One-pot synthesis, characterization, photocatalytic activity and biological studies of Co(II), Ni(II) and Cu(II) complexes of a tetraazamacrocyclic Schiff base

Abstract Template condensation between pentane-1,5-dial and triethylenetetramine in the presence of divalent metal salts in methanolic solution resulted in mononuclear 15-membered tetraazamacrocyclic Schiff base metal complexes, [M(C11H22N4)X2], where M = Co(II), Ni(II) and Cu(II), X = Cl− and C11H22N4 is the tetradentate macrocyclic ligand. The bonding and overall geometry of the complexes are inferred through elemental analysis, FT-IR, UV–vis, 1H/13C NMR, ESI-mass, molar conductance, magnetic susceptibility and TG measurements. On the basis of the absorption bands in UV–vis spectra and magnetic moment analyses, a six-coordinate octahedral geometry is proposed for all the metal complexes. The formulation of the metal chelates is compatible with the thermal decomposition profiles. Photocatalytic efficiency of the macrocyclic Ni(II) complex was studied using solar water-splitting. The in vitro antimicrobial activity of metal complexes has been tested against certain bacterial and fungal species. The scavenging ability of DPPH free radicals was used to test the antioxidant activity of the complexes. In addition, MTT assay was used to assess the cytotoxicity of Cu(II) chloro complex against L929 mouse fibroblast cell lines.


Introduction
Coordination chemistry of macrocyclic compounds have distinctive properties such as ionic and molecular recognition [1,2], biomimic functions [3,4], synthetic ionophores, models for studying magnetic exchange phenomena, therapeutic reagents in chelate therapy for metal intoxication, as a cornerstone in supramolecular chemistry [5,6], to study host-guest interactions and in phase transfer catalysis [7][8][9]. The synthesis of metal-containing macrocycles from an in situ one-pot template condensation is a broad subject of chemistry [10,11]. As a result, template reactions for synthesis of macrocyclic complexes have become ubiquitous, with transition metal ions serving as the templating agent in most cases [12,13]. Due to developments in bioinorganic chemistry, macrocyclic transition metal complexes have drawn more focus, being recognized as a variety of models for biologically significant species [14,15]. Particular interest has centered upon burgeoning applications of tetraazamacrocycles [N 4 ] and their transition metal complexes, including artificial metalloenzymes [16], as luminescent sensors for photodynamic therapy and biomedical diagnostics [17], mechanically interlocked molecular systems [18] and as chemical sensors [19,20]. The propensity of synthetic tetraazamacrocycles to bind biomedically relevant metal ions and serve as models for biological macrocyclic systems has stimulated the current research [21].
Design and synthesis of tetraazamacrocyclic Schiff base complexes in which a ligand comprises four nitrogen donor sites in a ringed structure around the metal ion represents an important objective in transition metals. Consequently, metal template cyclizations of dicarbonyl compounds with polyamines were analyzed. Extensive study has been conducted in many laboratories to characterize the complexation of Schiff base tetraazamacrocyclic systems since the discovery of their potential uses as dyes and pigments [22], stereospecific catalysts for hydrolysis and epoxidation [23], anticorrosives [24,25] and metal extractants in basic and applied sciences [26,27]. Numerous academic articles have also documentedSchiff base macrocycles , which are formed when different dicarbonyls condense with triethylenetetramine (TETA) along with its versatile bioapplications [28,29]. Polyamine derivatives, such as diethylenetriamine and triethylenetetramine are important pharmaceutical and chemical intermediates in supramolecular self-assembly, enzyme catalysis, macrocyclic complex synthesis and biological medicine because they are closely related to protein biological synthesis, cell growth, amine nucleic acid, hormone, lipid and sugar metabolism [30,31]. For these reasons, metal chelates of tetraazamacrocyclic Schiff bases are of keen interest.
This article is focused on the following objectives: synthesize 15-membered tetraazamacrocyclic Schiff base complexes of divalent cobalt, nickel and copper adopting a one-pot template condensation approach by using pentane-1,5-dial and triethylenetetramine (TETA), characterizing the structure of metal complexes by utilizing spectroscopic techniques, determining antibacterial and antifungal activities against certain microbial species, conducting photocatalytic study for the determination of photocatalytic efficiency of hydrogen production in the Ni(II) complex, achieving antioxidant and in vitro cytotoxic studies and performing thermal studies to understand temperature dependence of the prepared metal chelates.

