New Bi-Nuclear Nickel(II) Complex-Based Salen Schiff Base: Synthesis, Crystal Structure, Spectroscopic, Thermal, and Electrical Investigations

Abstract

In this study, a new bi-nuclear nickel complex [Ni₂HL₂(EtOH)₂](Cl)(EtOH) of a Schiff base ligand, 2-[3-[2-hydroxybenzylideneamino]propyliminomethyl]phenol, was synthesized and characterized using UV/Vis, IR, HRMS, and TGA/DTA analysis. The molecular structure of the obtained complex was corroborated by the single crystal X-ray diffraction technique. It was found in the complex that two molecules of the ligand coordinate with two nickel atoms through azomethine-N and phenoxy-O, resulting in 6-coordinate distorted octahedral geometry, in which two ethanol molecules occupy the axial positions. The dielectric and electrical properties of the obtained samples were studied by impedance spectroscopy at different frequencies (from 1 Hz to 1 MHz) in the temperature range 298–343 K. It is found that the electrical conductivity of the Ni(II) complex is lower than that of the free ligand H₂L, suggesting that the complexation traps the charge carriers contained in the ligand.

1. Introduction

Transition metal complexes derived from tetradentate Schiff bases (ONNO) have been extensively studied in virtue of their wide range of applications [1,2]. Indeed, these compounds are known for their anti-corrosion [3], catalytic [4,5], optoelectronic [6,7], electric [8], dielectric [9], magnetic [10,11], and biological properties [12,13].

Many previous studies have found that inorganic metal complexes might exhibit conducting properties [14,15], which are nowadays the subject of a promising research area. Thanks to their useful function in molecular electronics and vital biological life processes, conducting and semiconducting materials have drawn considerable attention [16]. Such conducting metal complexes stand on the threshold of a bright and exciting future. Hence, the study of the relationship between the chemical structures and electronic characteristics of such compounds still invites further and deeper exploration, as they have a tremendous potential for broader applications that has not been exhausted by their present uses.

Indeed, it is because of this wide range of applications possessed by bi-nuclear metal complexes that they have attracted much interest [17,18]. One of the most attractive properties of bi-nuclear complexes is the interaction exchange possibility between metal centers [19,20]. For they are also known as a catalyst in asymmetric synthesis [21,22], and have a unique catalytic property in the polymerization reaction of olefins compared with mononuclear complexes [23,24]. In addition, they are useful for mimicking bimetallic bio-sites in various proteins and enzymes [25]. Among metal complexes, nickel complexes have attracted great interest in various fields of chemistry [26,27]. Despite that salen and related Schiff’s base ligands react with transition metal ions through the deprotonated forms, which act as tetradentate chelating ligand [28,29], some reported works show that the oxygen atoms of the phenoxy groups are not deprotonated [30].

Keeping in view the abovementioned features, we report in this paper the synthesis of undescribed bi-nuclear nickel(II) complex-based 2,2′-((propane-1,3-diylbis(azaneylylidene))bis(methaneylylidene))diphenol Schiff’s base. Spectroscopic studies are performed for the ligand and its corresponding nickel complex to establish their structures and crystallographic studies were used to corroborate the structure of the obtained complex. The dielectric and electrical properties of the obtained compounds were also investigated.

2. Results and Discussion

The salen base Schiff ligand H₂L was prepared by the condensation of propane-1,3-diamine with two equivalents of salicylaldehyde using a common procedure described in the literature [31]. The nickel complex [Ni₂HL₂(EtOH)₂](Cl)(EtOH) was obtained through a simple reaction of a stoichiometric amount of nickel salt (NiCl₂, 6H₂O) with the ligand H₂L in refluxing ethanol.

The schematic representation of the ligand H₂L structure (Figure 1) and its nickel complex [Ni₂HL₂(EtOH)₂](Cl)(EtOH) (Figure 2) was established on the basis of the usual spectroscopic methods, viz.: FT-IR, ¹H and ¹³C NMR, and UV/Vis, as well as mass spectrometry. In addition, the structure of the nickel complex was confirmed by single crystal X-ray diffraction.

