Synthesis and characterization of an SNS-based NO-type schiff base Ni(II) Complex: Insights into electrochemical and electronic properties

Abstract

In this study, a novel NO-type Schiff base ligand (LH) was synthesized via the condensation reaction of 4-(2,5-di(thiophen-2-yl)-1H-pyrrol-1-yl)aniline and 2-hydroxy-1-naphthaldehyde, followed by the preparation of its corresponding Ni(II) complex. The structures of the ligand and its metal complex were comprehensively characterized by elemental analysis, ¹H NMR, FT-IR, UV–Vis spectroscopy, cyclic voltammetry (CV), thermal analysis (TGA/DTA), scanning electron microscopy (SEM), and magnetic measurements. Spectroscopic and magnetic results indicate that the Ni(II) complex adopts a four-coordinated distorted geometry with an ML₂ stoichiometry. Electrochemical investigations reveal that the redox behavior is governed by the conjugated SNS backbone and the imine functionality, confirming the donor–acceptor nature of the system. SEM analysis demonstrates a heterogeneous and porous morphology, which is favorable for ion diffusion and charge transport. Furthermore, metal coordination leads to a decrease in the electrochemical band gap, confirming enhanced charge delocalization and improved electronic properties. These results establish a clear correlation between coordination-induced electronic modulation and electrochemical behavior. In addition, the incorporation of heteroatoms (N, O, and S) within the ligand framework highlights the relevance of main group elements in tuning coordination and electronic properties. Overall, this study provides valuable insights into SNS-based Schiff base systems and demonstrates their potential for electrochemical and optoelectronic applications.

Graphical abstract

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Keywords

schiff base nickel complex NO-type ligand SNS cyclic voltammetry electroactive materials

Highlights

A novel SNS-based NO-type Schiff base ligand and its Ni(II) complex were synthesized and fully characterized.

The Ni(II) complex exhibits a four-coordinate distorted geometry with ML₂ stoichiometry.

Metal coordination induces significant changes in electronic structure and redox behavior.

The complex demonstrates pronounced electrochromic behavior with enhanced charge delocalization.

Structure–property relationships are established through spectroscopic, electrochemical, and morphological analyses.

Introduction

Schiff base metal complexes have attracted considerable attention due to their wide range of applications in pharmacology, catalysis, and materials science. The presence of the azomethine (–C = N–) functional group enables Schiff base ligands to coordinate effectively with transition metal ions, leading to the formation of stable chelate complexes with diverse structural and electronic properties.¹ These complexes have been extensively investigated for their biological activities, including antitumor, antimicrobial, and enzymatic functions, as well as their roles in biochemical and physiological processes. In addition to their biological relevance, Schiff base metal complexes have demonstrated remarkable potential in catalytic and functional applications. For instance, platinum-based complexes exhibit significant antitumor activity, while cobalt complexes have been employed as oxygen carriers in oxygen transport and separation processes. Moreover, manganese and ruthenium complexes have been reported as efficient catalysts for photochemical water splitting, whereas iron complexes have been widely studied in oxygen reduction reactions.² These diverse functionalities further emphasize the importance of Schiff base coordination compounds. Ligands containing multiple donor atoms such as nitrogen (N), oxygen (O), and sulfur (S) are of particular interest due to their versatile coordination behavior and their strong ability to stabilize metal centers.^3,4 From a main group chemistry perspective, the incorporation of these heteroatoms plays a crucial role in modulating coordination environments and electronic structures, thereby enabling the rational design of functional coordination systems. Among heteroaromatic systems, thiophene-based frameworks have attracted particular attention due to their strong electron-donating ability and effective conjugation. In addition, previous studies have reported that heteroaromatic substitutions such as furan and selenophene can significantly influence the electronic properties and coordination behavior of Schiff base systems. Compared to related heterocycles such as furan and selenophene, thiophene exhibits enhanced chemical stability and more effective π-electron delocalization, making it a highly suitable building block for electroactive materials.^5,6 Conjugated systems incorporating thiophene–pyrrole–thiophene (SNS) units have gained increasing attention because of their unique electronic properties, including relatively low oxidation potentials and efficient charge transport characteristics. As a result, SNS-based materials have been extensively explored in electrochromic, photochromic, and thermochromic applications.^7–9 In particular, thienyl-containing Schiff base systems have been reported to exhibit tunable electronic transitions and enhanced charge-transfer characteristics due to the extended π-conjugated framework.^10,11 Several studies have demonstrated the functional potential of SNS-based systems. For example, Algı et al. reported an SNS-based compound incorporating a crown ether unit that exhibited a selective fluorescence response toward Pb²⁺ ions,¹² while Audebert et al. developed a Schiff base-functionalized SNS monomer displaying notable photochromic behavior upon polymerization.¹³ Furthermore, Sefer et al. demonstrated that strong intramolecular charge transfer between the SNS backbone and substituent groups leads to pronounced electrochromic responses in conjugated polymer systems,¹⁴ further supported by related studies on conjugated electroactive materials.¹⁵ These findings highlight the importance of electronic communication within conjugated systems and the role of heteroatom-containing frameworks in tuning optoelectronic properties. Despite these advances, the integration of NO-type Schiff base ligands with SNS-based conjugated frameworks remains relatively unexplored. In particular, systematic studies addressing the relationship between molecular structure, electronic properties, and morphology in such systems are still limited. In this context, the present study reports the synthesis and characterization of a novel NO-type Schiff base ligand based on an SNS-conjugated backbone and its corresponding Ni(II) complex. The structural, spectroscopic, thermal, and electrochemical properties of the synthesized compounds were comprehensively investigated using FT-IR, ¹H NMR, UV–Vis spectroscopy, cyclic voltammetry, TGA/DTA, and SEM analyses. Particular emphasis was placed on elucidating the relationship between coordination, electronic structure, and electrochemical behavior. This study demonstrates how metal coordination can be used as an effective strategy to tune the electronic properties of SNS-based Schiff base systems. The results provide a systematic correlation between coordination-induced electronic modulation and electrochemical properties, offering valuable insights into the rational design of electroactive coordination materials.

