In our earlier studies [50,51,52], the same instruments and reagents were used for all characterization studies. DMF (Sisco Research Laboratories Pvt. Ltd.) and POCl₃ (Loba Chemie), were employed in the experimental work. 3,3′-Diaminobenzidine was obtained from TCI (Tokyo Chemical Industry), Japan. Dichloromethane (DCM) was procured from Avara Chemicals. Electronic absorption spectra of all compounds were measured on an Agilent Cary-60 UV–Vis spectrophotometer. Elemental analysis was conducted using the Elementar Excellence in Elements model, Unicube superuser V1.3.2 (065bdfa).

The formylated pyrazolone intermediates (1-3) were prepared according to the procedure reported in our earlier publications [16,17]. The purified formylated intermediates were further condensed with 3,3′-diaminobenzidine to yield the corresponding Schiff base ligands. Specifically, the formylated compound (4 mmol) was reacted with 3,3′-diaminobenzidine (1 mmol) in methanol under reflux for 4-5 h, resulting in the formation of quadridentate Schiff base ligands, as shown in Figure 1. We then cooled the reaction mixture to room temperature, filtered the solid products, washed them with cold methanol, and oven-dried them. The ligands were obtained in the form of yellow powders (85-87% yield, M.P. >200°C) and characterized by FT-IR (KBr, cm^-1). Attempts to record NMR spectra of the ligands were unsuccessful because the compounds displayed extremely limited solubility in various deuterated solvents, even upon heating and sonication. As a result, NMR data are not included for these ligands.

Complexes 1-3 were synthesized by refluxing a warm DCM solution (25 mL, 35-40°C) of ligand L₁-L₃ (2 mmol) with a DCM solution (15 mL) of Zn(OAc)₂·2H₂O (4 mmol) for 4-5 h. The precipitated solid was filtered after cooling, washed with cold ethanol, and dried. The synthetic route is shown in Figure 2. Each complex was isolated as a yellow powder in 90-95% yield with a melting point >200°C and characterized by FT-IR (KBr, cm^-1) and ¹H NMR (400 MHz, DMSO-d₆, δ, ppm).

In vitro anti-malarial activity against the chloroquine-sensitive Plasmodium falciparum (3D7 strain) was evaluated for all compounds at Microcare Laboratory and TRC (Surat, Gujarat). The experimental assay protocol has been described in detail in our previously published reports, and the same standardized procedure was followed in the present study [50,51,52].

The results of elemental analysis for complexes demonstrate a high level of agreement between experimentally determined and theoretically calculated values. The measured percentages of carbon, hydrogen, and nitrogen for all three complexes closely match their computed compositions, with only minor variations. These small discrepancies can be attributed to factors such as residual moisture in the samples, slight instrumental deviations, or sample handling, all of which remain within acceptable experimental limits. Overall, the strong concordance between theoretical and experimental data supports both the successful synthesis and the high purity of the prepared complexes. Theoretical and experimental elemental analysis data for complex-1, complex-2, and complex-3 (Table 1) and Figure S2 represent the actual elemental analysis data obtained directly from the instrument for the complexes. The percentage differences between the theoretical and experimental values were found to be very low, with deviations of 0.06%, 0.03%, and 0.00% for complex-1; 0.22%, 0.04%, and 0.00% for complex-2; and 0.07%, 0.03%, and 0.00% for complex-3 for N, C, and H, respectively, indicating excellent agreement between the calculated and observed values.

Proton NMR studies on all zinc(II) complexes (measured in DMSO-d₆) exhibit sharp, well-resolved signals that align closely with the anticipated coordination geometry and the aromatic architecture of the ligands. Due to inadequate dissolution in conventional NMR solvents, the ¹H NMR spectra of the free ligands are not reported. The ligands were insoluble in common organic solvents such as methanol, ethanol, chloroform, and acetone, but were slightly soluble in DMSO and DMF. Complex-1 shows two singlets at δ 2.38 and 2.42 ppm (each integrating for 6H), attributed to the methyl groups of the pyrazolone units. The aromatic region appears as a broad multiplet between δ 7.24-8.25 ppm (26H, Ar-H). The azomethine protons (–CH=N–) resonate distinctly as singlets at δ 8.89 and 9.09 ppm (2H each), confirming coordination through the imine nitrogen [53]. Complex-2 exhibits similar features with singlets at δ 2.39 and 2.43 ppm (6H each, CH₃). The aromatic protons appear as well-defined doublets and multiplets in the region δ 7.24-8.32 ppm, including characteristic doublets at δ 7.24-7.26 (d, 4H, J = 8 Hz), 7.47-7.51 (m, 4H), 7.69-7.71 (d, 2H, J = 8 Hz), 7.94-7.96 (d, 2H, J = 8 Hz), followed by additional downfield aromatic signals at δ 8.25-8.27 (d, 4H, Ar-H) and 8.32 (s, 6H, Ar-H), indicating para-substituted phenyl environments. The azomethine signals remain as sharp singlets at δ 8.87 and 9.06 ppm. Complex-3 displays two strong singlets at δ 2.38 and 2.41 ppm (12H each), consistent with additional methyl substituents. Its aromatic region spans δ 7.27-8.24 ppm, including characteristic doublets at δ 7.27-7.29, 7.67-7.69, 7.93-7.95, and 8.13-8.15 ppm (J = 8 Hz), and a singlet at δ 8.24 ppm (2H, Ar-H). The azomethine protons appear downfield as singlets at δ 8.88 and 9.07 ppm, similar to the previous complexes. All experimental spectra can be found in Figure S3, Figure S4, and Figure S5.