2.1. Crystal Structure Description

The crystal structure of the title compound is built from two metal ions bound by two organic ligands and two ethanol molecules to form a binuclear nickel(II) complex, in addition to two entity solvents, namely Cl⁻ and C₂H₅OH. The plot of the asymmetric unit of the new dimeric complex, [Ni₂HL₂(EtOH)₂](Cl)(EtOH), obtained using the Ortep 3 program [32], is shown in Figure 2. The specificity of the crystal structure of this compound is the disorder at the level of the ethanol solvent molecule. Indeed, the two carbon positions of the ethanol solvent are splitting and the refinement of the occupancy rate of these sites is set to the value obtained after refinement. This refinement requires a constraint on the distances O7–C39(A,B) and C39(A,B)–C40(A,B). Moreover, there is a void in the structure containing a disordered solvent molecule, which requires the use of SQUEEZE, as implemented in the PLATON and SHELXL programs [33,34].

Furthermore, each nickel atom is surrounded by two nitrogen and two oxygen atoms belonging to one ligand, which build a slightly distorted square plane. In addition, one of the axial positions is occupied by ethanolic oxygen, while the other is filled by oxygen belonging to the second ligand. The resulting Ni coordination polyhedra are two slightly distorted Ni₂N₄O₆ edge-sharing octahedra, as shown in Figure 3. As a matter of fact, the Ni₂N₄O₆ dimers are linked by hydrogen bonds through solvent molecules, forming chains parallel to the b direction with a Ni1–Ni2 distance between two chelated Ni equal to 3.148 Å, whereas the Ni belonging to neighboring molecules are far apart from each other by ligands. In addition, among the four Schiff base moieties of the title compound, only one (O1N1C1 to C7) is nearly planar, with the largest deviation from planes being −0.032(3) Å at N1, while the other three ((O2N2C11–C17), (O4N3C20–C26), and (O5N4C30–C36)) are inclined with respect to N2 of 0.250(2) Å, N3 of −0.356(3) Å, and N4 of −0.278(3) Å, respectively. The two phenyl rings connected to the chelated Ni1 are inclined at 62.12 (18) to each other and the dihedral angle between the both phenyl cycles linked to Ni2 is 55.92 (19)°.

In the crystal structure, the molecules are linked together by O−H…Cl hydrogen bonds involving the solvent molecules, namely Cl⁻ and the free ethanol (C₂H₅OH), forming chains parallel to the b axis, as shown in Figure 3. These chains are interconnected through C–H…π interaction between C8–H8A and the phenyl ring (C30 to C35). In addition, two other intramolecular C–H…π interactions are observed in this structure, as shown in Figure 4. Moreover, the complex presents a very strong intramolecular hydrogen bond (O5−H50…O1), which forms a bridge between two vertices of the edge-sharing octahedra Ni₂N₄O₆ (see Figure 4). The set of C–H…π interactions and hydrogen bonds are summarized in Table 4.

It would be interesting to compare the structure of the present binuclear dimers to those of other Schiff based dimers. Indeed, the structure of the binuclear complex of nickel (II) with Schiff base ligands reported by You et al. [35] is very close to that described in the present work. In both structures, the Ni atoms are in two edge-sharing octahedral sites having a base formed by two imino N atoms and two phenolate O atoms belonging to the Schiff base ligands, while the axial positions are occupied by one ethanol oxygen and one phenolate oxygen, as shown in Figure 2, while in the reported work, the axial positions are filled by one phenolate oxygen and one other oxygen belonging to water or methanol molecules. In the same work, the authors report the structure of the binuclear complex of zinc (II) with Schiff base ligands in which the two zinc atoms are located in a two edge-sharing trigonal–bipyramidal coordination. Moreover, the binuclear centrosymmetric copper (II) complex with the Schiff base ([Cu(H₂L)]₂) displays a distorted square pyramidal geometry surrounding the Cu^II+ [36]. The two copper atoms constituting the dimers are located in the two pyramids sharing a baso-apical edge. The Cu-O and Cu-N distances are between 1.9393 (17) A and 2.010 (2) A for the square plane and 2.3802(17) A for the CuO in the axial position. These values are slightly higher than those observed in the monomer complex of copper (II) with the same Schiff base [37]. In this complex, the copper (II) is surrounded by four donor atoms, two oxygen and two nitrogen atoms, forming a distorted square planar geometry. The Cu-O bond lengths are in the range between 1.894(2) and 1.956(2) Å, while the Cu-N distances are between 1.879(3) and 1.997(2) Å for C₂₂H₂₀CuN₂O₂. For more information, a review article on Schiff bases was recently reported by Aragón-Muriel et al. [38] and could be of interest to the authors.

