Synthesis, Characterization, Antibacterial Activity of Binuclear Metal Complexes with Macrocyclic Schiff Base Ligand and DFT Studies.

Islam, Marjan Marwan; Mohammed, Salwan Idress

doi:10.32894/kujss.2026.168996.1283

Synthesis, Characterization, Antibacterial Activity of Binuclear Metal Complexes with Macrocyclic Schiff Base Ligand and DFT Studies.

Document Type : Research Paper

Authors

Marjan Marwan Islam ¹

Salwan Idress Mohammed ²

¹ Department of Chemistry,Faculaty of Sciences ,University of Duhok ,Duhok ,Iraq

² Department of Chemistry, Faculty of Science, University of Duhok ,Duhok ,Iraq

10.32894/kujss.2026.168996.1283

Abstract

A new Series of binuclear macrocyclic Schiff base complexes with the general formula [M2LCl4], (M = Mn (Ⅱ), Fe(Ⅱ), Co(Ⅱ), Ni(Ⅱ), Cu(Ⅱ), Zn(Ⅱ))was synthesized and characterized, the macrocyclic ligand L = (2Z,21Z)-3,10,14,21-tetraaza-1,12(1,4)-dibenzenacyclodocosaphane-2,10,13,21-tetraene)was obtained through a 2:2 condensation reaction of Terephthalaldehyde with Hexamethylenediamine ,forming a stable N₄ macrocycle. The synthesized compounds are characterized by FT-IR, UV–Vis, atomic absorption spectroscopy (AAS), molar conductivity, magnetic susceptibility measurements, and ¹H and ¹³C NMR spectroscopy. The combined spectroscopic, analytical, and magnetic data are consistent with a tetrahedral environment around the metal centers for all complexes except the Cu(Ⅱ)complex which exhibits a square planar geometry. The complexes exhibit non-electrolytic behavior in solution, as inferred from molar conductivity measurements. The disc diffusion method was used to evaluate the antibacterial activity of the Schiff base ligand and its metal complexes. The result demonstrated that metal coordination significantly enhances the ligand's capacity to inhibit the growth of both Gram-positive and Gram-negative bacteria (Staphylococcus aureus, Escherichia coli and Klebsiella pneumoniae). Density functional theory (DFT) calculations were performed at the B3LYP level using the LANL2DZ basis set for the metal atoms and 6-311++G(d,p) for the non-metal atoms. The calculations provided optimized geometries, HOMO–LUMO energies, and thermodynamic parameters, which supported the experimental results and offered further insight into the metal–ligand interactions governing the stability and reactivity of the complexes.

Keywords

Macrocyclic Schiff base ligand

N4 donor

Antibacterial activity

DFT

Binuclear

Subjects

Inorganic Chemistry

1. Introduction:

Schiff bases constitute a broad class of organic compounds characterized by the presence of an imine (-C=N-) functional group [1], [2], [3]. They are typically formed through the condensation reaction between aliphatic or aromatic amines with aldehydes or ketones containing a reactive carbonyl group [4]. The formation of Schiff bases generally proceeds via nucleophilic attack of the amine on the carbonyl carbon, followed by elimination of water. Depending on the electronic properties of the substrate, this process can be accelerated by either an acid or base catalyst, or by the application of elevated temperature [5], [6]. The broad applicability of Schiff bases is primarily attributed to the strong donor ability of the azomethine nitrogen atom, which acts as an effective electron donor and enables Schiff bases to coordinate readily with metal ions, thereby attracting considerable scientific interest in both ligand design and metal-complex synthesis [7]. Schiff bases are substantial due to their stability, chelating properties, and biological applications [8]. The rise of antibiotic-resistant organisms has increased the importance of identifying new antibacterial medicines with improved selectivity and reduced toxicity [9]. Schiff bases and their metal complex have also been shown to have antibacterial, antifungal, and anticancer properties [10]. They are effective antibacterial agents, and metal complexation has been shown to enhance their biological activity and alter microbial response [11], [12].

