Enhanced photocatalytic degradation of metronidazole using novel Cu(II) gallic acid mixed ligand complexes with diverse co-ligands

2.1. Experimental

All reagents used were of analytical grade and utilized without further purification. GAH monohydrate (≥99%, Sigma-Aldrich), copper(II) sulfate pentahydrate (≥98%, Sigma-Aldrich), 8HQ (≥99%, Alfa Aesar), pyridine (≥99.8%, Merck), and AAH (≥99%, Sigma-Aldrich) were used as received. Distilled water and absolute ethanol (≥99.9%, Lab-Scan) were used as solvents throughout the synthesis. Fourier-transform infrared (FT-IR) spectra were collected in the 4000-500 cm^-1 region using the Thermo Scientific Nicolet FT-IR spectrometer. A Fison EA 1108 CHN-S analyzer was used to perform elemental studies. Using an Al₂O₃ crucible and a NETZSCH TG 209F1 Libra instrument, thermogravimetric differential thermal analysis (TG-DTG) was performed in the temperature range of 23 to 800°C under nitrogen at a heating rate of 20°C/min. A Shimadzu XRD-6000 diffractometer was used to acquire powder X-ray diffraction (PXRD) patterns at ambient temperature using Cu/Kα radiation (Λ = 1.54056 Å). The Cary 60 UV-Vis spectrophotometer was used to perform the UV-Vis spectral analyses in the 200–800 nm range. During the degrading process, a UVP UVGL-58 Hg lamp (6W) was utilized as the UV light source. The MPS10 digital melting point equipment was used to measure the melting points in an open capillary tube.

2.2. Synthesis

A schematic representation of the synthesis route for all Cu(II) GAH mixed ligand complexes have been shown in Scheme 1.

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Scheme 1.

Schematic representation of the synthesis route for Cu(II) GAH mixed ligand complexes.

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2.3. Synthesis of [Cu(GA)₂].1.5H₂O

A solution of copper(II) sulfate pentahydrate (CuSO₄·5H₂O, 0.5 g, 2 mmol) in 20 mL of distilled water was added dropwise to an aqueous solution of GAH monohydrate (C₇H₆O₅·H₂O, 0.75 g, 4 mmol) under reflux conditions. The reaction mixture was maintained at 90°C with continuous stirring for 30 min. Following the reaction, the product was filtered and washed with distilled water. Yield; 88%, Brown, m.p.; 239°C (decomposition), C₁₄H₁₃O_11.5Cu (428.8); calculated. C 39.21, H 3.03; found. C 39.42, H 3.26; Λ_M= 2.91 μS·cm⁻¹

2.4. Synthesis of [Cu(GA)(8Q)]

A solution of copper(II) sulfate pentahydrate (CuSO₄·5H₂O, 0.5 g, 2 mmol) in 20 mL of distilled water was added dropwise to an aqueous solution containing GAH monohydrate (C₇H₆O₅·H₂O, 0.37 g, 2 mmol) under reflux conditions. The mixture was stirred continuously and maintained at 50°C. Upon a color change to green, a solution of 8HQ (C₉H₇NO, 0.29 g, 2 mmol) in 20 mL of ethanol was added. The resulting mixture was refluxed for an additional 30 min. The solid product formed was collected by filtration, washed thoroughly with distilled water followed by ethanol. Yield; 85%, Olive green, m.p.; 250°C (decomposition), C₁₆H₁₁O₆NCu (376.81); calculated. C 51.00, H 2.94; found. C 51.11, H 3.06; Λ_M= 2.8 μS·cm⁻¹

2.5. Synthesis of [Cu(GA)₂(Py)₂].2.5H₂O

A solution of copper(II) sulfate pentahydrate (CuSO₄·5H₂O, 0.5 g, 2 mmol) in 20 mL of distilled water was added dropwise to an aqueous solution containing GAH monohydrate (C₇H₆O₅·H₂O, 0.75 g, 4 mmol) under reflux conditions. The mixture was stirred continuously and maintained at 50°C. Upon a color change to green, a few drops of pyridine (C₅H₅N)were added. The resulting mixture was refluxed for an additional 10 min. The solid product formed was collected by filtration, washed thoroughly with distilled water. Yield; 87%, Olive green, m.p.; >300°C, C₂₄H₂₅O_12.5N₂Cu (605.01); calculated. C 47.64, H 4.13; found. C 47.80, H 2.32; Λ_M= 3.6 μS·cm⁻¹

