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Recent Progress in Science and Engineering

(ISSN 3067-4573)

Recent Progress in Science and Engineering (ISSN 3067-4573) is an international peer-reviewed Open-Access journal published quarterly online by LIDSEN Publishing Inc. It aims to provide an advanced knowledge platform for Science and Engineering researchers, to share the recent advances on research, innovations and development in their field.

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Open Access Research Article

Novel Bis-Quadridentate Pyrazolone-Based Ligands and Zinc(II) Complexes: Spectroscopic Characterization and In Vitro Anti-Malarial Activity

Jignesh P. Sathvara , Rajendrasinh N. Jadeja *

  1. Department of Chemistry, Faculty of Science, The Maharaja Sayajirao University of Baroda, Vadodara 390002, India

* Correspondence: Rajendrasinh N. Jadeja

Academic Editor: Boris F. Minaev

Received: February 12, 2026 | Accepted: April 22, 2026 | Published: April 30, 2026

Recent Prog Sci Eng 2026, Volume 2, Issue 2, doi:10.21926/rpse.2602007

Recommended citation: Sathvara JP, Jadeja RN. Novel Bis-Quadridentate Pyrazolone-Based Ligands and Zinc(II) Complexes: Spectroscopic Characterization and In Vitro Anti-Malarial Activity. Recent Prog Sci Eng 2026; 2(2): 007; doi:10.21926/rpse.2602007.

© 2026 by the authors. This is an open access article distributed under the conditions of the Creative Commons by Attribution License, which permits unrestricted use, distribution, and reproduction in any medium or format, provided the original work is correctly cited.

Abstract

A new series of bis-quadridentate ligands (L1-L3) and their corresponding zinc(II) complexes (Complex 1-3) were rationally designed, synthesized, and characterized. The ligand framework features multiple donor sites that enable stable coordination to zinc(II), yielding well-defined metal complexes. The structures of the synthesized compounds were confirmed using a combination of analytical and spectroscopic techniques. Thermal gravimetric analysis revealed that all zinc complexes exhibit exceptional thermal stability, remaining stable up to 500°C, indicative of robust metal–ligand interactions. The in vitro anti-plasmodial activity of the ligands and their zinc complexes was evaluated against Plasmodium falciparum using a JSB staining-assisted microscopic method. The zinc complexes demonstrated enhanced activity relative to the free ligands, with half-maximal inhibitory concentration (IC50) values of 0.67 µg mL-1 (1), 0.53 µg mL-1 (2), and 0.59 µg mL-1 (3). In contrast, the free ligands exhibited IC50 values ranging from 0.69 to 0.84 µg mL-1.

Graphical abstract

Click to view original image

Keywords

Zn(II) complex; formyl pyrazolone; multidentate ligands

1. Introduction

Pyrazolone is a versatile synthon for constructing diverse molecular frameworks with applications in chemistry and materials science [1]. The pyrazolone framework is found in natural alkaloids and therapeutic agents, exhibiting significant biological activities and serving as a motif in enzyme inhibitors [2,3,4,5,6,7,8,9,10]. Substitution at the C4 position with donor groups enhances chelation and enables selective metal coordination.

Acyl-pyrazolones act as mono- or bidentate β-diketone ligands, but their denticity increases when the formyl or acyl group forms an imine via Schiff base condensation. This adds a nitrogen donor, enabling N,O,O- or N,O,N,O-chelation and creating rigid multidentate frameworks that stabilize higher-nuclearity metal complexes. Schiff base metal complexes themselves have attracted extensive interest for their structural flexibility and electronic properties, contributing to single-molecule magnetism, materials science [11], and catalysis [12]. They also possess significant industrial relevance [13], while group 12 complexes with d10 configuration continue to attract attention for their stability and functional versatility [14,15].