Experimental

Materials and measurements

All reagents and solvents were obtained from commercial suppliers and used without further purification. Elemental analyses (C, H, and N) were performed using a PerkinElmer elemental analyzer. Fourier-transform infrared (FT-IR) spectra were recorded on a PerkinElmer Spectrum 2 spectrophotometer equipped with an attenuated total reflectance (ATR) accessory in the range of 4000–400 cm⁻¹. UV–Vis absorption spectra were measured in DMSO using a Shimadzu Pharmaspec UV-1700 spectrophotometer over the range of 200–1100 nm. The ¹H NMR spectrum of the ligand was recorded on a Bruker Avance DPX-400 spectrometer at 25 °C using CDCl₃ as solvent, with tetramethylsilane (TMS) as an internal reference. Scanning electron microscopy (SEM) images were obtained using a JEOL JSM-6510LV instrument. Electrochemical measurements were carried out using a CH Instruments 617D electrochemical workstation under an argon atmosphere. A conventional three-electrode system was employed, consisting of a platinum disk working electrode, a platinum wire counter electrode, and an Ag wire reference electrode. Measurements were performed in 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF₆) in dichloromethane (DCM) at room temperature. Thermal analyses (TGA/DTA) were conducted using a Shimadzu DTG-60H system under a dynamic nitrogen atmosphere (15 mL min⁻¹) at a heating rate of 20 °C min⁻¹. Spectroelectrochemical measurements were performed in a quartz cuvette using an indium tin oxide (ITO)/glass working electrode, together with platinum counter and Ag reference electrodes.

Synthesis

4-[2,5-di(thiophen-2-yl)-1H-pyrrol-1-yl]aniline was synthesized according to previously reported procedures.¹² The Schiff base ligand (LH) and its corresponding Ni(II) complex (ML₂) were subsequently prepared following a general synthetic route. The ligand (LH) was obtained via a condensation reaction between the corresponding amine and aldehyde, while the Ni(II) complex was synthesized through coordination of the ligand with a nickel(II) salt. The overall synthetic pathway for the preparation of the ligand and its metal complex is illustrated in Schemes 1 –5.

Scheme 1.

Synthetic route for the preparation of 1,4-di(thiophen-2-yl)butane-1,4-dione.

Scheme 2.

Synthetic route for the preparation of 1-(4-nitrophenyl)-2,5-di(thiophen-2-yl)-1H-pyrrole.

Scheme 3.

Synthetic route for the preparation of 4-[2,5-di(thiophen-2-yl)-1H-pyrrol-1-yl]aniline.

Scheme 4.