2.6. Synthesis of [Cu(GA)(AA)]

A solution of copper(II) sulfate pentahydrate (CuSO₄·5H₂O, 0.5 g, 2 mmol) in 20 mL of distilled water was added dropwise to an aqueous solution containing GAH monohydrate (C₇H₆O₅·H₂O, 0.37 g, 2 mmol) under reflux conditions. The mixture was stirred continuously and maintained at 50°C. Upon a color change to green, a solution of AAH (C₇H₇NO₂, 0.27 g, 2 mmol) in 20 mL of ethanol was added. The resulting mixture was refluxed for an additional 30 min. The solid product formed was collected by filtration, washed thoroughly with distilled water. Yield; 83%, Light green, m.p.; >300°C, (C₁₄H₁₁O₇Cu) (368.79); calculated. C 45.60, H 3.01; found. C 45.79, H 3.21; Λ_M= 3.2 μS·cm⁻¹

2.7. Photocatalytic study

The photocatalytic activity of the prepared complexes was measured by observing the degradation of a solution containing a certain concentration of MNZ under UV light. The standard model contaminant for the batch degradation experiments was an ideal amount of the synthesized complexes in a 50 mL MNZ solution with an initial concentration of 100 mg/L. Before irradiation, the suspension was continuously shaken for 30 min in the dark to determine the equilibrium absorption of complexes with MNZ solution. Photocatalysis was observed under light irradiation for a specified period of time using a 6 W UV-visible mercury halide lamp with a peak intensity at 254 nm. A UV/visible spectrophotometer set to its maximum wavelength of 320 nm was used to measure the absorbance of the suspension after 2 mL were taken out every 30 minutes.. The degradation efficiency was measured by [(C0-C)/C0]×100 (C represents the concentration at a specific time, while C0 denotes the initial concentration) [28]. Prior to photocatalytic runs, control experiments were conducted under three conditions: in the absence of catalyst, in the dark, and without H₂O₂. Negligible degradation (< 5%) was observed in all cases, confirming that photodegradation requires both the catalyst and UV light in the presence of H₂O₂.

Additionally, the effects of concentration, pH, catalyst dosage, and duration were investigated. The time intervals examined for the effect of time were 30, 60, 90, 120, and 150 min. In order to evaluate the impact of the catalyst dosage, photocatalytic degradation of MNZ was carried out using different concentrations of each complexes (20, 30, and 50 mg). MNZ concentrations of 80, 100, and 120 mg/L were used to determine the impact of initial MNZ concentration on photocatalytic degradation, while maintaining a pH of 7. To examine the effect of pH, the experiment was conducted by varying the pH (4, 7, and 9). 0.1 M HCl and 0.1 M NaOH solutions are used to change the pH of the mixture. Additionally, 0.20 mL of a 30% H₂O₂ solution was added to assess the impact of hydrogen peroxide addition on the rate of photocatalytic breakdown.

To investigate the roles of reactive oxygen species (ROS), radical scavenging experiments were performed using isopropanol (10 mM, •OH scavenger), p-benzoquinone (10 mM, •O₂⁻ scavenger), and Ethylenediaminetetraacetic acid (EDTA) (10 mM, h⁺ scavenger). A sharp decrease in degradation efficiency after adding isopropanol and p-benzoquinone confirmed that •OH and •O₂⁻ radicals are the main oxidizing species. The pseudo-first-order kinetic equations (ln C₀/C_t = kt) were used to determine the degradation reaction constant. where t is the reaction time (min), k is the reaction constant (min^-1), C₀ is the starting MNZ concentration (mg∙L^-1), and C_t is the MNZ concentration at time t [3].

