Spectroscopic and Quantum Chemical Studies of some Novel Mixed-Ligand Complexes of Fe(II) and Cu(II) and Investigation of their Antimicrobial Actvities

Introduction

Schiff bases represent an important class of organic compounds formed via condensation reactions between aldehydes or ketones and primary amines, first reported by Hugo Schiff.^1-5 These compounds can coordinate with metals through nitrogen and oxygen atoms to form stable complexes^6-7. When a single type of ligand reacts with metal salts, homoleptic complexes are formed, whereas reactions involving two different ligands with metal salts yield heteroleptic complexes,⁸ which have important applications as antitumor, antiviral, antifungal, and antibacterial compounds.^{7, 9-10}

Furthermore, mixed ligand complexes of Cu(II), Ni(II), and Zn(II) were synthesized utilizing thiophene-2-carbonyl-isonicotinohy-drazone as the principal ligand, which is prepared from the reaction of isoniazid (INH)  with Thiophene-2-carbaldehydein which is mixed with 1,10-phenanthroline hyFor instance, Kudrat-E-Zahan reported the synthesis of Ni(II) and Cu(II) mixed-ligand complexes using 2,2′-bipyridine and a Schiff base ligand derived from isoniazid and anisaldehyde. Similarly, Abd El-Halim¹¹ prepared mixed-ligand complexes of Mn(II), Co(II), Cu(II), Fe(III), Ni(II), Zn(II), and Cd(II) using a Schiff base synthesized from quinoline-2-carboxaldehyde and 2-aminophenol (1:1 ratio), subsequently combined with 1,10-phenanthroline (1:1 molar ratio).¹² Shane M. Wilkinson described the preparation of Fe(III) and Cu(II) homoleptic complexes using a Schiff base derived from benzhydrazide and salicylaldehyde. Production of Cu(II) mixed ligand complexes utilizing a tridentate Schiff base ligand and 2,2′-bipyridine, which are used as an antibacterial compound, Zeinab Albobaledi.¹³ synthesized mixed-ligand complexes of Cu(II), Co(II), Ni(II), and Zn(II) using a Schiff base prepared from curcumin and 2-aminobenzothiazole, which was then combined with 1,8-diaminonaphthalene. Abdel-Rahman.¹⁴ Formation of mixed ligand complexes regarding Cu(II), Co(II), Ni(II), as well as Zn(II) through the reaction of a Schiff base ligand synthesized via curcumin with 2-amino benzothiazole and this ligand mixed with 1,8-diaminonaphthalene for preparation of mixed ligand complexes Chandrasekar.¹⁵ Preparation of complexes of Mn(II) , Fe(III) and Cu(III) its complexes have applications in antibacterial and anticancer, molecular docking, and ADMET studies Abdel-Rahman.¹⁶

drate is mixed Ashrafuzzaman, M.D.¹⁷ Schiff bases and their metal complexes serve dual roles as industrially valuable agents and biologically active compounds, notably as antifungals¹⁸ and demonstrate a wide array of biological activities, including antifungal properties.¹⁰ They also function as highly effective catalysts across various reactions, though their performance can be compromised by moisture.¹⁹

Recent research shows that coordinating metals to Schiff-base ligands strongly influences both their biological activity and chemical properties. Because Schiff bases can bind metals at multiple sites, their cooperative behavior is being intensely studied. Consequently, designing new Schiff-base ligands and their metal complexes as potential drugs is now a major focus in medicinal chemistry²⁰  In this article, we aim to compare homoleptic and heteroleptic complexes prepared from a Schiff base ligand and evaluate their antibacterial activities.

Experimental Methods

Instruments and Measurements

FT-IR spectra were recorded using a Shimadzu ATR-FTIR spectrometer equipped with a single-bounce diamond ATR accessory at a resolution of 4 cm⁻¹. UV-Vis spectra were obtained using a double-beam spectrophotometer (model 6705 UV, JENWAY, Japan) connected to UV Probe software and interfaced with a computer. All measurements were performed using 10 mm quartz cuvettes with a spectral bandwidth of 1 nm. Elemental analyses (CHNO) were carried out using a Thermo Fisher Eager 300 analyzer. Melting points were determined using an AELAB DMP-800 colorful touchscreen melting point apparatus with open-ended glass capillaries. Magnetic susceptibility measurements were performed at 296.15 K using a Sherwood Scientific magnetic susceptibility balance.

