Novel transition metal complexes of nickel(II), copper(II) and zinc(II) with a symmetrical tetradentate Schiff base, obtained by condensation of 1,3-diaminopropane and 3,5-diiodosalicylaldehyde, N,N′-bis(3,5-diiodosalicylaldehyde)-1,3-diaminoporpane (H2L), have been prepared and characterized by elemental (CHN), FT-IR, UV–Vis and 1H NMR spectroscopic techniques. Out of these, copper (Cu(L)) and zinc (Zn(L)) complexes were isolated in the form of single crystals and characterized by single-crystal X-ray diffraction studies. In the Cu(L) complex, the geometry was found to be slightly distorted square planar, whereas the Zn(L) complex adapted slightly distorted octahedral geometry due to the attachment of two pyridine rings. The antibacterial screening of the prepared compounds was carried out by taking two Gram-positive (Staphylococcus aureus and Bacillus cereus) and two Gram-negative (Escherichia coli and Pseudomonas aeruginosa) bacterial strains. The appearance of zones of inhibition clearly depicted that the complexes have more retardation potential as compared to the Schiff base ligand.
There are a large number of findings in the last few years about the synthesis and characterization of symmetrical tetradentate Schiff bases (S.Bs) having ONNO donor sites [1,2,3,4,5]. This is because of their versatile applications and characteristics properties like coordination ability, structural diversity, stability under different oxidative and reductive conditions, catalytic and biocidal potential against certain species of bacteria, fungi and tumors [6,7,8,9,10]. Another most promising feature is that they can be prepared easily in high percentage yield under normal reaction conditions by using ordinary apparatus [11,12,13,14,15,16,17,18,19]. These Schiff bases have also the ability to be used in magnetic and in optoelectronics fields [20, 21].
A huge collection of transition metals complexes with bi-, tri- and tetradentate S.Bs possessing O and N donor sites are of specific worth due to their unusual configuration and amazing pharmacological applications [22,23,24,25,26]. In coordination chemistry, the oldest and extensive source of tetradentate S.Bs is salen-type ligands which are remarkably used for making complexes with transition metals [27,28,29]. The salen-type S.Bs are produced by the condensation of aliphatic or aromatic diammines and aldehydes possessing hydroxyl group at ortho position. These salen-type S.Bs have the ability to make two covalent and two coordinate covalent bonds with metals. Thus, azomethine linkage and phenolic oxygen can play a key role in generating the coordinative mode to make stable transition metal complexes with breathtaking geometries . Owing to high coordination ability, the salen-based metal complexes have also been used extensively as catalysts in a variety of organic transformations and in modeling of enzymes [31, 32].
The nickel(II), copper(II) and zinc(II) complexes with the Schiff bases exhibit widespread applications in various fields. These complexes have been studied extensively because of their pharmacological potentials as anticancer, antitumor, antibacterial, antifungal, antiviral, antitubercular, enzyme inhibition, DNA and RNA binding activities [14, 22, 23, 27,28,29, 33]. In addition to this, these complexes are equally important from industrial point of view because of their use in catalysis due to their greater redox potential [12, 32].
In the view of enormous applications, in the current study, we are hereby reporting the synthesis, characterization and antibacterial potential of tetra-iodo salen-based symmetrical tetradentate Schiff base ligand (H2L) and its Ni(II), Cu(II) and Zn(II) complexes.
Materials and physical methods
Microanalysis of the synthesized compounds was done by using a Heraeus CHN-O-FLASH EA 1112 elemental analyzer. Infra-red spectra were recorded on a FT-IR Prestige 21 spectrophotometer from 400 to 4000 cm−1 using KBr pellets. The electronic spectra were recorded in DMSO on CARY 50 spectrophotometer from 200 to 900 nm. 1H NMR spectra were taken in DMSO-d6 (400 MHz, Bruker) by taking tetramethylsilane (TMS) as reference standard for the calibration of the chemical shift (δ) values reported in ppm.
