Microwave Assisted Synthesis of some Ln(III) Chloride Complexes from  4-Formyl Antipyrine Schiff Base :Structural Characterization and Antimicrobial Evaluation

Introduction

Schiff base complexes have gained significant recognition in coordination chemistry because of their versatile binding properties, diversity in structure, and potential biological activities.  Due to their ability to create complexation quickly, most Schiff bases make good ligands. The coordination of a variety of metallic ions with different geometries and oxidation states may be possible with Schiff bases acting as multifunctional ligands in the interim. The formation of complexes with Schiff bases is possible for lanthanides and d-block metals alike¹.

Antipyrine², a well-known antipyretic and analgesic drug, serves as a key moiety in the synthesis of Schiff bases². Antipyrine Schiff base complexes have gained recognition for their impressive biological properties. Antipyrine analogues demonstrate a wide spectrum of biological properties including antimicrobial^2–4, as anticancer⁵, antiviral ^6–8, antioxidant and anti-inflammatory properties ^9,10. Because they are used in so many diverse domains, new Schiff bases generated from Antipyrine analogues have received a lot of interest these days^11,12. It is known that their broad-spectrum biological activities have attracted a lot of attention, particularly in medicinal chemistry^10,13. The metal ion, the Schiff base ligand, and their structural properties all have a significant impact on the bio-activities of Schiff base complexes.

Ln(III)complexes, known for their unique optical and biological properties, are widely used in biology and medicine as diagnostic tools and fluorescence materials.¹⁴ , electroluminescent devices ¹⁵, fluorescent markers and tags in various biological systems ^16,17 . In this context, special emphasis is placed on the therapeutic properties of rare earth complex drugs ^17,18.

This paper explores the microwave-enhanced synthesis¹⁹, characterization, and assessment of the bio-activities of the Ln(III) chloride complexes^2,20 of Schiff base resulted from the reaction between 2-aminophenol with  4-antipyrine carboxaldehyde. The coordination behaviour of Ln(III) ions with L₁ has been investigated through elemental analysis, thermal analysis, molar conductance, FTIR, ¹H-NMR ,UV-Vis spectroscopic techniques as well as magnetic susceptibility measurements. These lanthanide (III) compounds might introduce a novel class of promising antimicrobial agents.

Experimental

The chlorides of lanthanides, LnCl₃were prepared using standard synthesis methods. Ln₂O₃ was dissolved in 60% (v/v) hot hydrochloric acid, with any residue filtered, and the filtrate was evaporated with a steam bath before the resulting rare earth chlorides were crystallized.². Due to their strong hygroscopic nature, the Ln(III) chloride salts were kept within a desiccator in a vacuum with phosphorus(V) oxide. 4-antipyrine carboxaldehyde (≥ 99.8%)² and 2-aminophenol (≥ 99%) were obtained from Sigma-Aldrich Chemie in Germany. solvents like methanol, DMF and DMSO were purchased  from E.Merck, India.

Carbon, hydrogen, and nitrogen contents ² were analysed using Elementar model Vario EL III CHNS Analyzer ^2,21. The percentile metal content in complexes were estimated gravimetrically as Ln₂O₃ by peaceful pyrolysis method. The chloride content in the complex was estimated by Volhard²² method. Using the Toshniwal Conductivity Bridge and a dip type conductance cell with a platinum electrode, the molar conductances of complexes were determined at room temperature DMF (10–3 M).  ², A Schimadzu IR 470 spectrophotometer was used to record the FTIR spectra of the complexes using the KBr pellet technique, and a Nicolet Magna 550 FTIR²³ instrument was used to record the far-IR spectra of metal complexes using polyethylene pellets in the 500–100 cm⁻¹ region²³. Hitachi 220 A UV-visible double beam spectrophotometer was used to acquire the electronic spectra, which are in the 190–900 nm range.^19,23 d6-DMSO was used as the solvent in an FT–NMR Bruker AvanceIII spectrometer (400 MHz) to record the ¹H NMR spectra. Sherwood-Scientific Gouy Balance was used to measure Magnetic susceptibility measurements.^19,23. TG analysis of the compounds was conducted using  thermal analysis system-TGA-DTA Perkin Elmer STA 6000 ²in the temperature 15°C to 900°C.².