Molar conductivities of 10⁻³ M solutions of the metal complexes were measured at 296.15 K using a TRANAS INSTRUMENTS BC3020 Professional Benchtop Conductivity Meter in ethanol containing a few drops of DMF. ¹H-NMR and ¹³C-NMR spectra were recorded at room temperature using a Bruker 300 MHz NMR spectrometer in DMSO-d₆, with tetramethylsilane (TMS) as the internal reference.

Material

All chemicals (2-aminophenol, 4-bromobenzaldehyde, CuCl₂, FeCl₃, ethanol, DMF) were obtained from Shanghai Macklin and used as received without further purification.

Synthesis of Ligand

The Schiff base ligand (L₁), 2-((4-bromobenzylidene)amino)phenol, was prepared by refluxing equimolar solutions of 4-bromobenzaldehyde (5 mmol, 0.925 g) and 2-aminophenol (5 mmol, 0.54 g) in ethanol (8 mL each) at 70 °C for 3 h. Cooling afforded yellow crystals that were filtered, washed with cold ethanol, and recrystallized from ethanol to give pure L₁ in 70 % yield, melting piont. 133–137 °C. The compound was fully characterized by UV-Vis, ¹H/¹³C NMR, FT-IR, and elemental analysis.²¹

Scheme 1: synthesis of Schiff base ligand (2-((4-Bromobenzylidene)amino)phenol (L1) Click here to View Scheme

Homoleptic Complexes (C₁ and C₂)

The homoleptic complexes of Cu(II) (C₁) and Fe(III) (C₂) were prepared by reacting the respective metal salts (CuCl₂ and FeCl₃) with the Schiff base ligand 2-((4-bromobenzylidene)-amino)phenol (L₁). In a typical synthesis, 1 mmol of the metal salt was dissolved in ethanol and mixed with 2 mmol (0.55 g) of L1 dissolved in ethanol. The reaction mixture was refluxed at 50 °C for 5 hours ²². The resulting complexes were filtered, washed with cold ethanol, and recrystallized from ethanol to ensure purity.

Synthesis Heteroleptic Complexes (C₃ and C₄)

Complex C₃: The heteroleptic Cu(II) complex (C₃) was synthesized by reacting CuCl₂ (1 mmol) dissolved in ethanol with a mixture of L1 (1 mmol, 0.275 g) and 8-hydroxyquinoline (L₂, 1 mmol, 0.145 g) dissolved in ethanol. The reaction mixture was refluxed at 50 °C for 5 hours²³, Complex C₄: The heteroleptic Cu(II) complex (C₄) was prepared by reacting CuCl₂ (1 mmol) dissolved in ethanol with a mixture of L₁ (1 mmol, 0.275 g) and ninhydrin (L3, 1 mmol, 0.178 g) dissolved in ethanol. The reaction mixture was refluxed at 50 °C for 5 hours.

All complexes were purified by recrystallization from ethanol. The synthetic route for all complexes is illustrated in Scheme 2. The yield, color, and melting points of the complexes are summarized in Table 3.

Scheme 2: Synthesis of mixed-ligand complexes of Cu(II) and Fe(II). Click here to View Scheme

Ultraviolet-visible spectra

The UV-Vis spectra of the ligand (L₁) and its metal complexes were recorded at ambient temperature in ethanol solution. The spectrum of the free ligand (L₁) exhibits three distinct absorption peaks: at 207 nm (assigned to n→π* transitions), 272 nm and 354 nm (both attributed to π→π* transitions) ²⁴ as shown in Figure 1. Upon complexation, the spectra of the metal complexes (C₁– C₄) show additional absorption bands in the visible region: C₁: 450 and 470 nm, C₂: 440 nm, C₃: 400 nm, C₄: 400 and 480 nm

These new bands correspond to metal-to-ligand charge transfer (MLCT) transitions, confirming coordination between the metal ions and the ligand. Furthermore, weak d-d transition bands are observed at approximately 500 nm (for C₁, C₃, and C₄) and 510 nm for (C₂), which are characteristic of transition metal complexes.