Synthesis of tetradentate Schiff base ligand (H2L)
The precursors and solvents employed in the current experimental work were of laboratory grade and obtained from famed suppliers. Schiff base ligand was prepared by following the procedure reported in the literature . For this, 2 mmol of 3,5-diiodosalicylaldehyde was dissolved completely in 25 mL of ethanol, to this 1 mmol of 1,3-diaminopropane was added with continuous stirring. The mixed contents were then refluxed for 1-3 h over a water bath to maintain uniform temperature. Upon the completion of the reaction, monitored by TLC, the contents were cooled to the room temperature to get the precipitates of the required product. The precipitates were then collected by filtration aided with pressure gradient and scoured thrice with absolute ethanol to get the targeted Schiff base in pure form.
General procedure for the synthesis of the complexes
For the synthesis of transition metal complexes, Ni(L), Cu(L) and Zn(L), 1 mmol of each, Ni(OAc)2.4H2O, Cu(OAc)2.H2O and Zn(OAc)2.2H2O, was dissolved separately in 25 mL of methanol. To this, a hot methanolic solution (25 mL) of H2L (1 mmol) was added dropwise with continuous stirring. The resultant mixture was refluxed for several hours to obtain the precipitates of the metal complexes. These precipitates were then filtered, washed rigorously with methanol and then finally dried in an oven.
[Ni(L)], yield: 76 %. Calculated for C17H12I4N2NiO2: C 24.23, H 1.44, N 3.32 %. Analysis found: C 24.42, H 1.51, N 3.23. FT-IR (KBr, cm-1): υ C-H(aromatic): 3043, υ C-H(aliphatic): 2933, 2870, υ C=N: 1612, υ C=C and C-N: 1564, 1494, 1435, 1371, υ C-O: 1317, υ Ni-N: 530, υ Ni-O: 412. 1H NMR [DMSO-d6, δ (ppm)]: 8.30 (s, 2 H, H-C=N), 7.46 (d, 2 H, J = 2.4 Hz, H arom.), 7.32 (d, 2 H, J = 2.4 Hz, H arom.), 3.61 (t, 4 H, CH2-N), 2.03 (br, 2 H, CH2-C). UV–Vis, λmax (nm) (DMSO): 243, 351, 412, 570.
[Cu(L)], yield: 77 %. Calculated for C17H12I4N2CuO2: C 24.09, H 1.43, N 3.31 %. Analysis found: C 24.37, H 1.56, N 3.22. FT-IR (KBr, cm-1): υ C-H(aromatic): 3047, υ C-H(aliphatic): 2922, 2854, υ C=N: 1610, υ C=C and C-N: 1568, 1494, 1435, 1382, υ C-O: 1321, υ Cu-N: 545, υ Cu-O: 422. UV–Vis, λmax (nm) (DMSO): 245, 371, 597.
[Zn(L)], yield: 63 %. Calculated for C17H12I4N2ZnO2: C 24.04, H 1.42, N 3.30 %. Analysis found: C 24.17, H 1.48, N 3.19. FT-IR (KBr, cm-1): υ C-H(aromatic): 3051, υ C-H(aliphatic): 2918, 2850, υ C=N: 1616, υ C=C and C-N: 1560, 1508, 1433, 1375, υ C-O: 1294, υ Ni-N: 542, υ Ni-O: 412. 1H NMR [DMSO-d6, δ (ppm)]: 8.18 (s, 2 H, H-C=N), 7.88 (d, 2 H, J = 2.3, H arom.), 7.49 (d, 2 H, J = 2.3, H arom.), 3.76 (t, 4 H, CH2-N), 1.94 (br, 2 H, CH2-C). UV–Vis, λmax (nm) (DMSO): 241, 363, 456.
For X-ray diffraction measurement of Cu(L) and Zn(L) complexes, Bruker Kappa APEXII CCD diffractometer was utilized having graphite monochromated Mo-Kα radiation. The brown and yellow single crystals of Cu(L) and Zn(L) appropriate for X-ray diffraction inspection were extracted by methanol and pyridine solution, respectively. Bruker Apex-II software  was employed in order to collect X-ray diffraction data. For the sake of structure solution, data refinement, SHELXS97  and SHELXL-2018/3  were employed, respectively. During refinement, anisotropic displacement parameters were used for non-H-atoms, whereas H-atoms were refined with relative isotropic displacement parameters. Data were integrated with the assistance of Bruker SAINT  software package. Mercury , Platon  and ORTEP-3  were employed for graphical representation of the X-ray diffraction results.