Instrumentation
Estimation of carbon, hydrogen, nitrogen and oxygen in the metal chelates was carried out by a CHNSO elemental analyzer (Elementar Vario EL III). FT-IR spectra of the complexes at room temperature were recorded as KBr pellets with a Perkin Elmer C96217 model spectrophotometer from 400 to 4000 cm À1 . Far-IR spectra of the metal complexes were acquired from 30 to 500 cm À1 using a 3000 Hyperion Microscope with Vertex 80 FTIR System. The solid absorption spectra were carried out on an UV À vis À NIR spectrophotometer (Agilent Technologies, Cary 5000) from 200 to 800 nm. Mass spectra of the solid complexes were recorded using a HREMI, Thermo Scientific Exactive system. To analyze the photocatalytic hydrogen production with a metal complex, a Solar Simulator (AAA Solar light line A1, Science Tech. Pvt. Ltd.) and a Perkin Elmer Gas Chromatograph Clarus 590 were employed. Thermal analysis was carried out by using a simultaneous thermal analyzer (Perkin Elmer, STA8000). Molar conductivity of the complexes was measured as 10 À3 M DMF solutions at 298 K using a Systronics model 303 direct-reading conductivity bridge. A Lakeshore 7410 model vibrating sample magnetometer was used to assess magnetic susceptibility of the samples at room temperature.

Synthetic protocol of the macrocyclic Schiff base metal complexes
Solid macrocyclic Schiff base complexes were synthesized by one-pot reactions of pentane-1,5-dial and triethylenetetramine in the presence of a metal(II) chloride salt (CoCl 2 Á2H 2 O/NiCl 2 Á6H 2 O/CuCl 2 Á2H 2 O). Metal(II) chloride (1.5 mmol) dissolved in methanol (15 mL) was added to a methanolic solution (15 mL) of triethylenetetramine (1.5 mmol). The solution was then refluxed for 0.5 h. Then, in the refluxing mixture, a methanolic solution (15 mL) of pentane-1,5-dial (1.5 mmol) was added, and the mixture was refluxed for another 5-7 h. The resultant solution was concentrated, cooled to room temperature and stored in a desiccator overnight. Colored precipitates formed after overnight cooling, were filtered out, washed in cold MeOH, diethylether and dried under vacuum over anhydrous CaCl 2 . Template syntheses of macrocyclic Schiff base complexes are shown in Scheme 1. [

Photocatalytic study of hydrogen production by water splitting
The photocatalytic performance of 2 is evaluated based on the rate of hydrogen generation from water under visible light irradiation. Hydrogen generation by solar water splitting is a promising and sustainable method of energy generation as it utilizes a renewable source of energy (sunlight) to produce hydrogen gas, which can be used as a fuel in various applications [32][33][34]. Several attempts have been undertaken to generate inorganic semiconductor photocatalysts as a sustainable method for producing hydrogen using solar energy [35][36][37]. The photocatalytic hydrogen generation process is carried out in a 50 mL glass reactor with a length of 6 cm and a diameter of 3 cm, at 25 C, using a Solar Simulator (AAA Solar light line A1, Science Tech. Pvt. Ltd.). Using the Solar Simulator, 5% (w/v) of the synthesized metal complex (sample) is immediately disseminated in 20 mL water under 1 Sun (100 mW cm À2 ) irradiation. The lamp is 12.5 cm away from the solution. Throughout the experiment, the solution is subjected to continuous magnetic stirring at 950 rpm to ensure homogeneous mixing. The generated hydrogen gas is collected and analyzed using a Hydrogen Gas Chromatograph (Perkin Elmer Gas Chromatograph Clarus 590) provided with openLAB software. High purity nitrogen gas (99.99%) is used to transport the hydrogen gas produced during the solar water splitting process through the chromatograph. The purpose of using high purity nitrogen gas as the carrier gas is to ensure that the hydrogen gas sample being analyzed is not contaminated by any other gases and to achieve accurate results in the analysis 2.5. Biological experiments 2.5.1. In vitro analysis of antibacterial activity In vitro antibacterial activity of 1-3 were preliminarily screened against pathogenic bacteria using the Agar well diffusion method [38]. Four bacterial strains, including gram-positive strains Staphylococcus aureus (MTCC 96) and Enterococcus faecalis (MTCC 439), and gram-negative strains Escherichia coli (MTCC 443) and Pseudomonas aeruginosa (MTCC 424), were chosen for their clinical importance in causing diseases in humans. The minimum inhibitory concentration (MIC) was determined by reported macrodilution tube method.
2.5.1.1. Preliminary screening. The bactericidal activities of 1-3 were tested at 50, 100 and 200 mg mL -1 concentrations on Mueller Hinton Agar (MHA) plates. Culture media (38 g) (MHA) was suspended in 1000 mL distilled water and gently heated to 100 C to completely dissolve the medium. It was placed into appropriate bottles, autoclaved at 15 lbs pressure (121 C) for 15 min, and thoroughly mixed. In a laminar flow hood, the medium was then put into sterilized flat bottomed petri plates that had been dried in a 37 C incubator. The medium was allowed to solidify before being kept at 4 C for future use. Four wells were cut using a sterile well bore with an 8 mm diameter. To obtain log phase cultures, freshly sub cultured bacteria strains were suspended in one mL nutrient broth and incubated at 37 C for 2 h. Opacity was also tested using McFarland turbidity standards of 0.5 (approximately 1 to 2 Â 10 8 Cfu mL -1 ). To obtain an even inoculum, 100 mg mL -1 of pure cultures of test organisms were swabbed equally over the surface of MHA plates using a sterile swab and allowed to dry for 5 min. Each well received 100 mg mL -1 of varying concentration of tested samples, 0.05% of DMSO (negative control) and 125 mg mL -1 of standard drug streptomycin (positive control). Following an 18-h incubation period at 37 C, the diameter of the inhibitory zone was measured in millimeters to assess antibacterial activity.

