nm with an ELIZA microplate reader, the mean background absorbance was automatically deducted and mean values of each drug concentration was calculated. The relation between drug concentration and surviving fraction is plotted to obtain the survival curve of breast tumor cell line for each compound [15, 16].

The percentage of cell survival was calculated as follows:

$$\mathrm{Survival fraction}=\frac{\mathrm{O}.\mathrm{D}. (\mathrm{treated cells})}{\mathrm{O}.\mathrm{D}.(\mathrm{control cells}).}$$

The IC50 values (the concentrations of the Schiff base ligand (H2L) or its metal complexes required to produce 50% inhibition of cell growth).

2.9 Computational Methodology

Gaussian09 suite of program was used for the electronic structure calculations of H2L and Zn(II) complex. DFT based B3LYP method along using the LANL2DZ basis set were employed for full optimization. In order to incorporate the effect of the solvent around the molecule, the TD-DFT method (along with LANL2DZ basic set) was used to estimate the electronic absorption spectra of the ligand and its zinc(II) complex. The contribution of molecular orbital on HOMO and LUMO were also calculated.

2.10 Molecular Docking

To find out the achievable binding modes of the best active compounds against Crystal structure of Staphylococcus aureus nucleoside diphosphate kinase complexed with ADP (PDB ID: 3Q8U), molecular docking studies were performed via MOE2008 software. It is rigid molecular docking software and is an interactive molecular graphics program for calculating and displaying feasible docking modes of a receptor and ligand and complex molecules. It necessitates the ligand and the receptor as input in PDB format. The amino acid chain was kept and the water molecules and co-crystallized ligands and chloride ions in out sphere were removed. The structure of ligand in PDB file format was created by Gaussian09 software. Crystal structure of Staphylococcus aureus nucleoside diphosphate kinase complexed with ADP (PDB ID: 3Q8U) was downloaded from the protein data bank (http://www.rcsb.org./pdb) [16].

3 Results and Discussion

3.1 Elemental Analysis

The synthesized Schiff base ligand (H2L), namely [1-(5-(1-((8-aminonaphthalen-1-yl)imino)ethyl)-2,4-dihydroxyphenyl)ethanone] was a brown solid and stable at room temperature. It was soluble in ethanol. The results obtained were in good agreement with those calculated for the suggested formula. The structure of the Schiff base ligand under study was shown in Fig. 1 which also illuminating the intra molecular hydrogen bonding.

Fig. 1
figure 1

Structure of Schiff base ligand (H2L) illustrating the intra molecular hydrogen bonding

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Metal complexes of H2L ligand were synthesized through the reaction of ethanolic solutions of Schiff base and each metal salt in 1:1 ratio and they had the composition of MH2L type. The results of experimental elemental analysis of H2L and its complexes were to a large degree in agreement with the theoretical calculations.

3.2 Mass Spectral Study

The full scan mass spectra of ligand, Fe(III), Cr(III) and Ni(II) complexes were shown in Fig. 2. The ligand mass spectrum exhibited a molecular ion peak at m/z = 334.36 amu equivalent to [M]+, which confirmed the proposed formula [C20H18N2O3]+. The intensity of these peaks gave an idea of calculated value at 334.39 amu.

Fig. 2
figure 2

The full‐scan mass spectra of a ligand and b Cr(III) complex c Ni(II) Complex

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The mass spectrum of Cr(III) complex revealed a molecular ion peak at m/z = 618.36 amu which was concurrent with the calculated weight of 618.39 amu. This result confirmed the stoichiometry of this complex as being of [MH2L] type. On the other hand, the peak of the parent ligand in the mass spectrum of the Cr(III) complex appeared at m/z 334.11 amu. The full scan mass spectra of ligand and Cr(III) complex were shown in Fig. 2.

The mass spectrum of Ni(II) complex revealed a molecular ion peak at m/z = 548.96 amu which was concurrent with the calculated molecular weight of 549.39 amu. This result confirmed the stoichiometry of this complex as being of [MH2L] type. On the other hand, the peak of the parent ligand in the mass spectrum of the Ni(II) complex appeared at m/z 334.67 amu.

The mass spectrum of Fe(III) complex revealed a molecular ion peak at m/z = 568.48 amu which was concurrent with the calculated molecular weight of 568.89 amu. This result showed expected m/z values for molecular ion peak and peaks of corresponding fragments of Fe(III) complex (Fig. 3).