2.2. Spectroscopic Studies

The FT-IR spectra of the ligand H₂L and its corresponding nickel complex are compared to attest the coordination of the ligand. The FT-IR spectrum of the ligand H₂L (Figure S1) reveals in particular the presence of a strong absorption band assigned to the vibration of the (C=N) bond at 1629 cm⁻¹. The absorption band at 3250 cm⁻¹ is attributed to the vibration of the (O-H) bond.

On the other hand, the FT-IR spectrum of the nickel complex (Figure S2) shows in particular the presence of a remarkable wideband characteristic of the (O-H) associated bond of the ethanol molecule. Moreover, the stretching vibration of (C=N) was identified at 1622 cm⁻¹, which corroborates the presence of Schiff’s base in the complex. Further evidence of chelation of the ligand with the nickel salt was shown by the bands at a range 444–643 cm⁻¹, assigned to Ni-O and Ni-N vibration in the nickel complex. These assignments are in good agreement with the literature data [39,40].

The proposed structure for the obtained nickel complex was further corroborated using high-resolution mass spectrometry. The mass spectrum recorded in DMSO shows a peak for the protonated molecular ion [([Ni₂HL₂](Cl))-H] at m/z = 805.01499 (Figure S3).

2.3. Electronic Spectra

The absorption spectral data for H₂L and [Ni₂HL₂(EtOH)₂](Cl)(EtOH) were obtained in freshly prepared ethanol solution (Figure 5). The ligand H₂L spectrum consists of two relatively intense bands at 264 nm and 295 nm, involving the π→π* transition, as well as two other low-intensity bands at 354 nm and 440 nm, involving π→π* excitation of the C=N bond and n→π* transition, respectively. Complexation with Ni(II) results in two significant changes of the spectrum. The two intense bands at 279 nm and 298 nm, as well as the relatively low-intensity band at 384 nm shifted to a longer wavelength, witnessed the coordination through azomethine nitrogen (C=N). The band observed at 447 nm could be attributed to the ligand–metal charge transfer transitions. The electronic absorption data of [Ni₂HL₂(EtOH)₂](Cl)(EtOH) suggest an octahedral structure [41], which was further confirmed by X-ray crystal structure analysis. The electronic spectral data are listed in Table 1.

2.4. Thermogravimetric Analysis

The purity and thermal stability of the free ligand H₂L and its nickel complex [Ni₂HL₂(EtOH)₂](Cl)(EtOH) were investigated by thermal gravimetric analysis (TGA) and differential thermal analysis (DTA) in the range of 0 to 600 °C and heating rate of 10 °C per minute in an open atmosphere. The free ligand H₂L reflected acceptable stability up to 200 °C, then started to decompose in a single step up to Toff ~ 228 °C and TDTA = 300 °C. The volatilized mass represents about 46%, which corresponds to the loss of nearly 2H₂O, 2NH₃, and 5(CO or CO₂), as shown in Figure 6.

However, the complex [Ni₂HL₂(EtOH)₂](Cl)(EtOH) showed a completely different decomposition pattern with two distinct pyrolysis steps (Figure 7). The first decomposition step was recorded between 90 and 143 °C and corresponds to the loss of the free solvent Cl⁻ and ethanol molecules from the structure ([Ni₂HL₂(EtOH)₂](Cl)(EtOH)) with TDTA = 118 °C. This loss represents about 9% of the complex mass. The second stage was recorded between 441 and 470 °C (TDTA = 451 °C) for the loss of 26% of the complex mass. This perdition is attributed to the loss of the two ethanol molecules and the decomposition of the ligand, as described above.