Determination of MIC.
The MIC is the lowest concentration of an antimicrobial agent that will inhibit visible growth of a specific microorganism after overnight incubation. The MICs of 1-3 were tested against bacterial strains through a macrodilution tube method [39]. The method involves preparing serial dilutions of 1-3 in sterile tubes at test concentrations ranging from 128 to 0.25 mg mL -1 in DMSO, then inoculating each tube with a fixed amount of the microorganism. Turbidity was observed after incubating the inoculated tubes at 37 C for 24 h. MIC was expressed as the lowest concentration of the tested complexes, which inhibited the growth of bacteria observed by lack of turbidity in the tubes. Streptomycin (positive control) was also tested for MIC.

Antifungal study by the tube method
Aspergillus niger and Penicillium chrysogenum were two fungal strains chosen for testing the antifungal efficacy of 1-3. On Sabouraud's Dextrose Agar (SDA), the fungal strains were examined. 65 g of SDA was dissolved in 1000 mL of double-distilled water. The process of sterilization was carried out in an autoclave at 121 C for 15-20 min. For testing, concentrations of 200 and 300 mg mL -1 extract were diluted in 1 mL of 0.05% DMSO. The positive and negative controls were imidazole (standard medication; 100 mg mL -1 ) and 0.05% DMSO, respectively.
The antifungal activities of 1-3 were tested against fungi using a Test tube technique [40]. The samples were prepared by adding the examined metal complexes directly to the molten media (SDA) with up to 1 mL in each tube. After thoroughly mixing with the medium, the samples were allowed to set up in slanting positions to harden. The test fungi were introduced into corresponding test tubes after the tubes were checked for sterility. The tubes were inoculated with A. niger and P. chrysogenum and incubated at room temperature for 14-21 days before the results were recorded.

Antioxidant activity by DPPH radical scavenging assay
The scavenging ability of DPPH free radicals was used to test the antioxidant activity of 1-3 [41]. Prior to UV measurements, a methanolic solution of DPPH (60 M) was prepared and 3.9 mL of this solution was then mixed with various concentrations of methanolic solution of 1-3 (2.5, 25, 50, 100 and 200 mg mL -1 ), which were maintained at room temperature for 15 min in the dark. Reduction of the DPPH radical was observed at 515 nm using a UV-vis spectrophotometer by measuring the decrease in absorbance. Ascorbic acid was used as a reference standard and a stock solution was prepared by diluting it in distilled water using the same concentrations of 1-3. A 3.9 mL methanolic solution of DPPH (60 M) was used as the control without adding any tested compounds and reference ascorbic acid. The 95% methanol was used as blank. The percent scavenging activity (% inhibition) of compounds was calculated using the following equation.
ð%Þ scavenging activity ¼ ðAbsorbance of Control À Absorbance of sampleÞ=Absorbance of ControlÞ Â 100 Drug activity was expressed as the 50% inhibitory concentration (IC 50 ). IC 50 values were calculated from regression lines, where x was the tested compound concentration in lg mL -1 and y was percent inhibition of the tested compounds.

