Synthesis, crystallography, and conducting properties of a new nickel(II) complex containing 4-(diphenylamino)benzaldehyde-4-(ethyl)thiosemicarbazone

2.1 Materials

All chemicals employed in this research were purchased from BDH (British Drug Houses), Acros, and Fluka. Elementar Unicube elemental analyzer (Elementar Analysensystem GmbH, Germany) was used to perform the elemental analysis. A Johnson Matthey Mark I MSB magnetic susceptibility balance, model MKIC, was used to measure magnetic susceptibility. The molar conductivity was measured using a Jenway 4320 conductivity meter fitted with a platinumized electrode immersion cell. The UV spectrophotometer Shimadzu UV-1800 was used to record electronic absorption spectra, and the samples were prepared as 1.0 × 10^-5 M solutions in dimethyl sulfoxide (DMSO).

2.2 Synthesis of NiL₂ complex

The NiL₂ complex was synthesized by refluxing 0.6 mmol of 4-(diphenylamino)benzaldehyde-4-(ethyl)thiosemicarbazone with 0.3 mmol of nickel(II) acetate in 40 mL of hot ethanol over a period of three hours under open-air conditions. After the reaction was complete, a dark brown precipitate was obtained, which was filtered, washed thoroughly with cold ethanol, and dried in a desiccator. Single crystals of the NiL₂ complex were produced using vapor diffusion, where a solution of the crude product was allowed to evaporate in a mixture of methanol and DMF in a ratio of 2:3 for 2 weeks until well-formed crystals were obtained. The general conversion has been illustrated in Scheme 1.

Close

Scheme 1.

Synthesis of NiL₂ complex.

Export to PPT

2.3 Single X-ray crystallography

A NiL₂ single crystal was placed on the goniometer, and diffraction data were measured at 296(2) K on a Bruker APEX II DUO CCD area-detector with MoKα radiation (λ = 0.71073Å) with both φ and ω scans. A combination of all frames produced 26,225 reflections, of which 4,862 were independent (41.8% with I > 2σ(F²)). The multi-scan absorption correction was performed through SADABS 2014/5 (Krause et al., 2014). The structure was solved by direct methods in SHELXTL (Bruker, 2009), and full-matrix least-squares refinement on F² was performed in SHELXTL (Sheldrick, 2014) using anisotropic displacement parameters on non-hydrogen atoms. SADABS (Bruker, 2009) was also used to correct diffraction intensities, and PLATON (Bruker, 2009) was utilized to perform geometric calculations. In addition, SHELXP (Bruker, 2009) was used to produce molecular graphics. Hydrogen atoms were positioned, calculated, and refined isotropically using a riding model, with Uiso fixed to 1.5 times the Ueq on terminal sp³ carbons and 1.2 times the Ueq on all other carbon-bound hydrogens. Data quality and refinement statistics are as follows: R_int = 0.1066, R_σ = 0.1165, R₁ = 0.0633, wR(F²) = 0.1831, and S = 0.991, with the largest residual electron density peak and hole of 0.26 and −0.33 e Å^-3, respectively.

2.4 Preparation of the film and conductivity measurements using electrochemical impedance spectroscopy (EIS)

The solution-casting method was used to prepare polymer electrolyte films. To start with, 1 g of carboxymethylcellulose (CMC) and 0.1 mL of polyethylene glycol (PEG) were dissolved in 20 mL of distilled water. The NiL₂ dopant and isophthalic acid (IA) were added to this homogeneous solution in different proportions (0-25 wt% each) and stirred until dissolved. The mixture was subsequently poured into Petri dishes and dried in an oven at 55-60°C to produce thin films. The thickness of the dried film was measured with a micrometer at several points on each film, and the average value was used to calculate the conductivity. Then, the dried films were cut into 2 cm diameter discs and pressed between two stainless steel electrodes under spring pressure. These electrode assemblies were then attached to a HIOKI 3532-50 LCR Hi-tester to perform electrochemical impedance spectroscopy (EIS). The impedance was measured at room temperature and between 303 and 353 K, and the ionic conductivity (σ) of the NiL₂-doped films was determined by the following equation (Ramlli et al., 2022):

Where t denotes the film thickness (cm), R_b is the bulk resistance determined from the intercept of X-axis of the Nyquist plot, and A represents the contact area between the film and the electrode. The activation energy (E_a) can be obtained by fitting the temperature dependence of the ionic conductivity to the Arrhenius equation (Ramlli et al., 2022):

Where σ₀ denotes the pre-exponential factor, k is the Boltzmann constant, and T represents the absolute temperature in Kelvin.