2.5. Dielectric Studies

The frequency dependence of the permittivity and the dielectric loss at room temperature of the ligand H₂L and the [Ni₂HL₂(EtOH)₂](Cl)(EtOH) complex are illustrated in Figure 8. First, we observed that the dielectric parameters of the two samples show a plateau at high frequencies and then increase at low frequencies. The permittivity of the ligand H₂L is about 2.8, while its dissipation factor is 0.07 at high frequencies. The decrease in frequency leads to an increase in dielectric parameters until they reach 7.5 for ε_r and 0.55 for Tanδ at 1 Hz. It is well known that the dielectric parameters correspond to the polarization phenomenon in the materials. The observed increase in dielectric parameters at low frequencies can be due to space charge and defects in the material [42]. In other words, the applied electrical field helps in the jump of space charge and defects along low energy sites and their accumulation at high barrier energy sites results in an enhancement in the polarization at lower frequencies [43]. At higher frequencies, these charges cannot follow the oscillation of the electric field, leading to a decrease in ε_r. It worth noting that, for a fixed frequency, one can observe that (i) the permittivity of the nickel(II) complex is lower than that of the free ligand H₂L and (ii) the dielectric loss of the complex is higher than that of the ligand. Thus, complexation of the ligand H₂L with Ni(II) results in a decrease in the permittivity and a slight increase in the dissipation factor. The decrease in the permittivity can be explained by the fact that the complexation reduces the space charge, defects, and electron density, which comes from the free nitrogen and oxygen atoms, of the H₂L ligand, in agreement with the above structure data, which showed that nitrogen and oxygen atoms are linked to nickel atoms in the nickel(II) complex.

The temperature dependence of the dielectric parameters ε_r and Tanδ of the studied compounds at 1 KHz are presented in Figure S4. It is found that the dielectric constant ε_r and the dielectric losses Tanδ increase slightly with temperature. An increase in the intensities of ε′ and tanδ at higher temperatures is apprehended on account of a reduction in bond energies in the materials. It seems that the orientation of small dipoles (polarons) and the electronic and ionic polarizations are facilitated with an increase in temperature.

2.6. Electrical Studies

2.6.1. Dc Electrical Conductivity

Figure S5a shows the Nyquist plot of the ligand H₂L measured at different temperatures and Figure S5b shows that of its corresponding nickel(II) complex. The extracted values of the dc conductivity (σ_dc) are tabulated in Table 2. From the analysis of the Nyquist plots, it is observed that the two samples present the same temperature behavior; the electrical resistance decreases by increasing temperature, suggesting that the electrical conductivity of the materials is thermally activated. The equivalent circuit of the conductivity in these samples can be described by a simple equivalent circuit of a parallel combination of a resistance R and a constant phase element (CPE). It is found, for each temperature, that the resistance of the ligand H₂L is lower than that of the bi-nuclear nickel complex, thereby the latter is more resistive.

The electrical dc conductivity of each sample was calculated by the following Equation (1).

(1)${\mathsf{\sigma }}_{\mathrm{dc}}=\frac{\mathrm{e}}{\mathrm{S}}\frac{1}{\mathrm{R}}$

where (e) is the thickness of the sample, (S) is the area of the sample–electrode contact, and (R) is the sample resistance at zero frequency (extracted from the Nyquist plot).

The variation in lnσ_dc as a function of 1000/T for H₂L and its nickel(II) complex is illustrated in Figure S6. It is found that the dc conductivity values throughout the temperature variation of H₂L are higher than those found for [Ni₂HL₂(EtOH)₂](Cl)(EtOH). The ligand H₂L exhibits a dc conductivity of 7.05 × 10⁻¹⁰ S·cm⁻¹ at RT and 1.94 × 10⁻⁸ S·cm⁻¹ at 343 K, while its nickel(II) complex presents a dc conductivity of 2.40 × 10⁻¹¹ S·cm⁻¹ at RT and 1.57 × 10⁻¹⁰ S·cm⁻¹ at 343 K. In addition, it is clearly observed that the dc conductivity increases linearly with the temperature for the two samples, which suggests that dc conductivity is a thermally activated process and follows an Arrhenius law defined as follows [44,45]:

σ_dc = σ₀ exp(−E_a/K_BT)(2)

where E_a is the activation energy, σ₀ is the pre-exponential factor, and K_B is the Boltzmann’s constant. The activation energy values E_a were calculated using the slopes as shown in Figure S6. It is found that the ligand H₂L exhibits an activation energy of 0.81 eV, while the nickel(II) complexation has E_a equal to 0.37 eV. It appears that the complexation of the ligand reduces the activation energy of the conduction process. Therefore, the observed shrinking of the electrical conductivity in the complex is due to the reduction in the charge carriers.