Fig. 3
figure 3

Mass spectrum and fragmentation pattern of Fe(III) complex

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3.3 FT-IR Spectral Study

The characteristic infrared vibrational frequencies of the free ligand showed broad band at 3402 cm−1 attributed to the stretching frequency of ѵ(NH2) and/or the phenolic group; ѵ(OH), band at 1637 cm−1 attributed to the stretching frequency of keto carbonyl group, band at 1590 cm−1 ascribed to the stretching frequency of –C=N– and band at 661 cm−1 attributed to NH2 bending vibration [7, 18].

IR spectra of the complexes were recorded to confirm their structures. The vibrational frequencies and their tentative delegations for H2L ligand and its transition metal complexes can justify by comparison of the IR spectra of the metal complexes with those of the free ligand. The data revealed that all complexes showed broad bands in the range 3376–3452 cm−1 which may be recognized to the stretching frequency of ѵ(NH2) and/or the phenolic group; ѵ(OH), a water molecules related with the complexes which may be further confirmed by 1H NMR and thermal analyses [7, 17, 18].

The band attributable to ν(C=N) azomethine stretching vibration of ligand that appeared at 1590 cm–1, was shifted in complexes and appeared at 1560–1600 cm–1 which can attribute to coordination of azomethine nitrogen to metal ions [6, 19, 20]. The keto carbony1 group stretching frequency (1637 cm−1 in the free ligand) was slightly shifted due to an intramolecular hydrogen bond which was formed between phenolic group proton and oxygen of the keto carbonyl [21, 22]. The NH2 bending of the ligand appeared at 661 cm–1, which was shifted in complexes and appeared at 620–697 cm–1 due to coordination of amino nitrogen to metal ions [23,24,25].

Finally, the above interpretation was supported by the appearance of the new bands at 506–593 and 450–488 cm−1 assigned to ѵ(M–O) and ѵ(M–N), respectively [7, 26]. Supporting that the bonding of the ligand to the metal ions was achieved by coordination of metal ions with one of the phenolic oxygen, amino and azomethine nitrogen atoms of the ligand so it behaved as neutral tridentate. These data will be supported by 1H NMR analysis [27].

3.4 1H NMR Spectrum

1H NMR spectral data (δ ppm) of the ligand proportionate to TMS (0 ppm) in DMSO-d6. The signals observed at 9.83 and 12.22 ppm may be assigned to the phenolic OHa and OHb groups, respectively. The difference observed in these values may be due to estimated H-bonding linking the hydrogen atom of the –OH group and the keto carbonyl group. These signals vanished in an existence of D2O, indicating that these protons are acidic and the phenolic groups can participate in the coordination with the metal ions [7, 27, 28]. The signal of the -NH2 groups was found at 5.45 ppm and disappeared in presence of D2O [27, 29]. The spectrum of free ligand (H2L) exhibited a multiple signals in 7.05–6.39 ppm region which may be assigned to naphthalene protons (m, 6H) [28]. In the aromatic region, benzene shown as two singlet signals at 7.81 and 8.40 ppm [28].

1H NMR spectral data (δ ppm) of Zn(II) and Cd(II) complexes showed a down-field shift in the NH2 resonance from δ 5.45 (free ligand) to 5.38 ppm (Zn(II) complex) and to 5.40 ppm (Cd(II) complex) indicating its coordination to the Zn(II) and Cd(II) ions, respectively [29]. The phenolic –OHa group showed its proton signal at 10.24 and 9.92 ppm in the Zn(II) and Cd(II) complexes, respectively, (9.83 ppm for H2L ligand). These data confirmed that OHa participated in coordination process without proton displacement. The other phenolic –OHb group not coordinating and still appeared in the same position as shown in the ligand spectrum [30, 31]. The singlet signal at 2.65 ppm corresponds to CH3 protons of methyl moiety in the free Schiff base ligand was found at 2.88 and 2.89 for Cd(II) and Zn(II) complexes, respectively.

