Degradation of dye wastewater by using cobalt, copper and nickel Schiff base complexes as catalysts: spectral, molecular modelling, catalytic activity and metal removal from aqueous solution

ABSTRACT Metal complexes of Co(II), Cu(II) and Ni(II) with the Schiff base ligand, 6,6ʹ-(([1,1ʹ-biphenyl]-4,4ʹ diylbis(azaneylylidene))bis(methaneylylidene))bis(2,4-dichlorophenol) (H2L) were synthesised. All synthesised compounds were identified and characterised. The conductivity measurements showed the non-electrolytic nature of all the complexes. The decomposition of all complexes ended with metal oxide. Octahedral geometries were proposed for all complexes. Photocatalytic degradation of methylene blue dye (MB) in the presence and absence of hydrogen peroxide was studied by using Co(II), Cu(II) and Ni(II) complexes as catalysts. Fluorescence studies have shown that the reported compounds can act as photoactive materials. Density functional theory was used to investigate the geometries of the ligand and its complexes. The adsorption of Co2+, Ni2+ and Cu2+ in an aqueous solution on H2L ligand under various conditions was studied. The optical properties of all complexes were studied. Antimicrobial activities of the ligand and its transition metal complexes were studied. The in-vitro cytotoxicity of ligand and its cobalt(II), nickel(II) and copper(II) complexes were evaluated towards the human Liver Carcinoma (HepG2) cell line and compared with standard cisplatin.


Introduction
The N 2 O 2 donor of Schiff base complexes has been considered as precursors for new polynuclear metallic complexes leading to rich coordination chemistry [1][2][3].Schiff base complexes containing one or more halogen groups have important applications such as antitumour [4], DNA cleavage [5], and electrochemistry [6].Also, Schiff base complexes were most extensively studied in medicinal and pharmaceutical fields due to their wide variety of pharmacological activities such as anti-inflammatory activity [7], antibacterial [8], analgesic [9], antitubercular [10], antioxidant [11], anthelmintic [12], and anticancer [13].Schiff base complexes derived from salicylaldehyde derivatives and various amines have been widely investigated because of their wide applications like biological activities [14] and may have the potential of chemotherapy [15,16].Salicylaldehyde-based Schiff base complexes of 2-amino-l,3,4-thiadiazole have been screened for antibacterial activity [17].Schiff base metal complexes were shown to have photoluminescent properties [18][19][20][21][22][23] and offer attractive light emission properties [24,25].The coordination complex derived from the condensation reaction of diamines and salicylaldehyde generally exhibited good luminescence properties due to the presence of a hydroxyl group, a coordinating nitrogen atom, and a delocalised π-system [26,27].
Dye wastes were one of the most troublesome pollutant groups because they were easily identified by the human eye and were not easily biodegradable.To minimise environmental pollution, it is necessary to destroy an organic dye such as methylene blue and methyl orange from industrial waste [37][38][39].Some complexes were found to have catalytic ability for the degradation of methylene blue under visible light irradiation [40].Adsorption was one of the physical-chemical methods for removing dyes from effluents that has been found to be the simplest and costeffective [41].
In the present study, we have described the preparation and characterisation of the 6,6ʹ-(([1,1ʹ-biphenyl]-4ʹdiylbis(azaneylylidene))bis(methaneylylide))bis(2,4-dichlorophenol) as a Schiff base compound to coordinate with transition metal ions Cu(II), Co(II) and Ni(II).The synthesised complexes have been using as a catalyst by a thermal decomposition method to evaluate their potential activity towards water treatment via photocatalytic degradation of methylene blue as an organic pollutant.Also, the molecular structures of the Schiff base and its complexes were optimised theoretically.Metal ion adsorption had also been studied from aqueous solutions using a synthetic organic ligand to test the decontamination of water.

Materials
Benzidine, 3,5-dichlorosalicylaldehyde, CoCl 2 .6H 2 O, NiCl 2 .6H 2 O and CuCl 2 .2H 2 O were obtained from BDH Chemicals Ltd, England.Methylene blue (MB) and EDTA-Na (sodium salt of ethylenediaminetetraacetic acid) were brought from sigma Aldrich company.All solvents were of analytical reagent grade (AR) and had the highest purity available.They were used without further purification.