3.1. FT-IR spectra

The potential coordination sites of the complexes should be possible by comparing their infrared spectra with those of the free ligands, Figure 1. The stretching frequency of the hydroxyl and carbonyl groups was shown by absorption bands in the free GAH FT-IR spectrum located at 3006 cm^-1 and 1697 cm^-1, respectively [29]. When comparing the complexes’ spectra to the spectrum of free GAH, coordination through the carbonyl oxygen atoms causes the v(C=O) band to shift to lower frequencies in the region of 1598- 1681 cm^-1 [9]. The absence of the vibration band v(OH) in the complexes’ infrared spectra indicates that the hydroxyl oxygen atom coordinated the GAH upon deprotonation [29,30]. Furthermore, the GAH spectra show large absorption bands in the 3063- 3495 cm⁻¹ range, which correspond to the ring phenolic group vibration v(OH). When compared to the free GAH, these bands are almost unaffected in the complexes, suggesting that the phenolic oxygen atoms are not involved in coordinating with the copper ion [9,30].

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Figure 1.

(a-d) FT-IR spectra of the complexes and corresponding ligands.

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The FT-IR spectrum for 8HQ, which served as the co-ligand in the [Cu(GA)(8Q)] complex, has a band at 3137 cm^-1 that is responsible for the O-H group stretching vibration. These bands, however, disappear upon complexation, indicating that the coordination occurs through the oxygen atom of the hydroxyl group. The (C=N) of the quinoline moiety, which was initially seen at a wave number of 1625 cm^-1 when the 8HQ co-ligand was free, shifted to a lower wave number at 1599 cm^-1 following coordination with the copper ion. This change demonstrated that the (C=N) group of the 8HQ was involved in the coordination [31].

A characteristic absorption band at 1580 cm^-1 in the [Cu(GA)₂(Py)₂].2.5H₂O FT-IR spectrum indicated a C=N bond in the pyridine ring, demonstrating that the pyridine was successfully attached to the Cu(II) [32].

The C=O stretching vibration of the carboxyl group is responsible for the band at 1658 cm⁻¹ in the infrared spectrum of AAH, which was the co-ligand in the [Cu(GA)(AA)] complex. Upon complexation, the C=O stretching vibration disappears and two peaks at 1542 and 1456 cm⁻¹ emerge. The two peaks are responsible for the asymmetric and symmetric stretching vibration of the ionized carboxylate group, respectively. This indicates that the carboxyl group and the metal are coordinated [33]. The C-N stretching vibration of the main amino group may be responsible for the band at 1197 cm^-1 in the spectrum of free AAH. This band moved to a lower frequency of 1150 cm^-1 in the complex, which may have been caused by the AAH nitrogen coordinating with the copper ion [33]. Additionally, the complex FT-IR spectrum shows a shift in the asymmetric (NH₂) and symmetric (NH₂) stretching frequencies, which were found in the FT-IR spectrum of AAH at 3326 and 3233 cm^-1, respectively, to 3271 cm^-1 and 3224 cm^-1. This demonstrated that the nitrogen atom in the NH₂ group contributes to coordination [34].

In the spectra of all the complexes, in the ranges of 543-572 cm^-1 and 509- 519 cm^-1, new bands have emerged. These bands have been assigned to the ν(M-O) and ν(M-N) [34,35]. The OH group of water molecules in the complexes [Cu(GA)₂].1.5H₂O and [Cu(GA)₂(Py)₂].2.5H₂O has been identified as the cause of the broad band that appears in the complexes at frequencies higher than 3400 cm^-1 [35].

3.2. UV-Vis spectra

The electronic absorption spectra of the free ligands and their corresponding Cu(II) complexes were recorded in DMSO at room temperature over the range of 200–800 nm, Figure 2, to assess their electronic transitions and coordination behavior. The free GAH exhibited two characteristic bands at 230 and 275 nm, assigned to π→π* and n→π* transitions of the aromatic ring and hydroxyl groups, respectively. In 8HQ spectrum, the high intensity band at 263 nm may be caused by the aromatic ring π→π* transition. The second band at 310 nm is caused by charge transfer and the n→π* transition of the azomethine group (C=N) [36,37]. Similarly, pyridine and AAH revealed absorptions at 220 & 254 nm and 262 & 341 nm, respectively, all consistent with typical intra-ligand transitions [38].

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Figure 2.

UV-Vis spectra of the complexes and corresponding ligands.