Table 1 Thermoanalytical results (TG and DTG) of Schiff base ligand (H2L) and its metal complexes
Full size table

3.5 Molar Conductance Measurements

The molar conductance values of the complexes were measured at room temperature. These values showed that Zn(II) and Cd(II) complexes have no conductance and they were considered as nonelectrolytes. Cu(II), Cr(III), Mn(II), Co(II) and Ni(II) complexes had molar conductivity values of 52.0, 58.0, 60.0, 74.0 and 57.0 Ω−1 mol−1 cm2, respectively. These data suggest that these complexes are ionic in nature and they were electrolytes. The Fe(III) complex had a molar conductivity value of 139.5 Ω−1 mol−1 cm2, which suggested that this complex was ionic and electrolytic in nature in nature [9].

3.6 Electronic Spectral Study

It is possible to draw up the electronic transitions and foretell the geometry with the support of magnetic moments of most metal ions. The main absorption bands of the ligand were at 233, 276 and 324 nm and they were ascribed to π → π* and n → π* transitions within the aromatic rings in the ligand, respectively. The bands in all metal complexes located at 223–277 nm and 316–324 nm region were ascribed for π → π* and n → π* transitions, respectively, which can be related to the binding of these coordination centers to the central metal ions. The UV–vis spectra displayed bands at 537 nm which can be attributed to d–d transition in the Cr(III)complex [32, 33].

The electronic spectrum of Cr(III) complex showed three spin allowed bands at 18,150, 24,010 and 26,150 cm−1. These bands may be assigned to 4A2g(F) → 4 T2g(F), 4A2g(F) → 4T1g(F) and 4A2g(F) → 4T1g(P) transitions implying the octahedral geometry of the complex. The magnetic moment was found to be 3.98 B.M. which approved octahedral geometry of Cr(III) complex [34].

The Mn(II) complex showed three spin allowed bands at 17,280, 21,130 and 25,950 cm−1. These bands may be assigned to 4T1g → 6A1g, 4T2g(G) → 6A1g and 4T1g(D) → 6A1g transitions implying the octahedral geometry of the complex. The measured magnetic moment was 5.64 B.M. which approved octahedral geometry of Mn(II) complex [9].

The Fe(III) complex showed three spin allowed bands at 22,460, 20,190 and 16,565 cm−1. These bands may be assigned to 4T2g(G) → 6A1g, 4T2g(G) → 6A1g and 4T1g(D) → 6A1g transitions implying the octahedral geometry of the complex. The magnetic moment was 5.33 B.M. which approved octahedral geometry of Fe(III) complex [9, 35].

The Co(II) complex showed three spin allowed bands at 22,950, 20,110 and 16,245 cm−1. These bands may be assigned to 4T1g(F) → 4T2g(F), 4T1g(F) → 4A2g(F) and 4T1g(F) → 4T2g(P) transitions implying the octahedral geometry of the complex. The magnetic moment was calculated and found to be 4.89 B.M. which confirmed octahedral geometry of Co(II) complex [35].

The Ni(II) complex showed three spin allowed bands at 28,100, 22,185 and 11,970 cm−1. These bands may be assigned to 3A2g → 3T1g(P), 3T1g(P) → 3A2g and 3A2g (F) → 3T1g (F) transitions implying the octahedral geometry of the complex. The calculated magnetic moment was found to be 3.11 B.M. which pointed out the presence of octahedral Ni(II) complex [34, 35].

The Cu(II) complex showed three spin allowed bands at 21,016, 17,258 and 14,181 cm−1 which can be assigned to 2B1g → 2A1g, 2B1g → 2Eg and 2B1g → 2B2g transitions, respectively, implying the octahedral geometry of the complex. The complex was found to has magnetic moment value of 2.09 B.M. which support its octahedral geometry [35, 36].

The Zn(II) and Cd(II) complexes were diamagnetic. According to the empirical formulae and an octahedral geometry for the Zn(II) and Cd(II) complexes was suggested [35].

3.7 Thermal Analysis

TG and DTG results of H2L and its metal complexes at a heating rate of 10 °C/min in nitrogen atmosphere over the range from ambient temperature to 1000 °C were summarized in Table 1 [39].

Table 2 The different Mulliken charges of H2L and [Zn(H2L)(H2O) Cl2]3H2O complex
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The Schiff base ligand with the molecular formula (C20H18N2O3) was thermally decomposed in two successive decomposition steps within the range from 30 to 900 °C with total mass loss of 99.51% (calcd. 99.90%). The first step within the temperature range of 30–125 °C was correlated with decomposition of CH6NO molecule with mass loss of 14.86% (calcd. 14.35%) with a maximum at 116 °C. The second step within the temperature range of 125–760 °C which correlated with decomposition of C19H12NO2 molecule with mass loss of 84.65% (calcd. 85.53%) with a maximum at 233 °C.