Figure 1: UV-Visible Spectrum of Schiff base ligand (L1). Click here to View Figure

Figure 2: Uv-visible Spectra of complexes (C1– C4) Click here to View Figure

Table 1: Electronic spectral assignment of ligand (L₁) and Complexes (C₁-C₄)

Compounds	Intra-ligandtransitionλmax nm	MLCTλmax nm	d-d transitionλmax nm	Assignment
L1	272,354, 207	–	–	–
C1	300	450,470	500	dxy → dX2y2
C2	200,220,250	440	510	dxy → dX2y2
C3	270, 340	400	500	dxy → dX2y2
C4	260,300, 340	400,480	500	dxy → dX2y2

 FT-IR spectra

FT-IR peaks as shown in figure 3 show the main peaks of uncoordinated ligand (L1), which is C=C 1581.82 cm^-1  and N=C 1621.82 cm^-1,  and C-H 3052.58 cm^-1 , stretching and also OH peak in 3292.48 Cm^-1  for (L1) ²², Upon complexation, notable shifts in these bands are observed, The OH region with in the infrared spectrum with the metal compounds appear in the higher wavenumber in C₁ and C₃ in (3321.1, 3319.77 ) cm^-1 and the shift of OH band to lower Wavenumber in (C₂ and C₄) (3200) cm^-1 denoting the coordination

within the atomic oxygen. The shift of C=N to lower wavenumber in (C₁-C₄): (1653.42, 1650, 1653.42, 1649.11) cm^-1 respectively , which is indicated the coordination on C=N ¹⁶, also peaks in fingerprint reagent for C₁– C₄ between (500-700) cm^-1, showed the L-M peaks which indicate the complexation reaction ¹⁷, as shown in table 2 and Figure 4.

Figure 3: FT-IR spectra of the Schiff base ligand (L1). Click here to View Figure

Figure 4: FT-IR spectrum of Homo and Heterolepetic complexes (C1– C4). Click here to View Figure

Table 2: FT-IR Data of the ligand (L₁) and complexes of (C₁-C₄)

Compounds	OH	C=C	C=N	C-H	L-Metal	C=O
L1	3292.48	1581.82	1693.64	3052.58	–	–
C1	3321.1	1581.60	1653.42	3052	544.44,673.72	–
C2	3200	1550	1650	–	518.58,599.02	–
C3	3319.77	1581.60	1653.42	3059.70	511.40,604.7	–
C4	3200	1588.78	1649.11	2936.22	551.62,629.19	1742.48

NMR spectra

¹H NMR (CDCl₃, δ, ppm) peaks for (L1) as shown in figure 5, a singlet is related to hydrogen of OH in 9.12 ppm also for HC=N give a peak in 8.72 ppm and also give ( triplet, triplet, doublet, doublet, singlet) in the reagent between (6-9) ppm which are related to hydrogen of benzene ring ²⁵. The ligand gives a ¹³C NMR peak at 158.7, which is related to carbon in C=N and peaks between (110-150), which is related to carbons of the benzene ring ²⁶ , as shown in Figure 6.

Figure 5: 1H-NMR spectrum of Schiff base ligand (L1). Click here to View Figure

Figure 6: 13C-NMR spectrum of Schiff base Ligand (L1) Click here to View Figure

Elemental analysis

Emental analysis of (L₁)  show amount of carbon, nitrogen, Hydrogen and also sulfur which amount of carbon is %55.326 and amount of hydrogen is %3.418 and amount of nitrogen is %5.032 and Sulphur is zero which all are close to theoretical amount, Theoretical amounts for all elements are {C (56.502), H (3.62), Br (28.91), N (5.07), S (0.00)} ²⁷. All measured elements are within 1 % absolute of the theoretical values, confirming the sample’s identity and high purity.