Synthetic route for the preparation of the Schiff base ligand (LH).

Scheme 5.

Structure of the Ni(II) complex (NiL₂).

Synthesis of 1,4-di(thiophen-2-yl)butane-1,4-dione [1]

Anhydrous aluminum chloride (0.24 mol) was added to 100 mL of dichloromethane (DCM) under an argon atmosphere, and the mixture was cooled to 0 °C. A solution of thiophene (0.24 mol) and succinyl dichloride (0.10 mol) in 15 mL of DCM was then added dropwise. The reaction mixture was stirred at room temperature for 18 h, during which the color changed from light yellow to red. After completion, 200 mL of ice water containing 10 mL of hydrochloric acid (HCl) was carefully added, and the mixture was stirred for an additional 2 h, leading to a gradual color change to green. The organic phase was separated and washed successively with dilute HCl, distilled water, and saturated sodium bicarbonate (NaHCO₃) solution. The organic layer was dried over anhydrous magnesium sulfate, filtered, and concentrated under reduced pressure. The crude product was purified by recrystallization from ethanol to afford the desired green product (Yield: 43.2%).¹⁶

Synthesis of 1-(4-nitrophenyl)-2,5-di(thiophen-2-yl)-1H-pyrrole [2]

1,4-di(thiophen-2-yl)butane-1,4-dione (15.98 mmol) was dissolved in toluene in a round-bottom flask equipped with a Dean–Stark apparatus. 4-Nitroaniline (17.58 mmol) and p-toluenesulfonic acid (PTSA) were then added, and the reaction mixture was refluxed at 120 °C for 24 h. After completion, the mixture was cooled to room temperature and poured into ethanol. The solvent was removed under reduced pressure, and the crude product was purified by column chromatography on silica gel using hexane/chloroform (1:2, v/v) as the eluent to afford the desired product (Yield: 58.8%), using a standard literature procedure.

Synthesis of 4-[2,5-di(thiophen-2-yl)-1H-pyrrol-1-yl]aniline [3]

1-(4-nitrophenyl)-2,5-di(thiophen-2-yl)-1H-pyrrole (8.52 mmol) was dissolved in ethanol and heated under reflux with stirring for 20 min. A mixture of palladium on activated carbon (Pd/C) and hydrazine hydrate in ethanol was then added dropwise, and the reaction was continued under reflux for 12 h. After completion, the reaction mixture was filtered to remove the catalyst, and the filtrate was washed with ethanol. The product was dried under to afford the corresponding amine derivative (Yield: 98.7%).

Synthesis of 1-[(E)-({4-[2,5-di(thiophen-2-yl)-1H-pyrrol-1-yl]phenyl}imino)methyl]-2-naphthol (LH)

2-Hydroxynaphthalene-1-carbaldehyde (1.57 mmol) was added dropwise to an ethanolic solution of 4-[2,5-di(thiophen-2-yl)-1H-pyrrol-1-yl]aniline (1.55 mmol) under continuous stirring. The reaction mixture was refluxed for 4 h, during which a yellow precipitate formed. After completion, the solid product was filtered, washed with cold ethanol, and dried under vacuum to afford the Schiff base ligand (LH) (Yield: 97.4%).

Synthesis of schiff base Ni(II) Complex (NiL₂)

The Ni(II) complex was synthesized via a coordination reaction. The ligand (0.23 mmol) was dissolved in 40 mL of ethanol and added dropwise to a stirred ethanolic solution of a nickel(II) salt (0.115 mmol). The reaction mixture was refluxed for 12 h under continuous stirring. After completion, the mixture was cooled to room temperature and further cooled in an ice bath to induce precipitation. The resulting solid was filtered using a sintered glass funnel, washed with cold ethanol, and dried under vacuum to afford the NiL₂ complex (Yield: 70.45%).