The [Cr(H2L)(H2O)Cl2]Cl·6H2O complex showed five decomposition steps within the range from 30 to 900 °C through total mass loss of 77.14% (calcd 77.95%) leaving ½Cr2O3 metal oxide contaminated by carbon atoms as residue. The first step within the temperature range of 30–140 °C was correlated by loss of six uncoordinated water molecules with mass loss of 17.26% (calcd. 17.45%) with a maximum at 85 °C. The second and third steps within the temperature range of 140–300 °C were correlated with loss of one coordinated water molecule and 3/2Cl2 with mass loss of 20.33% (calculated mass loss = 20.11%) with two maxima at 230 and 265 °C. The last two steps within the temperature range of 300–900 °C were assigned to loss of C15H18N2O1.5 molecule with mass loss of 39.55% (calcd. 40.39%) with two maxima at 350 and 845 °C.

The [Mn(H2L)(H2O)2Cl]Cl·4H2O complex showed six decomposition steps within the temperature range from 30 to 900 °C with total mass loss of 76.70% (calcd. 77.59%) leaving MnO metal oxide contaminated by carbon atoms as residue. The first three steps within the temperature range of 30–150 °C were correlated with loss of four uncoordinated water molecules with mass loss of 11.93% (calcd. 12.67%) with a maximum at 55, 100 and 115 °C. The fourth step within the temperature range of 150–250 °C correlated by loss of Cl2, 2H2O and CH4 molecules with mass loss of 21.01% (calcd. 21.64%) via a maximum at 205 °C. The last two steps within the temperature range of 250–900 °C were attributed to the loss of C14H18N2O2 molecule with mass loss of 43.76% (cald. 43.28%) via two maxima at 275 and 460 °C.

The [Fe(H2L)(H2O)3]Cl3·H2O complex showed three decomposition steps within the temperature range from 30 to 900 °C through total mass loss of 74.53% (calcd.75.33%) leaving ½Fe2O3 contaminated by carbon atoms as residue. The first step within the temperature range of 30–300 °C can be accounted to the loss of Cl2 and 4H2O molecules with mass loss of 24.8% (calcd. 25.14%) with a maximum at 200 °C. The last two steps within the temperature range of 300–900 °C were assigned to loss of ½Cl2 and C15H18N2O1.5 molecules with mass loss of 49.73% (calcd. 50.19%) with two maxima at 450 and 750 °C.

The [Co(H2L)(H2O)2Cl]Cl·4H2O complex showed five decomposition steps within the temperature range from 30 to 900 °C through total mass loss of 75.70% (calcd.76.34%) leaving CoO contaminated by carbon atoms as residue. The first and second steps within the temperature range of 30–130 °C were accounted to the loss of 3H2O uncoordinated water molecules with mass loss of 9.51% (calcd. 9.43%) with a maximum at 50 and 115 °C. The third and fourth steps within the temperature range of 130–350 °C were assigned to loss of 3H2O and Cl2 molecules by mass loss of 21.06% (calcd. 21.84%) with a maximum at 160 and 250 °C. The last step within the temperature range of 350–900 °C correlated with loss of C15H18N2O2 molecule with mass loss of 45.13% (calcd. 45.07%) with maximum at 650 °C.

The[Ni(H2L)(H2O)2Cl]Cl.6H2O complex showed six decomposition steps within the range from 30 to 900 °C with total mass loss of 75.99% (calcd. 75.76%) leaving NiO contaminated with carbon atoms as residue. The first step within the temperature range of 30–100 °C can correlated with loss of 4H2O uncoordinated water molecules by mass loss of 11.73% (calcd. 11.83%) by a maximum at 60 °C. The second and third steps within the temperature range of 100–280 °C can assigned to the loss of 4H2O, Cl2 and CH4 molecules with mass loss of 27.27% (calcd. 26.13%) with maxima at 140 and 235 °C. The last three steps within the temperature range of 280–900 °C can accounted to the loss of C13H14N2O2 molecule by mass loss of 36.99% (calcd. 37.80%) with maxima at 300, 475 and 700 °C.