Instrumentation
Carbon, hydrogen and nitrogen (C, H & N) analyses were carried out on a Perkin Elmer 2400 mode instrument.The conductivity measurements were performed in DMF (Dimethyl formamide) solution (1×10 −3 M), by using a Jenway 4010 conductivity metre.IR spectra were recorded using KBr pellets in the region 4000-400 cm −1 on FTIR spectrophotometer, Shimadzu 8201. 1 H NMR spectra were recorded by using DMSO (dimethyl sulphoxide) as a solvent and a BRUKER 500 MHz spectrophotometer using TMS as an internal reference.Mass spectra of the solid ligand and its complexes were measured on a JEOL JMS-AXE 500 spectrometer.The TG and DTG were recorded from ambient to 800 °C with a heating rate of 10°C/min under a nitrogen atmosphere, using a Shimadzu DT-50 thermal analyser.The ligand and its complex spectra were measured on a Shimadzu 3101 pc spectrophotometer.The photoluminescent properties of all the compounds were studied using a LS50B Jenway 6270 fluorimeter.

Synthesis of the complexes
The metal salt (2 mmol) in 10 mL ethanol and two drops of triethylamine were added dropwise to a solution of ligand H 2 L (1 mmol) in 25 mL methylene chloride.The reaction mixture was stirred under reflux for 6 h.The solid complexes were filtered and washed several times with ethanol, ether and then air-dried.

Metal ion uptake study
The adsorption of metal ions from an aqueous solution using a synthetic organic ligand (H 2 L) was studied by various factors such as time and concentration of metal ions.The adsorption process was performed by stirring 5 mg ligand and 5 mL of the prepared aqueous solution of each of CoCl 2 .7H 2 O, CuCl 2 .2H 2 O and NiCl 2 .6H 2 O (0.01 M, 0.001 M and 0.0001 M) in a flask at room temperature and filtered after different times, 1, 2 and 5 h.The cobalt, nickel, and copper ion content in the synthesised metal complexes was determined complexometrically by titration against the standard solution of EDTA-Na using a suitable indicator at a suitable pH value [42].The removal percentage % of metal was calculated using the following equation: Where: C o is the initial concentration of metal ions (mg/L).
C e is the concentration of metal ions at time t (mg/L).

Photocatalytic dye degradation
A mixture of 30 mL methylene blue (MB) at two different concentrations (11 ppm and 9 ppm) and 2 mg complexes as catalyst was placed in the dark for 30 minutes.Then, the mixture was exposed to a tungsten lamp 100 W at a different time in the absence and presence of H 2 O 2 (1 mL, 30%, 10 mmol) in a closed system.2.0 mL of supernatant was drawn from the mixture every 10 minutes and analysed by a UV-Vis spectrophotometer at a maximum absorbance wavelength of MB dye (665 nm).The percentage of degradation for MB was calculated by: where, A 0 = initial concentration of the dye, A t = dye concentration after time t.

Microbiological investigation
The antibacterial activities of the prepared ligand and its complexes were tested against two local Gram-positive bacterial species (Staphylococcus aureus and Bacillus subtilis) and two local Gram-negative bacteria (proteus vulgaris and Escherichia coli) on nutrient agar medium (NA).Also, the antifungal activities were tested against two local fungal species (Aspergillus fumigatus and Candida albicans).Gentamycin and ketoconazole were used as standard references for Gram-positive & Gram-negative bacteria and fungi, respectively.The test was done using the diffusion agar technique.The sample was tested at 10 mg/mL concentration in DMSO.The well was filled with the test solution using a micropipette and the plate was incubated 24 h for bacteria at 37 °C and 72 h for fungi at 30 °C.

