Cd (II) metal complexes of N, O donor Salicyloylpyrazole oxime Schiff bases: Synthesis, Spectroscopic Characterization and Antimicrobial Activities

 

Vishal G. Gaikwad¹, Sakshi M. Thorat¹, Rupali Endait-Malkar², Suresh T. More³,

Namdeo T. Dhokale¹*

¹Department of Chemistry, K. J. Somaiya College of Arts, Commerce and Science,

Savitribai Phule Pune University, Kopargaon-423601 (MS) India.

²Department of Chemistry, Radhabai Kale Mahila Mahavidyalaya,

Savitribai Phule Pune University, Ahmednagar (MS) India.

³Department of Chemistry, Arts, Commerce and Science College Kinhavali, Dist-Thane, (MS) India.

*Corresponding Author E-mail: namdeo.dhokale@gmail.com

 

 

INTRODUCTION:

Oximes are versatile ligands that readily form complexes with transition metal ions¹. Numerous transition metal complexes containing 2-hydroxyoxime ligands have been reported in the literature. The coordination chemistry of transition metal ions with 2-hydroxyoximes has attracted considerable interest owing to their wide range of applications in various fields^2,3.

 

Oxime-based complexes have been explored as cerebral and myocardial perfusion imaging agents^4,5. Furthermore, metal complexes of oximes exhibit distinctive structural features arising from intramolecular hydrogen bonding and specific packing arrangements, which contribute to their important applications as biochemical models^6-9.

 

Schiff base and their metal complexes show wide range of applications in agriculture, pharmaceutical and industrial chemistry^10-14. Phenolic oximes are versatile ligands and occupy an important position in the formation of chelate complexes with transition metals. Over the past few decades, the study of transition metal complexes has attracted increasing interest due to their wide range of applications. Transition metals are d-block elements characterized by partially filled d orbitals, which facilitate the formation of coordination complexes with various ligands. Phenolic oximes form stable complexes with a variety of transition metals by coordinating through the phenolic oxygen and imine nitrogen atoms¹⁵. These ligands are frequently employed as bridging units in the synthesis of manganese-based phenolic oxime complexes, which exhibit interesting magnetic properties arising from magneto-structural correlations in phenolic oxime compounds¹⁶. Tridentate copper complexes derived from hydrazone-based ligands containing 2-hydroxybenzoic acid have been synthesized and shown to exhibit superoxide dismutase activity as well as antiproliferative properties¹⁷. Transition metal complexes of gallium with bis-aminothiolate ligands have been utilized as myocardial perfusion imaging agents¹⁸. Schiff bases and their transition metal complexes play a significant role in the development of biochemistry, pharmacy, and medicinal chemistry due to their excellent properties, including antioxidant capacity¹⁹, antimicrobial activity^20,21,, antifungal activity²², and applications in semiconducting materials²³. In addition, pyrazole and its derivatives readily form metal complexes and are widely used as anticancer²⁴, antifungal²⁵, and anti-inflammatory²⁶ agents.

 

Pyrazole and its derivatives occupy an important position in the field of medicinal chemistry owing to their broad spectrum of biological activities²⁷. Numerous pyrazole-containing compounds have been reported to exhibit antibacterial²⁸, antimicrobial^29,30, anticancer^31,32, anti-inflammatory^33-35, insecticidal³⁶, fungicidal³⁷, and several other pharmacological properties^38,39. In particular, pyrazole-based copper and cadmium metal complexes have demonstrated remarkable antibiotic activity, showing superior performance compared to chloramphenicol, with minimum inhibitory concentrations as low as 5 µg /L⁴⁰.

 

Cadmium (Cd) is a toxic heavy metal that bioaccumulates over time and causes significant adverse effects on human health, primarily targeting the kidneys, liver, bones, and immune system⁴¹. Schiff bases derived from substituted amines and aryl aldehydes, along with their Cd (II) metal complexes, exhibit significant antimicrobial activity and inhibitory effects against the enzyme alkaline phosphatase⁴². Schiff base cadmium complexes exhibit significant antibacterial activity⁴³ and also demonstrate promising catalytic and antioxidant properties⁴⁴, DNA binding studies⁴⁵ highlighting their potential applicability in both biological and chemical processes⁴⁶.