Antimicrobial activity assay
All the prepared compounds were screened against various species of bacteria to investigate their inhibition potential. For this, four standard strains of bacteria were selected, out of which two were Gram-positive (Staphylococcus aureus PTCC1431 and Bacillus cereus PTCC1015) and remaining two were Gram-negative (Escherichia coli PTCC1394 and Pseudomonas aeruginosa PTCC1074). For the sake of comparison, two reference drugs, ampicillin (10 mg/disk) and erythromycin (15 mg/disk), were shortlisted. DMSO was taken not only for positive control but also as a solvent to dissolve the tested compounds (50 µg/mL). Bacterial strains, cultured in petri dishes, were treated with the tested compounds and incubated at 37 °C for 24 h. The antibacterial activity of the screened compounds was stipulated by the appearance of zones of inhibition around the specimens used. Every assay was executed thrice to get the final results which are presented in Table 5 and in Fig. 1.
Results and discussion
A salen-based tetradentate Schiff base ligand H2L was prepared by treating 1,3-diaminopropane with the corresponding 3,5-diiodosalicylaldehyde in nearly 68 % yield in ethanol. Reaction of H2L with corresponding metal acetates in a 1:1 ratio under reflux directed to the formation of nickel(II), copper(II) and zinc(II) complexes. The findings from spectroscopic characterization are in compliance with the proposed molecular formula of the synthesized compounds. Scheme 1 gives a comprehensive view of synthetic procedure adopted for the preparation of novel compounds.
Crystal structures of complexes
In the asymmetric unit of Zn(L), the 3-iminopropan-1-ol group A (C1/C6/C7/N1/O1) and 3,5-diiodophenyl ring B (C1-C6/I1/I2) are found to be planar with respective r.m.s deviation of 0.0041 Å and 0.0382 Å and the dihedral angle between group A and ring B is found to be 4.94 (4)°. This dihedral angle indicated that group A and ring B are almost parallel to each other. The C-atoms of ethyl group C (C8/C9) are at the distance of 0.0360 (1) Å and 1.4937 Å above the plane of group A. Experimental details of Zn(L) are specified in Table 1. The pyridine ring D (C10-C14/N2) is disordered over two sets of locations with occupancy ratio 0.58(3): 0.42(3). The major part E (C10A-C14A/N2A) is oriented at the angle of 30.1 (2)° with respect to minor part F (C10B-C14B/N2B). The dihedral angles A/C, A/D, B/C, B/D are found to be 74.5 (8)°, 83.6 (1)°, 72.1 (7)° and 79.4 (1)°, respectively. X-rays structure investigation revealed that ligand-to-metal ratio is 2:1. The central Zn-atom is hexa-coordinated by one N and O-atom of chelating ligand, one N and O-atom of symmetry related chelating ligand (x,x-y,1/2-z), N-atom of pyridine ring and N-atom of symmetry-related pyridine ring (x,x-y,1/2-z) as shown in Fig. 2. In the coordination sphere, axial positions are occupied by O-atoms, whereas equatorial positions are occupied by N-atoms. Bond length Zn-O is 2.012 (5) Å, and Zn-N bond length ranges from 2.155 (6) Å to 2.265 (1) Å, whereas bond angles range from 82.6 (5)° to 106.43 (3)°. The selected bond length and bond angles are specified in Table 2. The central Zn-atom (Zn1) is in the plane of (N1/N2A/N1iv/N2Aiv) and deviated from mean plane by 0.0691 Å as compared to deviation of 0.038 Å in related reported complex . A distorted octahedral geometry is formed by Zn(L), and similar geometry is found around the central Zn-atom in an already reported Zn-complex with a different ligand . C-H…O bonding and halogen bonding of the type C-H…I are responsible for crystal packing as shown in Fig. 3 and specified in Table 3. No other weak interaction is found in crystal packing.