Within this chemistry, formyl pyrazolones are valuable as they readily form imines, linking β-diketone coordination chemistry with Salen/Salophen-type Schiff bases. These derivatives act as tunable ligands with strong metal-binding ability and enhanced conjugation [16,17], enabling stable metal chelate formation [18]. When converted to Schiff bases, they generate Salen- or Salophen-type tetradentate N, O-donor frameworks [19,20,21,22]. Classic Salen/Salophen ligands, formed from salicylaldehyde and diamines, possess N2O2 donor sets that yield neutral, thermally stable complexes [23,24]. Their rigidity and chelating strength support broad catalytic activity, including oxidation, epoxidation, hydroxylation, hydrogenation, and CO2 fixation [25,26,27,28,29,30]. Marchetti et al. reported the synthesis of pyrazolone-based hydrazones and their Zn(II) and Cu(II) complexes, which exhibited significant biological activity against Trypanosoma brucei. Among them, the Zn(II) complex showed the highest activity and selectivity, and mechanistic studies suggested that the compounds interfere with nucleotide metabolism, possibly targeting CTP synthetase [31]. Gusev et al. reported Zn(II) Schiff base complexes based on the pyrazolone ligand 4-{(E)-[(2-fluorophenyl)imino]methyl}-5-methyl-2-phenyl-2,4-dihydro-3H-pyrazol-3-one (HL), exhibiting well-defined crystal structures and solvent-dependent luminescence. The complexes showed tunable emission behavior, highlighting their photophysical significance [32]. Baryshnikov et al. investigated Zn(II) complexes with 3-(pyridin-2-yl)-5-(arylideneiminophenyl)-1H-1,2,4-triazole ligands using DFT and QTAIM analysis. The study highlighted the influence of tautomeric forms on the electronic structure, stereochemistry, and luminescence properties of the complexes [33]. Gusev et al. reported Zn(II) Schiff base complexes based on pyrazolone-derived azomethine ligands, exhibiting high thermal stability and strong blue photoluminescence in both solution and solid state. The luminescence was attributed to intraligand charge-transfer, as supported by TD-DFT calculations. These complexes were successfully incorporated into organic light-emitting diodes (OLEDs), yielding good brightness and efficiency. The study highlights the potential of pyrazolone-based Zn(II) complexes as promising blue-emitting materials [34]. The compounds exhibit an unusual excitation-dependent emission behavior, characterized by a shift in emission color from blue to yellow with increasing excitation wavelength. Quantum chemical calculations suggest that supramolecular variations in the single-crystal architecture of the synthesized complexes play a significant role in governing this photophysical behavior, as reported for novel zinc complexes with pyridyltriazoles in the study Structure and excitation-dependent emission of novel zinc complexes with pyridyltriazoles [35].

Malaria remains a major global health burden. According to the World Malaria Report 2022, the 568,000 deaths reported before the COVID-19 pandemic increased to 625,000 in 2020 and slightly decreased to 619,000 in 2021. During the pandemic, an estimated 63,000 additional deaths were attributed to COVID-19-related disruptions. Moreover, 247 million cases were reported in 2021, up from 245 million in 2020 and 232 million in 2019 [36,37]. Plasmodium falciparum, the most virulent human malaria parasite, multiplies within red blood cells, and inadequate or delayed treatment leads to increased parasitemia and progression to severe malaria [38,39]. Malaria control is seriously challenged by increasing drug resistance, poor treatment compliance, and the widespread availability of falsified medicines [40,41]. Inside infected erythrocytes, the parasite degrades hemoglobin for growth, and its inhibition is lethal. Thus, falcipain-2, a cysteine protease active in trophozoite and schizont stages, is a key target for anti-malarial drug development [42,43,44,45,46].

Considering these factors, a binucleating Schiff base ligand based on a pyrazolone framework was designed. It was synthesized by condensing 3,3′-diaminobenzidine with formyl pyrazolone, along with its Zn(II) complexes, where the diamine core provides flexibility and strong metal-binding ability [47]. The rationale for employing zinc is supported by the literature, which highlights the therapeutic relevance of metal-based systems in anti-malarial research. Timothy et al. [48] reported that tetraazamacrocyclic complexes exhibit metal-dependent activity, with Zn2+ showing appreciable potency in the order Co < Ni < Cu < Zn < Fe ≤ Mn. Bagul et al. [49] found that the HPPHmCB ligand shows moderate activity against chloroquine-resistant Plasmodium falciparum (IC50 = 5.23 ± 0.09 µM), which significantly improves upon metal coordination. The Zn(PPHmCB)2 complex exhibited enhanced potency (IC50 = 2.39 ± 0.08 µM), supporting zinc-based complex design.

2. Experimental Procedure

2.1 Material and Instrumentation

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).

2.2 Synthetic Route of Ligands

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.

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Figure 1 Synthetic route of Schiff base ligands. Where, L1: R1 = R2 = H and L2: R1 = Cl, R2 = H and L3: R1 = H, R2 = CH3. L1; Mol. wt.: 951.06 (g/mol). FT-IR: 1663 (C=O pyrazolone), 1617 (C=N). EA calcd/found for C56H46N12O4(%): C 70.72/70.30; H 4.88/4.99; N 17.67/17.51. L2; Mol. wt.: 1088.83 (g/mol). FT-IR: 1663 (C=O pyrazolone), 1624 (C=N). EA calcd/found for C56H42Cl4N12O4(%): C 61.77/61.47; H 3.89/3.80; N 15.44/15.29. L3; Mol. wt.: 1007.17 (g/mol). FT-IR: 1662 (C=O pyrazolone), 1619 (C=N). EA calcd/found for C60H54N12O4(%): C 71.55/71.79; H 5.40/5.27; N 16.69/16.58. Figure S1 represents the actual elemental analysis data obtained directly from the instrument.

2.3 Synthetic Route of Zn(II) Complexes

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).