The[Cu(H2L)(H2O)2Cl]Cl·5H2O complex showed six decomposition steps within the temperature range of 30–700 °C with total mass loss of 87.6% (calcd. 86.64%) leaving CuO as residue. The first and second steps within the temperature range of 30–170 °C can correlated with loss of 5H2O hydrated water molecules with mass loss of 15.95% (calcd. 15.13%) via maxima at 40 and 125 °C. The third step within the temperature range of 170–235 °C was assigned to loss of 2H2O molecules by mass loss of 5.69% (calcd. 6.05%) with a maximum at 200 °C. The last three steps within the temperature range of 235–700 °C can accounted to the loss of Cl2 and C20H18N2O2 molecules by mass loss of 65.96% (calcd. 65.46%) with maxima at 300, 375 and 500 °C.

The [Zn(H2L)(H2O) Cl2]3H2O complex showed five decomposition steps within the temperature range of 30–900 °C with total mass loss of 79.43% (calcd. 78.36%) leaving ZnO contaminated by carbon atoms as residue. The first step within the temperature range of 30–100 °C can correlated with loss of 3H2O uncoordinated water molecules with mass loss of 10.47% (calcd. 9.96%) via a maximum at 50 °C. The second step within the temperature range of 100–200 °C can assigned to the loss of H2O and ½Cl2 molecules by mass loss of 11.73% (calcd. 11.83%) with a maximum at 160 °C. The third step within the temperature range of 200–350 °C can accounted to the loss of ½Cl2 and CH3 molecules with mass loss of 10.23% (calcd. 9.31%) with a maximum at 275 °C. The last two steps within the temperature range of 350–900 °C were assigned to the loss of C16H15N2O2 with mass loss of 49.5% (calcd. 49.23%) with two maxima at 500 and 800 °C.

The [Cd(H2L)(H2O)·Cl2] complex showed five decomposition steps within the temperature range from 30 to 900 °C with total mass loss of 77.42% (calcd. 76.52%) leaving CdO as residue. The first step within the temperature range of 30–270 °C can correlated with the loss of H2O, Cl2 and C10H10N2 molecules with mass loss of 46.57% (calcd. 46.10%) via a maximum at 230 °C. The last four steps within the temperature range of 270–900 °C can assigned to the loss of C10H11O2 molecule with mass loss of 30.85% (calcd. 30.42%) with two maxima at 300, 600, 678 and 805 °C.

The thermograms of the complexes confirmed their structures where the presence of uncoordinated water molecules in all metal complexes except Cd(II)complex was suggested and all complexes have decomposed with three to six steps [39].

3.8 Geometry Optimization

The completely optimized geometries of the ligand H2L and its Zn(II) complex were illustrated in Fig. 4. The values of the selected bond lengths and bond angles calculated for zinc(II) complex showed distorted octahedral geometry around the Zn(II) ion (Table 3). A slight elongation in bond lengths C5-N20, C11-N17, N20-C29 and O34-H35 was observed in the zinc(II) complex as ligand H2L coordinated via azomethine nitrogen, amino nitrogen and phenolic oxygen. The Zn44-O34, Zn44-N20 and Zn44-N17 bond lengths were found to be 2.08, 2.20 and 2.17 Å, respectively. The four equatorial positions were occupied through azomethine nitrogen, amine nitrogen, phenolic oxygen and coordinated water oxygen while two axial positions were occupied by two chloride ions [8, 16].

Fig. 4
figure 4

The optimized structure of a H2L and b its Zn(II) complex

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Table 3 The different optimized and quantum chemical parameters of H2L and its zinc(II) complex
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3.8.1 Molecular Electrostatic Potential (MEP)

In order to study the reactions, electrostatic potential V(r) maps were calculated which are known for the identification of the electronic charge distribution around molecular surface and subsequently to expect sites for the reactions. These maps were calculated by using the same basis set as for optimization. In the present study, 3D plots of MEP were drawn for the ligand and its Zn(II) complex (Fig. 5). Based on the MEP, one can generally order the electron-rich area which has red color on the map (favor site for electrophilic attack). However, the electron-poor region has blue color (favor site for nucleophilic attack). But the region with green color points to neutral electrostatic potential region. The H2L is stable having approximately uniform distribution of charge density. However, oxygen and nitrogen atoms are surrounded with a greater negative charge surface, making these sites potentially more favorable for electrophilic attack (red) (Fig. 5). The aromatic ring seems neutral in terms of electron density. This distribution of potential is in favor of the complexation reaction which is further confirmed by electrostatic potential distribution of zinc(II) complex, where a greater negative charge is surrounded to the metal center (Fig. 5b). The Mulliken electronegativity also indicated the increase of electronegativity of oxygen and nitrogen in zinc(II) complex than free H2L making them the favor site of electrophilic attack via metal ion. The charge of Zn atom decreased from (+ 2 to + 0.67) due to coordination sphere process (Table 2) [16].