Schiff-base macrocyclic ligands and their metal complexes represent an essential class of compounds that have been extensively investigated. Macrocyclic tetradentate Schiff base ligands are significant in coordination chemistry due to their four donor sites, which provide robust and versatile binding [13], as shown in Figure 1. The synthesis of tetraazamacrocyclic complexes has been significantly aided by condensation reactions between diketones and primary diamines in the presence of metal ions.The metal facilitates the process by directing the steric course toward cyclic molecules rather than polymeric structures. Macrocyclic complexes exhibit greater thermodynamic stability and enhanced selectivity as ion binders compared to their open-chain analogs [14], [15]. Their cyclic framework generates stable metal complexes with various structural features, making them significant in catalysis, bioinorganic chemistry, and medical applications [16].

Macrocyclic Schiff ligands, particularly when coordinated with transition metals, have attracted significant attention because of their structural diversity and their ability to influence the physicochemical and biological properties of the resulting metal complexes [17]. In the present study, a novel Schiff Base Ligand and symmetrical binuclear azomethine complexes of Mn(Ⅱ), Fe(Ⅱ), Co(Ⅱ), Ni(Ⅱ), Cu (Ⅱ), and Zn(Ⅱ), These complexes were prepared through the condensation of Terephthalaldehyde and 1,6-hexamethylenediamine (Figure 2). The tetradentate Schiff base ligand and all complexes (1-6) were identified and characterized using NMR, IR, UV-Visible spectroscopy, atomic absorption, molar conductivity, and magnetic moment measurement. They were examined for their antibacterial efficacy against Escherichia coli, Staphylococcus aureus, and Klebsiella Pneumonia to assess the influence of metal-ion identity and macrocyclic coordination on biological effectiveness.

Furthermore, Density Functional Theory (DFT) calculations were carried out using the B3LYP functional. The 6-311++G(d, p) basis set was used for non-metal atoms (C, H, and N), while the LANL2DZ basis set was employed for the transition metal ions. Gaussian 16 software was used for geometry optimization and the calculation of structural parameters, including bond lengths and bond angles. The frontier molecular orbitals (FMOs),(HOMOs and LUMOs)were also investigated. The global reactivity descriptors were derived from FMO energy-level analysis. Providing optimized geometries, HOMO–LUMO energy gaps, and electrostatic potential assessments of the molecules. The thermodynamic parameters were calculated in both the gas and solvent phases to evaluate the effect of solvation on the stability of the studied compounds.

Figure 1: The macrocyclic formation reaction, where A and B represent aliphatic or aromatic(dialdehyde and diamine),n is the number of moles of M(Ⅱ) transition metals.

2. Materials and Methods

All solvents and reagents used in this work were of analytical quality. Most chemicals were supplied by the University of Duhok, including MnCl₂·4H₂O, FeCl₂·4H₂O, CoCl₂·6H₂O, NiCl₂·6H₂O, CuCl₂·2H₂O, ZnCl₂, hexamethylenediamine, ethanol, and diethyl ether. Additionally, Terephthalaldehyde was purchased from Bide Pharmatech Ltd, glacial acetic acid from Scharlau, and DMF from Thomas Baker. The ¹H and ¹³C-NMR spectra of the synthesized ligand were recorded on a Bruker Ultra Shield 500 MHz NMR spectrometer in CDCl₃ at 298 K. FTIR spectra were obtained using a Shimadzu FTIR-8400S spectrophotometer in the 4000–400 cm⁻¹ range with KBr discs. Magnetic susceptibility measurements were performed in the solid state at 298 K using a Sherwood Scientific magnetic balance. Electronic absorption spectra were recorded at 25 °C in DMF using a Jenway 6800 double beam UV–Vis spectrophotometer 10⁻³ M with 1 cm quartz cells. The melting and decomposition temperatures of the ligand and its complexes were determined using an electrothermal melting point apparatus. Molar conductivity measurements were carried out using a 4520PH conductivity meter for 10⁻³ M DMF solutions at 25°C. Metal ion contents of the complexes were finally quantified using an AA-670G Atomic absorption spectrophotometer. The disc diffusion method was used to measure antibacterial activity. The geometries and energies of these compounds were assessed using Gaussian 16W.