 

 

A survey of the literature indicates that transition metal complexes containing pyrazole moieties and oxime groups are of considerable importance in the field of chemistry, with wide-ranging applications in agriculture, industry, and biological studies. In view of the extensive applications of transition metal complexes, the present study was undertaken to synthesize Cd (II) complexes with salicyloylpyrazole oxime ligands.

 

MATERIALS AND METHODS:

All chemicals used in the present work were commercially available and were used after appropriate purification. Distilled water and distilled alcohols were used throughout the experiments. The melting points of the synthesized compounds were determined using the open capillary method. Electrical conductivity measurements were carried out in 1 × 10^-3 M DMF solutions using an Elico digital conductivity meter (Model CM-180). FT-IR spectra of the ligands and their complexes were recorded on a Shimadzu FT-IR spectrophotometer in the range 4000–450 cm^-1. UV–Visible spectra were recorded on a Shimadzu double-beam spectrophotometer (UV-1800) in the range 700–190 nm at K. J. Somaiya College of Arts, Commerce and Science, Kopargaon. Thermogravimetric analysis (TGA) was carried out at the Department of Chemistry, New Arts, Commerce and Science College, Ahmednagar, in an air atmosphere at a heating rate of 10 ⁰C min^-1. The metal content in the complexes was determined by a volumetric method⁴⁷. The differently substituted salicyloylpyrazole oxime ligands were synthesized following procedures reported earlier⁴⁸.

 

General procedure for the synthesis of metal complexes:

An aqueous solution of cadmium sulfate (1 mmol) was prepared using distilled water. The metal salt solution was acidified with concentrated hydrochloric acid and gently warmed on a hot water bath. To this warm acidic solution, an alcoholic solution of the ligand (2 mmol) was added slowly dropwise with constant stirring. A slight excess of the ligand solution was added to ensure complete complexation. The reaction mixture was then rendered alkaline by the addition of alcoholic ammonia. The resulting solution was digested on a boiling water bath, leading to the formation of a colored precipitate of the metal complex. The precipitate was filtered, washed thoroughly with hot distilled water to remove unreacted metal ions, followed by washing with ethyl alcohol to remove excess ligand. The final product was dried under ambient conditions. The synthetic route for the Cd(II) complexes is illustrated in Scheme 1.

 

Scheme 1: Synthetic route for synthesis of Cd (II) complexes of salicyloylpyrazole oximes.

 

Table-1: Physical and analytical data of Cd (II) complexes.

Complex	Substituent			M.P.	% Yeild	% Element Found (Calc.)				Molar Conductance (Ω-1cm2mol-1)
	R1	R2	R3	(00C)		M	C	H	N
Cd-1	Cl	H	H	268-270	89	13.95 (14.68)	53.56(53.32)	3.32 (3.42)	10.88 (10.97)	14.3
Cd-2	CH3	H	H	282-284	78	15.21 (15.50)	59.86(59.63)	4.02 (4.45)	11.78 (11.59)	17.6
Cd-3	Br	H	H	252-254	86	12.89 (13.15)	47.21(47.77)	2.98 (3.07)	9.98 (9.83)	20.7
Cd-4	Cl	CH3	H	228-230	78	14.53 (14.16)	54.69(54.46)	3.51 (3.81)	10.85 (10.58)	18.6
Cd-5	Cl	H	Cl	272-274	68	13.12 (13.47)	48.75(48.92)	2.88 (2.90)	10.42 (10.07)	13.6

 

RESULT AND DISCUSSION:

All synthesized Cd (II) complexes of salicyloyl pyrazole oxime were prepared by the stoichiometric reaction of the metal salt with the Schiff base ligand in a 1:2 metal-to-ligand (M:L) ratio. The resulting complexes are  grey in colour, air-stable, and non-hygroscopic solids, distinctly different in appearance from the free ligand. Elemental analysis data were found to be in good agreement with the calculated values for the proposed molecular formulae, confirming an ML₂type composition for the complexes. The melting points of the complexes were determined using the open capillary method and are uncorrected. All complexes melt with decomposition at higher temperatures compared to the free ligand, further supporting successful complex formation. The synthesized complexes are soluble in polar solvents such as DMF and DMSO, but insoluble in water, ethyl alcohol, acetone, chloroform, and dichloromethane. The observed solubility behavior, together with elemental analysis results, suggests that the complexes are monomeric in nature.