Magnetic moment

Magnetic moment of complexes determined by determination of mass susceptibility of complex compounds by using this equation: molar susceptibility (emu mol⁻¹), Xmeas = Xg × M.Wt , µeff = 2.83 (XpT1/2)  (Bohr-magneton formula, T = 298 K), Calculated Bohr magneton for the complexes (C₁-C₄): (1.84), (1.7), (1.53), and (1.7) rspectvily ²⁸, These values are all in the narrow range 1.5–1.8 BM, which is markedly value for one unpaired electron (1.73 BM).

Table 3: Analytical and physical properties of ligand (L₁) and their metals complexes (C₁-C₄).

Compounds	Molecular formula	Sstructure	μeff  (B.M)	Color	Conductivity(S·cm²/mol)	% Yield	Melting Point
L1	C13H10NOBr	–	–	Yello	–	%70	133-137
C1	[Cu(L1)2Cl2]	Octahedral	1.84	Black	12.5	%60	>300
C2	[Fe(L1)2Cl2]Cl	Octahedral	1.7	Grey	2.1	%85	>300
C3	[Cu(L1)(L2)Cl2]	Octahedral	1.53	Green	6.1	%72	259-260
C4	[Cu(L1)(L3)Cl2]	Octahedral	1.7	Green	13.8	%40	>300

 DFT analysis

Quantum chemistry studies molecules’ bond formations, molecular structures, energy levels, and chemical reactivity using computational methods. The three-dimensional arrangements of the compounds

[Cu(L1)₂Cl₂] and [Fe(L1)₂Cl₂]Cl were created utilizing Gauss View 6.0 and further optimized utilizing the Gaussian 16 software suite. The study of geometry improvements and frequency calculations were performed in the gaseous phase utilizing the Becke, 3-parameter, Lee–Yang–Parr (B3LYP) operational.

The B3LYP/6-311++G(d,p) A basic set was employed for nitrogen, carbon, oxygen, and hydrogen atoms.

The computed frequencies were subsequently optimized to confirm that the resulting structures were in the minimum energy state.

The Gauss View molecule imaging program was utilized to visualize the input data files as well as determine the HOMO–LUMO energies²⁹

Table 4: Appendix X. The electronic structure of the selected investigated compounds is described by quantumchemical parameters computed using DFT at the B3LYP/6-311++G(d,p) theoretical value in the gaseous phase.

Quantum Chemical parameters	C1	C2	C3	C4
HOMO	-5.42241568	-5.57017358	-5.6346646	-5.71901994
LUMO	-2.71542561	-2.74535815	-5.44962708	-5.60663686
Ionization energy (eV) (1)	5.422416	5.570174	5.634665	5.71902
Electron Affinity (eV) (A)	2.715426	2.745358	5.449627	5.606637
Energy gap (eV) (ΔE)	2.70699	2.824815	0.185038	0.112383
Hardness (eV) (η)	1.353495	1.412408	0.092519	0.056192
Softness (eV)-1 (σ)	0.738828	0.708011	10.80862	17.79627
Electronegativity (eV) (χ)	4.068921	4.157766	5.542146	5.662828
Chemical potential (eV) (μ)	-4.06892	-4.15777	-5.54215	-5.66283
Electrophilicity (eV) (ω)	6.116061	6.119698	16.59954	28.53421
Nucleophilicity (eV)-1 (ε)	0.163504	0.163407	0.006024	0.003505
Back-donation (eV) (∆EBD)	-0.33837	-0.3531	-0.02313	-0.01405
Electron transfer (∆N)	1.082782	1.006166	7.878695	11.89834

Figure 7: Electrostatic potentials map of the selected compounds based on DFT at 6-311G++(d,p) basis set. Click here to View Figure

HOMO and LOMO energy

The HOMO energies: These values represent the energy of the highest occupied molecular orbital. The more negative the HOMO energy, the more stable the electrons in the compound. The HOMO energy for complexes (C₁-C₄): -5.422 Ev , -5.570 eV , -5.634 eV , -5.719 eV respectively.