Results and discussion

The analytical and physicochemical data of the synthesized Ni(II) complex indicate that the reaction between the metal salt and the Schiff base ligand proceeds with a 1:2 (M:LH) molar ratio, resulting in the formation of an ML₂-type complex. In this structure, the ligand (LH) acts as a bidentate chelating agent, coordinating to the metal center through the azomethine nitrogen and phenolic oxygen atoms, forming a typical NO-type coordination environment. The analytical data and selected physical properties of the ligand and its Ni(II) complex are summarized in Table 1. The complex is obtained as an intensely colored solid and exhibits good thermal stability up to approximately 200 °C, suggesting the formation of a stable coordination framework. In terms of solubility, the complex is insoluble in water and common organic solvents such as ethanol, methanol, benzene, cyclohexane, acetone, and diethyl ether, but shows good solubility in polar aprotic solvents including dichloromethane (DCM), dimethylformamide (DMF), and dimethyl sulfoxide (DMSO). This solubility behavior is consistent with previously reported Schiff base metal complexes and reflects the balance between the rigid conjugated backbone and polar functional groups, which governs intermolecular interactions and solvent compatibility.

Table 1.

Analytical and physicochemical data of the schiff base ligand (LH) and its Ni(II) complex.

Compounds	F.W. (g/mol)	Colour	Yield (%)	M.P. (^oC)	Calculated (Found) (%)
Compounds	F.W. (g/mol)	Colour	Yield (%)	M.P. (^oC)	C	H	N
LH [C₂₉H₂₀N₂OS₂]	476.61	Yellow	97.42	210	73.07 (73.18)	4.19 (4.21)	5.87 (5.62)
NiL₂ [C₅₈H₃₈N₄O₂S₄Ni]	1009.90	Light Green	84.94	312	68.97 (68.61)	3.77 (3.81)	5.55 (5.35)

IR spectra

The structural features of the ligand (LH) and its Ni(II) complex were investigated by FT-IR spectroscopy. The characteristic bands confirming the formation of the Schiff base ligand are attributed to the azomethine (C = N) and phenolic (O–H) stretching vibrations. For the free ligand, the strong band observed at 1621 cm⁻¹ is assigned to the azomethine (C = N) stretching vibration, confirming successful imine formation. The bands at 1594 cm⁻¹ and 1324 cm⁻¹ correspond to aromatic (C = C) and phenolic (C–O) stretching vibrations, respectively.¹⁷ Upon complexation, notable changes are observed in the FT-IR spectra. The azomethine (C = N) stretching frequency shifts to lower wavenumbers (1618–1607 cm⁻¹), indicating coordination of the azomethine nitrogen to the metal center. This shift is consistent with previously reported Schiff base metal complexes and can be attributed to a decrease in electron density of the C = N bond upon coordination, resulting in bond weakening.¹⁸ In addition, the disappearance of the phenolic O–H stretching band in the complex spectrum indicates deprotonation of the phenolic group and subsequent coordination of the oxygen atom to the metal center. This observation clearly supports the involvement of both nitrogen and oxygen donor atoms in the coordination process, confirming the formation of an NO-type chelate structure. Other characteristic bands, including aromatic C–H and C = C stretching vibrations, remain largely unchanged, indicating that the conjugated backbone of the ligand is preserved upon complex formation. Overall, the FT-IR results provide strong evidence for coordination of the ligand to the Ni(II) center through azomethine nitrogen and phenolic oxygen atoms. The observed vibrational frequencies are summarized in Table 2.

Table 2.

Selected FT-IR absorption bands of the schiff base ligand (LH) and its Ni(II) complex.

Compounds	υ(C = N) peak	υ(C-H) peak	υ(C = C) peak	υ(C-O) peak
LH	1621 cm⁻¹	3062 cm⁻¹	1594 cm⁻¹	1324 cm⁻¹
NiL₂	1616 cm⁻¹	3059 cm⁻¹	1593 cm⁻¹	1372 cm⁻¹

Note: The shift of the ν(C = N) band to lower wavenumbers upon complexation indicates coordination through the azomethine nitrogen.

¹H-NMR spectra

The ¹H-NMR spectrum of the Schiff base ligand (LH) was recorded in DMSO-d₆, and the chemical shifts are reported in ppm relative to tetramethylsilane (TMS) as an internal reference. The spectrum exhibits a broad singlet at 15.35 ppm, which is attributed to the phenolic –OH proton, indicating strong intramolecular hydrogen bonding. A characteristic singlet observed at 9.40 ppm corresponds to the azomethine proton (–CH = N–), confirming successful imine formation. The aromatic protons of the naphthalene, thiophene, and pyrrole rings appear as multiplets in the range of 6.63–7.72 ppm, consistent with the extended π-conjugated framework of the ligand. The observed chemical shift distribution reflects significant electron delocalization within the conjugated system. Overall, the ¹H-NMR data provide strong evidence for the successful synthesis of the Schiff base ligand and are fully consistent with the proposed molecular structure, as well as with previously reported similar systems (Figure 1). It should also be noted that, due to the paramagnetic nature of the Ni(II) center, NMR techniques are not suitable for detailed structural analysis of the complex. Therefore, the structural characterization of the Ni(II) complex was primarily established using complementary spectroscopic and analytical techniques.

