INTRODUCTION:

Schiff bases, generated through the condensation reaction between primary amines and aldehydes or ketones, represent a highly adaptable and widely explored class of ligands in coordination chemistry. Their ease of synthesis, structural tunability and capacity to stabilize multiple oxidation states make them indispensable for constructing metal complexes with tailored properties^1-3. The naphthaldehyde-derived Schiff bases have important class due to incorporation of the extends π-conjugation of naphthyl scaffold.

This extends π-conjugation increases molecular rigidity, and offers additional sites for functional modification, producing ligands that frequently deliver enhanced electronic communication and distinctive steric environments compared with simple salicylaldehyde analogues^4-6.

Scheme 1 Naphthaldehyde and its derivative

The structural diversity of Schiff bases spans simple bidentate N, O-donors to elaborate tridentate/tetradentate frameworks and fused systems, enabling the formation of mononuclear, multinuclear and polymeric architectures across the main group, d- and f-block elements^7-18. Such structural variability directly influences coordination geometry, electronic structure and intermolecular packing. These characteristics functionalities result in catalysis, sensing, biological activity and nonlinear optical (NLO) behaviors^19-22.

Synthetically, the core imine linkage is typically accessed under mild condensation conditions in alcoholic media, but recent work has emphasized greener and more efficient protocols, including solvent-free synthesis, microwave-assisted synthesis and aqueous or ionic-liquid media, which reduce reaction times and improve yields while aligning with sustainable chemistry goals^23-25. This methodological diversification has accelerated access to large ligand libraries and enabled rapid structure–property screening.

The applications of metal complexes derived from naphthaldehyde Schiff bases have expanded rapidly and now encompass several bio-medically oriented research. Recent investigations in antimicrobial, antioxidant and cytotoxicity studies has demonstrated that metal coordination frequently augments biological activity relative to the free ligands, although standardization of assay conditions remains necessary for cross-study comparisons.

The extended conjugation and strong donor–acceptor motifs accessible in substituted naphthaldehyde Schiff bases produce notable third-order and second-order NLO responses when appropriately functionalized and coordinated to metals. Recent reports combine crystal engineering, spectroscopic characterization and DFT-derived hyperpolarizability calculations to identify structure motifs that enhance NLO coefficients, pointing the way toward practical optoelectronic materials^{26, 27}.

The characterization of naphthaldehyde derived Schiff bases and their metal complexes draws on a complementary set of spectroscopic, thermal and structural techniques^18-20. IR and NMR spectroscopy remain indispensable for confirming imine formation and probing coordination, while UV–Vis spectroscopy and magnetic measurements assist assignment of metal oxidation states and geometry. Importantly, single-crystal X-ray diffraction studies have proliferated in the last five years, supplying precise bond-length and angular data that are essential for correlating coordination environment with reactivity and physical properties. Modern studies routinely combine these experimental techniques with thermal (TGA/DSC) and electrochemical analyses to construct a comprehensive physical picture.

This review therefore focuses on developments since 2020 in the synthesis, characterization and biological applications of naphthaldehyde derived Schiff base ligands and their metal complexes. It surveys synthetic innovations and green approaches, modern spectroscopic and structural methods and critically examines application domains where these materials show exceptional promise particularly antimicrobial therapeutics. By highlighting structure–property relationships and identifying persistent knowledge gaps, the review aims to guide the rational design of next-generation naphthyl Schiff base complexes for targeted functional applications.

Naphthaldehyde-Derived Schiff Base Ligands:

The naphthaldehyde-derived Schiff base ligands possess distinctive structural features that set them apart from conventional benzaldehyde- or salicylaldehyde-based analogues. These differences primarily arise from the extended π-conjugated naphthalene framework, the positional availability of the aldehyde and hydroxyl groups, and the strong tendency of these systems to engage in intramolecular interactions. Collectively, these characteristics play a decisive role in governing ligand stability, coordination behavior, and functional performance.

A key structural aspect of these ligands is the naphthaldehyde precursor itself, most commonly 1-hydroxynaphthaldehyde or 2-hydroxynaphthaldehyde. The relative position of the hydroxyl group with respect to the aldehyde strongly influences Schiff base formation and coordination chemistry. In ortho-hydroxynaphthaldehyde derivatives, the phenolic –OH group typically participates in intramolecular hydrogen bonding with the azomethine nitrogen after condensation with an amine. This interaction stabilizes the imine linkage and promotes planarity, which is favorable for π-delocalization and metal chelation. Such intramolecular hydrogen bonding also facilitates keto–enol tautomerism, enabling O, N-chelation upon deprotonation of the phenolic proton during metal coordination.