Fig. 5
figure 5

Molecular electrostatic potential map of a H2L and b [Zn(H2L)Cl2].The electron density isosurface is 0.004 a.u

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3.8.2 Molecular Parameters

Additional parameters such as the highest occupied molecular orbital energy (EHOMO), the lowest unoccupied molecular orbital energy (ELUMO), ∆E, absolute electronegativities, χ, chemical potentials, Pi, absolute hardness, η, absolute softness, σ, global electrophilicity, ω, global softness, S, and additional electronic charge, ∆Nmax have been estimated for the H2L free ligand and its zinc(II) complex and registered in Table 3. Electrophilicity index (ω) is one of the greatest essential quantum chemical descriptors in describing toxicity and the reactivity of various selective sites. The electrophilicity may quantify the biological activity of drug receptor interaction. Also, this index measures the stabilization energy when the system acquires extra negative charge from the environment. η and σ indexes, are the measure of the molecular stability and reactivity. Also, their concepts are related to each other. The softness indexes are the vice versa image for global hardness. These parameters are useful in order to support the suggestion structures. The mentioned quantum chemical parameters were calculated with the help of the following equations: [16]

$$\Delta E = E_{LUMO} - E_{HOMO}$$
(1)
$$\chi = \frac{{ - (E_{HOMO} + E_{LUMO} )}}{2}$$
(2)
$$\eta = \frac{{E_{LUMO} - E_{HOMO} }}{2}$$
(3)
$$\sigma = \frac{1}{\eta }$$
(4)
$$Pi = - \chi$$
(5)
$$S = \frac{1}{2\eta }$$
(6)
$$\omega = \frac{{Pi^{2} }}{2\eta }$$
(7)
$$\Delta N_{\max } = - \frac{Pi}{\eta }$$
(8)

The data calculated were presented in Table 4 and reflected the following notes:

  1. 1.

    The data of H2L and its zinc(II) complex had a great chance and priority for biological activity based on high ω value.

  2. 2.

    S and ω were the softness indexes while η is for hardness indication; a hard molecule had a high stability due to its high energy difference in-between the EHOMO and ELUMO than the soft molecule. So, the soft molecule was the reactive one having flexible donation towards the metal ions. Accordingly, the investigated H2L molecule was soft towards the coordination.

  3. 3.

    The positive electrophilicity index (χ) value and the negative electronic chemical potential (μ) value indicated that the H2L molecule capable of accepting electrons from the environment and its energy must decrease upon accepting electronic charge. Therefore, the electronic chemical potential must be negative [16].

  4. 4.

    The Zn(II) complex showed high values of dipole moments than the free ligand.

Table 4 Main calculated optical transition with composite ion in terms of molecular orbitals
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Both the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) were the main orbitals that participate in chemical stability. The HOMO represents the ability to donate an electron, LUMO as an electron acceptor represents the ability to obtain an electron. From the attained data (Table 5), it can assume that:

  1. 1.

    The energies of the HOMO and LUMO were negative values and more negative than free H2L, which showed the stability of isolated complex.

  2. 2.

    The EHOMO and ELUMO values of zinc(II) complex were calculated and showed a decrease than the free ligand which represents the strength of M–H2L bonds.

  3. 3.

    The total energy of zinc(II) complex was higher than free ligand, which indicated greatly the stability of the isolated solid complex.

  4. 4.

    The small energy gap can be associated with a high chemical reactivity, low kinetic stability and reflects to efficient electronic charge transfer interaction making the molecule highly polarizable.

  5. 5.

    The HOMO level was mostly localized on the azomethine nitrogen and oxygen of phenol group in the ligand, which indicated the preferable sites for nucleophilic attack to the central metal atom [16].