Anticancer activities
Cytotoxicity of the Schiff base ligand and its metal complexes were carried out on HepG-2 (human hepatocellular cancer cell line) which was obtained from VACSERA Tissue culture Unit using the Cisplatin standard at the Regional Center for Mycology and Biotechnology, Cancer Biology Department, Pharmacology Department, Azhar University.The cell was seeded in a 96-well plate at a cell concentration of 1 × 10 4 cell per well in 100 μl of growth medium.A fresh medium containing different concentrations of the test sample was added after 2 h of seeding.Serial two-fold dilutions of the tested chemical compound were added to confluent cell monolayers dispensed into 96-well, flat-bottomed microtiter plates using a multichannel pipette.The microtiter plates were incubated at 37°C in a humidified incubator with 5% CO 2 for a period of 24 h.Three wells were used for each concentration of the test sample.IC 50 , the concentration required to cause toxic effects in 50% of intact cells, was evaluated, and potency was calculated from the percentage of alteration of Vinblastine standard [43].

DFT study
Theoretical calculations were carried out to investigate the geometry of the ligand (H 2 L) and its complexes using the Gaussian 09 programme at the B3LYP/6-31 G(d) and B3LYP/ LANL2DZ level of theory for H 2 L and complexes, respectively [44].

Results and discussion
All metal complexes were stable and non-hygroscopic at room temperature.Analytical data, colour and yield for the Schiff base ligand and its complexes are listed in Table S1.
The analytical results demonstrated that Co(II) and Ni(II) complexes have 2:1 and 2:2 for Cu(II) complex (M:H 2 L) stoichiometry.

FT-IR spectra
The IR spectrum of the ligand H 2 L showed a strong band at 1615 cm −1 assigned to the azomethine (HC = N) linkage (Figure S1).This band shifted in the spectra of complexes to lower frequencies in the range (1595-1598 cm −1 ) due to the involvement of nitrogen of azomethine group in coordination (Table 1).Furthermore, the phenolic (C-O) vibration at 1377 cm −1 in the spectrum of the ligand was shifted to a higher frequency on complexation with Co(II), Ni(II) and Cu(II) ions indicating the coordination of these ions through phenolic oxygen of the Schiff base [45].The band at 3384 cm −1 due to phenolic OH in the ligand was shifted to a higher frequency in all the complexes indicating the coordination of phenolic oxygen to the metal ion without deprotonating [46].Finally, the spectra of complexes Co(II), Ni(II) and Co(II) exhibited new bands at the ranges 517-587 and 429-474 cm −1 assigned to the ʋ(M-O) and ʋ(M-N) stretching vibration modes, respectively [47].

1 H NMR spectrum
The 1 H NMR spectrum of ligand, H 2 L showed a significant azomethine proton signal (s, CH = N) at 9.06 ppm and the multiples corresponding to aromatic ring protons appeared in the region 6.68-7.76ppm (m, 12 H, ArH) [48] and the singlet signal at 10.10 ppm due to the O-H group [49].

Mass spectra
The mass spectra of the ligand (H 2 L) and its Co(II) and Ni(II) complexes showed molecular ion peaks at m/z 531.05, 861.91 and 861.20, respectively, which were consistent with their molecular formula (Figure S2).

Electronic spectra
The electronic spectra of the Schiff base ligand (H 2 L) and its metal ion complexes in DMSO were measured in the range of 200-800 nm (Figure 1).The UV-vis absorption spectrum of the H 2 L ligand showed a strong band at 296 nm due to π-π* transitions and two bands at 386 and 454 nm were assigned to n-π*, for the electrons localised on the C = N chromophore [50].The electronic spectra of Co(II), Ni(II) and Cu(II) observed bands at 282, 278 and 280 nm assigned to π -π* and 370, 374 and 398 nm, respectively, assigned to n-π* transitions of donating atoms oxygen and nitrogen, which were overlapped with the intermolecular CT from aromatic rings.The spectrum of the Cu(II) complex observed d-d transition bands at 560 and 750 nm corresponding to 2 B 1g (F)→ 2 A 1g (F) and 2 B 1g (F) → 2 A 2g (F), respectively.The electronic spectrum of the Ni(II) complex showed bands at 602 and 734 nm, which may be related to 3 A 2g (F)→ 3 T 1g (P) and 3 A 2g (F)→ 3 T 1g (F) transitions, respectively, in an octahedral geometry [51].However, in the Co(II) complex, no d → d transition was observed due to its low intensity [52].

