FT-IR and UV-Vis Spectroscopic studies of Cd(II), Hg(II) and Zn(II)  metal complexes of 2-methoxy-2'-hydroxychalcone

Adebayo T. Bale*; Wahab A. Osunniran; Mohammed H. Sadiya; Fausat O. Adebona

Kwara State University, Nigeria

*Correspondence: adebayo.bale@kwasu.edu.ng

DOI: https://doi.org/10.71431/IJRPAS.2025.4411

INTRODUCTION

Chalcones (1, 3-diphenyl-2-propen-1-ones) are opened chain flavonoids where two aromatic rings are joined by three-carbon, α, β-unsaturated carbonyl system, which are naturally occurring and synthetically produced compounds belonging to the flavonoid family [1] and which the major portion is derived from edible plants and nature in foods such as tea, vegetables, fruits, flavors and soy foods [2]. Most of the chalcones are known to exhibit a wide spectrum of biological activities such as anti-inflammatory, anti-oxidant, anti-cancer, anti-viral and anti-microbial activities. Additionally, chalcones are widely used in organic synthesis as precursors for several heterocyclic compounds [3]. Studies have shown that chalcones not only limit cancer proliferation but are also potent agents in vivo against skin cancer and show effects on tumor angiogenesis. Due to the presence of the C=C double bonds, Chalcones can be present in the E-(trans-) or Z-(cis-) configuration. In addition the C=C double bond may be oriented trans or cis in relation to the carbonyl group, resulting in s-trans or s-cis conformers. The E-isomer of chalcone is the most thermodynamically stable isomer due to its planarity and lack of torsion caused by steric effects [4].  Chalcone compounds with very good new pharmacological properties are obtained by changing of the molecular structure of chalcones and by combining different substituents, besides being used as food additives, chalcones are also used as additives in cosmetic products due to their UV protection properties.  In addition, they are used in the treatment of viral diseases, cardiovascular diseases, parasitic infections, gastritis cases, stomach cancer and as a pain reliever.

                                             Figure 1. Structure of chalcone       

Figure 2. Trans (E) structure of chalcone

                                           Figure 3. Cis (Z) structure of chalcone

The aim of this research is to synthesize and characterize 2-methoxy-2’-hydroxychalcone (2M2HC) and its metal complexes as viable additions to the library of chalcone derivatives.

MATERIALS AND METHODS

Materials

All reagents and chemicals are of analytical grade and were used as received without further purification. 2’-hydroxyacetophenone, 2-methoxybenzaldehyde; metal salts: Mercury(II) chloride, Zinc(II) chloride, and Cadmium(II) chloride; solvent: ethanol, methanol, acetone, water and chloroform. The reagents were obtained from Sigma-Aldrich (UK). The samples were analyzed using FT-IR Spectrophotometer (FTIR-8400S, Shimadzu, Japan), melting point apparatus (Stuart SMP10 Digital Melt Point), UV-Vis spectrophotometer (UV-1650PC, Shimadzu, Japan). Analytical balance (AR2130, Ohaus, USA), Thin Layer Chromatography (TLC) plate and UV lamp (254 nm).

Methods

Synthesis of the Chalcone Ligand

The method reported by Bale et al., (2022) [5] was adopted and modified for the synthesis of the chalcone. 2-methoxybenzaldehyde (0.136 g, 1 mmol) was weighed and 2’-hydroxyacetophenone (0.120 mL, 1 mmol) was measured. To a stirred solution of 2’-hydroxyacetophenone (1 mmol) in ethanol (10 mL), a solution of sodium hydroxide (60 %, 3 mL) was added drop-wise. The reaction mixture was initially stirred for 30 min at room temperature. 2-methoxybenzaldehyde (1 mmol) was added to the mixture and stirred at room temperature until crystals appeared. The reaction was monitored by thin-layer chromatography (TLC). Upon completion of the reaction, the resulting solids were filtered and washed with an excess of distilled water and air-dried to afford an analytical sample.

Scheme 1. The synthesized chalcone (2M2HC)

Synthesis of the metal complexes of the chalcone

The method reported by Rateb et al., 2009 [6] was adopted and modified for the synthesis of the chalcone metal complexes. The chalcone (E)-1-(2-hydroxyphenyl)-3-(2-methoxyphenyl)prop-2-en-1-one (0.254 g, 1 mmol) and Hg(II) chloride, Zn(II) chloride and Cd(II) chloride (0.136 g, 0.068 g, 0.092 g; 0.5 mmol) were weighed into a mortar and ground with pestle for 45 min. A small amount of n-hexane was added to allow the formation of powdered crystals. The product obtained was air-dried and kept in a desiccator to afford an analytical sample.

Scheme 2.  Metal complexes of the synthesized chalcone

Characterization of the chalcone and its metal complexes

A multi-technique approach, comprising Fourier Transform Infrared Spectroscopy (FT-IR), Utraviolet-Visible Spectroscopy and physical properties including melting point and solubility test were employed to characterize the synthesized chalcone and its metal complexes.

Solubility Test

The use of water and various organic solvents; ethanol, methanol and chloroform were utilized to determined the solubility of the chalcone and its metal complexes.

Melting point

The melting point of the chalcone and its metal complexes were determined using a melting point apparatus. A small amount of chalcone and its metal complexes were inserted into a capillary tube with one-end blocked, then placed in the apparatus, where temperature is increased until melting occurred.