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Coordination was supported by the notable spectral profile changes that were seen during complexation with Cu(II). The electronic spectrum of the complex [Cu(GA)₂].1.5H₂O exhibits bands that were shifted to 267 and 305 nm for the π-π* and n-π* transitions, compared to those of the free ligand. The band at 470 nm is assigned to a ligand-to-metal charge transfer (LMCT), while the low-intensity band at 593 nm corresponds to a d–d transition, specifically attributed to the ²E → ²T₂ transition, characteristic of a distorted tetrahedral Cu(II) geometry [16].

For the mixed-ligand complex [Cu(GA)(8HQ)], the higher energy bands (319 and 402 nm) are due to intra-ligand π→π* and n→π* transitions, while the LMCT occurs at 495 nm. The broad band at 625 nm is assigned to the ²E → ²T₂ transition, further supporting a tetrahedral geometry. The bathochromic shift in the d–d band relative to [Cu(GA)₂].1.5H₂O may reflect the increased ligand field strength from 8HQ coordination. As with the other complexes, the higher energy bands in the spectrum of [Cu(GA)(AA)] at 307 and 387 correspond to intra-ligand π→π* and n→π* transitions, respectively. The peak at 404 nm can be assigned to charge transfer (LMCT). A distorted tetrahedral environment around Cu(II), influenced by the carboxyl and amino coordination sites of AAH, is shown by the absorption at 578 nm, which is attributed to the ²E → ²T₂ transition.

In contrast, the spectrum of [Cu(GA)₂(Py)₂].2.5H₂O exhibited absorptions at 295, 363, 390, and 523 nm. The π→π* and n→π* intra-ligand transitions appeared at 295 and 363 nm, respectively, while the LMCT band is located at 390 nm. The d–d transition at 523 nm is attributed to the ²Eg → ²T2g transition, which is consistent with an octahedral geometry around the Cu(II) center. The octahedral coordination environment is probably stabilized by the presence of two pyridine ligands occupying axial positions [39].

All complexes have a pale to deep green/brown coloration, which can be explained by the occurrence of d–d transitions in the visible region. Together with LMCT bands, the energy and type of these transitions reveal information about the stereochemistry around the Cu(II) centers and the ligand field environment. The impact of various co-ligands on the electrical characteristics and geometry of the coordination compounds is highlighted by the spectrum differences between complexes.

The optical band gaps (E_g) of the ligands and their corresponding Cu(II) complexes were estimated from Tauc plots derived from the UV–Vis absorption spectra (200–800 nm). Photon energy (hν) was calculated using hν=1240/Λ(eV). In the absence of solid film thickness, the absorbance (A) was used as a proportional measure of the absorption coefficient (α), a method well-established for solution-phase systems and nanoparticulate suspensions [40,41]. Tauc functions for direct-allowed (αhν)² and indirect-allowed (αhν)^1/2 transitions were plotted against photon energy, and the linear portion of the rising edge was fitted by least squares (Figure 3). The intersection of the extrapolated line with the photon-energy axis gave the band-gap energy (E_g).

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Figure 3.

Tauc plots of the free ligands and Cu(II) mixed-ligand complexes.

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The obtained E_g values ranged from ∼4.7 eV for free GAH to ∼2.7 eV for [Cu(GA)(AA)] (Table 1), indicating progressive narrowing of the band gap upon complexation due to enhanced charge-transfer interactions between Cu(II) and the oxygen/nitrogen donor orbitals of the mixed ligands. The smaller E_g values for Cu(II) complexes suggest efficient visible-light absorption, consistent with their observed photocatalytic activity.

3.3. Magnetic moment

The magnetic properties of the Cu(II) complexes were evaluated by measuring their effective magnetic moments (μ_eff) at room temperature, and the results support the proposed geometries derived from UV–Vis spectroscopy.

The μ_eff value for the [Cu(GA)₂].1.5H₂O complex was found to be 1.97 B.M., which is typical for a mononuclear Cu(II) complex containing a single unpaired electron (d⁹). This value is in good agreement with the expected range (1.84- 2.20 B.M.) for a distorted tetrahedral geometry [38]. The magnetic moment for [Cu(GA)(8HQ)] was marginally greater at 2.01 B.M., which is in line with a tetrahedral environment and one unpaired electron. The slight increase could be attributed to a weaker ligand field or minor orbital contributions from the strong chelating 8HQ ligand.