2.6.2. Ac electrical Conductivity

The ac conductivity (σ_ac) of the under-study samples is calculated using the following equation:

σ_tot = (e/S)·(Z′/Z′2 + Z″2)(3)

The variation in ac conductivity (logσ_ac) as a function of the frequency for H₂L and [Ni₂HL₂(EtOH)₂](Cl)(EtOH) recorded at room temperature is shown in Figure 9. It can be seen that the alternative conductivity increases linearly with frequency. This dispersion of the conductivity is in agreement with the expectation of Jonscher’s power law [46]:

σ_ac = Aω^s(4)

where A is the pre-exponential factor, ω corresponds to 2πf, and s (varies between 0 and 1) is the power-law exponent.

The variation in this parameter with temperature can provide information about the conduction mechanism in the samples. In fact, based on the variation in the frequency exponent with temperature, many mechanisms for the conduction are proposed [43,46]:

• If the frequency exponent (s) is independent of temperature, the mechanism is the quantum mechanical tunnel model (QMT);

• If (s) decreases to a minimum and then increases with a further increase in temperature, the mechanism is the large polaron tunnel model (LPT);

• If (s) increases with increasing temperature, the mechanism corresponds to the small polaron tunnel model (SPT);

• If (s) decreases with increasing temperature, the conductivity origin is described by the correlated barrier hopping (CBH) model.

The values of the frequency exponent (s) of the studied samples at different temperatures were determined from the linear plots at high frequencies, and its variation as a function of temperature is plotted in Figure S7. From the analysis of these spectra, it is found that the frequency exponent(s) decreases with increasing temperature. Thus, the correlated barrier hopping (CBH) model is the appropriate mechanism to describe the conduction nature in the ligand H₂L and its nickel (II) complex. In this model, the charge carriers take place through the barriers separating the localized sites and the decrease in (s) values signifies a reduction in this energy barrier [47].

3. Materials and Methods

3.1. Experimental Section

All of the chemicals used in the experiment were obtained from Sigma-Aldrich, had an analytical grade, and were used as received with no further purification. The progress of the reactions was monitored by TLC and spots were visualized under UV light. The melting points were determined using a KOFLER BENCH. The infrared spectra were recorded at room temperature using a BRUKER VERTEX 70 spectrometer. The ¹H and ¹³C NMR spectra were recorded at room temperature on a BRUKER AVANCE II 300 MHz instrument. The spin multiplicities are reported as singlet (s), doublet (d), triplet (t), multiplet (m), doublet of doublets (dd), doublet of triplets (dt), and quintet (qu). The high-resolution mass spectrum was recorded on a Waters/Vion IMS-QTOF: Spectrometer, equipped with an electrospray ionization (ESI) source, operating in either positive or negative ion mode. The electronic spectra were recorded in ethanol using a UV-6300PC/VWR spectrophotometer.

3.1.1. Preparation of Ligand H₂L

A mixture of propane-1,3-diamine (1.11 g, 14.97 mmol) and salicylaldehyde (3.66 g, 29.95 mmol) was refluxed in absolute ethanol (10 mL) until the consumption of reagents. The completion of the reaction was monitored by thin-layer chromatography (TLC). Then, the solvent was evaporated and the residue was crystallized in ethanol. The yellow precipitate was isolated by filtration and washed with cold ethanol and dried in vacuo.