The imine (–C=N–) functional group is the primary coordination site in naphthaldehyde Schiff bases. Its sp² hybridization, conjugation with the naphthalene ring, and electron density are influenced by both the aromatic core and the nature of the amine used. Aromatic amines, heterocyclic amines, and aliphatic diamines have all been employed, yielding mono-, bi-, or polydentate ligands. Simple mono-condensation typically affords bidentate N, O-donor ligands, whereas diamines can generate symmetrical or asymmetrical tetradentate frameworks. This structural flexibility allows precise control over ligand denticity and bite angle, which in turn determines the preferred coordination geometry of the resulting metal complexes.

The combination of extended conjugation, tunable donor sets, and structural rigidity makes naphthaldehyde-derived Schiff bases highly attractive platforms for coordination chemistry and application-driven research.

Synthesis of naphthaldehyde derived Schiff base metal complexes:

B. Mohan and N. Shaalan synthesized a tetradentate Schiff base ligand with high yield through the reaction of 2-hydroxy-1-naphthaldehyde and 2-aminobenzhydrazide. This ligand was utilized to prepare complexes of manganese, cobalt, nickel, copper and zinc in 1:1 molar ratio³¹. TA Shah and his co-workers prepared and characterized a series of 2-hydroxy-1-naphthaldehyde derived Schiff base. These Schiff bases were used to prepare Cu(II) complexes³². S. Parvarinezhad et. al. reported a maganease, cobalt and zinc complexes featuring a N O donor Schiff base ligand prepared from 2-hydroxy-1-napthaldehyde and 2-methoxyethylamine³³.

The reaction between 2-hydroxy-1-naphthaldehyde and 2-amino-3-methylpyridine result in the formation of N O donor Schiff base ligand³⁴. This ligand was used to prepare Fe(II), Ni(II), and Co(II) complexes in 1:2 molar ratios. M. Ismael and co-workers synthesized and characterized novel Ni(II) ternary complexes incorporating a 2-hydroxy-1-naphthaldehyde Schiff base and N-heterocyclic co-ligands³⁵.

T. A. Alorini synthesized a novel Schiff base ligand via the reaction of p-phenylenediamine with 2-hydroxy-1-naphthaldehyde and benzaldehyde³⁶. This novel ligand was subsequently reacted with different 3d metal salts to yield transition metal complexes. S.Y. Lawan et. al. was synthesized Schiff base ligand via the acid-catalyzed reaction of 2-hydroxy-1-naphthaldehyde and 4-chloroaniline in in 1:1 molar ratio³⁷. This ligand was used to prepare manganese, nickel and copper complexes in a 1:2 (M:L) stoichiometric ratio.

S.D. Oladipo et. al. synthesized two halogenated Schiff bases via the reaction of 2-hydroxy-1-naphthaldehyde with 2,6-dichloroaniline and 4-bromo-2,6-dichloroaniline. These newly synthesized ligands were allowed to coordinate with Cu(II) ion to form mononuclear complexes of 1:2 metal to ligand molar ratio³⁸.

A. Abdou reported the two novel mixed-ligand metal complexes of nickel and copper³⁹. These compounds, formulated as $[Ni(NPH)(PDBZ)(Cl)]$ and $[Cu(NPH)(PDBZ)(Cl)]$ , were derived from the coordination of Ni(II) and Cu(II) ions with 2,2′-pyridine-2,6-diylbis(1H-benzimidazole) (PDBZ) and a p-toluidine and 2-hydroxy-1-naphthaldehyde derived Schiff-base (NPH). The spectroscopic and analytical investigations confirmed a distorted octahedral geometry for both complexes³⁹. Another mixed-ligand complexes of transition metals were reported by Al-Fakeh and co-workers. The reaction p-phenylenediamine with benzaldehyde, and 2-hydroxy-1-naphthaldehyde result in formation of biologically potent Schiff base. These newly synthesized Schiff base and 2-amino-4,6-dimethylpyrimidine were used to prepare mixed ligand transition metal complexes⁴⁰.

M. Yadav reported cobalt, nickel, copper and zinc complexes derived from four novel hydrazone-based Schiff ligands. These ligands were synthesized through the condensation of benzoic acid hydrazide or its 4-chloro derivative with oxy-functionalized 2-hydroxy-1-naphthaldehyde and 4-hydroxybenzaldehyde⁴¹. Structural elucidation was achieved using a comprehensive array of analytical and spectroscopic techniques. The results indicate that the ligands function as bidentate $NO$ donors, coordinating via the azomethine nitrogen and the enolic carbonyl oxygen⁴¹.