In vitro cytotoxic effect determination by MTT assay
The macrocyclic Schiff base Cu(II) complex was tested on L929 mouse fibroblast cells for cytotoxicity. 3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT) assay [42] was used to assess the cytotoxic activity of 3. To determine cell viability, the MTT assay uses mitochondrial dehydrogenase to reduce MTT, tetrazolium yellow dye, to insoluble formazan (purple color) in living cells. A suitable solvent is used to dissolve the insoluble purple formazan into a colored solution. At a wavelength of 570 nm, the absorbance of this colored solution can be measured. The dose response curves can be used to determine a drug's cytotoxic ability to cause cell death, while the amount of purple formazan produced by untreated control cells can be measured. The cell line was grown in DMEM (Dulbecco's Modified Eagle's Medium) supplemented with 10% FBS, sodium bicarbonate, L-glutamine and an antibiotic solution containing Penicillin (100 lg mL -1 ), Streptomycin (100 lg mL -1 ) and Amphoteracin B (2.5 lg mL -1 ) in a 25 cm 2 tissue culture flask. In a humidified 5% CO 2 incubator, cultured cell lines were incubated at 37 C. After 24 h, the growth medium was removed, 3 in 5% DMEM were five times serially diluted by two-fold dilution (100, 50, 25, 12.5, 6.25 mg in 100 mL of 5% DMEM) and each concentration of 100 mL were put in the respective wells in triplicates and incubated at 37 C in a humidified 5% CO 2 incubator. The entire plate was examined in an inverted phase contrast tissue culture microscope (Olympus CKX41 with Optika Pro5 CCD camera) after 24 h of treatment, and microscopic observations were recorded as photographs. Any visible changes in cell morphology, such as rounding or shrinkage of cells, granulation and vacuolization in the cytoplasm, were deemed indications of cytotoxicity. Control is prepared by the addition of a culture medium to the cells that are not treated with 3. These cells are seeded in the same way as the treated cells, and all the culture conditions are kept the same to ensure that any changes observed in the treated cells are not due to any inherent changes in the cells, but rather due to the cytotoxic effects of 3. The percent of cell viability was determined using the absorbance measurements as follows: Cell viability ð%Þ ¼ Mean OD Samples=Mean OD of control group Â 100 ðwhere OD ¼ optical densityÞ

Chemistry
A simple approach was used to produce 1-3, emphasizing the use of readily accessible starting compounds, low reaction temperatures and better product yields. The isolation of Schiff base tetraazamacrocyclic complexes was achieved by [1 þ 1] template condensation of pentane-1,5-dial with triethylenetetramine with Co(II), Ni(II) and Cu(II) ions using methanol as solvent. The complexes are colored, soluble in organic solvents including dimethylformamide, dimethylsulphoxide and dichloromethane, and stable to the atmosphere at room temperature. Complexes 1-3 were thermally stable to 203 C, beyond which they decomposed.

Electronic absorption spectral analysis and magnetic susceptibility measurements
UV-visible spectra and magnetic moments of macrocyclic complexes have been used to assign the geometry of 1-3 (Table 2). The solid absorption spectrum of [Ni(C 11 H 22 N 4 )Cl 2 ] exhibits absorption bands at 678 and 772 nm which may be ascribed to 3 B 1g ! 3 B 2g and 3 B 1g ! 3 E g , respectively, and are comparable with distorted octahedral geometry around nickel. Absorption at 392 nm can be assigned due to ligand-to-metal charge transfer band (LMCT) [43]. The intense higher energy band at 235 nm may be due to a p!p Ã transition of the (C ¼ N) group [44]. The effective magnetic moment (m eff ) per metal ion in 2 measured at room temperature was 3.12 BM [45]. The band gap of the Ni(II) macrocyclic complex is estimated to be 2.74 eV based on the solid absorption spectrum. The solid absorption spectrum and Tauc plot of 2 are depicted schematically in Figure 1(a,b), respectively.
Absorption with k max 232 nm can be assigned to intraligand transitions involving the azomethine functions in 3. A moderately intense band centered at 448 nm is assigned to ligand-to-metal charge transfer band [46]. These charge transitions are most likely from p-orbitals of azomethine to d-orbitals of metal ions. 3 also displays two absorption bands at 532 and 655 nm which can be attributed to 2 B 1g ! 2 E g and 2 B 1g ! 2 B 2g transitions, respectively, corresponding to distorted octahedral geometry (Supplementary Material, Figure S1(a)). It has a magnetic susceptibility of 1.78 BM [47], which is close to theoretical spin-only values for Cu 2þ (3d 9 system). As can be shown in Supplementary Material, Figure S1(b), the band gap of the solid copper complex is calculated to be 3.59 eV.
In the solid electronic spectrum of 1, intense band observed at 462 nm is assigned to charge transfer from the ligand to metal ion (LMCT). According to previous findings, for a d 7 octahedral geometry, three spin allowed transitions are expected: 4 T 1g (F) ! 4 T 2g (F), 4 T 1g (F) ! 4 A 2g (F) and 4 T 1g (F) ! 4 T 1g (P). However, in [Co(C 11 H 22 N 4 )Cl 2 ] only two absorption bands at 629 and 704 nm are identified, which could be owing to the masking of the high intensity charge transfer band. The magnetic moment of 4.88 B.M. for 1 established an octahedral geometry [47]. Focusing on the solid absorption spectrum, it has been determined that the measured band gap for 1 is 3.52 eV. In Supplementary Material, Figures S2(a) and (b), the UV-vis spectrum and Tauc plot of 1 are displayed.