Yellow solid; yield: 3.97 g, 94%; m.p. = 112 °C; FT-IR: ν(O-H): 3450 cm⁻¹, ν(C=N): 1629 cm⁻¹, ν(C=C): 1620 cm⁻¹, 1579 cm⁻¹, 1495 cm⁻¹, 1457 cm⁻¹, ν(C–O): 1277 cm⁻¹; ¹H NMR (CDCl₃, 300 MHz) (δ, ppm): 2.13 (qu, 2H, CH₂, ³J = 6.6 Hz), 3.73 (t, 4H, ³J = 6.6 Hz), 6.91 (td, 2H, Ar-H, ⁴J_m = 1.2 Hz, ³J_o = 7.8 Hz), 7.00 (d, 2H, Ar-H, J = 8.4 Hz), 7.27 (dd, 2H, Ar-H, J_m = 1.8 Hz, J_o = 7.8 Hz), 7.34 (td, 2H, Ar-H, J_m = 1.5 Hz, J_o = 7.5 Hz), 8.39 (s, 2H, -CH=N), 13.47 (s, 2H, Ar–OH); ¹³C NMR (CDCl₃, 75 MHz) (δ, ppm): 31.72 (CH₂), 56.83 (CH₂-CH=N), 117.00, 118.66, 118.77 (-C_Ar-CH=N), 131.30, 132.30, 161.14 (-C_Ar-OH), 165.48 (-CH=N). UV/Vis in ethanol, λ_max nm [ε_max (L·mol⁻¹·cm⁻¹)]: 264 (32,567), 295 (12,943), 354 (4569), 440 (1344).

3.1.2. Synthesis and Crystallization of Nickel Complex [Ni₂HL₂(EtOH)₂](Cl)(EtOH)

To an ethanolic solution of the ligand H₂L (0.268 g, 0.95 mmol), a solution of NiCl₂ and 6H₂O (0.226 g, 0.95 mmol) in ethanol was added dropwise through a dropping funnel. Then, the mixture was refluxed for 4 h and left to cool to room temperature. The formed precipitate was isolated by filtration and washed with diethyl ether and cold ethanol. The obtained complex is insoluble in water, ether, chloroform, methylene chloride, and hexane. In contrast, it is soluble in methanol, ethanol, tetrahydrofuran, acetonitrile, dimethylformamide, and dimethyl sulfoxide. Therefore, it was recrystallized in ethanol and green crystals were obtained by slow evaporation of the ethanolic solution.

Green crystals; yield: 0.648 g, 80%; m.p. > 260 °C; FT-IR (neat): ν(C=N): 1610 cm⁻¹, ν(C–O): 1192 cm⁻¹, ν(Ni–O): 560 cm⁻¹, ν(Ni–N): 444 cm⁻¹; UV/Vis in ethanol, λ_max nm [ε_max (L·mol⁻¹·cm⁻¹)]: 279 (16,062), 298 (14,139), 384(2419), 447(1827); ESI-QTOF-MS (m/z): mass calculated for [([Ni₂HL₂](Cl))-H]⁺: 805.57555, found: 805.01499.

3.1.3. X-Ray Data Collection and Crystal Structure Determination

A single crystal of bi-nuclear nickel(II) complex derived from 2,2’-((propane-1,3-diylbis(azaneylylidene))bis(methaneylylidene))diphenol Schiff base was mounted on a Brucker D8 VENTURE four circles diffractometer equipped with a CCD bi-dimensional detector and INCOATEC IμS micro-focus source MoKα monochromatic radiation (λ = 0.71073 Å) [48]. Data were corrected for Lorentz and polarization effects and for absorption [49]. The structure was solved by direct methods using SHELXT [50] and refined (by weighted full-matrix least-square on F2 techniques) to convergence using the SHELXL2016 program [33]. A summary of the measurement parameters is presented in Table 3. Hydrogen atoms were localized in a Fourier map or placed geometrically and included in the final cycles of refinement with isotropic thermal parameters, yielding the final R values summarized in Table 3. The final atomic coordinates and isotropic or equivalent isotropic displacement parameters are given in Table S1. The inter-atomic bond lengths and angles are summarized in Table S2. The hydrogen bonds O–H…Cl and the C–H…π interactions are reported in Table 4. The cif file containing the atomic positions, the anisotropic displacement parameters, the interatomic bonds and angles (supplement materials), and the measured and calculated intensities is deposited in the crystallographic Data Centre under the number CCDC 1904847. These data can be obtained free of charge via the Cambridge Crystallographic Data Centre, https://www.ccdc.cam.ac.uk (accessed on 30 August 2022), or e-mail: [email protected].

