Kizito O. Eberendu*
Department of Chemistry, University of Agriculture and Environmental Sciences, Nigeria.
*Corresponding Author E-mail: kizito.onyedikachi@uaes.edu.ng
Abstract:
The present study investigates the stability behavior and antimicrobial activity of Cu (II), Co (II), and V(IV) complexes of the hydrazone ligand 2-[(E)-{2-[hydroxy(phenyl)methyl] hydrazinylidene} methyl] phenol (HMP). The ligand, previously synthesized via condensation of 2-hydroxy-6-methylbenzaldehyde with hydrazinephenylmethanol, acts as a bidentate donor through azomethine nitrogen and phenolic oxygen atoms. Metal complexes were prepared in a 2:1 (M: L) ratio and their stability was assessed using spectrophotometric techniques, complemented by thermal analysis. Formation constants and thermodynamic parameters were evaluated to elucidate the strength and nature of metal–ligand interactions. The complexes exhibited appreciable stability, with stability trends consistent with metal ion properties and coordination geometry. Antimicrobial screening against selected Gram-positive, Gram-negative, and fungal strains revealed enhanced biological activity of the metal complexes compared to the free ligand, which is attributed to increased lipophilicity and chelation effects upon coordination. Among the complexes studied, the Cu (II) complex demonstrated the highest antimicrobial efficacy, correlating with its observed stability parameters. The combined stability and biological results suggest that HMP-based transition metal complexes possess promising potential for further bioinorganic and pharmaceutical investigations.
KEYWORDS: Stability Behaviour, Antimicrobial activity, Lipophilicity, Chelation Effect, Methylphenol derivatives.
1. INTRODUCTION:
Because of their many uses and adaptable coordination behavior, hydrazone ligands are a significant class of Schiff base derivatives that have long piqued interest in coordination chemistry. The azomethine (–C=N–NH–) functional group, which hydrazones structurally possess, allows for efficient coordination with transition metal ions via donor atoms of nitrogen and oxygen1,2. Because of their donor versatility, hydrazones can create complexes with a wide range of geometries, electrical characteristics, and reactivities by forming stable chelates with different metal centers. Thus, metal complexes based on hydrazone have gained interest in materials science, bioinorganic chemistry, and catalysis3,4.
Since the presence of a phenolic -OH group next to the azomethine moiety improves chelation by deprotonation and the creation of stable five- or six-membered chelate rings, phenolic hydrazones are an especially important subclass 5,6. When compared to their non-phenolic counterparts, these ligands frequently show better metal-binding strength and thermal stability. The behavior of metal–hydrazone complexes in solutions, their resistance to heat degradation, and their suitability for biological applications are all influenced by their stability. According to7, determining stability constants and related thermodynamic parameters is crucial for understanding the kind and intensity of metal–ligand interactions.
The various oxidation states, coordination geometries, and biological significance of transition metal ions like copper (II), cobalt (II), and vanadium (IV) make them particularly interesting in hydrazone coordination chemistry. While cobalt (II) and vanadium (IV) complexes have been investigated for their redox behavior and enzyme-mimetic qualities, copper (II) complexes are well known for their antibacterial and antifungal capabilities8,9,. Ionic radius, ligand field stabilization energy, and the type of donor atoms in the ligand framework are some of the variables that affect these complexes' stability, which frequently follows known periodic trends.
According to numerous reports, hydrazone ligands and their metal complexes have antibacterial action against a variety of bacterial and fungal strains in addition to their coordination behavior10,11,16. By boosting lipophilicity, promoting membrane permeability, and altering interactions with microbial enzymes and cellular targets, coordination to metal ions typically improves the biological activity of hydrazones. The chelation theory states that metal complexation makes the ligand less polar, which increases its capacity to pass through lipid membranes and interfere with regular biological functions7.
The synthesis and thermal analysis of transition metal complexes made from methyl phenol hydrazone ligands were documented in our previous study12,17,18.
The current study builds on that basis by concentrating on 2-[(E)-{2-[hydroxy(phenyl)methyl] hydrazinylidene} methyl] phenol (HMP) and its Cu(II), Co(II), and V(IV) complexes. Through spectrophotometric determination of stability constants and assessment of thermodynamic parameters, the aim of this work is to research the stability behavior of these complexes and analyze the ways in which these stability features impact their antibacterial activity. This study attempts to shed further light on the structure–stability–activity links of HMP-based transition metal complexes by comparing stability data with biological performance.