Table 5 Biological activity of Schiff base ligand and metal complexes
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3.8.3 UV–Vis Spectra

An interpretation of the photochemistry of transition metal compounds requires knowledge of the properties of molecular orbitals and appropriate excited states. Frontier orbitals played pertinent role in such systems, because they rule the electronic excitations and the transition characters. With the aid of TD-DFT calculations, it is possible to have comments about the contributions of the ligand and metal orbitals to molecular orbitals. It is not practical to analyze all the electronic transitions and the orbitals; therefore, some limitations were used. By reason of high accuracy and low computational cost of TD-DFT, it has been popularized for theoretical investigation of electronic spectra of molecules. The present exploration for low laying excited state on optimized ground state structures of H2L and its zinc(II) complex have been performed at TD-DFT/B3LYP/LANL2DZ level of theory for thirty singlet state. In Table 5, the experimental and theoretical electronic spectra were presented. The TD-DFT calculations have been evaluated in the N,N-dimethyl-formamide solvent background and matched with the experimental data. The transitions between interfrontier orbitals for wavelengths corresponding to maximum oscillator strength of simulated results with contemporary experimental observations were presented in Fig. 6a, b. For example, the electronic transitions for H2L obtained at calculated 321 nm correspond to experimental peak at 324 nm. This transition has been majorly contributed from HOMO to LUMO + 2 transitions which was primarily n → π* in nature. The different transitions and its experimental counterpart of the free ligand and its zinc(II) complex have been summarized in Table 4 [16, 17].

Fig. 6
figure 6

a Theoretical electronic absorption transitions for H2L in ethanol solvent. b Theoretical electronic absorption transitions for Zn-H2L complex in DMF solvent

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3.9 Structural Interpretation

On the basis of the various physicochemical and spectral data presented and discussed above, the structures of the Cr(III), Mn(II), Fe(III), Co(II), Ni(II), Cu(II), Zn(II) and Cd(II) complexes were shown in Fig. 7.

Fig. 7
figure 7

Structures of metal complexes

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3.10 Antimicrobial Activity

Antibacterial activity of H2L ligand and Cr(III), Mn(II), Fe(III), Co(II), Ni(II), Cu(II), Zn(II) and Cd(II) complexes were tested against different bacterial species: two Gram(+) Staphylococcus aureus and Bacillus subtilis and two Gram(−) pseudomonas aeruginosa and Escherichia coli by using modified agar diffusion method.

The results of antibacterial activity were tested in vitro as shown in Table 5. The metal complexes showed that the highest activity was found for Ni(II) and Cd(II) complexes against Bacillus Subtilis and the lowest activity against Bacillus Subtilis was observed for Zn(II) complex.

The Co(II) complex showed high activity while Cu(II) complex was the lowest active against Staphylococcus aureus [37].

The Cd(II), Cu(II) and Cr(III) complexes were highly active while Zn(II) complex was the lowest active against Escherichia coli.

The highly active was Cd(II) complex and the lowest active was Zn(II) complex against Pseudomonas aeruginosa.

The biological activity of these complexes was given graphically in Fig. 8.

Fig. 8
figure 8

Biological activity of Schiff base ligand and its metal complexes against different bacterial species

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3.11 Antitumor Activity

The in vitro anticancer activities of the synthesized Schiff base ligand and its complexes studied on human MCF-7 (breast cancer) cell line. The Ni(II), Cd(II), Mn(II) and Cu(II) complexes possess significant antitumor activity against MCF-7 proposing these compounds as potential antitumor agents (Fig. 9). It was believed that the ability of such compounds to inhibit tumor production was due to their chelating capability. The structure of the ligand and complexes played an important role in the anticancer activity. Thus, attachment of hydrophobic group in rigid ligands showed a better anticancer ability [8, 38, 39].

Fig. 9
figure 9

Anticancer activity of Schiff base ligand (H2L) and its metal complexes

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IC50 values of Ni(II), Cd(II) and ligand were found to be 50, 46 and 24.5 µg/ml, respectively. The Mn(II) and Cu(II) complexes need more than 50 µg/ml to decrease validity of 50% of cancer cells. So the ligand had higher anticancer activity [38].

3.12 Molecular Docking Studies

Molecular docking analysis has been widely used to know the interactions of small drug molecules with bio targets like DNA and protein molecules. Additionally, binding site of the target specific region of the biomolecules can be predicted (Fig. 10) [39].