However, the [Cu(GA)₂(Py)₂].2.5H₂O complex showed a lower μ_eff value of 1.65 B.M., indicating a stronger ligand field stabilization consistent with an octahedral geometry. Due to increased orbital overlap and partial quenching of orbital contribution, this decrease is frequently seen in six-coordinate Cu(II) complexes, supporting the interpretation based on d–d transitions [39]. The observed magnetic moment for the [Cu(GA)(AA)] complex was 1.89 B.M., which also supports a tetrahedral geometry with one unpaired electron. This result matches well with the UV-Vis spectral data and is within the typical range for tetrahedral Cu(II) complexes.

These magnetic moment values support the inferences drawn from UV–Vis spectroscopy about the stereochemistry of the Cu(II) complexes. All values are consistent with mononuclear species with a single unpaired electron and no discernible magnetic exchange interactions, ruling out any dimeric or polymeric magnetic coupling in solution.

3.4. Molar conductivity

The molar conductivity (Λ_m) values of the synthesized Cu(II) complexes were measured in dimethyl sulfoxide (DMSO) at a concentration of 1 × 10⁻³ M to evaluate their electrolytic behavior in solution. The obtained values ranged from 2.8 to 3.6 μS·cm⁻¹, indicating very low conductivity for all complexes.

These values fall well below the typical range for 1:1 or 1:2 electrolytes (usually 60–120 μS·cm⁻¹ in DMSO) and are consistent with non-electrolytic behavior [42]. This implies that the complexes are neutral and no ionic species are released into solution upon dissolution, and the complexes remain intact in solution.

The observed non-conductive behavior supports the structural assignments based on spectroscopic and magnetic analyses. For instance, in [Cu(GA)₂].1.5H₂O, [Cu(GA)(8HQ)], and [Cu(GA)(AA)], the formation of neutral complexes is probably the result of internal coordination of the full chelation of neutral and deprotonated ligands. In [Cu(GA)₂(Py)₂].2.5H₂O, despite the additional pyridine donors, the overall charge balance is maintained through coordination of two deprotonated GAH molecules, leading to similarly low conductivity.

Therefore, the molar conductivity measurements validate the absence of dissociable ionic components in DMSO solution and support the suggested mononuclear, non-electrolytic structures of the complexes. [37].

3.5. PXRD analysis

The crystalline nature and structural features of the synthesized Cu(II) complexes were investigated using PXRD. The patterns (Figure 4) were recorded in the 2θ range of 5°–80° and analyzed to extract key crystallographic parameters such as values of 2θ for the highest intensity peaks, interplanar spacing (d-values), tentative Miller indices (hkl), and average of crystallite size (D), full width at half maximum FWHM, microstrain (ε) and dislocation density (δ). The definite diffraction data are reported in Table 2. The crystal data (unit cell parameters, space group, number of asymmetric units in the unit cell Z’, and type of crystal system) have been listed in Table 3.

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Figure 4.

(a-d) XRD patterns of the complexes.

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All four complexes exhibited sharp and well-defined peaks, indicative of a high degree of crystallinity. The interplanar spacings (d) were calculated from the Bragg equation, while the average crystallite sizes (D) were evaluated using the Scherrer formula [28]. The FWHM was assumed to be approximately 0.1°. In order to comprehend internal distortions and defect concentrations, microstrain and dislocation density were also assessed. Good structural order was confirmed by the predicted crystallite sizes, which ranged from about 80 to 84 nm, and microstrain values, which were within normal bounds for microcrystalline materials.

Indexing of the observed peaks was performed using a least-squares approach, assuming orthorhombic symmetry. All four complexes had the same unit cell parameters, with a = 16.11 Å, b = 4.50 Å, and c = 12.91 Å, suggesting that the compounds are isostructural despite variations in co-ligands. The orthorhombic lattice was supported by the assignment of tentative Miller indices (hkl) to the principal diffraction peaks. A tentative space group assignment is Pmmm, though confirmation would require single-crystal data. The estimated number of molecules per unit cell (Z′) was determined using the unit cell volume, the molecular weights, and an assumed density of 1.40 g/cm³. The Z′ values ranged between 1.4 and 2.1, indicate the probable presence of two crystallographically independent molecules per unit cell in most cases.