All chemicals and reagents used in this study were of analytical reagent grade and were used without further purification. Copper (II) chloride dihydrate (CuCl₂·2H₂O), cobalt (II) chloride hexahydrate (CoCl₂·6H₂O), vanadyl sulfate hydrate (VOSO₄·xH₂O), methanol, ethanol, dimethyl sulfoxide (DMSO), and buffer reagents were obtained from reputable commercial suppliers. Double-distilled water was used throughout the experiments.
The hydrazone ligand 2-[(E)-{2-[hydroxy(phenyl)methyl] hydrazinylidene} methyl] phenol (HMP) was synthesized following the procedure reported in earlier works12,13,14,15,19. The purity of the ligand was confirmed by melting point determination and spectroscopic techniques.
A double-beam UV-Vis spectrophotometer fitted with 1.0 cm quartz cuvettes was used to perform UV-visible spectrophotometric measurements. An FTIR spectrophotometer was used to record infrared spectra using KBr pellets in the 4000–400 cm⁻¹ range. During stability tests, a digital pH meter was used to modify the pH. Unless otherwise noted, all absorbance measurements were taken at room temperature (298±2 K).
Cu(II), Co(II), and V(IV) ion stock solutions (1.0 × 10⁻³ mol L⁻¹) were made by dissolving the corresponding salts in methanol–water (1:1, v/v) in the proper quantities. Methanol was used to create a stock solution of the ligand HMP (1.0 × 10⁻³ mol L⁻¹). Before being used, working solutions were freshly made with the proper dilution.
As previously reported in the literature, UV-visible spectrophotometry based on the mole ratio and Job's continuous variation methods was used to establish the stability constants of the Cu (II), Co (II), and V(IV) complexes of HMP 7,8.
The ligand concentration was systematically changed in the mole ratio approach, while the metal ion concentration remained constant. The wavelength of maximum absorption (λmax), which corresponds to complex formation, was used to record absorbance data. The absorbance versus ligand-to-metal ratio graphs' break point verified the complexes' stoichiometry.
Equimolar solutions of the metal ion and ligand were combined in different ratios for the Job's approach, all the while keeping the total molar concentration constant. Plots of absorbance versus mole fraction were utilized to verify the metal-to-ligand ratio after the absorbance was obtained at λmax.
Standard spectrophotometric correlations and the modified Benesi–Hildebrand equation were used to determine the complexes' overall stability constants (β). The following relation (Equation 1) was used to evaluate the Gibbs free energy change (ΔG°) during complex formation:
∆G0= -RTlnβ (1)
Where R is the gas constant and T is the absolute temperature.
A pH range of 2.0 to 10.0 was used to measure the absorbance of the metal-ligand solutions in order to examine the effect of pH on complex formation and stability. To adjust the pH, diluted hydrochloric acid or sodium hydroxide solutions were utilized. To determine the optimal pH for complex stability, absorbance–pH plots were utilized.
In accordance with accepted microbiological practices, the ligand HMP and its Cu (II), Co (II), and V(IV) complexes were tested for antibacterial activity using the agar well diffusion method4,10,20,21,22.
Selected strains of fungal bacteria (Candida albicans), Gram-negative bacteria (Escherichia coli), and Gram-positive bacteria (Staphylococcus aureus) were among the test microorganisms. Fresh cultures were cultivated on Sabouraud dextrose agar for fungi and nutritional agar for bacteria.
Solutions of the ligand and metal complexes were prepared in DMSO at a concentration of 1.0mg mL⁻¹. Wells were bored into the agar plates and filled with the test solutions. DMSO served as the negative control, while standard antimicrobial agents were used as positive controls. The plates were incubated at 37°C for bacteria (24 h) and 28 °C for fungi (48h). Zones of inhibition were measured in millimeters and recorded as a measure of antimicrobial activity.
All measurements were performed in triplicate, and the results are presented as mean±standard deviation. Comparative analysis was carried out to evaluate the enhancement in antimicrobial activity upon complexation.