FT-IR spectra and mode of binding
The IR spectral observations were used to provide a preliminary identification of the formation of metal chelates. A strong intensity band was present in the IR spectrum of triethylenetetramine at 3400-3300 cm À1 which corresponds to V(NH 2 ) groups that were not found in IR spectra of 1-3. Likewise, there was no strong absorption band at 1740-1710 cm À1 , indicating that the carbonyl (>C ¼ O) stretching frequency of pentane-1,5-dial moiety was absent. The condensation of the amino group of triethylenetetramine and the carbonyl group of pentane-1,5-dial and the formation of the Schiff base macrocycle are confirmed by disappearance of these bands and the appearance of a new strong absorption band near 1629-1620 cm À1 , which could be ascribed to V(C ¼ N) [48]. Table 3 shows that the value of stretching vibration of azomethine group (C ¼ N) was less (1620-1585 cm À1 ) than expected (1690-1650 cm À1 ) [49]. This decreased value of (C ¼ N) stretching can be rationalized by a drift of lone pair density of azomethine nitrogen toward the metal ion, implying that coordination occurs via nitrogen of (C ¼ N) groups. IR spectra of the complexes also revealed a band at   [29]. The presence of these bands suggest coordination of azomethine nitrogen with the metal ion. Bands at 300-320 cm À1 may be assigned to m(M-Cl) vibrations [50]. IR spectral data of 1-3 are provided in Table 3 along with their tentative assignments. Figure 2. In the metal free macrocyclic Schiff base compounds, a signal appeared at 7.90-7.98 pm due to azomethine protons (-CH ¼ N). This peak was shifted downfield to 7.67-7.76 ppm in the Schiff base macrocyclic metal complexes . Likewise, in the NMR spectrum of 2, Figure 2, the azomethine CH protons (-CH ¼ N; 2H) are at 7.68 ppm [28] due to the coordination of nitrogen atom to nickel. The methylene protons adjacent to the azomethine group (-CH ¼ N) are a triplet at 2.31 ppm in the NMR spectrum. There are no resonances related to the -NH 2 or > C¼O groups, implying that the suggested macrocyclic skeleton was produced via the interaction of pentane-1,5-dial with TETA. The NH signal is at 3.52 ppm, indicating the participation of the NH group in chelation without proton displacement [28]. The methylene protons in the vicinity of secondary amine are responsible for a multiplet at 3.43 ppm. The methylene protons of the triethylenetetramine moiety (4H, 2CH 2 ) are represented by another multiplet at 2.67 ppm. Moreover, the pentane-1,5-dial moiety has methylene protons (6H, 3CH 2 ), in the range of 1.93-1.54 ppm.

H NMR spectrum of [Ni(C 1H 22 N 4 )Cl 2 ] is shown in
The experimental proton decoupled 13 C NMR spectrum of the Ni(II) chloro complex is illustrated in Supplementary Material, Figure S9. There are six different types of signals, indicating six different carbon environments in 2. At 167.50 ppm, the iminic carbon atom in the azomethine group (-CH ¼ N-) produces a downfield signal. The -CH 2 carbon in the triethylenetetramine moiety is associated to a signal at 52.71 ppm. The -CH 2 carbon atom in the proximity of secondary amine is responsible for a signal at 50.38 ppm. The methylene protons in the region of the azomethine group are accountable for a signal at 49.5 ppm. In the measured spectrum, signals in the range 19.00-23.00 ppm are from -CH 2 carbon of the dialdehyde moiety. Table 3. IR spectral assignments of macrocyclic Schiff base metal complexes (cm À1 ).