Infrared spectral studies of the ligands and their complexes clearly indicate successful metal–ligand coordination, providing convincing evidence of complex formation. The free ligands L₁-L₃ exhibit a characteristic carbonyl stretching band ν(C=O) at 1662-1663 cm^-1, along with azomethine ν(C=N) vibrations in the range 1617-1624 cm^-1. Following ligand-metal binding, the ν(C=O) band disappears completely in all complexes, indicating enolization of the pyrazolone unit followed by coordination to Zn(II) through the enolate O-atom. A new absorption band appears in the region 1333-1337 cm^-1 in all complexes, corresponding to the ν(C–O) stretching of the coordinated enolate group, further confirming metal–oxygen bond formation [53]. Additionally, the azomethine stretching frequency ν(C=N) shifts slightly to lower wavenumbers (1618-1623 cm^-1) in complexes 1-3 compared to their parent ligands, which is consistent with Zn(II)–N(imine) bond formation. Overall, the combined disappearance of the C=O band, the occurrence of the new C–O band, and the shift in the C=N stretching frequency unequivocally Validate the formation of the Zn(II) complexes. The broad band in the region 3437-3423 cm^-1 is weak and poorly defined in the free ligands but becomes more prominent in the corresponding complexes, which may be attributed to O–H/N–H stretching vibrations associated with intermolecular hydrogen bonding or adsorbed moisture rather than coordinated or lattice solvent molecules, as supported by TGA analysis. New bands in the regions 559-561 cm^-1 and 446-447 cm^-1 are assigned to ν(Zn–O) and ν(Zn–N), respectively. The spectra of all compounds are presented in Figures S6-S11. Table 2 presents a comparison of the FT-IR frequencies of ligands with complexes.

The electronic spectra of the ligands exhibit multiple bands within the 300-400 nm region, which can be ascribed to intraligand π → π* and n → π* transitions arising from the aromatic rings and azomethine functionalities (Figure 3). The L₂ spectrum exhibits a clear vibrational progression in the 374-475 nm region, indicating strong vibronic coupling in the excited state. The presence of multiple resolved peaks suggests that the electronic transition is accompanied by excitation of specific vibrational modes, likely due to a significant change in equilibrium geometry between the ground and excited states. This structured band contrasts with the broader, less resolved features in the other spectra and suggests a more rigid or well-defined chromophore environment for L₂. Upon coordination with Zn(II), notable changes are observed in the spectral profiles of the complexes. All zinc(II) complexes display an absorption band around ~300 nm and ~350 nm, corresponding primarily to retained intraligand transitions within the phenyl and heterocyclic moieties. A relatively sharp band appearing in the 400-450 nm region is significantly more pronounced in the complexes. It is indicative of ligand-to-metal electronic interaction or enhanced π-conjugation resulting from coordination-induced electron delocalization [54,55]. Relative to the corresponding free ligands, the complexes show distinct red shifts (bathochromic shifts) in both major absorption bands. These shifts reflect a lowering of the energy gap between the HOMO and LUMO levels [53,56], which is commonly observed when ligands coordinate to metal centers, forming more rigid, conjugated frameworks. Overall, the UV–Vis results confirm the formation of complexes and highlight the modifications in the electronic structure upon metal coordination.

The thermogravimetric analysis (TGA) curves of the complexes (Figure S11, Figure S12, and Figure S13) show that all complexes possess good thermal stability. Complex 1 shows a very small, gradual weight loss of about 4.35% at lower temperatures, which can be assigned to the loss of physisorbed water or minor surface effects rather than the loss of coordinated solvent or water molecules. No clear mass-loss step related to volatile components is observed. Complex 2 exhibits a gradual mass loss up to approximately 500°C. Beyond this temperature, a sharp weight loss of about 34.9% is observed, indicating the temperature at which significant decomposition of the organic ligand framework begins. Similarly, complex 3 remains stable up to around 500°C, after which a major weight loss (~45.48%) occurs, corresponding to the temperature at which breakdown of the coordinated ligand structure starts.

The fact that major decomposition begins only at high temperatures for all complexes confirms strong metal-ligand bonding and good thermal stability. Figure S12, Figure S13, and Figure S14 show TGA curves indicating high thermal stability of the complexes up to ~500°C.

The synthesized ligands (L₁-L₃) and their corresponding zinc(II) complexes were evaluated for their in vitro anti-plasmodial efficacy against the chloroquine-sensitive Plasmodium falciparum 3D7 strain via a modified Rieckmann assay. All tested compounds exhibited notable activity, with IC₅₀ values ranging from 0.53 to 0.84 µM. A key observation was the significantly improved inhibitory potency of the Zn(II) complexes relative to their free ligands. For comparison, reference drugs chloroquine (CQ) [57] and quinine (QN) [58] gave IC₅₀ values of 0.063 µM and 0.826 µM, respectively. While the synthesized compounds were less potent than CQ, the observed improvement in activity upon zinc coordination suggests that these metal-based scaffolds may be promising candidates for further investigation as anti-malarial agents. The complete biological data are compiled in Table 3. The biological activity of the synthesized ligands and their three zinc(II) complexes was evaluated and compared with our previously reported study on related bidentate acylpyrazolone metal complex. A structurally related complex, [Zn₂(L₂C)₂(HQ)₂] (C₅₂H₃₄Cl₂N₈O₁₀Zn₂), has been reported to exhibit significant anti-malarial activity with an IC₅₀ value of 0.79 µM. The anti-malarial activity observed for the present zinc(II) complexes is promising and comparable to that reported in our earlier publication and with some of the most active zinc complexes described in the literature, as summarized in Table 3.