FT-IR and UV-Vis Spectroscopic studies of Co(II), Cu(II) and Mn(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.2025.4412  

INTRODUCTION

Chalcones also known as 1, 3-diphenyl-2-propen-1-ones are found naturally in numerous edible and medicinal plant, and are also manufactured artificial substance that belongs to the flavonoid family. The term chalcone originated from the Greek name ‘Chalcos’ which means “bronze”, they are active aromatic compounds serving as parent molecules for various bioorganic precursors in medicinal chemistry. Chalcones are recognized as benzyl acetophenone possessing a highly electrophilic three-carbon α, β-unsaturated carbonyl system [1]. Chalcones are mostly found at higher portions in plants parts such as petals, heartwood, leaves, barks, fruits, and roots [2] with their valuable importance such as anti- oxidants which prevents chronic diseases from entering the human body, destroying the DNA and proteins of different type of cancer disease, cardiovascular, and neurological diseases. They have great applications and wide range of biological activities such as anti- inflammatory, xanthine oxidase inhibitors, anti- histaminic, anti-malarial, anti-viral, anti- microbial, anti- viral and anti- diabetic effects [3 & 4]. Besides being used as food additives, chalcones are also used as additives in cosmetic products due to their UV protection properties.

 Chalcones have conjugated double bonds and a fully delocalized π-electron system on both benzene rings. Molecules of such system have low redox potentials, making them more likely to undergo electron transfer reactions.

       Figure 1. Structure of chalcone

Chalcones come in two different forms, Trans (E) and Cis (Z) isomers. The most common chalcones are the Trans isomers because they are generally more thermodynamically stable, whereas the Cis isomers are unstable because of their huge steric repulsion interactions between the carbonyl group and the phenyl ring [5 & 6]. In higher plants, enzyme called chalcone synthase converts p-coumaroyl-CoA and malonyl-CoA via the phenylpropanoid pathway, which produces the chalcone aromatic B-ring and the bridge in between.

                                     Figure 2. Isomers (Trans and Cis) of Chalcones

Chalcones undergoes various metabolic pathway in plants, resulting in the production of aurones, glycosyl conjugates and naringenin. These compounds play crucial ecological roles, such as acting as signaling molecules in plant-microbe symbioses and serving as biological regulators for plant dispersal. Glycosyl conjugated chalcones are commonly found in flowers and are essential for pollination. The chalcone family has garnered significant attention due to its wide spectrum of biological activity and interest from synthesis and biosynthesis perspectives. The most prevalent chalcone derivatives in plants are those that have been hydroxylated or methoxylated. The aim of this research is to synthesize and characterize Co(II), Cu(II) and Mn(II) metal complexes of 2-methoxy-2’-hydroxychalcone (2M2HC) 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: cobalt(II) chloride, copper(II) chloride, manganese(II) chloride; solvent: ethanol, methanol, acetone 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 Rateb et al., 2009 [7] was adopted and modified for the synthesis of the chalcone ligand. A solvent-free synthesis method was used which includes grinding of the hydroxy ketone with aromatic aldehyde at room temperature. The mixture of appropriate 2’-hydroxyacetophenone (0.120 mL, 1 mmol) was measured and reacted with 2-methoxybenzaldehyde (0.136 g, 1 mmol) in an open mortar at room temperature and ground with pestle for about 2-3 min until the mixture melts. Then 3 g of NaOH was added into the mixture in the mortar and ground together for about 45 min until a solid mass with yellow colour was obtained. A little drop of n-hexane was added for precipitation and to allow the formation of a yellow powder crystal. The product was air-dried and placed in a desiccator to afford an analytical sample.

             Scheme 1. Synthesis of the chalcone ligand (2-methoxy-2’-hydroxychalcone)

Synthesis of the metal complexes of the chalcone

The method reported by Rateb et al., 2009 [7] was adopted and modified for the synthesis of the chalcone metal complexes. (E)-1-(2-hydroxyphenyl)-3-(2-methoxyphenyl) prop-2-en-1-one (0.254 g, 2 mmol)) and Mn(II) chloride, Cu(II) chloride and Co(II) chloride (0.126 g, 0.135 g, 0.129 g; 1 mmol respectively) were weighed into a mortar and ground with a pestle for 45 min. A small amount of n-hexane was added to allow the formation of powdered crystal. The products obtained were air-dried and kept in a desiccator to afford analytical samples.

Scheme 2. Synthesis of the chalcone metal complexes

Characterization of the synthesized chalcone and its metal complexes

The chalcone and its metal complexes were characterized by melting point, Fourier Transform Infrared Spectroscopy (FT-IR) and Ultraviolet-Visible Spectroscopy.

Solubility Test

Water and some common organic solvents; acetone, ethanol, methanol and chloroform were used to determine the solubility of the chalcone and its metal complexes.

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 instrument as background. Then small amount of chalcone and metal(II) complexes were mixed with KBr and were pelletized using hydraulic press, then inserted into the instrument and scanned in transmittance mode at a frequency range of 4000–400 cm^{- 1}.