Magnetic measurements
At room temperature, the measured magnetic moment values were found to be consistent with the suggested geometry around the metal ions.
The magnetic susceptibility, X g , and the effective magnetic moment, µ eff , were calculated using the following: X g : gram susceptibility, L: length of tube, M: mass of sample, R and R 0 : reading before and after applying the magnetic field, C: constant equals to 2.086, X M : molar susceptibility, Mol.wt: molecular weight, µ eff : effective magnetic moment, T: absolute temperature.
The measured value of the Co(II) complex was found to be 2.47 B.M., which agreed with the presence of three unpaired electrons.The value of the effective magnetic moment of 2.89 BM for the Ni(II) complex confirmed the presence of high spin Ni(II) in the octahedral environment.The Cu(II) complex has a µ eff value of 1.85 B.M. characteristics of an octahedral environment around the metal ion [53].

Fluorescence spectra
The fluorescence spectra of the ligand and its complexes (Figure S3) were investigated in DMSO solution at room temperature.The ligand exhibited an emission band at 482 nm upon excitation at 386 nm.On complexation, the fluorescence emission spectra of Co(II), Ni(II) and Cu(II) complexes exhibited high-intensity bands upon excitation at 370, 374 and 280 nm, respectively, assigned to the (π→π*) intraligand transitions.The enhancement of fluorescence for complexes may be attributed to the dinuclear connection of metal atoms to ligand, which enhanced the conformational rigidity of complexes [54].Therefore, these synthesised compounds can serve as potential photoactive materials.

Thermal analysis
The thermogravimetric behaviour of the metal complexes has been studied by using TG and DTG analyses, and the results are represented in Figure S4.
The TG curve of [Co 2 (H 2 L)(H 2 O) 4 Cl 4 ] complex was thermally decomposed in a single step in the temperature range of 288 to 780°C with an estimated mass loss of 82.89% (calcd.82.66%) and leaving 2CoO 17.11% (calcd.17.38) as the product of decomposition.
The TG curve of [Cu 2 (H 2 L) 2 Cl 4 ].3H 2 O complex decomposed in three steps.The first step occurred in the temperature range 75-157°C with a weight loss of 9.04% (calcd.9.03%) equivalent to the loss of three hydrated water molecules and Cl 2 .The second step occurred in the range 157-577°C with a mass loss 51.49% (calcd.51.93%) due to the loss of C 26 H 16 N 2 Cl 10 .The last step in the range of 577-750°C with a mass loss of 28.25% (calcd.28.04) leaving 2CuO 11.25% (calcd.11.49) as a final residue.
The thermodynamic activation parameters of the decomposition processes of complexes, namely activation energy (E), frequency factor (A), enthalpy of activation (ΔH*), the entropy of activation (ΔS*) and Gibbs free energy change of the decomposition (ΔG*), were evaluated graphically by employing the Coats-Redfern relation [55].
where W ∞ is the mass loss at the completion of the decomposition reaction, W is the mass loss up to temperature T, R is the gas constant and ϕ is the heating rate.Since 1-2RT/E* ≈ 1, the plot of the left-hand side of Eq. ( 1) against 1/T gave a straight line.E* was then calculated from the slope, and the Arrhenius constant, A, was obtained from the intercept.
The kinetic parameters are listed in Table 2. From the obtained results, it was found that the entropy of activation has negative values for all metal complexes, which indicates that decomposition reactions proceed with a lower rate than normal ones.Also, all values of ΔG* were positive which indicates that all steps were nonspontaneous.The activation energies of the decomposition were found to be in the range of 16.97-90.39kJ mol −1 indicating the high thermal stability of the complexes [56].Hence, we can state that the order of thermal stability was observed as the sequence Cu(II) > Ni(II) > Co(II) according to two factors.Initially, according to the mass of the complex, it was found that the copper complex has a higher greater stability than nickel and cobalt complexes due to its molecular mass [57].The second factor was the activation energy, where the Cu(II) complex showed the highest value for activation energy (90.39 kJ mol −1 ) due to the slower decomposition process [57].
The results of the spectroscopic analysis were in full alignment with the suggested complex structures (Figure 2).