3.1 Synthesis

In theory, the free ligand (LH) is predominantly present in the thione form, but when coordinated with Ni²⁺, deprotonation occurs, and the ligand converts to the thiolate form, which then binds to the Ni²⁺ metal center. The complexation was prepared by direct reactions between Ni²⁺ and LH in a molar ratio of 1:2, which corresponds to the chemical formula of NiL₂. The average yield was 87.48% ± 3.37% when repeated five times, and the melting point was 201.5°C. The calculated elemental composition for C₄₄H₄₂N₈NiS₂ was C 65.43%, H 5.49%, N 13.87%, and S 7.94%, whereas the experimental analysis gave C 65.93%, H 4.94%, N 13.00%, and S 7.78%. The acceptable difference between the experimental and calculated Carbon, Hydrogen, Nitrogen and Sulphur (CHNS) values (<1%), indicates that the complex formed has a ratio of 1:2 ratio (metal:ligand) and is consistent with the proposed molecular formula, C₄₄H₄₂N₈NiS₂ (NiL₂). The NiL₂ complex was found to be diamagnetic at ambient temperature with an experimental magnetic moment of ≈0.00 BM. This lack of unpaired electrons in the split d-orbitals verifies a square planar geometrical arrangement around the nickel center (Osman et al.,2021). Moreover, the molar conductivity values of 13.7 S cm² mol⁻¹ in 1x10^-3 M DMSO solutions indicate a non-electrolytic behavior comparable to previous reports (Ekennia et al., 2019).

3.2 Infrared spectra

The FT-IR spectrum of the present NiL₂ dopant has been provided in Fig. S1 and is compared with its similar free ligand (Anwar et al., 2025) to study the binding mode. The FT-IR spectrum of the free ligand shows a band in the 1322 cm^-1 region, which is assigned to the thioamide bands v(C=S) (Anwar et al., 2025). However, this band disappeared and a new band attributed to v(C-S) appeared at 639 cm^-1 in NiL₂. The peak for the functional group v(NH^a) was absent in the NiL₂ dopant due to depronotation of -SH by tautomerism (Hassan et al., 2024), while it appeared in the free ligand at 3157 cm^-1 (Anwar et al., 2025). Moreover, the v(CH=N) band shifted to a lower frequency (1578 cm^-1) upon complexation, compared to 1684 cm^-1 for the free ligand (Anwar et al., 2025). The shift to a lower frequency is due to the C=N bond becoming weaker, as the electron density around the nitrogen atom has been transferred to the Ni(II) ion (Lavanya et al., 2021). Thus, the FT-IR spectrum of NiL₂ confirmed the free ligand was coordinated through the azomethine nitrogen and the thione sulfur donor atoms. All other bands have appeared at the expected peaks, as previously reported (Farooq et al. 2024; Seifunnisha et al., 2021; Tarai et al., 2023).

3.3 Electronic spectra

The UV-Vis spectrum of the NiL₂ dopant (Fig. S2) was measured in DMSO. The UV-Vis spectrum of the NiL₂ dopant is very similar in shape to that of the similar free ligand (Anwar et al., 2025) and differs only in the values of the molar extinction coefficient (ε). In similar the free ligand, two distinct peaks are observed at 299 nm (21,700 L mol^-1cm^-1) and 369 nm (31,700 L mol^-1cm^-1), which correspond to the π→π* and n→π* transitions of the conjugated triphenylamine and the lone pairs in the nitrogen and sulfur atoms (Anwar et al., 2025). In comparison, the UV-Vis spectrum of the NiL₂ dopant also shows two peaks at 301 nm (ε = 45,300 L mol^-1cm^-1) and 395 nm (88,700 L mol^-1cm^-1) but at slightly higher absorption peak values than the free ligand, as seen in many previous reports (Zhang et al., 2024; Muthuramalingam et al., 2019).

Three bands for the ¹A_1g→¹A_2g, ¹A_1g→¹B_1g and ¹A_1g→¹E_g transitions were expected at 400-600 nm for diamagnetic and square planar of Ni(II) ions, but due to the strong inter-ligand transition, these transitions are obscured and result in a weak shoulder in this region (Zangrando et al., 2015). Thus, a new shoulder band appeared at 445 nm in the present NiL₂ doping, which can be assigned to the metal-ligand charge transfer transition (MLCT) indicating the complexation occurred (Arafath et al., 2023).