Melting Point

Melting point was determined using a Stuart model SMP10 digital melting point apparatus. Small amount of chalcone and the metal(II) complexes were inserted into a capillary tube in which one end was blocked. Then the capillary tube with samples were inserted into the melting point apparatus till it melts or decompose.

UV-Visible 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 give constant absorbance value. Absorbance was plotted against wavelength (nm) to obtain 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 (2M2HC) was 85 % while that of the Mn(II), Cu(II)and Co(II) complexes were 31 %, 52 % and 94 % respectively. The interaction between 2’-hydroxyacetophenone and 2-methoxybenzaldehyde gave a yellow-coloured chalcone [7]. The Mn(II), Cu(II) and Co(II) complexes were orange-red, light-green and dark-brown in colour respectively. The purity and stability of the chalcone and the metal complexes were established by the observance of sharp melting points. The melting point of the chalcone ligand was 117 ºC, while the Mn(II), Cu(II) and Co(II) complexes exhibited  258 ºC, 230 ºC and 244 ºC respectively which is an indication of thermal stability.

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

Table 3. FT-IR analysis of the chalcone ligand and its metal complexes

Table 4. UV-Visible analysis of the chalcone ligand and its metal complexes

FT-IR Analysis of the Chalcone Ligand and its Metal Complexes

The FT-IR spectra of the chalcone ligand (2M2HC) and the Mn(II), Cu(II) and Co(II) complexes were presented in figures 5-8 respectively. The FT-IR showed an absorption band at 3487 cm^-1 attributed to v(O-H) vibration frequency of the chalcone. In the spectra of the metal complexes, this band shifted towards lower wavenumber indicating coordination of hydroxyl group to the metal atom. Notably, the Mn(II) complex exhibits a band at 3456 cm^-1, indicating a slight shift, while the Co(II) complex, a broader shift was observed around 3450 cm^-1. The Cu(II) complex showed a band at 3451  cm^-1 confirming coordination through the O-H group [8]. The chalcone exhibited a strong absorption band at 1638 cm^-1 attributed to the v(C=O) as reported by Bale et al., 2022 (1655 cm^-1) [9]. In the metal complexes, this band shifted to slightly higher wavenumbers which indicates the coordination of keto (C=O) and hydroxyl groups to the metal atom in the complexes. The absorption band which appears at 2372 cm^-1 was assigned to v(C-H) aromatic in the ligand. The sharp band at 1251 cm^-1 was attributed to v(C-O) stretching vibration in the ligand. The absorption band which appears at 1552 cm^-1 was attributed to v(C=C) stretching vibration in the ligand (Devi et al., 2008, 1572 cm^-1; Bale et al., 2022, 1579 cm^-1) [10 & 9]. The metal-ligand (M-O) interaction was confirmed by the new bands at the lower region 574 cm^-1, 573 cm^-1 and 467 cm^-1 for Mn(II), Cu(II) and Co(II) complexes respectively. This observation indicates successful complexation of the ligand with the metal ions.

Figure 5. FT-IR Spectrum of the Chalcone Ligand (2M2HC)

Figure 6. FT-IR Spectrum of Mn(II) complex (2M2HCMn)

Figure 7. FT-IR spectrum of Cu(II) complex (2M2HCCu)

Figure 8. FT-IR Spectrum of Co(II) complex (2M2HCCo)

Ultraviolet-Visible Spectroscopic Analysis of the Chalcone and its Metal Complexes

Table 4 presents the maximum wavelength of the chalcone and its complexes on the UV-Vis spectra in figures 9-12. The UV-Vis spectrum of the Chalcone ligand (figure 9), exhibited strong absorptions at 292 nm and 362 nm. The electronic transition at 292 nm was assigned to π-π* (conjugated C=C). This electronic transition was observed at a lower wavelength (250-278 nm) for the metal complexes. The absorption band observed at 362 nm corresponds to the n-π* transition of the carbonyl group (C=O) in the ligand. This electronic transition was observed at a higher wavelengths (365-366 nm) for the metal complexes. These shifts indicate the coordination of the metal ions to the carbonyl group of the chalcone ligand [8]. The UV-Vis absorption spectra indicated the presence and modification of the ligand and the metal complexes absorption bands.

Figure 9. UV-Visible spectrum of the chalcone ligand (2M2HC)

Figure 10. UV-Visible spectrum of Mn(II) complex (2M2HCMn)

Figure 11. UV-Visible spectrum of Cu(II) complex (2M2HCCu)

Figure 12. UV-Visible spectrum of Co(II) complex (2M2HCCo)

CONCLUSION

2-methoxy-2’-hydroxychalcone (2M2HC) and its metal complexes were synthesized and characterized by physical and spectroscopic techniques. The results obtained supports the structures deduced for the ligand (2M2HC) and the metal complexes (2M2HCMn, 2M2HCCu and 2M2HCCo).

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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