Mass spectral analysis
For more structural information, see Figure 3 which depicts the ESI mass spectrum of 2. 1-3 have intact molecular ion peaks, indicating the formation of a six-coordinate species. Calculated m/z ratios have been used to make the assignments. The proposed molecular formulas of 1-3 were validated by correlating their molecular weight with m/z values in the ESI mass spectra of the complexes. The ESI mass spectrum of Ni(II) chloro complex ( Figure 3) showed a molecular ion peak [Ni (C 11

Thermal analysis (TG/DTA)
Thermal analysis was carried out to further clarify the structural formulas of 1-3, and the data are shown in Table 4. TG/DTA of the macrocyclic Schiff base metal complexes was measured in a N 2 atmosphere at a heating rate of 10 C min -1 . Thermograms of 1-3 show similar decomposition patterns, including one and two stages and resulting in the formation of metal oxide (Figure 4 and Figures S12 and S13 of Supplementary Material).
[Ni(C 11 H 22 N 4 )Cl 2 ] is stable up to 237 C ( Figure S12, Supplementary Material) and shows a continuous weight loss up to 525 C, according to the TG and DTG curves. Metal oxide (NiO) is left behind as a residue when it decomposes completely in one step at 424 C with a weight loss of 78.02%. These findings demonstrated that coordinated water was not present in Ni(II) chelate, proving the six-coordinate geometry suggested by chemical analyses and FT-IR data.
The TG curve (Figure 4) of 3 showed thermal decomposition in two steps from 204 to 662 C. During the first stage of thermal decomposition, two molecules of HCl with a mass loss of 20.83% (calcd. 21.15%) are lost. The loss of C 11 H 20 N 4 is shown by the large mass loss occurring between 321 and 662 C in the second step of decomposition (weight loss found ¼ 59.58%, theoretical weight loss ¼ 60.37%). This mass loss is identified by DTA at 400 C. CuO is considered as the final residue of the thermal pyrolysis of 3 and the total mass losses during the decomposition processes are revealed to be 80.41% (calculated as 81.52%). The amount of metal observed in the oxide residue was found to be in consistent with values calculated using the composition formula suggested based on the elemental analysis data.
[Co(C 11 H 22 N 4 )Cl 2 ] also reveals two stages of decomposition between 223 and 760 C (Supplementary Material, Figure S13). Two HCl molecules and C 7 H 5 N were lost, which resulted in an estimated mass loss of 50.55% (theoretical weight loss ¼ 51.75%) for the first step between 223 and 442 C with a DTA at 261 C. Within the temperature range of 442-760 C, the loss of C 4 H 15 N 3 (weight loss found ¼ 30.64%, theoretical weight loss ¼ 30.92%) relates to the second stage of decomposition. According to the information in Table 4, the total weight losses from the decomposition stages are determined to be 80.75% (calcd.82.67%), and CoO is recognized as a residue.

Structural interpretation
According to the IR spectra of the macrocyclic complexes, the metal ion is coordinated to two azomethine N and two secondary amine NH of the macrocyclic ring. The molar conductance (K M ) values for the prepared complexes are 12-18 X À1 cm 2 mol À1 , showing a non-electrolytic nature with no ion present outside the coordination spheres of the complexes; 1-3 are neutral. UV-vis spectra and magnetic susceptibility measurements stipulate that the Schiff base macrocyclic complexes exhibited distorted octahedral geometry. Thermal analysis was also consistent with [M(C 11 H 22 N 4 )X 2 ] for 1-3.