In the asymmetric unit of Cu(L) shown in Fig. 4 and specified in Table 1, the 3-iminopropan-1-ol group A (C1/C6/C7/N1/O1) and 3,5-diiodophenyl ring B (C1-C6/I1/I2) are found to be planar with respective r.m.s deviation of 0.0121 Å and 0.0213 Å and the dihedral angle between group A and ring B is found to be 3.73 (1)°. This dihedral angle indicated that group A and ring B are nearly parallel to each other. The C8 and C9 atoms are disordered over two sets of locations with occupancy ratio of 50% and 25% for each part, respectively. The other half molecule is generated symmetrically through symmetry code –x, y, 1/2-z. The central Cu-atom is coordinated by two N-atoms and two O-atoms of chelating ligands. The selected bond length and bond angles are displayed in Table 2, and these bond lengths and bond angles are within the range reported for relevant Schiff base complexes [42, 43]. The geometry of four-coordinated complexes can be computed by the utilization of geometric index τ4=[360-(α+β)]/141, where α and β are the two largest angles around the metal center . The value of τ4 is zero for perfect square planar geometry and one for perfect tetrahedral geometry. In our case, α=172.82, β=172.82 and value of τ4 is found to be 0.0071 which is very close to zero resulting in the formation of slightly distorted square planar geometry around Cu-atom. The configuration of the molecular structure is stabilized by intramolecular C-H…N bonding. The molecules are connected with each other through C-H…O, C-H…N and C-H…I hydrogen bonding to form infinite chains along  crystallographic direction as shown in Fig. 5 and specified in Table 3.
The 1H NMR spectra of ligand were taken in deuterated chloroform (CDCl3), while DMSO-d6 was used to record the spectra of synthesized complexes and the data are given in detail in the experimental section. The most eminent signals in the spectrum of H2L appeared at δ =14.71 ppm, which are attributed to the resonances of phenolic protons, which faded away in the spectra of nickel and zinc complexes as obvious from Fig. 6. This is a clear-cut indication of the dibasic nature of Schiff base ligand upon complexation. Another prominent signal visible in the 1H NMR spectra of the nickel and zinc complexes at δ = 8.30 and 8.18 ppm, respectively, is assigned to the azomethine protons (H-C=N-). These resonance values appear to be slightly shifted downfield as compared to the spectrum of the free Schiff base ligand. Thus, shifting of signal is also an indication about the point of attachment of metal ions with the Schiff base ligand. The signals for aromatic protons in the spectra of nickel and zinc complexes popped up in the range of δ = 7.32–7.58 ppm and δ = 7.38–7.88 ppm, respectively. The –CH2–N protons of the Ni(L) and Zn(L) complexes emerged as a triplet at δ = 3.61 and 3.76 ppm, accordingly. The broad signals at δ = 2.03 and 1.94 ppm are attributed to the –CH2–C protons of the Ni(L) and Zn(L) complexes, consecutively.
The comparison between the selected bands in the FT-IR spectra of Schiff base ligand and its metal complexes is presented in Table 4 and in Fig. 7. It is obvious from the data that in the FT-IR spectra of metal complexes, the bands for ν (H-C=N-) and ν (C‒O) were shifted to lower and higher wavenumbers, respectively, in comparison with its corresponding free ligand, thereby indicating a coordinative interaction between the azomethine nitrogen and phenolic oxygen atoms with central metals ions. This coordination could also be confirmed by the appearance of weak bands located at the low wavenumbers which were assigned to (M‒N) and (M‒O) at 530-542 cm-1 and 412-422 cm-1, respectively.
UV–Vis spectral studies
The UV–Vis absorption spectrum of the Schiff base ligand shows bands at 254, 329 and 414 nm. The band at 254 nm can be attributed to the π–π* transition of the benzene rings. The absorption band at around 329 nm is due to a π–π* transition of the C=N group. A low-intensity band in the lower-energy region at 414 nm is assigned to the n–π* transition of non-bonding electrons on the nitrogen of azomethine group in the free ligand [45, 46]. Maximal wavelength (λmax) due to the π–π* transition of the benzene rings and π–π* transition of the C=N groups in the metal complexes is increased. These red shifts in the UV–Vis spectra of the complexes are ascribed to the hydroxyl deprotonation of the Schiff base ligand during complexation, and formation of coordination bonds. The intense bands in the range of 412–597 nm were corresponded to π–π* azomethine and d–π* transition . For example, a comparison of the ligand and nickel complex spectra is given in Fig. 8.
