Impedance Spectroscopy as a Powerful Tool for Researching Molybdenum-Based Materials with Schiff Base Hydrazones

Molybdenum coordination complexes are widely applied due to their biological and pharmacological potential, as well as their performance in different catalytic processes. Parent dioxidomolybdenum Schiff base complexes were prepared via the reaction of [MoO2(acac)2] with a hydrazone Schiff-base tetradentate ligand. A new hydrazone-Schiff base (H2L1 and 2) and its corresponding mononuclear and polynuclear dioxidomolybdenum(VI) complex were synthesized and characterized by spectroscopic methods and elemental analyses, and their thermal behavior was investigated by thermogravimetry. The crystal and molecular structures of H2L2 ligands and the complexes [MoO2(L1)(H2O)], [MoO2(L2)(H2O)], [MoO2(L1)(MeOH)]∙MeOH, [MoO2(L1)(EtOH)]∙EtOH, [MoO2(L1)(2-PrOH)]∙2-PrOH, and [MoO2(L1)]n were determined by single-crystal X-ray diffraction. Using the in situ impedance spectroscopy method (IS), the structural transformations of chosen complexes were followed, and their electrical properties were examined in a wide range of temperatures and frequencies.


Introduction
The principal interest in the chemistry of molybdenum(VI) complexes with Schiff-base ligands arises from their biological and pharmacological importance [1,2]. To a greater extent, molybdenum compounds have been widely applied and researched in different catalytic processes, such as sulfoxidation [3,4], olefin epoxidations [5][6][7], alcohol oxidations [8], etc. The capability of molybdenum to adapt and easily convert between different oxidation states signals further applications. For instance, it has been shown that transition metal complexes with Schiff bases have potential in electrochemical or molecular materials with nonlinear optical properties [9]. Furthermore, vanadium complexes with quadridentate Schiff bases demonstrate semiconducting properties [10]. Schiff-base N,Nbis-(furaldehyde)-1,2-phenylenediamine-dipicolyl metallic complexes of Cu(II), Co(II), and Hg(II), and the N,N -bis-(furaldehyde)-1,2-ethylenediamine-dipicolyl Cu(II) complex, incorporated into a plasticized PVC membrane, were used to prepare highly selective thiocyanate electrodes. The mechanism for selectivity towards SCN − was supported by AC impedance measurements [11]. Dielectric and conductivity properties of Co(II), Cu(II), Ni(II), and Cd(II) complexes of azines derived from benzophenonehydrozone with different aldehydes were reported [12], as well as the conducting performance of Co(II), Ni(II), and Cu(II) derived from imidazole-2-carboxaldehyde and glycylglycine [13]. Furthermore, Joseyphus et al. reported on the dielectric properties and conductivity studies of tetradentate Co(II), Cu(II), and Ni(II) Schiff-base complexes [14]. Based on the above studies, the research aims of the present study were to prepare a new class of molybdenum materials containing Schiff-base ligands and to study them further by impedance spectroscopy. To the best of our knowledge, the electric and conducting properties of these kinds of molybdenum materials have lybdenum materials containing Schiff-base ligands and to study them furt ance spectroscopy. To the best of our knowledge, the electric and conduct of these kinds of molybdenum materials have not been reported so far, mak tigation novel and promising. The Schiff-base ligands used for the prepara lybdenum complexes are presented in Scheme 1. The ligands were chosen previous studies with similar types of ligands in order to correlate the sub aldehyde ring and its potential influence on the type of coordination com thermore, the Cambridge Structural Database (CSD 5.43, with updates up 2022) revealed, besides the structure of H2L 1 ligand (CSD, ref ADEWOW) examples of coordination compounds with the ligands used in this study: metal center (CSD, ref BOTPIK) [17], and H2L 2 with a Zn metal center, (CSD, [18]. In order to enrich the chemistry of molybdenum, the present study molybdenum mononuclear and polynuclear complexes with the above-me of ligands.

Preparation of the Ligands
The H2L 1,2 ligands were obtained by a condensation reaction of 5-nitro (0.4315 g, 2.836 mmol) and 2-hydroxy or 4-hydroxybenzhydrazide (0.4739 g in a molar ratio of 1:1, in ethanol (30 mL). Suitable crystals of the H2L 2 lig tained, and the molecular and crystal structures were determined.