Photocatalytic water splitting for hydrogen evolution
Solar water splitting was used to investigate the photocatalytic efficiency of hydrogen generation in 2 [32][33][34][35]. Light absorbing semiconducting materials containing metal ions are commonly used in photocatalytic hydrogen evolution by water splitting reactions. The photocatalytic capabilities of the metal complex depend on how it behaves as a semiconductor [37]. A semiconducting property is indicated by the band gap (E g ) of the Ni(II) complex, which is determined from the Tauc plot as 2.74 eV (Figure 1(a)). The photoactive organic ligands are primarily responsible for the semiconducting characteristics of metal chelates. Under solar irradiation, the tetradentate macrocyclic ligand (C 11 H 22 N 4 ) in [Ni(C 11 H 22 N 4 )Cl 2 ] could act as an antenna to absorb light in the visible region. The light excitation of the produced photocatalyst typically takes place in the visible region (>420 nm). The electrons move to the conduction band while the holes are left in the valence band of the photocatalyst, resulting in formation of pairs of negative electrons (e -) and positive holes (h þ ), as shown in Figure 5. The energy difference between the valence band and the conduction band is referred to as the "band gap", which is also known as the "photo-excited" state of the photocatalyst. In order for the light to be absorbed by the photocatalyst, this must match the wavelength of the light. The excited electrons and holes separate and move to the photocatalyst's surface after photoexcitation. Here, they serve as a reducing agent and an oxidizing agent to produce O 2 and H 2 , respectively, in the photocatalytic water-splitting reaction [51]. A schematic view of the principle of the produced photocatalytic system for water is illustrated in Figure 5. Under solar irradiation from a suspension of 2 in distilled water, photocatalytic water splitting was examined. A catalytic quantity of 0.1 g Ni(II) complex in 20 mL distilled water generates 2.4 mL hydrogen in 30 min when subjected to sunlight. The findings of the experiment are summarized in Table S1 (Supplementary Material) by varying the catalytic amount of the Ni(II) complex (0.2, 0.5, 0.75 and 1.0 g) and the volume of hydrogen produced. After 30 min of sunlight irradiation, the amount of hydrogen evolved rises up to 0.75 g Ni(II) complex, then falls. With a 0.75 g catalytic loading, a maximum volume of 9.8 mL of H 2 is produced. As a result, 0.75 g is chosen as the optimal catalytic loading. The produced photocatalyst was filtered from the reaction mixture after irradiation and dried in air.
The experiment is repeated with a catalytic loading of 0.75 g for 1 h and the rate of hydrogen production for irradiation every 10 min is also investigated; the data are displayed in Figure 6. The volume of hydrogen evolved increases until 40 min and then gradually drops. A maximum volume of 5.6 mL is seen in Figure 6 at 30-40 min. The decrease in Ni(II) photocatalyst activity after 40 min could be attributable to an increase in the rate of charge recombination. Charge recombination is a key competitive mechanism within the photocatalyst that has a significant impact on the efficiency of the photocatalytic water splitting reaction [52].

Antibacterial activity
Complexes 1-3 were screened for their in vitro antibacterial activities to assess their growth-inhibiting potential against gram-positive bacterial strains S. aureus, E. faecalis and gram-negative bacterial strains E. coli, P. aeruginosa and then compared with the standard antibiotic streptomycin. The antibacterial activities were assayed by measuring the diameter of the inhibition zone in millimeters. The data are listed in Table S2 and depicted in Figures S14-S16 (Supplementary Material). The bar diagram in Figure  7 shows that 1-3 exhibit varying degrees of inhibitory effects on the growth of bacterial strains. Bacteriological studies revealed that among 1-3, 1 showed the highest inhibition zone of 19 mm at 200 mg mL -1 , which is almost equal to streptomycin against P. aeruginosa when compared to 2, but 3 was inactive with P. aeruginosa and S. aureus. At 100 mg mL -1 , the inhibition zone of [Ni(C 11  showed inhibition zones of 12 and 10 mm at 200 mg mL -1 , respectively, toward E. faecalis, although 2 had none. With increasing 2 concentration, the antibacterial activity against P. aeruginosa increased and was higher than that against E. coli and comparable to that of streptomycin. Each complex was effective at some dilutions. The MICs of 1-3 were determined by macrodilution tube method in which the effectiveness was observed at lower concentrations (Table S3). Comparisons between the MIC values of 1-3 and streptomycin were made. Standard antibiotic streptomycin had MICs of 2, 2, 5 and 5 mg mL -1 against  E. coli, E. faecalis, S. aureus and P. aeruginosa, respectively. The MIC of 1 against P. aeruginosa was 9 mg mL -1 which is close to MIC of antibiotic against similar bacterial strain. Compound 2 had a MIC of 128 mg mL -1 against E. coli and a MIC of 32 mg mL -1 against P. aeruginosa. Comparatively low activity was seen for 3 against most of the bacterial strains. It can be concluded that the antimicrobial activity possessed by metal complexes is an intricate blend of various factors [53].

Antifungal activity
The in vitro antifungal activities of 1-3 were investigated against fungal strains P. chrysogenum and A. niger using imidazole as standard drug by test tube method. The photograph of the tubes are shown in Figures S17-S19 and the results are listed in Table S4. 1-3 show effective antifungal activity against P. chrysogenum and A. niger, with growth inhibited at the tested concentrations of 200 and 300 mg mL -1 . 300 mg mL À1 for [Co(C 11 H 22 N 4 )Cl 2 ] and 200 mg mL -1 for [Ni(C 11 H 22 N 4 )Cl 2 ] against P. chrysogenum and A. niger, respectively, exhibited only mild fungal growth as compared to imidazole. However at both concentrations, the Cu(II) complex had the highest levels of fungal growth inhibition against P. chrysogenum. When compared to the standard, 2 and 3 failed to regulate the growth of P. chrysogenum and A. niger when used at the tested concentrations of 300 and 200 mg mL -1 .

