Synthesis and biological evaluation of transition metal complexes of thiosemicarbazone schiff bases

Thiosemicarbazones occupy a special position in bioinorganic chemistry because they chelate transition metal ions through a tridentate N, N, S donor set, generating complexes whose biological activity frequently exceeds that of the free ligand by an order of magnitude. This research reports the synthesis and characterisation of seven Schiff base thiosemicarbazone ligands (TC-1 to TC-7) derived from aromatic aldehydes and thiosemicarbazide, together with their Cu(II), Ni(II), and Co(II) complexes. All compounds were characterised by elemental analysis, IR, UV-Vis, ¹H NMR (ligands), molar conductivity, and magnetic susceptibility. The complexes adopted square-planar (Cu and Ni) or octahedral (Co) geometries based on spectroscopic data. in vitro cytotoxicity against MCF-7 breast carcinoma cells was assessed by MTT assay; antimicrobial activity by CLSI broth microdilution. The Cu(II) complex of TC-4 (2-chloro-5-nitrobenzaldehyde-derived ligand) showed the highest cytotoxic potency (IC₅₀ 9.8 μM vs. MCF-7) and the lowest MIC of 3.9 μg/mL against Candida albicans. Bubble chart and heatmap analyses demonstrated a clear correlation between increasing logP and decreasing IC₅₀ across the series. These results confirm that the Cu(II)-thiosemicarbazone scaffold is a productive lead for further anticancer optimisation.

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Coordination Geometry ComparisonClose ×

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IC₅₀ MCF-7 (μM) / —: TC-1 Cu(II) vs TC-2 Cu(II) vs TC-3 Cu(II) vs TC-4 Cu(II)Close ×

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S. aureus and E. coli and more: TC-3 Cu(II) vs TC-4 Cu(II) vs TC-7 Cu(II)Close ×

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Introduction

The coordination chemistry of thiosemicarbazones has attracted sustained interest since the pioneering work of French and Blanz in the 1960s established that the biological activity of these ligands is markedly enhanced upon metal complexation. The free thiosemicarbazone acts as an inhibitor of ribonucleotide reductase by chelating the iron cofactor of the enzyme's tyrosyl radical centre; when pre-chelated with exogenous metal ions-particularly Cu²⁺-the complex is taken up more efficiently by cells through lipophilicity-mediated membrane permeation and may exercise additional cytotoxic mechanisms including DNA intercalation, reactive oxygen species generation, and proteasome inhibition. The Schiff base variant of thiosemicarbazones-formed by condensation of thiosemicarbazide with aromatic aldehydes carrying electron-withdrawing or electron-donating substituents-offers a convenient structural handle for tuning the electronic and steric environment of the coordinated metal. Despite extensive literature on individual thiosemicarbazone-metal systems, systematic comparison of seven structurally diverse ligands paired with three biologically relevant metal ions (Cu, Ni, Co) under matched synthesis and assay conditions, combined with logP-activity correlation analysis, has been infrequently reported. This research provides this dataset, establishing structure-activity trends that can guide further lead optimisation.

Materials and Methods

Materials and Methods

Materials

Materials

Methods

Ligand synthesis: Thiosemicarbazide (1 mmol) and aldehyde (1 mmol) were refluxed in ethanol (10 mL) with 2 drops glacial acetic acid for 3 h. Crystalline product was filtered, washed with cold ethanol, dried in vacuo. Metal complexes: Metal acetate (0.5 mmol) in water:ethanol (1:1, 5 mL) was added dropwise to ligand (1 mmol) in ethanol (10 mL) at 60 °C with stirring; precipitate was collected after 2 h reflux. Characterisation: Elemental analyses (C, H, N, S) by PerkinElmer 2400 Series II; IR (KBr, 400-4000 cm⁻¹, PerkinElmer Spectrum 65); UV-Vis in DMSO (Shimadzu UV-2600); molar conductance in DMSO (10⁻³ M); magnetic moments by Gouy method. MTT assay: MCF-7 cells (5 × 10³/well, 96-well plate) treated with 0.1-100 μM compound for 48 h; IC₅₀ by nonlinear regression. Antimicrobial MIC by CLSI M07 broth microdilution in MHB (bacteria) and RPMI-1640 (C. albicans) over 24 h/48 h.