 

Solution Conductivity:

The solution conductivities of all synthesized metal complexes were measured using an Elico digital conductivity meter (Model CM-180) at the Department of Chemistry, K. J. Somaiya College, Kopargaon. A known amount of each solid metal complex was dissolved in DMF to prepare 1 × 10^-3 M solutions in a standard 25 mL volumetric flask. The prepared solutions were transferred to clean and dry 100 mL beakers, and molar conductance values were recorded at room temperature. The observed molar conductance values for all complexes indicate their non-electrolytic and covalent nature⁴⁹. This non-electrolytic behavior supports the formation of chelated structures in the complexes. The molar conductance values are summarized in Table 1.

 

IR spectra:

The IR spectra of the synthesized free ligands and their Cd(II) metal complexes were recorded using a Shimadzu FT-IR spectrometer. A comparative analysis of the spectra of the free ligands and the corresponding Cd(II) complexes was carried out to elucidate the coordination behavior. The free ligands exhibit a broad absorption band in the range 3391–3310 cm^-1, which is attributable to the ν(O–H) stretching vibration of the free phenolic group. The absence of this band in the spectra of the metal complexes indicates deprotonation of the phenolic –OH group, followed by the formation of a metal–oxygen bond between the Cadmium ion and the phenolic oxygen atom of the ligand⁵⁰. Another absorption band observed in the free ligands in the range 3166–3131 cm^-1 is assigned to the hydrogen-bonded ν(O–H) stretching vibration of the oxime group. This band undergoes a slight shift in the metal complexes and appears in the range 3178–3140 cm^-1, suggesting coordination through the oxime nitrogen atom along with the presence of strong hydrogen bonding. Further evidence for this mode of coordination is provided by the shift of the azomethine ν(C=N) stretching band from 1563–1542 cm^-1 in the free ligands to 1543–1516 cm^-1 in the Cd(II) complexes. Additionally, the ν(N–O) stretching vibration observed at 1282–1226 cm^-1 in the free ligands is shifted to 1268–1218 cm^-1 upon complexation. These shifts in the characteristic vibrational frequencies upon coordination can be attributed to conjugation of the C=N group with the metal–ligand bonds in the complexes^51,52.

Medium to strong absorption bands corresponding to ν(C–O) were observed in the free ligands in the range 1097–1073 cm^-1. In the spectra of the metal complexes, these bands shifted to lower frequencies, appearing in the range 1057–1031 cm^-1. This downward shift upon chelation supports the coordination of the metal ions to the phenolic –OH group after deprotonation^53,54. Further evidence for the coordination of the salicyloylpyrazole oxime ligands to the metal ions is provided by the appearance of weak bands in the low-frequency region at 560–535 cm^-1 and 485–468 cm^-1, which are assigned to ν(M–O) and ν(M–N) vibrations, respectively^55,56. These bands are observed exclusively in the spectra of the metal complexes and are absent in the spectra of the free ligands, confirming the participation of the oxygen and nitrogen atoms in coordination.

 

The significant FT-IR absorption bands of the synthesized ligands and their metal complexes are summarized in Table 2.

 

Powder XRD spectra:

XRD pattern of zinc metal complexes were obtained in solid form. The powder X-ray diffraction of some selected synthesized metal complexes were scanned on Goniometer powder diffraction PW 3050/60 with Cu-K-alpha-1 radiation (λ=1.5406 Å). The powder XRD patterns were measured in 2 theta range between 5.0084 and 89.9744 with step size 0.0170. The X-ray diffractogram of all complexes showed broad peak, which indicate amorphous nature⁵⁷. Though amorphous nature of complexes was observed they were generally not soluble in non-polar solvents.