Antioxidant potency
The scavenging ability of DPPH free radicals was used to test the antioxidant activity of 1-3 at various concentrations. The observed variations in the free radical scavenging abilities on the basis of percent inhibition are provided in Figure 8 and the results are listed in Table S5. The concentration-dependent curve in Figure 9 displays the ability of 1-3 to scavenge free radicals. As illustrated in Figure 8, the scavenging potential of compound was concentration dependent and percent inhibition increased as the concentration of the tested drug increased. The scavenging abilities of 1-3 were comparable to that of ascorbic acid. By comparing their IC 50 values, this was further validated. From Figure 8, the IC 50 value of [Cu(C 11 H 22 N 4 )Cl 2 ] was 22.14 mg mL -1 , which is comparable to that of ascorbic acid (IC 50 value 16.02 mg mL -1 ). Among the examined complexes, [Cu(C 11 H 22 N 4 )Cl 2 ] demonstrated the highest scavenging potential, which results in significant antioxidant activity and can be promoted as a potential treatment for cancer and other disorders. The redox characteristics may be responsible for the difference in antioxidant activity values of 1-3. The redox properties of metal complexes are influenced by axial ligation, ring size of chelate and degree of unsaturation in the chelate ring [54].

Determination of cytotoxic activity
The anticancer potential of drugs can be determined through cytotoxic studies employing cell lines. The criteria for anticancer pharmaceutical candidates is that they destroy proliferating cancer cells while causing no harm to normal cells. In this work, MTT assay was used to assess the cytotoxicity of [Cu(C 11 H 22 N 4 )Cl 2 ] against L929 fibroblast cell lines. Cell lines have been handled with varied concentrations (6.25-100 mg mL -1 ) for 24 h. Microscopically captured cell morphology images of control and 3 at five different concentrations are presented in Supplementary Figure S20. The in vitro cytotoxic activity of 3 is shown in Figure 10 as a function of % viability against concentration and the data are provided in Table S6. The percentage viability of 3 diminishes as the concentration of the compound increases. L929 cells had a cell viability of 47.44% at 100 mg mL -1 . It is evident from the results that the LC 50 or the concentration required to inhibit viability by 50% for [Cu(C 11 H 22 N 4 )Cl 2 ] is 89.19 mg mL -1 . According to these findings, [Cu(C 11 H 22 N 4 )Cl 2 ] is less harmful to L929 fibroblast cell lines, probably as a result of strong antioxidant capability.
Compound 3 exhibited higher potential cytotoxic activity than similar macrocyclic compounds reported earlier. In particular, A. Palanimurugan and A. Kulandaisamy synthesized fully conjugated 14-membered macrocyclic pentaza Schiff base cationic solid complexes with copper(II), nickel(II), cobalt(II), vanadyl(II) and zinc(II) ions [55]. The anticancer activities of macrocyclic Schiff base and 3 were tested against the growth of breast cancer (human) cell line (MCF-7) using MTT assay method. At 100 mM, Schiff base inhibits cells by 57.64%, and 3 by 62.45%. The higher % cell inhibition value for  [CuL]Cl 2 showed that copper(II) complex has greater cytotoxicity toward the cells compared to the Schiff base.

Conclusion
This study provides new insight into Schiff base tetraazamacrocyclic Co(II), Ni(II) and Cu(II) complexes derived from pentane-1,5-dial and TETA in the presence of metal chlorides. Spectral, analytical and physicochemical analyses demonstrated six-coordinate octahedral geometries around the metal ion in 1-3 as monomers. The thermal studies validate that no coordination or lattice water is present. The template approach used in the current work is suitable for preparation of the stable and catalytic [Ni(C 11 H 22 N 4 )Cl 2 ] for efficient photocatalytic hydrogen evolution. Microbial studies revealed that 1 strongly hampers the growth of P. aeruginosa with good efficacy on clinical resistant pathogens. Inhibition of fungal growth against P. chrysogenum is also at its maximum in 3 at both tested concentrations. According to antioxidant and in vitro cytotoxicity studies, 3 showed highest scavenging potential, which results in strong antioxidant activity and is less harmful to L929 fibroblast cell lines. We therefore hope that the findings of this work will be beneficial in developing new metal complexes as a viable medication for the treatment of cancer and bacterial infections.