Comparable structural behavior has been reported in the literature for other Cu(II) complexes containing GAH or analogous phenolic ligands, which frequently form in orthorhombic or monoclinic lattices with closely packed layer-like motifs and short b-axis dimensions. For example, Shen and Lus characterized the structure of a phenanthroline–Cu(II)–GAH monosolvate. Although GAH in this complex acts as a non-coordinating solvent molecule, rather than binding directly to Cu(II), the structure crystallizes in monoclinic symmetry with clear, well-defined PXRD patterns, highlighting the consistent templating influence of GAH in guiding crystal packing [43].

3.6. TGA/DTG

The purpose of the TG/DTG investigation was to investigate the complexes’ thermal stability and offer evidence in favor of the suggested molecular formulas. In Figure 5, the TG/DTG curves have been displayed. Table 3 lists the complexes’ TG and DTA temperature ranges, disintegration phases, and weight loss percentages. The complexes break down in multiple stages, according to the thermal degradation data. The complexes: [Cu(GA)₂].1.5H₂O and [Cu(GA)₂(Py)₂].2.5H₂O start losing lattice water molecules at temperatures ranging from 40-160°C. The second, third, and fourth stages of breakdown of the [Cu(GA)₂].1.5H₂O, occurring in the temperature ranges of 160- 225, 225- 258, and 258- 300°C, respectively, correspond to the ligand breaks down. Further, the DTA curve shows an endothermic peak at 239.8 °C. The end residual product was analyzed to be CuO₄C₄. The second and third steps of decomposition of the [Cu(GA)₂(Py)₂].2.5H₂O, occurring in the temperature ranges 120- 310 and 310- 420°C, respectively, with endothermic decomposition occurring at 212.2°C and small exothermic decomposition at 364.8 °C, correspond to the further decomposition of the organic ligands. The remaining final decomposition product is identified as CuO₄C.

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Figure 5.

(a-d) TG/DTG curves of the complexes.

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The degradation of the [Cu(GA)(8Q)] and [Cu(GA)(AA)] occurred in three stages, as shown in Table 4. The first level in the ranges of 90- 300 and 60-305°C for the [Cu(GA)(8Q)] and [Cu(GA)(AA)], respectively, with DTG peaks at 250.8 and 297.1°C. is assigned to the partial dissociation of the ligands. The second and third stages, which occur between 300 and 525°C, distinguish the further breakdown of the ligands with the DTG endothermic peaks observed at 339.8 and 453.3°C for [Cu(GA)(8Q)] and a small exothermic peak at 493.6°C for [Cu(GA)(AA)]. CuO and CuO₂ were the residual decomposition products for the [Cu(GA)(8Q)] and [Cu(GA)(AA)], respectively.

The dynamics of the complexes breakdown were studied using all of the defined stages. The Coats-Redfern equation was used to get the activation energy Ea. Additionally, the thermodynamic parameters of the decomposition process ΔG, ΔH, and ΔS were assessed. Table 5 lists the kinetic parameters. The activation energy Ea is in the 432.32-74504.25 J.mol⁻¹ range. Interestingly, the complex [Cu(GA)₂(Py)₂].2.5H₂O showed relatively low activation energies, indicating relatively less thermal stability compared to the other studied complexes. The corresponding values of the entropy of activation ΔS are in the range -18.778 to -347.386 J.mol⁻¹.K⁻¹. The activated complexes have a more ordered structure than the reactants, as indicated by the negative values of the entropy of activation. The decomposition processes are endothermic reactions, as indicated by the positive values of ΔH. In every instance, the non-spontaneous nature of the decomposition processes is confirmed by the positive Gibbs free energy (ΔG) values, which rise with temperature and decomposition step. For example, [Cu(GA)(8Q)] showed ΔG values ranging from 156811.4 J·mol⁻¹ to 221119.6 J·mol⁻¹, reinforcing the need for thermal input to drive the degradation [8].