L. M. Aroua et. al. synthesized Schiff base ligand via the reaction of 2-hydroxy-1-naphthaldehyde with (1H-benzimidazole-2-yl)methanamine. This ligand was used to prepare chromium, manganese and zinc complexes⁴². E. Mousa reported a novel Schiff base complex derived from 1,8-diaminonaphthalene and 2-hydroxy-1-naphthaldehyde. This ligand was allowed to coordinate copper ion for synthesis of metal complex.

Characterization and Structure:

Comprehensive characterization is essential for establishing the structure–property relationships of naphthaldehyde-derived Schiff base ligands and their transition metal complexes. Recent studies increasingly rely on a combination of spectroscopic, structural, thermal, and electrochemical techniques to confirm ligand formation, coordination modes, geometry around the metal center, and physicochemical stability. The integration of multiple techniques provides complementary evidence that is particularly important for multifunctional systems intended for biological, catalytic, or materials applications.

The naphthaldehyde-derived Schiff base ligands and their metal complexes are comprehensively characterized using elemental analysis, UV-Vis, FT-IR, $1$ H, 13C NMR, mass spectrometry, EPR, XRD, HRTEM, FESEM, and TGA etc^31-43. The FT-IR and UV–Vis spectroscopy confirm imine formation, coordination modes, and electronic transitions, enabling ligand field analysis and geometry assignment. NMR and EPR provide detailed insights into ligand structure and metal–ligand bonding for diamagnetic and paramagnetic systems, respectively. Single-crystal X-ray diffraction offers definitive structural information. The mass spectrometry and elemental analysis verify composition and purity. Thermal (TGA/DSC) and electrochemical (cyclic voltammetry) studies assess stability and redox behavior relevant to catalytic and biological applications.

Naphthaldehyde-derived Schiff base ligands display versatile coordination behavior due to their N, O-donor sets, enabling stabilization of diverse metal geometries. Square planar complexes are commonly formed with d⁸ metals such as Ni(II) and Pd(II), while tetrahedral arrangements occur for Zn(II), Cd(II), and some Co(II) systems. Octahedral and distorted octahedral geometries dominate for Cu(II), Co(III), Fe(III), and Mn ions, often influenced by Jahn–Teller effects and ligand sterics. These ligands exhibit flexible chelation modes and can also adopt bridging configurations, leading to polynuclear structures.

Biological Applications:

Naphthaldehyde based Schiff base have appeared as potent therapeutic agents because of their enhanced stability and high coordination affinity. The complexes of these ligands exhibit significant antimicrobial activity by disrupting microbial cell walls through chelation.

Recent research highlights their anticancer potential, antioxidant properties, ability to interact with DNA makes them excellent candidates for targeted drug delivery. Their diverse biological profile, often superior to the parent ligands, continues to drive innovation in medicinal chemistry.

In vitro antimicrobial screening via the agar well diffusion method revealed that both the ligand and its complexes exhibit significant activity against bacterial strain such as K. pneumonia, S. aureus, and C. albicans (fungi)³¹. The inhibitory activity of some synthesized complexes were found worthy, with IC50 values from IC₅₀ = 3.0 ± 0.7 µM to IC₅₀ = 19.2 ± 0.8 µM. Hirshfeld surface analysis further elucidated the role of intermolecular hydrogen bonding and π–π interactions. Molecular docking studies confirmed that complexation enhances binding potency compared to the free ligands and standard inhibitors (Saccharic acid and Uronic isofagomine). Researcher identifies Cu(II) complexes as promising scaffolds for β-glucuronidase inhibition, offering a foundation for the development of potent therapeutic agents³². The molecular docking studies against the B-cell lymphoma protein (PDB ID: 4LXD) demonstrated high binding affinities, suggesting that transition metal complexes may serve as effective inhibitors for liver cancer therapy³³. A naphthaldhyde derived Schiff base and its metal complexes demonstrated antibacterial and antifungal activity against various strains, in such as Escherichia coli, Staphylococcus aureus, and Candida albicans³⁴. The SAR model, derived from combined theoretical and experimental data, offers a strategic framework for designing potent metal-chelate antimicrobials. The antimicrobial screening complexes showed that the mixed-ligand system outperformed the binary Ni(II) analogs against B. cereus, E. coli, and A. fumigatus and the in silico drug-likeness parameters suggested high bio-potential³⁵.