Preparation of the Ligands
The H 2 L 1,2 ligands were obtained by a condensation reaction of 5-nitrobenzaldehyde (0.4315 g, 2.836 mmol) and 2-hydroxy or 4-hydroxybenzhydrazide (0.4739 g, 2.836 mmol), in a molar ratio of 1:1, in ethanol (30 mL). Suitable crystals of the H 2 L 2 ligand were obtained, and the molecular and crystal structures were determined. The complexes were heated to 290 • C and left at the corresponding temperature for 1 h. After they had cooled to room temperature, further analyses were performed.

Impedance Spectroscopy Measurements
The electrical and dielectric properties of the compounds [MoO 2 (L 1 )( ], and [MoO 2 (L 1 )] n were studied via impedance spectroscopy (IS). Complex impedance was measured over a wide range of frequencies (0.01 Hz to 1 MHz) and temperatures (30-210 • C, step 30 • C) using an impedance analyzer (Novocontrol Alpha-AN Dielectric Spectrometer, Novocontrol Technologies GmbH & Co. KG, Hundsangen, Germany). The temperature was controlled to ±0.2 • C. The measurements were performed on polycrystalline powder samples pressed into cylindrical pellets with a diameter of 5 mm diameter and a thickness of 1 mm under a uniform load of 2 10 3 kg using a hydraulic press. For the electrical contact, gold electrodes with a diameter of 3.8 mm were sputtered onto both sides of the pellets using an SC7620 sputter coater from Quorum Technologies (Laughton, UK). The experimental data were analyzed by electrical equivalent circuit (EEC) modeling using the complex nonlinear least-squares (CNLLSQ) fitting procedure and WinFIT software (version 3.2, Novocontrol Technologies GmbH & Co. KG, Hundsangen, Germany).

Physical Methods
Elemental analyses were conducted by the Analytical Services Laboratory of the Ruder Bošković Institute, Zagreb. Thermogravimetric (TG) analyses were performed using a Mettler TGA/DSC3+ thermobalance in aluminum crucibles. All experiments were performed in an oxygen atmosphere with a flow rate of 200 cm 3 min −1 and with heating rates of 10 K min −1 . Differential scanning calorimetry (DSC) measurements were carried out using a Mettler-Toledo DSC823e calorimeter and analyzed with Mettler STARe 9.01 software. IR-ATR spectra were recorded on a Perkin Elmer Spectrum Two spectrometer in the spectral range between 4500-450 cm −1 .
Single-crystal X-ray diffraction: Single crystals of H 2 L 2 , [MoO 2 (L 1 )( ]·EtOH, and [MoO 2 (L 1 )(2-PrOH)]·(2-PrOH) of suitable quality were selected for the diffraction experiments. Data were collected using a Rigaku XtaLAB Synergy-S diffractometer armed with a Dualflex source (Cu Kα radiation, λ = 1.54184 Å) and a HyPix detector. Data were collected via ω-scans at 170 K. Data were processed with the CrysAlis program package [21]. A summary of the general crystal data, along with the data collection and structure refinement parameters, is presented in Table S1 (Supplementary Materials). The structures were solved via dual-space methods using SHELXT [22]. The refinement procedure was accomplished via full-matrix least-squares methods based on F 2 values against all reflections, including the anisotropic displacement parameters for all non-H atoms. Hydrogen atoms bound to carbon atoms were placed in geometrically idealized positions and refined by using the riding model, with U iso = 1.2 U eq of the connected carbon atom, or as ideal CH 3 groups, with U iso = 1.5 U eq . Hydrogen atoms attached to heteroatoms were located in the different Fourier maps in the final stages of the refinement procedure. Their coordinates were refined freely but with restrained N−H distances of 0.86 (2) and O-H distances of 0.84 (2) A. All refinements were conducted using SHELXL [23]. The SHELX programs were operated within the Olex2 suite [24]. Geometrical calculations were performed and molecular graphics were produced using Mercury [25]. The structure for H 2 L 2 was solved as a two-component non-merohedral twin (for details, see the Supplementary Materials), with the two twin domains being present in a ratio of 0.436:0.564. The data for [MoO 2 (L 2 )(H 2 O)] were treated for twinning. The twin law was found to be (−1, 0, 0, 0, −1, 0, 0, 0, 1), while the two twin components were present in the 0.212:0.788 ratio.