Microwave-assisted synthesis of  Schiff base L₁¹⁹

Equimolar amounts (1:1) of 4-formyl antipyrine and 2-aminophenol were dissolved in 8–10 mL methanol. The reaction mixture was thereafter subjected to 4-5 minutes of 350W microwave radiation. An orange-red crystalline solid formed when the solution was gradually cooled. After filtering, the solid was washed with cold methanol and crystallised from the methanol.

Yield: 85%, mp: 169°C. The ligand’s purity was checked by TLC, infrared² spectrum and elemental analysis. Yield: 82%. mp:169^o. Elemental analysis data for C₁₈H₁₇N₃O₂ (molecular weight: 307.35) were found (calculated) as follows: C, 70.34% (70.1%); H, 5.58% (5.28%); N, 13.67% (13.73%) ¹⁹.

Scheme 1: Microwave-assisted Synthesis of  Schiff base L119Click here to View Scheme

Microwave assisted synthesis of  Chloride complexes of Lanthanides [Ln(L₁)₂(Cl)₂]Cl.4H₂O ( where Ln = La(C1), Sm(C2), Eu(C3) and Dy(C4)¹⁸

A mixture of LaCl₃·6H₂O (0.371 g, 1 mmol) and Schiff base ligand L₁ (0.615 g, 2 mmol) was ground thoroughly to form a uniform blend. The blend was dissolved in 8-10 mL of ethanol and subjected to microwave irradiation at 350W for 7 minutes. After monitoring reaction completion by TLC, the resulting solid complex (C1) filtered, recrystallized in hot ethanol, dried over anhydrous CaCl₂ under vacuum.

The other complexes Sm(C2), Eu(C3), and Dy(C4)), were synthesized similarly by reacting ligand L₁ with the respective lanthanide chloride, LnCl₃·6H₂O (Ln = Sm, Eu, and Dy), with a metal-to-ligand ratio of 1:2

Scheme 2: Microwave-assisted Synthesis route for Ln (III) chloride complexes of L1Click here to View table

Antimicrobial studies

The antimicrobial efficacy of the compounds tested in vitro for six bacteria— B. subtilis,  A. hydrophila, E. coli, K. pneumoniae, V.cholerae  and S. aureus—using the disc diffusion method  through the agar medium, and three fungi—C. albicans , A. fumigatus and A. flavus —cultured on potato dextrose agar. Stock solution was prepared by dissolving 1 mg of each test compound into 1 mL of dimethylformamide (DMF). 5mm filter paper discs saturated in 20 microlitres of each compound placed on the seeded plates for testing.The plates were incubated at 37°C for a duration of 24 hours for bacterial strains while 72 hours in the case of  fungal species¹⁹. Streptomycin (20 µg/disc) served as the reference standard for antibacterial activity, while Amphotericin-B (20 µg/disc) was used for antifungal activity. DMF was employed as the control solvent. ^18,19,24.

Results and Discussion

All the synthesised complex are  coloured solids  stable in air, non-hygroscopic, soluble in acetone, methanol, ethanol, DMF, and DMSO; it is sparingly soluble in ethyl acetate but insoluble in ether, water, benzene, and carbon tetrachloride. The physical characteristics  and elemental analysis results of L₁²⁵ and its complexes are consistent with the calculated values (Table 1). The complexes are 1:1 electrolytes according to the molar conductance measurements in DMF (10^–3 M). ^24,26. [Ln(L₁)₂Cl₂]Cl·4H₂O, is a general formula supported by the data, where Ln represents La(III), Sm(III), Eu(III), or Dy(III).