3.4 ¹H NMR

The ¹H NMR spectrum of the diamagnetic NiL₂ dopant (Fig. S3) shows similar peaks to those reported for its free ligand (Anwar et al., 2025). However, the peaks are shifted upfield, with a maximum shift of 2.07 ppm (Table S1), due to coordination with diamagnetic metal atoms (Rajeev A. et al., 2024), supporting the formation of the NiL₂ complex. Additionally, the proton signal appears for one unit of ligand (L) due to the symmetric environment coordinated to the nickel(II) centre. The NH^a signal disappears due to deprotonation. The CH=N proton is strongly deshielded because it is close to an electron-withdrawing nitrogen atom and appears as a singlet peak at δ_H = 9.77 ppm, followed by a triplet peak at δ_H = 7.71-7.73 ppm for the NH proton. The aromatic ring protons appear at δ_H = 6.81-8.01 ppm as multiple peaks. The most weakly deshielded are the -CH₂- and -CH₃ protons, which appear at δ_H = 3.21-3.14 and δ_H = 1.09-1.05 ppm, respectively.

3.5 Crystal structure of NiL₂

The solid-state molecular arrangement of the dopant was unambiguously determined by single-crystal X-ray diffraction. Comprehensive crystallographic parameters, along with refinement details, are compiled in Table S2. Whereas Table S3 provides the experimental bond lengths and angles. The dopant crystallized in the low-symmetry triclinic P-1 space group, where its asymmetric units assemble through intermolecular interactions to form a continuous three-dimensional network (Fig. 1). Each asymmetric unit consists of two molecules of the deprotonated ligand (L) and the crystallographically independent Ni(II) ion. The Ni(II) ion adopts a tetracoordinated NiN₂S₂ core with a square planar geometry that is slightly distorted, as reflected by the τ ₄ value of 0.107 (10.7°), which lies much closer to the ideal square-planar limit (τ ₄ = 0) than the tetrahedral limit (τ ₄ = 1). In the NiN₂S₂ coordination core, Ni²⁺ is coordinated to two N-atoms (N3, N3A) and S-atoms (S1, S1A) of bidentate ligands, leading to the formation of two five-membered chelate rings. The deviation of the ideal square planar geometry can be seen in the bond angles: S1–Ni1–N3 (85.84(11)°), N3–Ni1–S1A (94.16(11)°), S1A-Ni1-N3A (85.84(11)°) and N3A–Ni1–S1 (94.16(11)°), which deviated slightly from the ideal 90°, similar as previous report (Biswas et al., 2024). The bond lengths for Ni1–N3, Ni1–N3A, Ni1–S1 and Ni1–S1A were 1.898(3), 1.898(3), 2.1276(13), and 2.1276(13) Å, respectively, which are consistent with other distorted square planar geometries of Ni(II) complexes (Osman et al., 2021).

Close

Fig. 1.

ORTEP structure of NiL₂ complex showing 50% probability ellipsoids.

Export to PPT

The neighboring NiL₂ chains are connected by the C2A–H2AB···S1A and C12A–H12B···S1A, forming a 3D scaffold structure with parallel layers along the b axis (Fig. 2). A summary of all observed hydrogen bonding geometries can be seen in Table S4. In addition, the crystal packing is further stabilized by π···π stacking interactions involving Cg1···Cg3 [symmetry code: 1−x, −y, 1−z] and Cg2···Cg3 [symmetry code: 1+x, y, z] contacts, both with centroid–centroid distances of 3.710 (2) Å. Here, Cg1 corresponds to the five-membered chelate ring (Ni1–S1–C3–N2–N3), Cg2 to its symmetry-related counterpart (Ni1–S1A–C3A–N2A–N3A) and Cg3 to the aromatic ring (C5–C6–C7–C8–C9–C10). Thus, the existence of intermolecular hydrogen bonds and π···π stacking interactions acts as a driving force in supramolecular compositions and crystal stability (Fig. 3).

Close

Fig. 2.

The crystal packing shows C-H^…S interactions of the NiL₂ complex along the b axis.

Export to PPT

Close

Fig. 3.

The crystal packing shows π···π interactions of the NiL₂ complex.

Export to PPT

3.6 Conductivity analysis

The impedance measurements were performed on CMC-PEG films with different concentrations of NiL₂ dopant (system 1) and IA (system 2) at ambient temperature. The Nyquist plots for both systems have been shown in Fig. 4, respectively. Table 1 presents the ionic conductivity values for all specimens, expressed in S/cm.

Close

Fig. 4.

(a) Nyquist plots for samples with various NiL₂. (b) Nyquist plots for samples with various IA concentrations at room temperature.

Export to PPT

In system 1, the incorporation of the newly synthesized dopant, NiL₂, in concentrations ranging from 5 wt% to 25 wt% reveals a distinct trend of increasing ionic conductivity, with the highest conductivity observed at 15 wt% of the dopant. This enhancement in conductivity can be attributed to the dopant’s role in promoting electron mobility within the molecular structure, as the incorporation of the metal center becomes more effective when an electric field is applied to the film (Wu et al., 2024). The improved ion mobility within the dopant molecules contributes to the formation of more efficient ionic conduction pathways. The conductivity reaches its maximum value of 1.81 × 10⁻⁸ S cm⁻¹ at 15 wt%, representing an optimal balance where adequate electron pathways are established without excessive obstruction or molecular aggregation. Beyond 15 wt.%, however, the conductivity begins to decrease due to the excess dopant, creating a bulkiness of ions on the film, which reduces the ion mobility in the film system (Koopmans et al., 2020).