The LUMO energies: These values represent the energy of the lowest unoccupied molecular orbital. The more negative the LUMO energy, the more difficult it is to excite an electron to this orbital. The LOMO energy for complexes (C₁-C₄): -2.715 ev , -2.745 Ev , -5.449 eV ,  -5.606 Ev respectively

The Energy gaps: These values represent the difference between the HOMO and LUMO energies. A larger energy gap indicates a more stable compound. The energy gape for complexes (C₁-C₄):  2.706 Ev , 2.824 Ev , 0.185 eV , 0.112 eV respectively the compound C₂ has the largest energy gap, indicating the most stable compound and C₄ has the most negative HOMO , LOMO and has the heights.

Figure 8: HOMO and LUMO complexes of (C1 and C2). Click here to View Figure

Antibacterial Activity

The Schiff base ligand (L1) and its corresponding complexes were evaluated for antibacterial activity against two bacterial strains: Salmonella (Gram negative) and Staphylococcus (Gram positive) ³⁰, The disk diffusion method was employed, with the complexes tested in powder form, and the inhibition zones were measured in millimeters (mm) and the results listed in table 5.

The homoleptic complexes (C₁ and C₂) exhibited strong antibacterial activity against both strains. The complex (C₁) displayed inhibition zones of 25 mm for Salmonella and 26 mm for Staphylococcus, while (C₂) showed even greater efficacy, with 30 mm inhibition zones for both bacteria. In contrast, the heteroleptic complexes (C₃ and C₄) demonstrated significantly reduced activity, suggesting that the homoleptic form of this ligand is more effective against these pathogens. Notably, the ligand (L₁) showed no inhibitory effect on either bacterial strain.^31-33

Table 5: Antibacterial activities of Ligand (L₁) and complexes (C₁-C₄) quantified in millimeters

Compounds	Salmonella (-)	Staphylococcus (+)
C1	25 mm	26 mm
C2	30 mm	30 mm
C3	25 mm	25 mm
C4	22 mm	14 mm
L1	–	–

Figure 9: Investigation antibacterial activity of Ligand (L1) and their complexes (C1-C4). Click here to View Figure

Conclusion

This study confirms the successful synthesis of 2-(4-bromobenzylidene)aminophenol ligands and their Fe(II) and Cu(II) complexes, which were thoroughly characterized using elemental and spectral techniques. The complexes were synthesized in a 1:1 metal to ligand (M:L) ratio and characterized by melting point, UV-Vis, FT-IR, and molar conductivity Octahedral geometry was established by magnetic and UV–Vis data, and molar conductance confirmed the complexes are non-electrolytes in solution. The Gauss View molecule imaging program was utilized to visualize the input data files and determine the HOMO–LUMO energies.

The synthesized complexes exhibited strong antibacterial activity against Salmonella and Staphylococcus. Notably, complexes (C₃ and C₄) showed reduced efficacy against both bacterial strains, suggesting that homoleptic complexes of this ligand possess enhanced antibacterial properties. In contrast, the free ligand (L1) displayed no antibacterial activity.These findings underscore the importance of metal coordination in enhancing the biological activity of amide-based compounds, providing a foundation for further exploration of their therapeutic and pharmacological applications.

Acknowledgment

The paper’s authors express their sincere appreciation to the Research Center at Raparin University for offering spectroscopic capabilities. Gratitude is also expressed to the Chemistry Department, College of Science, Raparin University, for providing crucial investigation facilities as well as assistance during this research project.

Funding Sources

The author(s) received no financial support for the research, authorship, and/or publication of this article.

Conflict of Interest

The author(s) do not have any conflict of interest.

Data Availability Statement

This statement does not apply to this article.

Ethics Statement

This research did not involve human participants, animal subjects, or any material that requires ethical approval.

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