3.1.4. Electrical Measurements

The electrical measurements of the ligand H₂L and its corresponding nickel complex [Ni₂HL₂(EtOH)₂](Cl)(EtOH) were studied by impedance spectroscopy using a Modulab MTS data acquisition system equipped with a Linkam temperature control system. The powders of H₂L and [Ni₂HL₂(EtOH)₂](Cl)(EtOH) were pressed to form pellets of 1 mm in thickness and 10 mm in diameter. Then, the pellets were electroded with silver lacquer and placed in the Linkam-type sample holder. The conductance and capacitance measurements were collected at different frequencies (1 Hz–1 MHz) in the temperature range from room temperature (RT) to 343 K. From the collected data, the dielectric parameters (permittivity (εr) and dielectric loss (Tan (δ))) and the electrical parameters (dc and ac conductivity and activation energy (Ea)) were determined.

4. Conclusions

In summary, we prepared a new bi-nuclear nickel complex based on 2,2′-((propane-1,3-diylbis(azaneylylidene))bis(methaneylylidene))diphenol Schiff base. The structures were evidenced without doubt through spectroscopic and single crystal X-ray diffraction analysis, which showed that the chelation occurs through the deprotonated phenolic oxygen and azomethine nitrogen atoms of the ligand. The coordination behavior shown for the H2L ligand, in this case, is totally different from that shown in previously published work. Hirshfeld surface analysis was performed to explore the intermolecular interactions and packing patterns in the crystal structure. The electrical conductivity of the H₂L ligand is diminished with nickel(II) coordination. The complex [Ni₂HL₂(EtOH)₂](Cl)(EtOH) exhibits a dc conductivity of 2.40 × 10⁻¹¹ S·cm⁻¹ at room temperature, which indicates the electrical insulating nature of this complex. The frequency dependence of the conductivity showed that the conductivity of both compounds follows Jonscher’s universal power law. The variation in the frequency exponent (s) with temperature demonstrates that the conduction nature in the samples is described by the correlated barrier hopping (CBH) mechanism.

Author Contributions

Experiments; writing—original draft preparation, B.E.-S.; crystallographic studies, M.S., L.E.A., L.B. (Linda Bejaoui) and S.K.; electrical studies, E.M.H. and L.B. (Lahcen Bih); spectroscopic analysis, M.E.Y. and A.N.; writing—review and editing, M.B., L.B. (Linda Bejaoui) and S.K; methodology; supervision, M.F. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

Not applicable.

Acknowledgments

The authors thank the Faculty of Science, Mohammed V University in Rabat, Morocco for the X-ray measurements; City of Innovation, Sidi Mohamed Ben Abdallah University of Fez, Morocco for spectroscopic accomplishments.

Conflicts of Interest

The authors declare no conflict of interest.

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Figures and Tables

Figure 1. Structure of the ligand H2L.

Figure 2. Plot of the [Ni2HL2(EtOH)2](Cl)(EtOH) molecule with the atom-labelling scheme. Displacement ellipsoids are drawn at the 50% probability level. H atoms are represented as small circles.

Figure 3. The Ni2N4O6 dimers linked by hydrogen bonds through solvent molecules.

Figure 4. Crystal packing for the [MHL2(EtOH)2].(Cl).(EtOH) showing molecules linked by O–H…Cl hydrogen bonds as dashed blue lines and C–H…π interaction as green lines.

Figure 5. UV/Vis spectra of ligand H2L and its nickel complex.

Figure 6. TGA and DTA of the free ligand H2L.

Figure 7. TGA and DTA of the [Ni2HL2(EtOH)2](Cl)(EtOH) complex.

Figure 8. Frequency dependence of the permittivity (εr) and the dielectric loss tan (δ) of H2L and [Ni2HL2(EtOH)2](Cl)(EtOH) at room temperature.

Figure 9. Frequency dependence of the conductivity (log σac) of H2L and [Ni2HL2(EtOH)2](Cl)(EtOH) at room temperature.

Table 1

UV/Vis spectral data for H₂L and [Ni₂HL₂(EtOH)₂](Cl)(EtOH).

Entry	Wave Lengths (nm)	Assignment
H2L	264, 295	π→π* (phenolic chromophore)
	354	n→π*
	440	π→π* (C=N)
[Ni2HL2(EtOH)2](Cl)(EtOH)	279, 298	π→π*
	384	n→π*
	447	d→ π* charge transfer (CT)

Table 2

Room temperature values of the dielectric and electrical parameters for H₂L and the [Ni₂HL₂(EtOH)₂](Cl)(EtOH) complex at 1 KHz.