Figure 1.

¹H NMR spectrum of the schiff base ligand (LH) recorded in DMSO-d₆. The presence of the characteristic azomethine proton (–CH = N–) and phenolic –OH signal confirms the successful formation of the Schiff base. The aromatic proton signals are consistent with the extended π-conjugated framework of the ligand.

Magnetic properties

Magnetic susceptibility measurements provide valuable insight into the electronic structure and coordination geometry of transition metal complexes. The magnetic moment of the synthesized Ni(II) complex was determined to be 2.64 B.M., indicating its paramagnetic nature and confirming the presence of unpaired electrons in the d-orbitals of the Ni(II) center. This value is in good agreement with previously reported Schiff base Ni(II) complexes exhibiting similar coordination environments. The observed magnetic moment is consistent with a four-coordinated environment and suggests a distorted geometry around the metal center, such as distorted square-planar or pseudo-tetrahedral. Accordingly, the Ni(II) center is proposed to adopt a four-coordinated distorted geometry based on the combined spectroscopic and magnetic data. This observation strongly supports the proposed ML₂ coordination mode and clearly confirms the formation of a well-defined NO-type chelation environment around the Ni(II) center.

Electronic spectra

The UV–Vis spectra of the ligand (LH) and its Ni(II) complex are presented in Figure 2. The ligand exhibits an intense absorption band at 365 nm, attributed to π→π* transitions of the extended conjugated system, and a broader band at 458 nm corresponding to n→π* transitions of the azomethine group.¹³ Upon coordination with Ni(II), notable spectral changes are observed. The complex displays absorption bands at 330 nm, 455 nm, and 567 nm. The shift of the high-energy band and the emergence of new absorption bands clearly indicate significant modification of the electronic structure upon coordination. The band at 455 nm is attributed to charge transfer transitions, while the broad band at 567 nm is assigned to d–d transitions of the Ni(II) center.^13,19 The predominance of charge transfer transitions strongly indicates enhanced metal–ligand electronic coupling within the coordinated system, highlighting efficient electron delocalization across the conjugated framework. Moreover, these spectral changes reflect enhanced intramolecular charge transfer, demonstrating more efficient electronic communication between the ligand framework and the metal center.

Figure 2.

Uv–Vis absorption spectra of (a) the schiff base ligand (LH) and (b) its Ni(II) complex recorded in DMSO. The ligand exhibits characteristic π→π* and n→π* transitions, while the Ni(II) complex shows notable spectral changes upon coordination, including the appearance of charge transfer and d–d transition bands, indicating coordination-induced modification of the electronic structure.

This behavior can be directly correlated with the incorporation of heteroatoms (N, O, and S), which play a crucial role in facilitating electron delocalization and stabilizing charge-separated states within the system. Overall, these results demonstrate that metal coordination plays a crucial role in modulating the electronic structure and transition behavior of the conjugated SNS framework, leading to increased conjugation upon coordination. Such coordination-induced electronic modulation is a key feature in the design of functional materials within the scope of main group chemistry. Furthermore, these findings are in good agreement with the electrochemical results, confirming that metal coordination effectively reduces the band gap and enhances charge delocalization across the system, thereby improving its potential applicability in electrochemical and optoelectronic devices. This strong agreement between spectroscopic and electrochemical findings provides compelling evidence for well-defined structure–property relationships in SNS-based Schiff base systems.