 

Thermal Analysis:

The thermal stability of the synthesized metal complexes was investigated by thermogravimetric analysis (TGA) using powdered samples, with heating carried out at a rate of 10 °C min^-1 in an air atmosphere. The weight loss was recorded as a function of temperature over the range 30–900 °C. The thermograms of the Cd (II) complexes show no significant weight loss up to 240 °C, indicating high thermal stability and the absence of both lattice and coordinated water molecules. Beyond 240 °C, the complexes undergo gradual decomposition up to the temperature range of 600–800 °C. This slow weight loss can be attributed to the elimination of gaseous products such as CO₂, NH₃, and other volatile fragments arising from the decomposition of the salicyloylpyrazole oxime ligand, resulting in the formation of a fine powder. At higher temperatures, no further weight loss is observed, suggesting the formation of a stable metal oxide as the final decomposition product. The residual mass percentages observed for Cd-1, Cd-2, Cd-3, Cd-4, and Cd-5 complexes are 16.11% (calcd. 16.95%), 17.58% (calcd. 17.88%), 14.93% (calcd. 15.21%), 16.01% (calcd. 16.35%), and 15.81% (calcd. 15.57%), respectively, which are in good agreement with the calculated values.

 

Table- 2: The significant peaks in FTIR spectra of free ligand and its Cd (II) complexes.

 

The TGA curves of the synthesized complexes are presented in Fig. 2.

 

Fig.-2: TGA profile of complex Cd-1 and Cd-2

 

Electronic Absorption Spectra:

Electronic absorption spectral study of ligands:

Molecular absorption of compounds in UV-Visible region is dependent on the electronic structure and excitation of electrons from different energy levels. The electronic transitions which are involved in ultra violet and near visible region are of different type. Such electronic transitions in organic molecules are σ→σ*, π→σ*, n→π* and π→π*. Among these only n→π* and π→π* transitions are more significant to chemists dealing with coordination compounds⁵⁸.

 

In this present study, the electronic spectra of ligands were measured to a known concentration (1 x 10^-4 M) in DMF over a range 200- 700 nm. The instrument was calibrated with a solution of 0.04 g potassium dichromate in 0.05 M KOH solution before use. The absorption spectra of ligands exhibit two bands in the region 277- 337 nm. The band in the region 277-295 nm is attributed to π→π* transitions of phenyl ring while the band in the region 333-337 nm is assigned to n→π* of azomethine (-CH=N-) group^59,60. The observed λ_max values for ligands are found to be nearly same. The λ_max values for synthesized ligands are summarized in Table and spectra are represented in Figure.

 

Electronic absorption spectral study of complexes:

The electronic absorption spectra of synthesized complexes were also measured in solution to a known concentration (1 x 10^-4 M) by using DMF as solvent over a range 200- 700 nm. The absorption band of ligand at λ_max nm due π→π* and n→π* for azomethine linkage group and phenolic –OH group was shifted to lower frequencies in the spectra of complexes⁶¹. This shift in the spectra of synthesized complexes supports the coordination of the ligand to Cd (II) ion. The Cd (II) metal complex because of its d¹⁰ configuration does not show d-d transition and hence it is possibly having tetrahedral geometry⁶².

 

Fig. 3: Electronic absorption spectrum of complex Cd-1

Fig. 4: Electronic absorption spectrum of complex Cd-2

 

Antimicrobial study:

In the present study, antibacterial activity of salicyloylpyrazoleoxime (Ligands) and Cd(II) complexes of salicyloylpyrazoleoximes was carried out. Highly purified samples of synthesized ligand and its Cd (II) metal complexes were tested for their in vitro antibacterial activity against various bacterial strains like Staphylococcus aureus, Bacillus subtilis, Actinomycetes, Klebsiella and Pseudomonas aeruginosa. The solutions of different compounds under test concentration of 100 µg/mL in DMF were poured in the cup plate/ well of bacteria seeded agar plates. These plates were incubated at 37 ⁰C for 24 hours for above bacteria. The activity was reported by measuring the diameter of zone of inhibition in mm. The standard antibiotic used was Penicillin. The solution without standard or compound i.e. only DMF was used as control.