2.1.Synthesis of Schiff Base Ligand [C₂₈H₃₆N₄]:

The synthetic route of the macrocyclic Schiff base ligand by mixing (0.02mol, 2.6822g) Benzene-1,4-dicarboxaldehyde in 70ml of ethanol with (0.02 mol, 2.323g) hexamethylenediamine in 50ml of ethanol, under continuous magnetic stirring. Subsequently, three droplets of glacial acetic acid were added as a catalyst. The reaction mixture was refluxed for 8 hours at 80 °C. The precipitate formed was then filtered, thoroughly washed with ethanol and diethyl ether, and dried in vacuo over anhydrous CaCl₂. A pale-yellow solid was obtained, yielding 4.0751 g (95.09%) and a melting point of 176 °C. H-NMR, 500 MHz; CDCl₃) δ 8.27 (s,1H), 7.747 (s, 2H), 3.62 (t, 2H), 1.77 – 1.35 (m, 4H).¹³C-NMR (125 MHz, CDCl3) δ 160.28, 138.07, 128.23, 77.03, 76.77, 61.83, 30.81, 27.17.

Figure 2:Synthesis of Schiff Base Ligand.

2.2.Synthesis of Binuclear Metal (II) Complexes:

2.2.1.Synthesis of [Ni₂(C₂₈H₃₆N₄) Cl₄] Complex:

The complex was prepared by reacting (0.001 mol,0.428 g) of the ligand in 15 mL of Chloroform, which was then placed in an ultrasonic bath for approximately 20 minutes. An ethanolic solution of NiCl₂·6H₂O (0.002mol,0.4753 g) was added dropwise to the ligand solution. The mixture was refluxed for approximately 9 hours at 80°C. The olive- green resultant solid complex was filtered, thoroughly washed with ethanol and diethyl ether successively, and subsequently dried at room temperature. Yield (0.6076 g), (88.35%), m.p (224) Dec.

2.2.2. Synthesis of the additional metal ions complexes [M₂(C₂₈H₃₆N₄) Cl₄]:

A series of transition metal complexes with the general formula [M₂LCl₄], M=Mn(Ⅱ), Fe(Ⅱ), Co(Ⅱ), Ni(Ⅱ), Cu(Ⅱ), Zn(Ⅱ),were synthesized following the same procedure described for the nickel complex. The corresponding metal salts were used in 0.002 mol quantities: M=MnCl₂.4H₂O (0.3985g), FeCl₂.4H₂O (0.3976g), CoCl₂.6H₂O (0.475g), CuCl₂.2H₂O (0.340g), ZnCl₂ (0.2725g). Each metal salt solution was added dropwise to a chloroform solution of the ligand (0.001mol) that had been previously subjected to ultrasonication, followed by reflux. The solids were filtered, washed with Ethanol and diethyl ether, dried at room temperature. All complexes were subsequently subjected to physicochemical and some spectroscopic characterization.

Figure 3: Synthesis of Schiff Base Complexes, where M=Mn(Ⅱ), Fe(Ⅱ), Co(Ⅱ), Ni(Ⅱ), Cu(Ⅱ), Zn(Ⅱ), n= 0,2,4,6.

2.3. Computational aspects:

In chemistry and physics, density functional theory (DFT) is a valuable technique for determining the electronic structure of atoms, molecules and solids [18]. Electronic structure calculations play a crucial role in elucidating chemical structure and reactivity [19].By advancing the development of computational chemistry, density functional theory (DFT) has been widely employed owing to its accuracy and low computational cost in calculating a broad spectrum of molecular properties, yielding reliable results that align with experimental data [20]. The geometries of the free ligand and its transition-metal complexes were optimized using the Gaussian 16 program. Geometry optimization calculations were carried out within the framework of density functional theory (DFT) employing the B3LYP functional. The 6-311++G (d,p) basis set was applied to the ligand atoms (C, H, and N), whereas the LANL2DZ basis set was used for the metal centers (Mn, Fe, Co, Ni, Cu, and Zn). This computational level was selected because it has been widely employed in studies of organic, coordination, and organometallic systems, offering a practical balance between computational cost and accuracy in describing ground-state geometries and electronic structures[21],[22].