Fourier Transform Infrared Spectroscopy (FT-IR) Analysis

FT-IR analysis was carried out using Shimadzu equipment. KBr (Potassium bromide, spectroscopy grade) was ground into (powdery form) pellets (with hydraulic press) and scanned with the instrument as background. Then small amount of chalcone and the metal complexes were mixed with the KBr pellets and were pelletized using hydraulic press, inserted into the instrument and scanned in transmittance mode at a frequency range of 4000–400 cm^{- 1}.

UV-Vis Spectroscopy

The UV-Vis spectra of the chalcone and its complexes were taken in ethanol (1x10^-4 M). The UV-Vis spectrophotometer was calibrated with ethanol, then the absorbance of the chalcone and its metal complexes in ethanol were measured at 320 nm till it gives a constant absorbance value. Absorbance was plotted against wavelength (nm) to derive the maximum wavelength (⋋max).

RESULTS AND DISCUSSION

Table 1. Physical properties of the chalcone (2M2HC) and its metal complexes

The percentage yield of the chalcone was 85 % and the metal complexes Hg(II), Zn(II), and Cd(II) were 42 %, 74 % and 57 % respectively. The interaction between 2-methoxybezaldehyde and 2’-hydroxyacetophenone gives yellow-coloured chalcone. The Hg(II), Zn(II), and Cd(II) complexes were brown, light yellow and light grey in colour respectively. The purity and the identity of the chalcone and its metal complexes were determined by the observance of sharp melting points. The melting point of the chalcone was 117 % while the Hg(II), Zn(II) and Cd(II) metal complexes were 196 %,  190 % and 175 %  respectively which is an indication of the effect of intermolecular forces and thermal stability.

Table 2. Solubility test of the chalcone and its metal complexes

Solubility is influenced by the nature of the solute, the type of  bonding within the solute, and the characteristics of the solvent [7]. The solubility test of the synthesized compounds were presented in Table 2. The solubility test was carried out in ethanol, methanol, water and chloroform. The chalcone was found to be soluble in water and partially soluble in all polar solvents used while the metal complexes were found to be soluble, partially soluble and insoluble in all the polar solvents. This is because polar substances dissolve in polar solvents due to the attractive forces between the polar molecules.

Table 3. UV-Vis diagnostic absorption bands of the chalcone and its metal complexes

Table 3 showed the diagnostic absorption bands of the chalcone and its metal complexes in the UV-Vis spectra in figures 4-7. The UV-Vis spectrum of the chalcone ligand (figure 4) exhibited strong absorption at 297 and 362 nm. The absorption band at 362 nm corresponds to the n-π* transition of the carbonyl group and the absorption band at 297 corresponds to π-π* transition of the conjugated C=C double bonds. The π-π* electronic transitions observed for  mercury zinc and cadmium complexes were found at a lower or higher wavelengths (239-297 nm). The n-π* electronic transitions of the mercury, zinc and cadmuim complexes were also observed at a lower or higher wavelengths (278-366 nm). The displacements resulted from the coordination between the carbonyl group and the metal. The UV-Vis absorption spectra indicated the presence and modification of the ligand and the metal complexes absorption bands.

Figure 4. UV-Vis spectrum of the chalcone (2M2HC)

Figure 5. UV-Vis spectrum of the Hg(II) complex (2M2HCHg)

Figure 6. UV-Vis spectrum of the Zn(II) complex (2M2HCZn)

Figure 7. UV-Vis spectrum of the Cd(II) complex (2M2HCCd)

Table 4. FT-IR analysis of the synthesized chalcone and its metal complexes

The FT-IR spectra of the chalcone and its Hg(II), Zn(II) and Cd(II) complexes were presented in figures 8-11 respectively and the FT-IR spectra interpretation and major bands of the chalcone ligand and its metal complexes were presented in Table 4. The FT-IR spectra of the ligand and metal complexes shows two diagnostic absorption bands corresponding to v(C=C) and v(C=O) (1552-1634 cm^-1 and 1615-1639 cm^-1) respectively. These are in the same range with the values obtained by Habib et al. (2011) [8]; Syam et al., (2012) [9] and Hussien et al., (2017) [10]. An intense absorption band appeared at 1252 cm^-1 assigned to v(C-O) in the ligand is shifted to a lower wavenumber (1118–1245 cm^-1) in the infrared spectra of the metal complexes (Bale et al., 2022; 1243-1250 cm^-1). The ligand has a broad band at 3487 cm^-1 corresponding to v(O-H). In the spectra of the metal complexes, the band was observed at a lower or higher wavenumber (3408-3624 cm^-1) (Tabti et al., 2018; 3210-3400 cm^-1) [11]. A new absorption band [v(M-O)] appeared in the infrared spectra of the metal complexes (Hg, Zn and Cd), but absent in the spectrum of the chalcone.

Figure 8. FT-IR spectrum of the chalcone (2M2HC)

Figure 9. FT-IR spectrum of the mercury complex (2M2HCHg)

Figure 10. FT-IR spectrum of the zinc complex (2M2HCZn)

Figure 11. FT-IR spectrum of the cadmium complex (2M2HCCd)

CONCLUSION

2-methoxy-2’-hydroxychalcone (2M2HC) and its metal complexes were synthesized and characterized by physical and spectroscopic techniques. The results obtained support the structures deduced for the ligand (2M2HC) and the metal complexes (2M2HCHg, 2M2HCZn and 2M2HCCd).

ACKNOWLEDGEMENT

The authors appreciated the support of all members of staff in the laboratory at the Department of Chemistry and Industrial Chemistry, Kwara State University (KWASU), Malete, Kwara State, Nigeria.

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