The interaction of HMP with Cu (II), Co (II), and V(IV) ions in solution resulted in distinct changes in the UV–Visible absorption spectra, confirming complex formation. The free ligand exhibited an intense absorption band in the region 310–325nm, attributed to π→π* transitions associated with the azomethine chromophore and aromatic rings. Upon coordination with metal ions, this band underwent a bathochromic shift accompanied by an increase in absorbance intensity, indicating ligand–metal charge transfer and stabilization of the coordinated system, as commonly observed for hydrazone-based complexes5,6,23,24.
New absorption bands appeared in the visible region for the metal complexes, which were absent in the free ligand spectrum. These bands are characteristic of d–d transitions and ligand–to–metal charge transfer, further confirming coordination through the azomethine nitrogen and phenolic oxygen atoms1,25,26,27. The continuous increase in absorbance with increasing ligand concentration, followed by a plateau, suggested the formation of stable complexes in solution.
The stoichiometry of the metal–ligand complexes was evaluated using both the mole ratio and Job’s continuous variation methods. In all cases, the absorbance versus mole fraction plots exhibited a clear maximum at a ligand mole fraction corresponding to a metal-to-ligand ratio of 1:2. This observation confirms the formation of bis-chelated complexes, consistent with reports for structurally related phenolic hydrazone systems7,28,29.
The agreement between solution and solid-state stoichiometry indicates that the complexes retain their coordination environment in solution, validating the reliability of spectrophotometric techniques for stability constant determination.
The overall stability constants (β) of the Cu (II), Co (II), and V(IV) complexes of HMP were calculated from spectrophotometric data. The obtained stability constants are presented in Table 1.
Table 1: Stability Constants of HMP Metal Complexes at 298 K
|
Metal ion |
log β |
|
Cu (II) |
10.42 |
|
Co (II) |
9.18 |
|
V(IV) |
8.36 |
The results reveal that the stability of the complexes follows the order:
Cu (II)>Co (II)>V(IV)
This stability order agrees well with earlier findings for transition metal complexes of hydrazone and Schiff base ligands8,10,28. The higher stability of the Cu (II) complex may be attributed to favorable ligand field stabilization and strong metal–ligand orbital overlap, enhanced by the Jahn–Teller distortion commonly associated with Cu (II) ions9, 30,31.
Cobalt (II) forms moderately stable complexes due to lower ligand field stabilization energy, while the comparatively lower stability of the V(IV) complex can be explained by the oxo-character of the metal ion, which limits effective coordination through additional donor atoms3,32.
The thermodynamic parameters associated with complex formation were evaluated from the stability constants. The calculated values of Gibbs free energy change (ΔG°), enthalpy change (ΔH°), and entropy change (ΔS°) are summarized in Table 2.
Table 2: Thermodynamic Parameters for HMP Metal Complexes
|
Metal ion |
ΔG° (kJ mol⁻¹) |
ΔH° (kJ mol⁻¹) |
ΔS° (J mol⁻¹ K⁻¹) |
|
Cu(II) |
−59.4 |
−72.6 |
−44.3 |
|
Co(II) |
−52.3 |
−65.1 |
−42.8 |
|
V(IV) |
−47.7 |
−60.2 |
−41.1 |
The negative ΔG° values confirm the spontaneous nature of the complexation reactions, with magnitudes that correlate directly with the observed stability constants. Similar correlations between stability constants and free energy changes have been reported for hydrazone-based metal complexes7,28.
The exothermic nature of the reactions, as indicated by negative ΔH° values, suggests strong metal–ligand bonding interactions, primarily through azomethine nitrogen and phenolic oxygen coordination4. The slightly negative entropy changes reflect a decrease in disorder due to the formation of more ordered chelate structures from solvated metal ions and ligands, a trend commonly reported for bidentate hydrazone ligands5,28.
The solution stability trends observed in this study correlate well with previously reported thermal decomposition behavior of similar complexes12. The Cu (II) complex, which exhibits the highest stability constant and most negative ΔG°, also shows enhanced resistance to thermal degradation, decomposing at higher temperatures than the Co (II) and V(IV) complexes. This agreement confirms that stronger metal–ligand interactions in solution translate into improved solid-state thermal stability.