Theoretical Background

Thiosemicarbazones coordinate metals as monoanionic tridentate ligands via the azomethine nitrogen (N1), the terminal nitrogen (N4), and the sulphur atom (S), forming a five-membered N1-N2-C-S chelate ring fused to a fivemembered N2-N1-C-N4 ring in the thione tautomer. On complexation, the C = S stretching frequency in IR shifts from approximately 840 cm⁻¹ in the free ligand to 780-810 cm⁻¹, confirming S coordination; the C = N azomethine stretch shifts from approximately 1600 cm⁻¹ to 1570-1585 cm⁻¹, confirming N coordination. Square-planar Cu(II) and Ni(II) complexes typically show molar conductance values of 8-12 Ω⁻¹ cm² mol⁻¹ in DMSO, consistent with nonelectrolytes. The observed magnetic moment of Cu(II) complexes around 1.73 BM (one unpaired electron, spinonly) and Ni(II) close to 0 BM for diamagnetic squareplanar species further confirm the proposed geometries.

Performance Evaluation

Performance of complexes was ranked using a composite biological activity score (BAS) calculated as the geometric mean of three normalised metrics: (1) normalised IC₅₀ against MCF-7 (lower = better); (2) normalised geometric mean MIC across four organisms (lower = better); and (3) selectivity index (SI = IC₅₀ against Vero normal cells/IC₅₀ against MCF-7, higher = better; Vero IC₅₀ estimated from literature analogue data). TC-4 Cu(II) achieved the highest BAS (0.84 out of 1.00), followed by TC-7 Cu(II) (0.76) and TC-3 Cu(II) (0.71). All Co(II) complexes scored below 0.50, consistent with their lower cytotoxic potency and higher MIC values compared with corresponding Cu and Ni analogues.

Results

All 21 compounds (7 ligands + 14 complexes) were isolated in 68-87% yield as coloured solids. Elemental analyses matched calculated values within±0.4%. IR data confirmed tridentate coordination through bathochromic shifts of ν(C = S) and ν(C = N) relative to free ligands. Molar conductance values (8-14 Ω⁻¹ cm² mol⁻¹ in DMSO) confirmed nonelectrolyte character. Magnetic moments confirmed squareplanar Cu(II) and Ni(II) and octahedral Co(II) geometries. MTT cytotoxicity data (Figure 1, Table 1) showed TC-4 Cu(II) as the most potent compound (IC₅₀ 9.8 μM), with IC₅₀ values decreasing as logP increases across the series. The MIC heatmap (Figure 2, Table 2) showed C. albicans as the most susceptible organism and P. aeruginosa the least, consistent with intrinsic resistance patterns.

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Figure 2Close ×

Heatmap of MIC values (μg/mL) for thiosemicarbazone complexes against four pathogenic microorganisms. Darker colour indicates

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Figure 1Close ×

Bubble chart of IC₅₀ (μM) vs logP for thiosemicarbazone metal complexes; bubble size proportional to molecular weight.

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Compound	S. aureus	E. coli	C. albicans	P. aeruginosa
TC-3 Cu(II)	15.6	31.2	7.8	62.5
TC-4 Cu(II)	7.8	15.6	3.9	31.2
TC-7 Cu(II)	15.6	31.2	7.8	62.5
Ciprofloxacin (ref)	0.25	0.5	—	0.5
Fluconazole (ref)	—	—	0.5	—

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Table 2Close ×

Antimicrobial MIC (μg/mL) of Selected Complexes (CLSI M07)

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Compound	IC₅₀ MCF-7 (μM)	LogP (calc.)	MW (g/mol)	Geometry
TC-1 (free ligand)	—	1.84	248.3	—
TC-1 Cu(II)	38.4±2.3	1.84	412	Square-planar
TC-2 Cu(II)	24.7±1.8	2.31	487	Square-planar
TC-3 Cu(II)	14.2±1.1	2.97	543	Square-planar
TC-4 Cu(II)	9.8±0.8	3.42	618	Square-planar
TC-3 Ni(II)	18.6±1.4	2.74	521	Square-planar
TC-4 Co(II)	31.7±2.1	3.42	624	Octahedral

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Table 1Close ×

Cytotoxicity (IC₅₀ vs MCF-7, μM) and Physicochemical Data

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Discussion

The positive correlation between logP and cytotoxic potency (Figure 1) is consistent with the established principle that metal complex uptake by cancer cells via passive diffusion is enhanced by increased lipophilicity, up to the threshold where aqueous solubility becomes limiting. TC-4, carrying both chloro and nitro groups on the aryl ring, has the highest logP

Acknowledgement

Acknowledgements Funding SourcesInstitutional research funds from the Sushruta Institute of Pharmaceutical Research, Dehradun, supported this investigation without external grants.

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