Entry	εr	Tan δ	σdc × 10−10(S·cm−1)	σac × 10−8(S·cm−1)	Ea (eV)
H2L	2.8	0.07	7.05	1.44	0.81
[Ni2HL2(EtOH)2](Cl)(EtOH)	0.9	0.09	0.24	0.33	0.37

Table 3

Crystal data, data collection, and structure refinement details for the [Ni₂HL₂(EtOH)₂](Cl)(EtOH) complex.

Crystal data	Chemical formula	C38H45N4Ni2O6·C2H6O·Cl
	M r	852.71
	Crystal system, space group	Orthorhombic, Pca21
	Temperature (K)	296
	a, b, c (Å)	17.5795 (12), 11.8562 (9), 20.2065 (13)
	V (Å3)	4211.6 (5)
	Z	4
	Radiation type	Mo Kα
	μ (mm−1)	1.01
	Crystal size (mm)	0.36 × 0.25 × 0.18
Data collection	Diffractometer	Bruker D8 VENTURE Super DUO [48]
	Absorption correction	Multi-scan [49]
	Tmin, Tmax	0.638, 0.746
	No. of measured, independent, and observed [I > 2σ(I)] reflections	108,380, 10,452, 8643
	R int	0.047
	(sin θ/λ)max (Å−1)	0.667
Refinement	R[F2 > 2σ(F2)], wR(F2), S	0.029, 0.069, 1.01
	No. of reflections	10,452
	No. of parameters	517
	No. of restraints	59
	H-atom treatment	H atoms treated by a mixture of independent and constrained refinement
	Δ⟩max, Δ⟩min (e Å−3)	0.22, −0.22
	Absolute structure	Flack x determined using 3701 quotients [(I+) − (I−)]/[(I+) + (I−)] [51]
	Absolute structure parameter	−0.002 (4)

Table 4

Hydrogen-bond geometry (Å, °) for the [Ni₂HL₂(EtOH)₂](Cl)(EtOH) complex.

D–H…A	D−H	H…A	D…A	D−H…A
O5–H50…O1	0.82	1.66	2.401 (3)	148
O7–H7A…Cl1	0.82	2.28	3.060 (7)	160
O3–H3O…Cl1	0.86 (4)	2.24 (5)	3.082 (3)	165 (4)
O6−H6O…Cl1 i	0.86 (5)	2.25 (5)	3.102 (3)	171 (5)
C8−H8A…Cg11 ii	0.97	2.78	3.722 (4)	163
C10−H10A…Cg10	0.97	2.94	3.825 (4)	152
C19−H19A…Cg9	0.96	2.74	3.616 (5)	152

Symmetry code: (i) x, y + 1, z; (ii) −1/2 + x, 1 − y, z. Cg9: centroid of (C11–C16); Cg10: centroid of (C20–C25); Cg11: centroid of (C30–C35).

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/chemistry4040080/s1, Figure S1: FT-IR spectrum of the ligand H₂L; Figure S2: FT-IR spectrum of the [Ni₂HL₂(EtOH)₂](Cl)(EtOH) complex; Figure S3: Mass spectrum of the [Ni₂HL₂(EtOH)₂](Cl)(EtOH) complex; Figure S4: Temperature dependence of the permittivity (ε_r) and the dielectric loss tan (δ) of H₂L and [Ni₂HL₂(EtOH)₂](Cl)(EtOH) at 1KHz; Figure S5: Cole-Cole diagrams of (a) H₂L and (b) [Ni₂HL₂(EtOH)₂](Cl)(EtOH) at different temperatures; Figure S6: ln (σ_dc) versus 10³/T plots of H₂L and [Ni₂HL₂(EtOH)₂](Cl)(EtOH); Figure S7: Temperature dependence of the exponent (s) of H₂L and [Ni₂HL₂(EtOH)₂](Cl)(EtOH); Table S1: Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų) of [Ni₂HL₂(EtOH)₂](Cl)(EtOH) complex; Table S2: Atomic displacement parameters (Ų) for [Ni₂HL₂(EtOH)₂](Cl)(EtOH) complex; Table S3: Selected bond distances (Å) and angles (°) for the [Ni₂HL₂(EtOH)₂](Cl)(EtOH) complex.

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