Table 1: Physicochemical and Molar conductivity data of L₁ & its Ln(III) complexes

Compound	Colour	Formulaweight	m.pºC	Yield (%)	Experimental(Calculated) %					Ʌm(DMF)W-1 mol-1cm2
					C	H	N	Cl	Ln
L1C18H17N3O2	orange red	307.35	169	85	70.1(70.34)	5.28(5.58)	13.73(13.67)	–	–	–
[La(L1)2Cl2]Cl.4H2O (C1)	Reddish brown	931.27	318	82	45.63(46.3)	4.75(4.51)	8.68(9.01)	10.8(11.4)	14.35(14.9)	79
[Sm(L1)2Cl2]Cl.4H2O (C2)	Yellowish Brown	1053.96	327	81	44.1(45.8)	4.75(4.45)	8.58(8.9)	10.83(11.28)	15.35(15.9)	81
[Eu(L1)2Cl2]Cl.4H2O (C3)	Brown	1057.29	312	80	44.73(45.7)	4.39(4.4)	8.26(8.89)	10.78(11.2)	15.68(16.1)	82
[Dy(L1)2Cl2]Cl.4H2O (C4)	Yellowish Brown	1063.4	318	79	44.57(45.24)	4.2(4.4)	8.47(8.79)	10.73(11.1)	16.39(17.02)	69

FT-IR spectra

Table 2 summarizes the characteristic FTIR bands observed for L₁ and its Ln(III) chloride complexes. Two distinctive bands may be seen in FTIR spectra of L₁ at 1651 and 1591 cm⁻¹, respectively. These bands represent the vC=O (carbonyl group stretching vibration) and the νC=N (azomethine group stretching vibration)^2,27.In complexes C1 to C4, the carbonyl and azomethine bands are observed to shift to shorter wavenumbers, suggesting the coordination between these groups and the central lanthanide ion. The existence of two low intensity bands in the region ¹⁹ 457–467 cm⁻¹ and 501–505 cm⁻¹, which are attributed to ¹⁹ Ln-N and Ln-O bond stretching¹⁹. L₁ also displays a broad band within the region of 3200–3423 cm⁻¹, which is assignable to  stretching vibration of  phenolic OH group having intramolecular hydrogen-bond ^2,19,28. In the complexes, this broad-band shifts, indicating that the phenolic oxygen participates in coordination without undergoing deprotonation.^27,29,30. Further evidence for this comes from the shifting of the phenolic C-O frequency of  L₁ from 1272 cm⁻¹ to lower^19,25,31 frequencies, frequencies, ranging from 1264 to 1256 cm⁻¹ in all complexes.¹⁹. Furthermore, a medium band in the 321–328 cm⁻¹ region is visible in the far-IR spectra of lanthanide complexes; this band is due to v_Ln–Cl vibrations and indicates the establishment of Ln–Cl bond. ^2,32,33. All complexes exhibit an IR spectral band in the 3050–3340 cm⁻¹ range due to the stretching vibrations in water molecules³⁴ in the lattice which may overlap with the broad band of the phenolic OH group. Through TG analysis, it is verified that the complexes contain hydration water.³⁵.

Table 2: FTIR Spectral Data for L₁ and Lanthanide Chloride Complexes

Compound	ν (O-H)	ν (C=O)	ν(C=N)	ν (C-O)	ν (Ln-N)	ν (Ln-O)	ν (Ln-Cl)
L1	3423b	1651s	1591s	1272m	–	–	–
C1	3340b	1636s	1577s	1264m	459w	501m	326m
C2	3348b	1641s	1573s	1258m	467w	502m	328m
C3	3343b	1639s	1575s	1256m	457w	502m	321m
C4	3341b	1636s	1568s	1261m	457w	504m	321m

s: strong, m: medium, b: broad

Electronic spectral and magnetic moment data

Table 3 summarizes the major bands discovered in the Electronic spectra of the L₁ and C1 to C4¹⁹. The ligand L₁ exhibits strong absorption bands at 294 and 375 nm, which are associated with the π→π* and n→π* electronic transitions in the azomethine linkage and carbonyl group, respectively. Both of these transitions are seen to be somewhat red-shifted in these complexes, suggesting that azomethine nitrogen and carbonyl oxygen have coordinated with the lanthanide ion^19,32. Along with the prominent ligand bands, weak f-f transitions are also observed in the spectra of complexes of Sm(III), Eu(III), and Dy(III)ions. ^17,19,36. Table 3 provides the prominent  f-f ¹⁹ transition with tentative assignments^2,19.

Table 3 also shows the magnetic moment values of the complexes. As anticipated, lanthanum complex(C1) is diamagnetic, while the other complexes display paramagnetic behaviour. The magnetic moment values for complexes C1 to C4 are close to the Van Vleck values, suggesting³⁷ that the lanthanide ions’ 4f electrons are largely shielded from the chemical environment ^[27].