 

The newly synthesized compounds exhibited varying degree of inhibitory effect (low to moderate) on the growth of tested bacteria. Among these, HL-5, Cd-2 and Cd-4 showed considerable zone of inhibition against test bacteria Staphylococcus aureus compared with standard drug. The compounds like HL4 and Cd-4 showed promising zone of inhibition against Bacillus subtilis. The synthesized compounds HL1, Cd-2, Cd-3 and Cd-5 showed promising antibacterial activity against test bacteria Actinomycetes compared with standard drug. The compounds HL-1, HL-3, HL-4, Cd-1, Cd-4 and Cd-5 showed considerable zone of inhibition against test bacteria Klebsiella and the compounds like HL1, HL4 and Cd-1 showed good zone of inhibition against test bacteria Pseudomonas aeruginosa.

 

However, antimicrobial screening of synthesized Schiff base metal complexes were found comparatively much more active than free ligands. The antimicrobial activity of metal complexes showed considerable zone of inhibition compared to that standard drugs. Most of the synthesized complexes showed antimicrobial activity against Actinomycetes and Pseudomonas aeruginosa. After coordination with metal ion, the compounds which are biologically inactive becomes active and biologically less active compounds becomes more active^63-64.

Table 3: Antimicrobial activity of ligand and Ni(II) Metal complexes.

Compound	Substituents				Bacteria
					Gram-positive				Gram-negative
	R1	R2	R3	R	Sa	Bs	Am	Ks	Pa
HL1	Cl	H	H	F	17	19	25	22	27
HL2	CH3	H	H	F	10	18	14	11	12
HL3	Br	H	H	F	18	18	17	23	--
HL4	Cl	CH3	H	F	17	18	--	25	26
HL5	Cl	H	Cl	F	27	16	15	15	--
Cd-1	Cl	H	H	F	17	19	16	23	29
Cd-2	CH3	H	H	F	23	19	24	15	--
Cd-3	Br	H	H	F	12	11	26	14	21
Cd-4	Cl	CH3	H	F	25	17	--	27	22
Cd-5	Cl	H	Cl	F	14	13	24	28	23
Standard					32	38	33	36	35
Control					8	6	7	9	10

 

CONCLUSION:

In this present study new Cd (II) metal complexes of salycyloylpyrazoleoxime are reported. The newly synthesized complexes are characterized by physical methods and elemental analysis. The formations of metal complexes are confirmed by UV-Visible, IR and electronic absorption spectroscopy. The spectral data suggest that the oxygen atom of phenol group and nitrogen atom of imine group are involved in coordination with metal ion. The formation of metal complexes is also confirmed by thermal methods of analysis. All the metal complexes are stable below 240⁰C and decompose slowly after 240⁰C giving formation of corresponding metal oxide. The complexes are insoluble in water, alcohol and other organic solvents but good solubility in DMF and DMSO. The electronic absorption spectra suggest probable tetrahedral geometry. The molar conductance values of these complexes suggest the non-electrolytic nature. The XRD spectra suggest the amorphous nature of the synthesized complexes. The antibacterial study revealed that the most of the ligands and metal complexes possess moderate antimicrobial activity.

 

CONFLICT OF INTEREST:

The authors have no conflicts of interest regarding this investigation.

 

ACKNOWLEDGMENTS:

We Thank to Principal, K. J. Somaiya College of Arts, Commerce and Science Kopargaon for providing necessary facilities. The author is also thankful to S. P. Pune University Pune (MS) for financial support under Minor Research Project under ASPIRE Research Mentorship Grant. The authors are also grateful to Department of Biotechnology –Star College Scheme, New Delhi (File No.: HRD-11011/18/2024-HRD-DBT) For providing financial assistance to purchase the required chemicals for this research work.

 

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Received on 28.01.2026      Revised on 23.04.2026 Accepted on 25.06.2026      Published on 04.07.2026 Available online from July 30, 2026 Asian J. Research Chem.2026; 19(4):395-402. DOI: 10.52711/0974-4150.2026.00059 ©A and V Publications All Right Reserved
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