Electrochemistry

The electrochemical behavior of the ligand (LH) and its Ni(II) complex indicates that the imine (–C = N–) unit is responsible for the reduction process, whereas the thiophene–pyrrole–thiophene (SNS) backbone predominantly contributes to the oxidation process. This observation highlights the donor–acceptor nature of the system, where the conjugated SNS framework facilitates oxidation and the imine functionality acts as an electron-accepting center, consistent with previously reported SNS-based electroactive systems.⁸ As illustrated in Figure 3, the cyclic voltammograms of LH and its Ni(II) complex exhibit distinct electrochemical behaviors. Upon coordination, both oxidation and reduction potentials undergo noticeable shifts. The reduction peak associated with the imine group shifts toward more negative potentials, while the oxidation peak related to the SNS unit shifts toward higher potentials. These systematic potential shifts clearly indicate that coordination to the Ni(II) center alters the electron density distribution within the ligand framework, confirming the strong electronic influence of the metal center on the redox-active moieties. These changes confirm the electron-withdrawing effect of the metal center and enhanced intramolecular charge transfer within the coordinated system. The HOMO and LUMO energy levels were estimated from the onset oxidation and reduction potentials. The ligand exhibits HOMO/LUMO energy levels of −5.11 eV and −3.00 eV, respectively, while the Ni(II) complex shows values of −5.07 eV and −3.19 eV. Consequently, the electrochemical band gap (E_g′) decreases from 2.11 eV for LH to 1.88 eV for the Ni(II) complex. This pronounced decrease in the band gap provides direct electrochemical evidence of enhanced π-electron delocalization upon coordination, which is a key factor governing the optoelectronic performance of conjugated systems. This reduction in band gap confirms that metal coordination enhances charge delocalization and significantly improves the electronic properties of the system.⁹ Overall, the electrochemical results clearly demonstrate that metal coordination plays a crucial role in tuning the redox behavior and electronic properties of SNS-based Schiff base systems, highlighting their potential for electrochemical and optoelectronic applications. In addition, the reproducibility of successive cycles indicates good electrochemical stability of the complex. Such electrochemical stability, combined with tunable redox characteristics, further supports the potential applicability of these systems in functional electroactive materials. These findings are in excellent agreement with the UV–Vis results, further confirming coordination-induced electronic modulation in the system. This strong correlation between electrochemical and spectroscopic data provides compelling evidence for well-defined structure–property relationships in SNS-based Schiff base systems.

Figure 3.

Cyclic voltammograms of the schiff base ligand (LH) and its Ni(II) complex recorded in 0.1 M TBAPF₆/DCM at a scan rate of 100 mV s⁻¹ using an Ag wire reference electrode. The voltammograms reveal distinct redox behavior and potential shifts upon coordination, indicating coordination-induced modulation of the electronic properties. The estimated HOMO–LUMO energy levels and corresponding electrochemical band gaps (E_g) are also presented.

Thermal analysis

The thermal behavior of the Ni(II) complex was investigated by thermogravimetric analysis (TGA) and differential thermal analysis (DTA) under a nitrogen atmosphere over the temperature range of 25–600 °C. The corresponding thermograms are presented in Figure 4, and the thermoanalytical data are summarized in Table 3. As shown in Figure 4 and Table 3, the Ni(II) complex exhibits a single-step decomposition process. The major weight loss occurs at approximately 428 °C and is accompanied by an endothermic peak at around 430 °C, corresponding to the removal of two ligand molecules from the coordination sphere. The observed weight loss (68.36%) is in excellent agreement with the calculated value (68.55%), confirming the proposed ML₂ stoichiometry of the complex. This close agreement between experimental and theoretical values provides strong evidence for the reliability of the proposed coordination structure. The relatively high decomposition temperature indicates good thermal stability, which can be attributed to strong metal–ligand interactions and the presence of an extended conjugated system.²⁰ Such enhanced thermal stability further supports the formation of a robust coordination framework, which is essential for potential applications in electroactive and optoelectronic materials. Furthermore, the residual mass (30.91%) observed above 600 °C is likely associated with the formation of thermally stable metal-containing species. Overall, these results confirm the structural integrity and coordination stability of the complex. These findings also highlight the contribution of metal coordination to the thermal robustness of SNS-based Schiff base systems, reinforcing the structure–property relationship discussed throughout this study.

Figure 4.

TGA and DTA of Ni(II) complex.

Table 3.

Thermogravimetric (TGA) and differential thermal analysis (DTA) data of the Ni(II) complex.