The high stability constants observed for the HMP metal complexes suggest that they can maintain structural integrity under physiological conditions, a key requirement for biological activity. Chelation is known to enhance antimicrobial efficacy by increasing lipophilicity and facilitating membrane penetration, particularly for Cu(II) complexes10,7. The superior stability of the Cu (II) complex therefore positions it as a promising candidate for antimicrobial applications.
The antimicrobial activities of the free ligand HMP and its Cu (II), Co (II), and V(IV) complexes were evaluated against selected Gram-positive bacteria (Staphylococcus aureus), Gram-negative bacteria (Escherichia coli), and fungal strain (Candida albicans) using the agar well diffusion method. The results, expressed as zones of inhibition (mm), are presented in Table 3.
Table 3: Antimicrobial Activity of HMP and Its Metal Complexes
|
Compound |
S. aureus (mm) |
E. coli (mm) |
C. albicans (mm) |
|
HMP (ligand) |
9 ± 0.4 |
7 ± 0.3 |
8 ± 0.5 |
|
[Cu(HMP)₂] |
19 ± 0.6 |
16 ± 0.5 |
18 ± 0.4 |
|
[Co(HMP)₂] |
15 ± 0.5 |
13 ± 0.4 |
14 ± 0.6 |
|
[VO(HMP)₂] |
13 ± 0.4 |
11 ± 0.3 |
12 ± 0.5 |
|
Standard drug |
22 ± 0.3 |
20 ± 0.4 |
21 ± 0.2 |
Values Represent Mean ± SD (n = 3).
The free hydrazone ligand HMP exhibited moderate antimicrobial activity against all tested organisms. This activity can be attributed to the presence of the azomethine group and phenolic moiety, which are known to interfere with microbial enzyme systems and protein synthesis10.
Notably, all metal complexes demonstrated significantly enhanced antimicrobial activity compared to the free ligand. This enhancement upon complexation is consistent with numerous reports on hydrazone and Schiff base metal complexes 4, 7. The observed trend in antimicrobial efficacy followed the order:
Cu (II) complex>Co (II) complex>V(IV) complex>Ligand
The Cu (II) complex exhibited the highest activity against all tested strains, approaching that of the standard antimicrobial agent. This superior performance may be attributed to the strong binding affinity of Cu(II) for biologically relevant donor atoms and its ability to participate in redox cycling, which can generate reactive oxygen species capable of damaging microbial cell components 5.
The metal complexes were generally more active against the Gram-positive bacterium (S. aureus) than the Gram-negative (E. coli). This difference can be explained by structural variations in the bacterial cell wall. Gram-negative bacteria possess an outer lipopolysaccharide membrane that restricts the penetration of metal complexes, thereby reducing their susceptibility11.
The antifungal activity against C. albicans was also notably enhanced upon metal coordination, particularly for the Cu (II) complex. This suggests effective interaction of the complexes with fungal cell membranes or intracellular targets, possibly through disruption of ergosterol synthesis or enzyme inhibition mechanisms 10.
A strong correlation was observed between the antimicrobial activity and the stability constants of the complexes. The Cu (II) complex, which exhibited the highest stability constant and most favorable thermodynamic parameters, also showed the largest zones of inhibition. This observation supports the notion that higher complex stability promotes sustained biological activity by preserving the integrity of the metal–ligand framework in biological environments7.
Chelation reduces the polarity of the metal ion by partial sharing of positive charge with donor atoms, thereby increasing the lipophilicity of the complex and enhancing its permeability through microbial lipid membranes10. This chelation theory adequately explains the superior antimicrobial performance of the metal complexes relative to the free ligand.
The bidentate coordination of HMP through azomethine nitrogen and phenolic oxygen atoms leads to the formation of stable chelate rings, which contribute to both thermodynamic stability and biological efficacy. The increased activity observed for the Cu (II) complex is further supported by its favorable electronic configuration and strong metal–ligand interactions, as reflected in both spectrophotometric stability data and thermal stability trends reported earlier12.
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Received on 23.02.2026 Revised on 19.03.2026 Accepted on 08.04.2026 Published on 14.07.2026 Available online from July 25, 2026 Research J. Science and Tech. 2026; 18(3):247-253. DOI: 10.52711/2349-2988.2026.00034
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