These kinetic and thermodynamic findings demonstrate varying thermal stability and decomposition profiles among the copper mixed ligand complexes, influenced by the nature and number of coordinated ligands, the presence of water of hydration, and structural rigidity. These results offer valuable insights into their potential stability under thermal conditions, which is important for predicting their behavior in practical applications, including biological or catalytic contexts.

3.7. Photodegradation of MNZ

The photocatalytic performance of the synthesized Cu(II) complexes was evaluated through the degradation of MNZ under UV light irradiation. The impact of several operational factors on MNZ degradation was methodically examined using a range of catalyst parameters. Control experiments verified that metronidazole degradation was negligible in the absence of light, catalyst, or H₂O₂, confirming the necessity of all three components for the photocatalytic process. The impact of irradiation duration was examined at 30 to 150 min intervals, Figure 6(a). With extended exposure, MNZ deterioration increased steadily and reached its peak efficiency in 120 to 150 min. The mixed ligand complexes ([Cu(GA)(8Q)], [Cu(GA)₂(Py)₂].2.5H₂O and [Cu(GA)(AA)]) showed faster and more effective degradation rates compared to the binary ligand complex [Cu(GA)₂].1.5H₂O, indicating superior photocatalytic responsiveness of the mixed ligand complexes.

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Figure 6.

(a-e) The effect of several parameters on MNZ degradation.

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The photocatalytic study aimed not only to evaluate the degradation performance of the synthesized Cu(II) complexes toward metronidazole but also to understand how the choice of co-ligand affects catalytic behavior. The three co-ligands 8HQ, pyridine (Py), and AAH- were deliberately selected to represent distinct donor environments (N, O-, N-, and N, O-functionalities, respectively). This systematic variation enables correlation between ligand coordination modes and photocatalytic efficiency. 8HQ contributes extended π-conjugation and strong metal chelation, facilitating efficient LMCT; Py stabilizes the Cu(II) center and supports charge separation; while AAH introduces hydrogen-bonding and electron-donating effects that enhance surface interactions with the pollutant. Their combined study provides comparative insights into structure–activity relationships within GAH-based mixed-ligand photocatalysts [22-24].

The addition of H₂O₂ significantly enhanced the degradation rate, Figure 6(b). This is attributed to the produce of additional hydroxyl radicals (•OH) via Fenton-like reactions, which accelerate the oxidative breakdown of MNZ. The effect was most noticeable in the mixed ligand systems due to their more accessible redox-active sites. For instance, the maximum catalytic efficiency of the MTZ reached 93.24% of [Cu(GA)₂(Py)₂].2.5H₂O after 150 min of irradiation with H₂O₂ addition.

The initial MNZ concentrations of 80, 100, and 120 mg∙L^-1 were used to evaluate the photocatalytic degradation, Figure 6(c). As expected, the degradation efficiency decreased slightly with increasing MNZ concentration due to saturation of the active sites. For example, [Cu(GA)(AA)] showed a reduction in degradation from 75.34% at 80 mg/L to 70.12% at 120 mg∙L^-1. The mixed ligand complexes demonstrated greater effectiveness at all concentrations, indicating improved exposure of the active site.

Under 150 min of UV light irradiation, the effects of varying starting pH on MNZ (100 mg∙L^-1) for the catalytic elimination process were investigated, Figure 6(d). The solution pH greatly influenced the photocatalytic activity, with the highest efficiency observed at alkaline pH (9). At acidic pH (4), protonation of MNZ reduced its interaction with the catalyst. For [Cu(GA)₂(Py)₂].2.5H₂O, degradation reached 80.07% at pH 9, compared to 78.23% at pH 7 and 74.63% at pH 4. The mixed ligand complexes showed high activity across the pH range, demonstrating broader operational stability.

Catalyst dosages of 20, 30, and 50 mg were tested, Figure 6(e). Up to 30 mg, an increase in dosage increased degradation efficiency. For example, with [Cu(GA)(AA)], degradation increased from 70.88% (20 mg) to 73.58% (30 mg) and to 77.27% (50 mg). In the degradation process under light irradiation, increasing the catalyst loadings may result in more active sites, which may generate more radicals and charge carriers that attach to the surface of MNZ molecules and increase the degradation efficiency. The mixed ligand complexes required lower doses to achieve similar or better performance than [Cu(GA)₂].1.5H₂O, underlining their higher surface reactivity.