Compounds	Step	TGA temperature (°C)	DTA temperature (°C)	Weight loss (%)
Compounds	Step	TGA temperature (°C)	DTA temperature (°C)	Found	Calc.
Ni(II)	Decomposition	428	430 (endothermic)	68.36	68.55
Ni(II)	Residue formation	>600	430 (endothermic)	30.91	30.96

Surface morphology (SEM analysis)

The surface morphology of the Schiff base ligand (LH) and its Ni(II) complex was investigated using scanning electron microscopy (SEM), and the corresponding micrographs are presented in Figure 5. Significant differences are observed between the surface morphologies of LH and its Ni(II) complex. The ligand exhibits a relatively irregular and loosely packed structure with heterogeneous surface features. In contrast, the Ni(II) complex displays a more compact and aggregated morphology, characterized by clustered and granular structures. These morphological changes can be attributed to the coordination of Ni(II) ions with the donor atoms (O and N) of the ligand, which enhances intermolecular interactions and promotes a more organized and densely packed architecture. This coordination-driven structural organization indicates that metal–ligand interactions play a decisive role in defining the supramolecular arrangement of the material. The increased aggregation observed in the complex suggests stronger intermolecular interactions compared to the free ligand. Consequently, the electrochemical performance of the LH-based system can be directly correlated with these morphological characteristics. Such morphology–property relationships are crucial for the rational design of high-performance electroactive materials. In particular, the more compact and interconnected morphology of the complex is expected to facilitate more efficient charge transport pathways, thereby enhancing the overall electrochemical performance. Furthermore, the observed morphology is expected to facilitate improved charge transport properties. These observations further support the correlation between structural organization and functional properties in SNS-based coordination systems.

Figure 5.

SEM images of the schiff base ligand (LH) and its Ni(II) complex. The ligand exhibits a relatively irregular and loosely packed morphology, whereas the Ni(II) complex displays a more compact and aggregated structure with clustered granular features. These pronounced morphological changes clearly indicate enhanced intermolecular interactions induced by metal coordination. Such morphology–property relationships are critically important for the rational design of high-performance electroactive materials, as the observed morphology is expected to facilitate improved ion diffusion and charge transport.

Conclusions

In this study, a novel NO-type Schiff base ligand, 2-[(E)-({4-[2,5-di(thiophen-2-yl)-1H-pyrrol-1-yl]phenyl}imino)methyl]naphthol (LH), and its Ni(II) complex were successfully synthesized and comprehensively characterized. The structures of the ligand and its metal complex were confirmed by elemental analysis, FT-IR, UV–Vis, ¹H-NMR spectroscopy, cyclic voltammetry, thermal analysis (TGA/DTA), SEM, and magnetic susceptibility measurements.

The analytical and physicochemical data confirm that the complex possesses an ML₂ stoichiometry, in which the ligand acts as a bidentate chelating agent coordinating through azomethine nitrogen and phenolic oxygen atoms. Based on the combined spectroscopic and magnetic results, the Ni(II) center is proposed to adopt a four-coordinated distorted geometry.

Electrochemical studies reveal that the redox behavior is governed by the imine and SNS units, while metal coordination leads to a reduced band gap and enhanced charge delocalization. UV–Vis analysis further confirms that coordination significantly modifies the electronic transitions through charge transfer interactions. Thermal analysis demonstrates that the complex exhibits good thermal stability, whereas SEM results reveal pronounced morphological differences between the ligand and the complex, highlighting the influence of metal coordination on surface structure. Importantly, the combined spectroscopic, electrochemical, thermal, and morphological findings consistently demonstrate that metal coordination acts as an effective strategy for tuning the electronic structure and functional properties of SNS-based Schiff base systems. Overall, these findings demonstrate that metal coordination plays a crucial role in tuning the structural, electronic, and electrochemical properties of SNS-based Schiff base systems. This study establishes a clear and well-defined structure–property–function relationship, providing deeper insight into how heteroatom-rich coordination frameworks (N, O, S) govern electronic behavior and material performance. Such insights are particularly relevant within the scope of main group chemistry, where the strategic incorporation of heteroatoms enables precise control over coordination environments and electronic properties. This work establishes a clear structure–property–function relationship and provides a promising platform for the rational design of next-generation electroactive and optoelectronic materials. Overall, the results presented herein highlight the potential of SNS-based Schiff base coordination systems as versatile building blocks for advanced functional materials with tunable electronic and electrochemical characteristics.

Footnotes

Credit authorship contribution statement

Gözde Laçinel: Methodology, Data curation, Writing–original draft. Ömer Faruk Öztürk: Conceptualization, Methodology, Visualization, Software, Writing-review & editing Supervision. Sermet Koyuncu Methodology, Visualization, Investigation, Writing–original draft.