We employed the computational protocol to determine the optimal bond lengths, bond angles, thermodynamic parameters and frontier molecular orbital energies (FMOs). Examining the energy of the (HOMO and LUMO)is especially crucial as variation in their energy separation illustrates how metal coordination influences the reactivity and stability of the complex[23],[24]. This is particularly relevant when considering the HOMO–LUMO energy gap associated with these orbitals. To identify the electrophilic and nucleophilic regions, the molecular electrostatic potential is employed[25].

2.4Antibacterial efficacy:

Using clinical isolates acquired from Sheryan Hospital, we evaluated the antibacterial efficacy. The antibacterial activity of the free ligand and its complex was evaluated against Staphylococcus aureus (a Gram-positive bacterium), Klebsiella pneumoniae, and Escherichia coli (two Gram-negative bacteria). Following CLSI recommendations, the test was performed using the disc diffusion method [26]. Each bacterial strain was injected into 5 mL of Brain Heart Infusion Broth using a single well-isolated colony. The cultures were then cultured at 37°C for 18 to 24 hours. To cover the surface of the Muller-Hinton agar (MHA) plates with bacteria in three different orientations we used a sterile cotton swab that had been submerged in the bacterial suspension. The plate was rotated 60 degrees between streaks, and the edges were cleaned thoroughly to ensure the germs were spread evenly. The ligand and its complexes were dissolved in DMF (10^-3 M), and 5 mm filter paper discs were dipped into the solutions, which were then allowed to dry in a sterile environment. To avoid inhibitory zone overlaps, the discs were placed on the contaminated agar plates with a 30-36 mm interval between each one using sterile forceps. Every single dish maintained a precise temperature of 37 degrees Celsius for more than 24 hours in an incubator. The circumference of the inhibition zone encircling the disc was gauged after the incubation period [27].

3.Results and Discussion:

Experimental and theoretical results on the synthesis of a macrocyclic Schiff base ligand and its transition metal complex are presented. Synthesis of the coordination complexes is confirmed by spectroscopic, physicochemical, and theoretical investigations. The ligand was synthesized via a conventional condensation reaction between 1,6-hexamethylenediamine and Terephthalaldehyde in a 2:2 molar ratio under acid-catalyzed conditions. They also show how metals alter ligand structure and electrical properties. The combination of the applied characterization techniques provides insight into metal–ligand interactions and the geometries adopted by the complexes. Density functional theory simulations generate optimized molecular structures, frontier orbital distributions, and reactivity descriptors. The calculated energies and thermodynamic parameters were interpreted in a comparative manner within the same computational framework to evaluate relative trends among the studied compounds. We measured the molar conductivity of the metal complexes in dimethylformamide (DMF) with values ranging from 19.45 to 34.30 Ω⁻¹.cm². mol⁻¹. These low conductivity values indicate that all the compounds behave as non-electrolytes in solution [28].

Table (1) presents key physicochemical data, including melting points, molecular weight, metal percentages, and other critical properties.

Compounds	M.wt	Color	m.p (°C)	Molar conductivity Ω^-1.cm².mol^-1	M% Cal Found	Yield %
Ligand	428.6	Pale yellow	176	---	---	85.09
[Mn₂LCl₄]	680	Yellow	279 *	30.4	16.15 15.66	70.56
[Fe₂LCl₄]	682.1	Light brown	253 *	22.1	16.37 15.97	65.01
[Co₂L Cl₄]	688.3	Light green	292 *	30.0	17.12 17.06	66.59
[Ni₂ LCl₄]	687.8	Olive green	224 *	29.4	17.07 16.90	73.95
[Cu₂LCl₄]	697.5	Green	285*	19.45	18.22 17.8	69.73
[Zn₂LCl₄]	701.2	Light Orange	207 *	34.3	18.65 18.32	81.93