Table 3: Electronic Spectral data (DMF )and magnetic moment values of L₁ and Ln(III) complexes C1 –C4 ²⁰

Compounds	Absorption bands (nm)	Tentative assignments	µeff(BM)
L1	(294, 375)	(π→ π*,  n→ π*)	–
C1	(308, 383)	(π→π*, n→ π*)	–
C2	(323, 378), 417	(π→ π*, n→ π*), 6H5/2→(6P, 4P )5/2	1.23
	442, 476, 498	6H5/2→4M17/2, 6H5/2 →4I11/2 6H5/2→4M15/2
C3	(314, 374), 531	(π→ π*, n→ π* ), 7F0→7D1	3.42
C4	(308, 378), 437,469	(π→ π*, n→ π*), 6H15/2→4G11/2, 6H15/2→4F9/2	10.08

¹H NMR spectra

¹H NMR spectrum of L₁ was obtained in DMSO-d6 (Figure1).It showed peaks in the range in the range 8.76 ppm and 8.3ppm corresponding to phenolic hydroxyl and azomethine protons respectively.². The spectrum of L₁ also shows as sharp singlet (3H) at 2.72 and 3.32 of the groups  =C-CH₃ and N-CH₃ of the pyrazolone respectively. The peaks of the azomethine and phenolic hydroxyl protons in the 1H NMR spectra of lanthanum complex C1 show a slight downfield shift, indicating that the azomethine nitrogen and oxygen atoms of the phenolic hydroxyl group coordinate without deprotonation.^17,27. 

Figure 1: IH NMR spectrum of ligand L1Click here to View Figure

Thermogravimetric analysis

The thermal behavior of complexes C1 to C4 were examined using thermogravimetric analysis over the temperature ranging 30–900°C under a nitrogen atmosphere³⁵. All the complexes exhibit similar decomposition patterns in three main stages. Table 4 presents the TG and DTG data for complexes C1 to C4.

In complex C1, the initial decomposition stage takes place between 52°C and 103°C, where four lattice water molecules are lost, resulting in an estimated mass reduction of 8.1%. The second stage, occurring between 207°C and 534°C, involves the breakdown of one ligand molecule, leading to a weight loss of around 31.8%. The third stage, spanning from 538°C to 778°C, agrees to decomposition of the remaining ligand molecule, with a reduction in weight of approximately 31.2%.³⁸, leaving anhydrous lanthanum(III) chloride as the residue above 778°C.^39,40.  All other lanthanide complexes, C2 to C4, show similar thermal behavior with three stages of decomposition, ultimately yielding anhydrous lanthanide chloride as the final residue at approximately 800°C.

Table 4: Thermal Decomposition data of Complexes C1 to C4

Complex	Stage ofdecomposition	TGTemperature  Range (oC)	DTG peak(oC)	Mass loss %Observed(calculated)	Assignments	FinalResidue(%)
(C1)	IIIIII	52-103207-534538-778	73289541	8.1(7.7)31.8(32.8)32.5(32.8)	Four  lattice waterOne ligand moleculeSecond ligand molecule	LaCl327.6(26.7)
(C2)	IIIIII	56-102212-464469-752	76302516	8.1(7.6)32.4(32.6)31.7(32.6)	Four  lattice waterOne molecule of L1.Second molecule of L1.	SmCl327.8(27.2)
(C3)	IIIIII	53-103217-472475-748	78308498	8.1(7.6)32.1(32.5)31.9(32.5)	Four  lattice waterOne molecule of L1.Second molecule of L1	EuCl328.2(27.4)
(C4)	IIIIII	46-106234-506508-778	74427523	8.1(7.5)31.8(32.1)31.6(32.1)	Four  lattice waterOne ligand moleculeSecond ligand molecule	DyCl328.6(28.2)

Based on the results from the analytical results , a tentative structure (Figure 2) of Ln(III)¹⁹ chloride complexes, [Ln(L₁)₂Cl₂]Cl·4H₂O (where Ln = La, Sm, Eu, and Dy),with a coordination number eight  has been proposed.