Evaluation of antimicrobial activity
In vitro evaluation of antibacterial potential of synthesized Schiff base ligand and its corresponding metal complexes was carried out by screening them against a panel of microorganisms. The tested microorganisms were the standard strains of two Gram-positive (Bacillus cereus PTCC1015 and Staphylococcus aureus PTCC1431) and two Gram-negative (Pseudomonas aeruginosa PTCC1074 and Escherichia coli PTCC1394) bacteria. It was found from the results (Table 5) that all the investigated compounds were potentially active against the tested strains. The sections of inhibition measured for the synthesized compounds ranged from 12 to 13 mm (H2L), 13 to 15 mm (Ni(L)), 14 to 15 mm (Cu(L)) and 14 to 15 mm (Zn(L)) for Gram-negative bacteria. Moreover, the zones of retardation against Gram-positive bacteria for the synthesized compounds ranged from 21 to 25 mm (H2L), 23 to 27 mm (Ni(L)), 22 to 27 mm (Cu(L)) and 24 to 28 mm (Zn(L)). Cu(L) showed the maximum activity against E. Coli followed by Zn(L). Against P. aeruginosa, Ni(L) and Zn(L) complexes showed maximum activity while minimum inhibition was spotted for H2L. For S. aureus and B. cereus Zn(L) showed maximum inhibition.
It is crystal clear from the appeared values that all the synthesized compounds are active against both strains of Gram-positive and Gram-negative bacteria in comparison with the standard drugs and hence proved that these are very suitable candidates for the search of alternative drugs in addition to available antibiotics.
It is believed that every compound has its own specificity in destroying the particular strains of bacteria. The presence of oxygen of the hydroxyl group (OH), nitrogen of the azomethine part (H-C=N), well-oriented iodo groups and in addition to this, specific metal ions may be correlated for the interferences in the process of proper cell division and hence further growth of the microbes is stopped . This once again confirms our earlier proposal that antibacterial activity is dependent on the molecular structure of the compounds and the bacterial strain .
On the basis of zones of inhibition that appeared in the current study, it can be concluded that the synthesized compounds have more inhibition potential toward Gram-positive bacteria in contrast with Gram-negative bacteria. This might be because of variance in the constitution of their cell walls . The better sensitivity of Gram-positive bacterial strains may be attributed to the presence of lipopolysaccharides in their cell wall which fends off the deposition of tested moieties in the cell membrane .
We have synthesized a series of Ni(L), Cu(L) and Zn(L) complexes with a salen-type Schiff base derived by the condensation of 3,5-diiodosalicylaldehyde and 1,3-diaminopropane by using alcohol as solvent. The complexes were characterized using various physico- and spectrochemical techniques. Furthermore, the molecular structures of Cu(L) and Zn(L) complexes have been determined by single-crystal X-ray analysis. The coordination sites are azomethine nitrogens and phenolic oxygens as evident from 1H-NMR, FT-IR and UV–Vis. The X-ray data revealed that the geometry around Cu(L) complex was a slightly distorted square planar, whereas in Zn(L) complex, the apical sites were involved in coordination with pyridine molecules to increase the coordination number from four to six to make distorted octahedral geometry. The antibacterial screening tests disclosed that the number of live bacterial strains was greatly reduced after adding the complexes compared to free Schiff base ligand alone. The antibacterial data revealed that the toxicity of metal complexes toward Gram-positive strains is high relative to Gram-negative strains.
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Kargar, H., Adabi Ardakani, A., Munawar, K.S. et al. Nickel(II), copper(II) and zinc(II) complexes containing symmetrical Tetradentate Schiff base ligand derived from 3,5-diiodosalicylaldehyde: Synthesis, characterization, crystal structure and antimicrobial activity. J IRAN CHEM SOC (2021). https://doi.org/10.1007/s13738-021-02207-x
- Schiff base complex
- X-ray structure
- Antibacterial activity