Click to view original image

Figure 2 Synthetic route of complexes. Where, complex-1: R1 = R2 = H and complex-2: R1 = Cl, R2 = H and comlpex-3 R1 = H, R2 = CH3. Complex-1; EA calcd/found for C56H42N12O4Zn2(%): C 62.41/62.43; H 3.93/3.93; N 15.60/15.61. FT-IR: 1621 (C=N), 1334 (C-O pyrazolone). 1H NMR: 2.38 (s, 6H, CH3), 2.42 (s, 6H, CH3),7.24-8.25 (m, 26H, Ar-H), 8.9 (s, 2H, =CH), 9.1 (s, 2H, =CH). Complex-2; EA calcd/found for C56H38Cl4N12O4Zn2(%): C 55.33/55.31; H 3.15/3.15; N 13.83/13.86. FT-IR: 1623 (C=N), 1337 (C-O pyrazolone). 1H NMR: 2.39 (s, 6H, CH3), 2.43 (s, 6H, CH3), 7.24-7.26 (d, 4H, (J = 8 Hz), ArH), 7.47-7.51 (m, 4H, , ArH), 7.69-7.71 (d, 2H, (J = 8 Hz), ArH), 7.94-7.96 (d, 2H, (J = 8 Hz), ArH), 8.25-8.27 (d, 4H, ArH), 8.32 (s, 6H, ArH), 8.9 (s, 2H, =CH), 9.1 (s, 2H, =CH). Complex-3; EA calcd/found for C60H50N12O4Zn2(%): C 63.56/63.58; H 4.44/4.44; N 14.82/14.83. FT-IR: 1618 (C=N), 1333 (C-O pyrazolone). 1H NMR: 2.38 (s, 12H, CH3), 2.41 (s, 12H, CH3), 7.27-7.29 (d, 8H (J = 8 Hz), Ar-H), 7.67-7.69 (d, 2H, (J = 8 Hz), ArH), 7.93-7.95 (d, 2H (J = 8 Hz), Ar-H), 8.13-8.15 (d, 8H (J = 8 Hz), 8.24 (s, 2H Ar-H), 8.88 (s, 2H, =CH), 9.07 (s, 2H, =CH).

2.4 Anti-Malarial Activity

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].

3. Result and Discussion

The structural and spectral features of the synthesized compounds are discussed below. The bulky and flexible ligand architecture results in low solubility in typical organic solvents.

3.1 Elemental Composition

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.

Table 1 Theoretical and experimental elemental analysis data for complexes.

3.2 NMR Interpretation

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.

3.3 FT-IR Spectral Interpretation

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.

Table 2 Comparison of FT-IR frequencies of ligands with complexes.

3.4 UV-Visible Spectra Analysis

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.

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Figure 3 UV-Vis spectra of ligands and Zn(II) complexes.

3.5 TGA Analysis

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.

3.6 In Vitro Anti-Malarial Activity

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.

Table 3 Plasmodium inhibition by the developed compounds [μM].

4. Conclusion

A novel series of binuclear bis-quadridentate ligands and their corresponding zinc(II) complexes were successfully synthesized and characterized by elemental analysis, TGA, UV-Vis, NMR, and IR spectroscopy. Complex formation was confirmed by UV-Vis spectroscopy through the appearance of characteristic metal–ligand charge-transfer bands. All complexes exhibited notable in vitro anti-malarial activity against Plasmodium falciparum. Compared to the free ligands, the zinc(II) complexes demonstrated enhanced biological activity, suggesting that increasing zinc coordination from mononuclear to binuclear frameworks improves anti-malarial efficacy. However, due to the bulky, rigid nature of these compounds, solubility limitations prevented further characterization.

Acknowledgments

The authors would like to acknowledge CSIR-HRDG (Council of Scientific and Industrial Research) for providing CSIR-Senior Research Fellowship to JPS. We are thankful to the Head of the Department of Chemistry at the Maharaja Sayajirao University of Baroda for offering the centres and resources required to complete this work.

Author Contributions

Rajendrasinh N. Jadeja: Conceptualization, reviewing and supervision for the synthesis and characterization have been done with his help. Jignesh P. Sathvara: He handled the synthesis, characterization, data analysis, and writing for the initial draft.

Funding

No funding information is available for this work.

Competing Interests

The authors emphasize that they are not aware of any personal or financial conflicts that might have appeared to affect the research described in this paper. It is unnecessary to disclose any conflicts.

Additional Materials

The following additional materials are uploaded at the page of this paper.

  1. Figure S1: The actual elemental analysis data obtained directly from the instrument.
  2. Figure S2: The actual elemental analysis data obtained directly from the instrument for the complexes.
  3. Figure S3: NMR spectrum of complex-1.
  4. Figure S4: NMR spectrum of complex-2.
  5. Figure S5: NMR spectrum of complex-3.
  6. Figure S6: FT-IR spectrum of L1.
  7. Figure S7: FT-IR spectrum of L2.
  8. Figure S8: FT-IR spectrum of L3.
  9. Figure S9: FT-IR spectrum of complex-1.
  10. Figure S10: FT-IR spectrum of complex-2.
  11. Figure S11: FT-IR spectrum of complex-3.
  12. Figure S12: TGA curve of the synthesized complex-1.
  13. Figure S13: TGA curve of the synthesized complex-2.
  14. Figure S14: TGA curve of the synthesized complex-3.

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