In system 2, the optimum conductivity of the film (with 15 wt% NiL₂) was further investigated by adding different concentrations of IA, as a proton donor, which protonated the synthesized NiL₂ dopant. The addition of 5 wt% to 20 wt% leads to an increase in conductivity, with the highest value of 1.58 × 10^-7 Scm^-1. The increase in ionic conductivity that was observed is mainly due to the protonation reaction between the dopant and IA, whereby the protons of IA are transferred to the nitrogen atom (N) of the dopant molecule. The result of this interaction is the creation of hydrogen bonds in the system, which subsequently allows the more effective circulation of electrons throughout the material (Teixeira et al., 2024). The introduction of IA also enhances the concentration of mobile protons in the polymer matrix, which also leads to the overall enhancement of ionic conductivity. This observation is in line with the results of other studies on the topic (Vignesh et al., 2022) that also showed that proton donors like IA can improve conductivity by increasing proton mobility and intermolecular interactions. However, when the IA concentrations were higher than 20%, the ionic conductivity started to decrease as the excess of protons in the system led to aggregation of the dopant, which hinders proton movement and ionic circulation (Luo et al., 2018), forming a thicker film with weak structural integrity.

Fig. 5 shows Nyquist plots for samples with 20 wt% IA at different temperatures. Table 2 summarizes the ionic conductivity values at different temperatures with the R_b derived from the plots in Fig. 5. The results demonstrated that the maximum ionic conductivity for this system was recorded at 4.60 × 10⁻⁷ S cm⁻¹ at a temperature of 353 K. This enhancement in conductivity with rising temperature can be attributed to the increased mobility of ions within the CMC polymer matrix, as elevated temperatures reduce the viscosity of the medium and facilitate easier ion transport. As the temperature increases, the polymer chains gain additional energy, which leads to a more dynamic movement of the segments, increasing their flexibility and decreasing the stiffness of the polymer structure (Mohapatra et al., 2015). This allows the ions to move more freely and move closer together (Park et al., 2021). In addition, higher temperatures reduce the intermolecular forces within the matrix, lowering the energy required for ion transport. This combined effect of increased chain flexibility and reduced energy barriers significantly improves the overall ionic conductivity of the system (Zhou et al., 2020).

Close

Fig. 5.

Nyquist plots for a sample containing 20 wt% IA at different temperatures.

Export to PPT

Fig. 6 shows the linear relationship between the ionic conductivity and the temperature for the sample with 20 wt% IA, which exhibits Arrhenius characteristics. From the slope, the activation energy (E_a) is 0.144 eV, calculated from Eq. 2. This implies that the conduction process in the film has a low thermal energy needed to excite charge carriers. This is indicated by the low activation energy, which suggests a high charge transport efficiency, even at comparatively low temperatures. The R² = 0.9878 shows that the linear correlation is strong, and the Arrhenius equation is a good approach to explain the temperature-dependent conductivity behavior of the synthesized material. The low thermal activation energy observed suggests that this material may be suitable for use in low-power electronic devices, where minimal energy is required to activate the charge carriers (Lococciolo et al., 2021). This makes the film suitable for various temperature environments in which it can maintain its conductivity even at lower thermal energy levels.

Close

Fig. 6.

Temperature-dependent ionic conductivity for samples containing 20 wt% IA.

Export to PPT

The Nyquist plots shown in Fig. 5 can be translated into an analog circuit model that provides valuable insight into the electrochemical properties of the film system. The equivalent circuit for 20 wt% IA at different temperatures has been shown in Fig. 7. The figure shows that R_b represents the resistance to the ion flow within the mass of the polymer electrolyte. It results from the rearrangement of ions within the free volume of the polymer matrix and indicates how the ions migrate and interact within the materials comprising the film system. The bulk capacitance (C_b) reflects the capacity of the material to retain electrical charge. This can be related to the polarization effects of the immobile polymer chains, which can affect how charges accumulate and dissipate in the film material. The non-ideal capacitive behavior of the system is considered by the constant phase element (CPE). It is the capacitive effects at the electrode-electrolyte interface and is commonly applied to represent low-frequency spikes (Muchakayala et al., 2017). This equivalent circuit has also been used in previous reports (Ramlli et al., 2022).

Close

Fig. 7.

Equivalent circuit for samples containing 20 wt% IA at various temperatures.

Export to PPT