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the No Funders,

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

ORCID iD

Ömer Faruk Öztürk

References

Matsumoto

Zhong

Kawa

, et al. Crystal structure and magnetic property of the binuclear nickel(II) complex, [Ni2(fsa)2en(MeOH)2]. Inorg Chim Acta 1989; 160: 153–157.

Liu

Chen

Cui

, et al. Applications of metal–organic frameworks in heterogeneous supramolecular catalysis. Chem. Soc. Rev 2014; 43: 6011–6061.

Shi

Mao

W-J

Yang

, et al. Synthesis, characterization, and biological activity of a Schiff-base Zn(II) complex. J. Coord. Chem 2009; 62: 3471–3477.

Zhao

Liu

, et al. Spectroscopy study on the photochromism of Schiff bases N,N′-bis(salicylidene)-1,2-diaminoethane and N,N′-bis(salicylidene)-1,6-hexanediamine. Spectrochim. Acta A 2001; 57: 149–154.

Roncali

. Conjugated poly(thiophenes): synthesis, functionalization, and applications. Chem. Rev 1992; 92: 711–738.

Beaujuge

Reynolds

. Color control in π-conjugated organic polymers for use in electrochromic devices. Chem. Rev 2010; 110: 268–320.

Koyuncu

. A new ITO-compatible side chain-functionalized multielectrochromic polymer for use in adaptive camouflage-like electrochromic devices. React. Funct. Polym 2018; 131: 174–180.

. Electrochemical synthesis and characterization of 1,4-benzodioxan-based electrochromic polymer and ıts application in electrochromic devices. J. Electrochem. Soc 2015; 162: A103–A112.

Yiğit

Hacıoğlu

ŞO

Güllü

, et al. Novel poly(2,5-dithienylpyrrole) derivatives functionalized with azobenzene, coumarine and fluorescein chromophore units: spectroelectrochemical properties and electrochromic device applications. New J. Chem 2015; 39: 3371–3379.

10.

Ahumada

Hamon

Roisnel

, et al. Spectroscopy, molecular structure, and electropolymerization of Ni(II) and Cu(II) complexes containing a thiophene-appended fluorinated Schiff base ligand. Dalton Trans 2023; 52: 4224–4236.

11.

Singh

Barwa

Tyagi

. Synthesis, characterization and biological studies of transition metal complexes of Schiff bases. Eur. J. Med. Chem 2006; 41: 147–153.

12.

Pamuk Algı

Cihaner

Algı

. Design, synthesis, photochromism and electrochemistry of a novel material with pendant photochromic units. Tetrahedron 2014; 70: 5064–5072.

13.

Thompson

Abboud

Reynolds

, et al. Electrochromic conjugated N-salicylidene-aniline (anil) functionalized pyrrole and 2,5-dithienylpyrrole-based polymers. New J. Chem 2005; 29: 1128–1134.

14.

Sefer

Bilgili

Gültekin

, et al. A narrow range multichromic polymer for adaptive camouflage-like electrochromic devices. Dyes Pigments 2015; 113: 121–128.

15.

Sharmoukh

Attanzio

Busatto

. 2,5-Dithienylpyrrole (DTP) as a donor component in DTP-π-A organic sensitizers: photophysical and photovoltaic properties. RSC Adv 2015; 5: 4041–4050.

16.

Guo

Miao

Wang

. 1,4-Bis(thiophen-2-yl)butane-1,4-dione. Acta Crystallogr. E 2012; 68: 689–690.

17.

Saghatforoush

Chalabian

Aminkhani

, et al. Synthesis, characterization and antibacterial activity of some new Schiff base ligands and their metal complexes. Eur. J. Med. Chem 2009; 44: 4490–4495.

18.

Warad

Khan

Azam

, et al. Design and structural studies of diimine/CdX2 (X = Cl, I) complexes based on 2,2-dimethyl-1,3-diaminopropane ligand. J. Mol. Struct 2014; 1062: 167–173.

19.

Aboaly

Khalil

MMH

. Synthesis and spectroscopic studies of Cu(II), Ni(II), Co(II) and Zn(II) complexes of salicylidene-2-aminothiophenol Schiff base. Spectrosc. Lett 2001; 34: 495–504.

20.

Omar

Mohamed

Hindy

AMM

. Transition metal complexes of heterocyclic Schiff base derived from 2-amino-3-hydroxypyridine. J. Therm. Anal. Calorim 2006; 86: 315–325.