Biological evaluation of transition metal complexes indicated intermediate antibacterial activity for the Co and Zn complexes against Salmonella typhi, while the V(III) complex shows superior antifungal potency against Candida albicans. Notably, in vitro cytotoxicity assays against SKOV3, PC-3, and HeLa cell lines demonstrated that the complexes possess higher antitumor activities than standard drugs, including cisplatin, estramustine, and etoposide³⁶. The antioxidant potential of some compounds was evaluated using the in vitro DPPH radical scavenging assay against an ascorbic acid standard. Notably, Mn-complex exhibited superior radical scavenging activity compared to the free ligand, other complexes, and the standard. However, at higher concentrations, the free ligand outperformed complexes³⁷. In vitro antidiabetic assays showed that some complexes possess significant inhibitory potential. For α-amylase, complexes 1 and 2 IC50 values of 148.126 mM and 107.786 mM outperformed the reference drug acarbose 171.559 mM. Antioxidant screening (DPPH, FRAP, and NO assays) further demonstrated that the complexes, particularly in NO scavenging, surpass the standard vanillin. While antibacterial activity against various strains was moderate, the metal complexes consistently exhibited superior biological potential compared to the ligands. The pharmacokinetic predictions confirmed that all compounds adhere closely to Lipinski’s Rule of Five³⁸. In vitro screening demonstrated that the metal complexes possess superior anti-inflammatory, antifungal, antibacterial, and antioxidant activities compared to the free ligands. Furthermore, the therapeutic potential was evaluated through CT-DNA binding studies and molecular docking simulations, which revealed significant binding affinities within target enzyme active sites. These results underscore the critical role of metal-ligand coordination in modulating pharmacological efficacy, offering a strategic framework for the development of new metallopharmaceuticals³⁹. The antioxidant potential of transition metal complexes was evaluated using DPPH radical scavenging assays. Furthermore, in vitro cytotoxicity was tested against A-549 and MRC-5 cancer cell lines; notably, the Cu(II) and Pd(II) complexes exhibited high potency comparable to cisplatin-based drugs. The complexes also demonstrated significant antimicrobial activity against various fungi and bacteria⁴⁰.

The in vitro antimicrobial assays of first row transition metal Schiff base complexes against several Gram-positive/negative bacterial and fungal strains revealed that chelation significantly enhances the toxicity of the compounds compared to the free ligands, with Copper(II) complex exhibiting the highest potency⁴¹. Additionally, antioxidant screening via DPPH assays demonstrated that the complexes—particularly the copper(II) derivatives IC₅₀ values (2.04–2.56 µM)) possess superior radical scavenging efficiency. Antimicrobial evaluation demonstrated that Cr(III), Mn(II) and Zn (II) complexes possess significant inhibitory potential against Bacillus subtilis and Escherichia coli, alongside moderate antifungal activity against Aspergillus niger³². The Mn(II) complex emerged as the most potent cytotoxic agent during in vitro anticancer screening, exhibiting $IC-50$ values of 6.7, 1.1 and 0.7 μg for MCF-7 (breast), HepG2 (liver) and HCT 116 (colorectal) human cancer cell lines, respectively. The docking simulations at the ERK2 active site confirmed high binding affinities for both the Schiff base and the manganese complex. Furthermore, larvicidal assays against Aedes aegypti revealed that the manganese and chromium complexes are highly toxic, with $LC-50$ values of 4.764 and 3.458 ppm, respectively⁴². A copper complex demonstrated high ionic conductivity and a temperature-dependent insulator-to-metal transition, suggesting its utility in adaptive signal modulation and thermally triggered drug delivery systems⁴³. Biological assessments underscored its dual-functionality: it exhibited robust antimicrobial activity against B. subtilis (30 mm inhibition zone) and significant cytotoxicity toward MCF-7 breast cancer cells line IC₅₀ of 18.4 μg/mL. These combined properties establish the complex as a versatile material for stable, infection-resistant bioelectronic platforms⁴³.

CONCLUSION:

Recent research has significantly advanced the synthesis, characterization, and application of naphthaldehyde-derived Schiff base transition metal complexes. Green and accelerated synthetic strategies have enabled rapid access to diverse ligand architectures, while spectroscopic, structural, and computational studies have clarified coordination behavior and structure–property relationships. Key insights highlight the role of the naphthalene framework in enhancing stability, conjugation, and metal-binding versatility, with metal coordination often improving biological, catalytic, and photophysical performance. The future efforts should focus on systematic structure–activity correlations, standardized evaluation methods, and scalable sustainable syntheses to expand applications in catalysis, sensing, and medicinal chemistry.

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Received on 05.01.2026 Revised on 10.02.2026

Accepted on 12.03.2026 Published on 10.04.2026

Available online from April 13, 2026

Asian J. Research Chem.2026; 19(2):137-142.

DOI: 10.52711/0974-4150.2026.00023

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