Photocatalytic activity study
Photocatalytic degradation of organic dye is a useful technique for the treatment of industrial pollutions because dyes are one kind of main contaminations of water [58].
The effect of time and two concentrations for MB (11 and 9 ppm) on the percentage of degradation of MB in the presence and absence of hydrogen peroxide is shown in Figure 3(a-c).The decrease in intensity of absorbance of dye after irradiation at different times gave the efficiency of photocatalytic degradation of the studied complexes.The decolouration percentage of MB by using Co(II), Ni(II) and Cu(II) complex catalysts was observed at 97.96, 97.96 and 95.56%, respectively, which means that the percentage of decloration was increased with time in the presence of H 2 O 2 by using MB concentration, 9 ppm.This can be attributed to the decrease in OH radicals [59].
The percentage of decolouration of MB by using Co(II), Ni(II), Cu(II) and Zn(II) complexes that derived from 4-aminoantipyrine derivative were found at 68.3, 71.8, 76.2, 81.4 and 77.5%, respectively, at 60 min [60].The influence of the time and temperature on MB (9 ppm) removal in the presence of hydrogen peroxide at three different temperatures, namely, 313 K, 323 K, and 333 K was studied (Figure 3(d-f)).It can be seen that the degradation percentages of MB dye increased with increasing temperature due to the interaction of the hydroxyl radicals with the dye molecules [61].In the presence of hydrogen peroxide, the rate of decolouration was influenced by the type of catalyst.The order of catalysts towards decolouration of MB (9 ppm) after 65 min at room temperature was found Cu(II) >Co(II) > Ni(II) complex (Figure 3 (g)).Also, the decolourisation efficiency follows the order: Cu(II) > Co(II) > Ni(II) complexes at constant MB concentration (9 ppm) and constant temperature (333 K) after 30 min as represented graphically (Figure 3(h)).
The rate constants of MB (9 ppm) decolouration were estimated by the equation: ln (A/A 0 ) = -k t , based on a pseudo-first-order kinetic, where k is the reaction rate constant, A/A 0 is the ratio of absorbance values corresponding to the characteristic peaks of dye before and after photocatalysis, and t is the reaction time at different temperatures of 313, 323 and 333 K in presence of hydrogen peroxide [62].Linear plots of ln A /A 0 vs. time for the decolouration of MB at different temperatures by using complex catalyst are shown in Figure S5.The reaction rate constant k of complexes and the thermodynamic parameters such as the standard Gibbs free energy (ΔG*), entropy (ΔS*) and enthalpy (ΔH*) changes of adsorption were calculated by plotting ln K versus 1/T, which  is known as Van't Hoff plots in Figure S6.The thermodynamic parameter values are listed in Table S2.The negative values of enthalpy change indicated the adsorption of MB exothermically [63].

Mechanism
Under the light radiation, electrons are excited from the valence band (VB) to the conduction band (CB) to produce holes (Figure S7).The adsorbed O 2 molecules at the catalyst surface combined with the electrons to generate superoxide radical anions.Then, H 2 O 2 was present in the medium combined with superoxide radical anions to produce more ( ▪ OH) radicals.These free radicals reacted with the dye molecule and decompose into CO 2 and H 2 O. MB molecules were attacked and decomposed by hydroxyl radical ( ▪ OH) which is a powerful oxidant generated from the photolysis of H 2 O 2 by light [64].
The following equations (Eq.1-7) showed the steps of the mechanism.