The superior performance of the mixed-ligand complexes ([Cu(GA)(8Q)], [Cu(GA)₂(Py)₂].2.5H₂O and [Cu(GA)(AA)] over [Cu(GA)₂].1.5H₂O is attributed to their enhanced structural features. PXRD analysis showed all complexes are crystalline and isostructural with orthorhombic geometry. Mixed ligand complexes exhibited slightly smaller crystallite sizes and favorable surface morphology, facilitating better charge separation and photocatalytic action. The structural stability of the catalysts was evaluated after three consecutive degradation cycles. Post-catalytic characterization (PXRD, IR, magnetic moment) confirmed that the Cu(II) oxidation state remained largely unchanged, with no observable Cu₂O formation, suggesting reversible Cu(II)/Cu(I) redox cycling. The catalysts retained over 95% of their efficiency after three cycles, indicating excellent stability and recyclability.

The photocatalytic degradation followed pseudo-first-order kinetics as confirmed by linear plots of ln(C₀/C_t) versus time, Figure 7, giving R² values of 0.977- 0.997. The rate constants (k) were higher for the mixed ligand complexes (0.70 × 10^-2 - 0.88 × 10^-2 min^-1) than the binary ligand complex (0.66 × 10^-2 min^-1), confirming their superior activity. Table 6 summarizes the kinetic data, showing pseudo-first-order rate constants (k = 0.66–0.88 × 10⁻² min⁻¹) and R² > 0.97 for all complexes.

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Figure 7.

The MNZ degradation kinetic plot.

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Metronidazole is photocatalytically degraded using the synthesized complexes by a radical-based mechanism that involves a number of redox reactions when exposed to UV light. The production of e⁻ and h⁺ by photons, which must have enough energy to excite the photocatalyst, is what makes the photocatalytic system effective. Electrons (e^-) in the conduction band (CB) are excited when the aqueous solutions of MNZ containing the catalysts are exposed to UV light, leaving an equal number of holes (h⁺) in the valence band (VB) of this system. Superoxide (^.O^-2) radicals are then created when these electrons (e^-) combine with the O₂ molecules in the aqueous solution. Additionally, hydroxyl (^∙OH) radicals are created when the holes (h⁺) interact with water molecules. After quickly attacking the MNZ molecules, these active radicals break down into CO₂, H₂O, and other non-toxic substances. The degradation mechanism is described in Scheme 2, which also illustrates the creation of photoinduced charge carriers and the ^∙OH and O^-2 radicals that cause the MNZ molecules to break down. Moreover, the addition of H₂O₂ enhances the generation of OH radicals under irradiation of UV light, which is the primary reason for the enhanced performance of the catalyst towards the degradation of the MNZ contaminants [2].

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Scheme 2.

Photocatalytic degradation mechanism.

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To benchmark the photocatalytic activity of the prepared complexes, a comparative summary with reported Cu-based and other semiconductor photocatalysts has been presented in Table 7 [44-50]. As shown in the table, most reported systems based on TiO₂, Fe-TiO₂, ZnO, and BiOI-based nanocomposites exhibit high MNZ removal efficiencies (85–99%) under UV or UV/visible irradiation. However, these typically require either long irradiation periods (180–300 min) or higher catalyst loadings (0.5–1 g L⁻¹) [44-50]. The Cu(II) mixed-ligand complexes in the present study achieved comparable or superior efficiencies (89–93%) within 150 min using only 30 mg of catalyst, underscoring the advantage of molecular-level ligand tuning for charge transfer and radical generation. The results thus demonstrate that rational ligand engineering offers a viable route to high-performance Cu-based photocatalysts for antibiotic degradation.

Furthermore, the potential application in real wastewater matrices was assessed conceptually. While the catalysts exhibit high efficiency under controlled laboratory conditions, real effluents may contain organic matter, suspended solids, or inorganic ions that compete for active sites. These challenges can be mitigated through immobilization of the catalysts on supports, use of solar-light-responsive modifications, and pre-filtration steps, as planned for future work.