*=Decomposition, M% = metal percent, Cal =Calculated, M.wt = Molecular weight

3.1 FTIR analysis:

Infrared spectra in the range 4000-400 cm^–1 have been obtained for the ligand and its complexes, with the measured frequencies assigned to distinct group vibrations by comparison with spectra of analogous complexes. The characteristic infrared frequencies of the Schiff base and its associated compounds are detailed in Table 2. The magnitude of displacement depends on the interatomic binding energy. Although a medium-to-broad band appears at 3425 cm^-1, the absence of the bending vibration (H-O-H) at around 1600-1630 cm^-1 confirms that no molecular or coordinated water is present. Therefore, the observed band is attributed to moisture introduced by the instrument environment rather than to hydration of the Schiff base. The aromatic stretching band C-H weak peak was found at 3032-3066cm^-1[29].

Vibrational frequency ranging (2920-2849) cm⁻¹respectively, were for the symmetric and asymmetric stretching vibrational spectra for(C-H)aliphatic[30]. The Ligands IR spectra exhibited no signals around 3300-3200 cm^-1 for the ν(NH₂) amino group or around 1716cm^-1 for the C=O group[31]. This indicated that the NH₂ group had fully reacted with the C=O group, hereby inducing the formation of a macrocyclic structure [32]. Within this spectrum, the Ligand displayed an azomethine stretching frequency at 1639 cm⁻¹, indicating the production of a Schiff base[33],[34].The infrared spectra of the complexes exhibited ligand bands with associated shifts indicative of complex formation. The imine stretching band observed in the free Schiff base shifts to a lower frequency, appearing in the range 1587-1608 cm⁻¹ in the complexes, signifying the involvement of the azomethine nitrogen atom in coordination with the metal ions [35]. Aromatic C=C stretching vibrations are commonly observed between 1504 and 1579 cm^⁻1 in Schiff bases and their metal complexes [36].

The CH₂bending shows between (1456-1498)cm^-1[34].At lower frequencies, the complexes displayed new bands at 418-466 in their vibrational spectra, which were attributed to v(M-N) frequencies respectively [26], [37]. The coordinated chloride ions cannot be detected as they fall outside the instrument's scale. The silver nitrate test indicates the absence of uncoordinated Cl- ions, as supported by the conductivity measurement [38]. The gas-phase infrared spectra of the synthesized compounds, calculated at the B3LYP/6-311++G(d,p) and LANL2DZ basis sets, are presented in Table 2. The significant similarity between the real and computed spectra suggests that the molecular structures of the ligand and its corresponding metal complexes are largely equivalent in both the solid and gas phases. The obtained results align well with previous research investigations [39], [40].

Table 2: The experimental and calculated FT-IR spectra of the SB ligand and its compounds.

Comp

ν(C=N)

Exp

Cal

ν_ar (C=C)

Exp

Cal

ν_ar (C-H)

Exp

Cal

ν_ali (C-H)

Exp

Cal

δ C-H

Exp

Cal

M-N

Exp

Cal

1639

1650

1566

1588

3032

3067

2849,2920

2976

1456

1497

[Mn₂LCl₄]

1581

1596

1579

1582

3045

3056

2856,2929

3031

1488

1512

422

434

[Fe₂LCl₄]

1602

1606

1560

1573

3066

3078

2854,2933

3017

1475

1491

449

451

[Co₂LCl₄]

1608

1622

1564

1540

3014

3045

2860,2931

3014

1498

1512

418

425

[Ni₂LCl₄]

1606

1628

1564

1580

3008

3036

2856,2927

2996

1475

1508

466

461

[Cu₂LCl₄]

1604

1622

1573

1559

3040

3084

2854,2927

2997

1465

1488

422

428

[Zn₂LCl₄]

1600

1624

1569

1540

3060

3084

2858,2923

2995

1461

1488

424

428

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