Metal uptake
The effect of contact time (1, 2 and 5 h) on removal percentage of metal from different concentrations of its aqueous solution (0.01, 0.001 and 0.0001 M) by H 2 L ligand was studied.It was found that the removal percentage of metal ions increased with time (Figure 4) .The highest removal percentage of Co(II), Ni(II) and Cu(II) were found to be 28.9, 13 and 36% at 0.01 M initial metal ion concentration, 58.3, 37.5 and 79.17% at 0.001 M initial metal ion concentration and 66.6, 44.4 and 86.1% at 0.0001 M initial metal ion concentration, respectively, after 5 h.Also, the results showed that the removal percentage of metal ions increased as the concentration of the metal ion solution decreased.The order of removal percentage of metal ions was as follows : Cu 2+ > Co 2+ > Ni 2+ .The complexation of Cu(II) ions was higher than that of the other metal ions.This may be due to the optimum distribution of the ligand function for satisfying the stereochemical requirement for Cu(II) ion complexation [65].

Optical properties
The number and positions of the peaks in the absorbance spectra of coordination compounds depend on many factors such as the identity of the metal, oxidation state of metal and geometry of the complex.The optical band gap (Eg) was determined from the electronic spectra of complexes by using the following Tauc equation: Where α is the absorption coefficient and was calculated from the relation α = A/d (where A is the absorbance and d is the thickness of the cell), B is the optical constant, h is Planck's constant and ʋ is incident light frequency [66].The optical band gap (Eg) can be obtained by extrapolation of the linear portion of the plot of (αhν) 2/n versus hν (Figure 5).For a direct transition (n = 1), Tauc equation became (αhν) 2 = B (hν-E g ).From the curve, the values of the direct band gap (E g ) equal 2.80, 2.83 and 2.98 eV for Co(II), Cu(II) and Ni(II) complexes, respectively.These values suggested that the synthesised complexes are semiconductors and lie within the same range of highly effective photovoltaic materials [67].

Optimisation of the structures
The optimised structure of H 2 L and its metal complexes along with the labelling of atoms were shown in Figure S8, and some selected bond lengths and bond angles are displayed in Tables S3-4.The bond lengths N1-C13 and N2-C14 (1.280 Å) azomethine, C 17 -O 1 , C 23 -O 2 (1.344 A°) for ligand were longer than all dinuclear complexes due to the coordination of ligand to metal through two azomethine nitrogen atoms and two oxygen of hydroxyl groups [68].The bond distances between the metal and both the chloro and aqua ligand were in the range 2.118-2.485A° and 1.782-1.980A°, respectively.These values are in the reported range.The bond angles in the coordination sphere of Co(II), Ni(II) and Cu(II) complexes, N2-Co2-O1, N1-Co1-O1, O2-Ni2-N2, O1- Ni1-N1, N3-Cu1-O3, N4-Cu2-O4, N2-Cu1-O2, N1-Cu2-O1 were found approximately near to the perpendicular values.The bond angles (89.6-96.2A°) and bond lengths (1.778-1.981A°) as obtained from DFT studies reveal the octahedral geometry for these complexes.

Molecular electrostatic potential (MEP) of ligand
The MEP maps of the ligand H 2 L are shown in Figure S9.The negative regions were concentrated over the oxygen atoms (deep red/yellow) of the OH groups and nitrogen atoms of imine groups.However, the positive regions were observed around the hydrogen atoms of phenolic OH groups (blue/green).Therefore, this molecule has four possible sites for coordination through two imine-N and two phenolic OH groups.The red region is best suited for electrophilic attack, while the blue region is best suited for nucleophilic attack.The different values of the electrostatic potential at the surface were represented by different colours and increases in the order: red < orange < yellow < green < blue [69].

Mulliken atomic charge
All the hydrogen atoms have a positive charge and both (H11) and H12 atoms have the highest positive atomic charge (0.328e).This is due to the presence of oxygen atoms (O1 and O2) in the phenolic group.In addition, the charge of the donor oxygen atoms in phenolic groups and nitrogen atoms (N1 and N2) in imine groups exhibited a negative charge (Figure S10).