Figure 2: Proposed Structure of (C1 to C4)Click here to View Figure

[Ln (L₁)₂Cl₂]Cl.4H₂O (where Ln = La, Sm  Eu and  Dy )

Biological studies

Antibacterial activity

All of the tested compounds demonstrated significant biological activity against various bacteria. The results, detailed in Table 5, show that the complexes exhibited notably superior activity against V. cholerae (VC), E. coli(EC) and A. hydrophila (AH),  with inhibition zones significantly larger than those observed for the parent ligand and the standard drug, streptomycin¹⁹. Complexes C1 and C4 displayed exceptional activity in opposition to V. cholerae, Escherichia. coli and A. hydrophila with inhibition zones of 30 mm or greater. Notably, complex C4 exhibited the largest inhibition zone of 34 mm against V. cholerae. The complexes demonstrated moderate activity against S.aureus(SA) and B. subtilis(BS)²⁵ but showed minimal activity against K. pneumoniae(KP).

Table 5 presents the zones of inhibition of the compounds against various microorganisms. Figure 3a shows the image of  inhibition zones of the tested compounds against A. hydrophila, while Figure 3b shows a bar diagram representing the antibacterial activity of L₁ and complexes C1 to C4. Chelation, which raises the lipophilic nature  of the central metal atom. This makes it easier for the metal to get through the lipid layers of the bacteria,leading to more aggressive destruction and limiting further microbial growth.^41,42. This could be the reason for the increased bioactivity that lanthanide complexes showed in comparison to the free ligand. ⁴³

Antifungal activity

Regarding antifungal activity, all of the chloride complexes and the ligand L₁ exhibit very strong activity with a large inhibition zone of 19-26mm against C. albicans. Among these complexes, C4 exhibited the highest inhibition zone of 26 mm against this fungus much higher than the standard drug Amphotericin B. The complexes C1 and C4 only showed against A. flavus and A. fumigatus with an inhibition zone of 11-14mm^44,45.The observations are detailed in Table 5 and visually depicted in Figure 3c.

Table 5: Average inhibition zone (mm) of antimicrobial activities for L₁ and its Ln(III) Chloride complexes¹⁹

Compound	Inhibition zone (mm)
	Activity against bacteria						Activity against fungi
	EC	KP	VC	AH	SA	BS	CA	A.flavus	A.fumigatus
L1	–	9	–	10	–	10	21	–	–
C1	32	10	30	30	15	13	22	14	11
C2	24	8	15	12	8	9	19	–	–
C3	33	–	25	23	–	10	23	14	11
C4	32	8	34	22	11	15	26	–	–
DMF	0	0	0	0	0	0	0	0	0
Streptomycin(Std.)	18	20	6	18	21	23	–	–	–
Amphotericin-B	–	–	–	–	–	–	18	16	15

DMF(negative control), Streptomycin: (standard antibacterial drug) and Amphotericin-b(standard antifungal drug) ^2,19 

Figure 3a: image showing Inhibition zone  of complex against A. hydrophila   Click here to View Figure

Figure 3b: Graphical depiction of of L1 and complexes C1-C4Click here to View Figure

Figure 3c: Graphical depiction of the antifungal activity of L1 and complexes C1–C4Click here to View Figure

Conclusion

In this study, some Ln(III) chloride complexes of antipyrine schiff base L₁, formulated as [Ln(L₁)₂Cl₂]Cl·4H₂O (where Ln = La, Sm, Eu, and Dy), were synthesized by microwave irradiation method. Physicochemical and various spectral techniques were used to elucidate the structure of complexes. The antimicrobial properties of L₁ and the complexes were assessed. The metal complexes showed a remarkable improvement in antimicrobial activity concerning the free ligand. With further investigation into their biological properties, these complexes could potentially serve as new antibacterial agents. Notably, the Dysprosium complex exhibited exceptional activity against the majority of the microorganisms that were examined .

Acknowledgement

The authors, Jincy EM and Kaladharan PV also thank the Sophisticated Test and Instrumentation Centre, CUSAT Kochi, MG University, Kerala, School of Marine Sciences, CUSAT and PG and Research Department of Chemistry at Maharaja’s College, Ernakulam for offering the necessary facilities.

Conflicts of Interest

No possible conflicts of interest were declared by the writers.

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