Fig. 10
figure 10

The relation between the negative lowest binding energy of the ligand and their metal complexes with 3Q8U receptors

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In the present study, 3Q8U was chosen as the model protein and the theoretical docking studies were demonstrated with Schiff base ligand and its metal complexes using patch dock web server tool [8]. One of the guides of molecular docking is to obtain an optimized conformation for each of the drug and protein with relative orientation between them such that the free energy of the overall system is minimized. The calculated structure of the Schiff base ligand and its metal complexes was docked with biological targets: the receptors of crystal structure of Staphylococcus aureus (PDB ID: 3Q8U). Also, the binding energies of the ligand and its metal complexes with receptors were calculated using computational docking studies. These energy values were listed in Table 6. The results showed that there are possible interactions between Schiff base ligand (H2L) and its metal complexes with 3Q8U receptors. The docking results detected that the main interaction force of the Schiff base ligand with the active sites were H-acceptor. The lowest binding energies of the ligand and its metal complexes with 3Q8U receptors were calculated and the results represented graphically in Fig. 11. From the docking study for the ligand and its metal complexes, it was concluded that the ligand was favorably interacted with 3Q8U receptors. The strongest binding of H2L with 3Q8U receptor, as it had the lowest binding energy value. By comparing the experimental and theoretical, it was found that H2L had the highest activity with the lowest binding energy (− 8.50 kcal mol−1) and the order of activity of other complexes was very near to the data obtained from experimental study [40,41,42].

Table 6 Energy values obtained in docking calculations of H2L and there metal complexes with receptors Crystal structure of Staphylococcus aureus nucleoside diphosphate kinase complexed with ADP(3Q8U)
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Fig. 11
figure 11figure 11

The interaction between H2L and its metal complexes with receptor 3Q8U a H2L, b [Cr(H2L)(H2O)Cl2]Cl·6H2O, c [Mn(H2L)(H2O)2Cl]Cl·4H2O, d [Fe(H2L)(H2O)3]Cl3·H2O, e [Co(H2L)(H2O)2Cl]Cl·4H2O, f [Ni(H2L)(H2O)2Cl]Cl·6H2O, g [Cu(H2L)(H2O)2Cl]Cl·5H2O, h [Zn(H2L)(H2O)Cl2]3H2O, i [Cd(H2L)Cl2(H2O)]

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4 Conclusions

Series of Cr(III), Mn(II), Fe(III), Co(II), Ni(II), Cu(II), Zn(II) and Cd(II) complexes were prepared with tridentate N,N and O donor novel Schiff base ligand (H2L) resulted from the condensation of 4,6-diacetylresorcinol (4,6-DAR) with 1,8-naphth-alenediamine. They were characterized by various physicochemical techniques. The spectroscopic data showed that the Schiff base ligand acted in all metal complexes as neutral tridentate ligand through the azomethine nitrogen, amino nitrogen and the oxygen atom of phenolic group. Their molar conductivity values confirmed that Zn(II) and Cd(II) complexes were nonelectrolytes while Cr(III), Mn(II), Co(II), Cu(II) and Ni(II) were 1:1 electrolytes, while, Fe(III) complex was 1:2 electrolyte. The geometry of the complexes is assigned as octahedral. From the data of elemental analyses, the complexes have composition of the MH2L type with general formula [M(H2L)(H2O)2Cl]Cl.nH2O (M = Mn(II) (n = 4), Co(II) (n = 4), Cu(II) (n = 5), Ni(II) (n = 6)), [M(H2L)(H2O)Cl2]xCl.nH2O (M = Zn(II) (x = 0, n = 2), Cd(II) (x = 0, n = 0), Cr(II) (x = 1, n = 3)), respectively, and [Fe(III)(H2L)(H2O)3]Cl3. The new complexes were subjected to study their biological potentials. The antimicrobial studies suggested that the Schiff base ligand and its metal complexes have significant biological activity. Hence, from all these extensive observations, it was concluded that the Schiff base ligand (H2L) and its newly synthesized metal complexes give the remarkable, versatile and valuable information of coordination compounds and also, they may be used as good biological agents. The docking study showed that the H2L and Mn(II) complex have the highest activity with the lowest binding energy of − 8.50 and − 5.80 kcal mol‐1, respectively, and the order of activity of other complexes was very near to the data obtain from experimental study. The results of anticancer activities of the synthesized Schiff base ligand and its complexes studied on human MCF-7 (breast cancer) cell line showed that ligand, Cd(II) and Ni(II) complexes had higher anticancer activity. The DFT study showed that Zn(II) complex showed high values of dipole moments than the free ligand.