Frontier molecular orbitals and global quantities
The highest occupied molecular orbital and lowest unoccupied molecular orbital have been calculated for ligand H 2 L and its metal complexes.The energy gap between these orbitals was used to explain the stability and chemical reactivity of a molecule.The green and red colours in the diagram indicate the negative and positive parts of orbitals as can be seen in Figure 6.The energy gap ΔE (E LUMO -E HOMO ) is a significant stability index, which helps to describe the kinetic stability and chemical reactivity.Molecules with a small energy gap are more polarised and reactive than hard ones since they easily offer electrons to an acceptor.The energy gaps between [LUMO−HOMO] for H 2 L and its Co(II), Ni(II) and Cu(II) complexes are −3.501,−0.438, −1.263 and −1.885 eV, respectively.An electronic system with a larger HOMO-LUMO gap should be less reactive than having a smaller gap.The energy gap ΔE of the ligand was greater than cobalt, nickel and copper complexes.These indicate that the complexes have more chemical activity than ligands.Also, Co(II) complex is more stable and highly reactive than both the Schiff base ligand and the other complexes.The HOMO and LUMO energies were used to find the global descriptors as given in Table S5.The quantum chemical parameters such as ionisation potential (I), electron affinity (A), electronegativity (X), globa hardness (η), softness (S) and electrophilicity index (ω) were calculated from the energies of frontier molecular orbitals (E HOMO , E LUMO ).

Antimicrobial activities
The comparison of the biological activity of the Schiff base ligand and its Co(II), Cu(II) and Ni(II) complexes with the standard bactericide and fungicide are shown in Figure 7 and the activity index of H 2 L and its metal complexes showed in Figure S11.The results revealed that: • The ligand has no activity against all antimicrobial species except E. coli.
• The Ni(II) complex has moderate activity against S. aureus, B. subtilis, E. coli bacteria and C. albicans and A.fumigatus Fungi.• Also, the Ni(II) complex has no activity against p. vulgaris bacteria.
• The antibacterial activity of the complexes was found to follow the order: Ni

Disclosure statement
No potential conflict of interest was reported by the author(s).

Figure 1 .
Figure 1.The UV-Vis spectra of synthesised ligand and its metal ion complexes.

Figure 2 .
Figure 2. The proposed structure of metal complexes.

Figure 3 .
Figure 3.Effect of time, concentration and temperatures of MB on the percentage of degradation of MB by catalysis in presence and absence of H 2 O 2 .(a-f).The order of photocatalytic degradation of MB with Cu(II), Co(II) and Ni(II) as catalysts at constant concentration of MB (9 ppm) after time 65 min (g) and at temperature (333 K) after time 30 min (h).

Figure 4 .
Figure 4. Percentage of removal Cu(II), Co(II) and Ni(II) at different concentrations by H 2 L at constant time (5 hrs.).

Figure 6 .
Figure 6.The frontier molecular orbitals of the ligand and its complexes.
Three complexes with molecular formulas [Co 2 (H 2 L)(H 2 O) 4 Cl 4 ], [Ni 2 (H 2 L)(H 2 O) 4 Cl 4 ] and [Cu 2 (H 2 L) 2 Cl 4 ].3H 2 O were isolated from the reaction of Co(II), Cu(II) and Ni(II) metal ions with H 2 L. The ligand acted as tetradentate in all prepared complexes.The electronic spectra of the complexes showed remarkable shift in π→π* and n→π* compared to the free ligand.Fluorescence studies indicated that the synthesised complexes can serve as potential photoactive materials.The activation thermodynamic parameters, such as (Ea, A, ∆H*, ∆S* and ∆G*) were calculated using Coats-Redfern method.The antibacterial activity of the complexes was found to follow the sequence: Ni (II) > Co (II) > Cu for S. aureus bacteria and Ni (II) > Cu (II) > Co (II) towards B. subtilis bacteria and E. coli.The theoretical calculations were carried out by the

Figure 7 .
Figure 7. Antimicrobial activity of the H 2 L and its metal complexes.

Table 1 .
Most important IR spectral bands of the ligand and complexes.

Table 2 .
Thermodynamic data of the thermal decomposition of the complexes.