Preparation and Characterization of Molybdenum Complexes
Hydrazone ligands were prepared via the reaction of 2-hydroxy-5-nitrobenzaldehyde with 2-hydroxybenzhydrazide or 4-hydroxybenzhydrazide, 1:1 ratio, in methanol. High yields were obtained: 71% for H 2 L 1 and 93% for H 2 L 2 . DSC analysis gave insight into the ligands' melting points and purity (Figures S1 and S2). For the H 2 L 1 ligand, an endothermic peak at 302 • C was attributed to the melting process and was followed by exothermic decomposition. For the H 2 L 2 ligand, the melting onset occurred at 304 • C. The IR spectra for both ligands are similar; characteristic C=O absorption bands are present at 1651 cm −1 for H 2 L 1 and 2 , while bands at 1601 cm −1 H 2 L 1 and 1588 cm −1 H 2 L 2 are characteristic for C=N. C-O phenyl bands are present at 1275 cm −1 for H 2 L 1 and at 1238 cm −1 for H 2 L 2 .
Further reactions of the prepared ligands with the molybdenum starting compound [MoO 2 (C 5 H 7 O 2 ) 2 ] in different solvents provided dioxidomolybdenum(VI) coordination complexes. When H 2 L 1 reacted with [MoO 2 (C 5 H 7 O 2 ) 2 ] in methanol, ethanol, or 2-propanol, the mononuclear complex of the general formula [MoO 2 (L 1 )(alcohol)]·alcohol was obtained. While standing at room temperature, the solvent molecule spontaneously left the crystal structure for the complexes obtained from methanol and ethanol, and the complexes were analyzed as were the [MoO 2 (L 1 )(alcohol)] complexes ( Figures S3 and S4). The complex obtained from 2-propanol remained as the crystallized solvent molecule, even after a longer period, which can be clearly seen in the thermogram ( Figure S5 The samples were exposed to the solvent vapors for 24 h, after which IR spectra of the samples were recorded and compared to the initial sample ( Figure S7, up and down). Additionally, the existence of the corresponding compound was confirmed by TGA as well. As expected, the complexes obtained from the H 2 L 1 ligand could be easily transformed into other mononuclear analogs, while the complexes obtained from H 2 L 2 were more rigid (Table 1), which also correlates with the results obtained from their solution-based synthesis. An unusual feature was noticed in the complex [MoO 2 (L 1 )(MeOH)] as exposed to EtOH vapors since [MoO 2 (L 1 )(H 2 O)] was formed, which was not in accordance with the results obtained from the solvent-based synthesis.

Mo Complex MeOH Vapors EtOH Vapors 2-PrOH Vapors
In all of the obtained complexes, the ligands were coordinated to the MoO 2 2+ core as a doubly deprotonated one, through ONO atoms, which was indicated by the IR spectra as the band at~1340 cm −1 , assigned to the C-O group of the hydrazone moiety, and the bands at~1600 cm −1 and 1250 cm −1 , belonging to C=N imine and C-O phenyl groups. Furthermore, in all of the mononuclear complexes, {MoO 2 } 2+ core vibrations appeared around~920 and 900 cm −1 . In the spectra of [MoO 2 (L 1or 2 )(MeOH)], a new band appeared at~1017 cm −1 due to the C-O MeOH vibrations in the spectra of the [MoO 2 (L 1 )(EtOH)] band at 1042 cm −1 , which is characteristic for C-O EtOH vibrations, and in the spectra of [MoO 2 (L 1 )(PrOH)]·2-PrOH band at 943 cm −1 , which is characteristic for C-O PrOH vibrations. In the polynuclear complexes [MoO2(L 1 )] n , polymerization occurred through the terminal O atom, assigned to the bands at 829 and 810 cm −1 in the spectra [26,27].

Description of Molecular and Crystal Structures
The molecular and crystal structures of the H 2 L 2 ligand and the complexes [ ·2-PrOH, as described in this work, were elucidated via singlecrystal X-ray diffraction (SC-XRD) experiments. Basic crystallographic data and refinement parameters are given in the Supplementary Materials in Table S1. A detailed description of the molecular and crystal structure of H 2 L 2 is given in the Supplementary Materials ( Figures S8 and S9 and Table S2). The suitable single crystals of the H 2 L 2 ligand and the com- In all of the examined complexes, the ligand (H 2 L 1 or H 2 L 2 ) is coordinated in its doubly deprotonated form of the imino tautomer [28,29], thus affording a tridentate ONO coordination mode ( Figure S10, Table S4). Coordination geometry around molybdenum atoms in all of the complexes can be described as a distorted octahedral one, being fulfilled by the cis-arranged oxo ligands, and, except for [MoO2(L 1 )] n (Figure 1a), an ancillary water or alcohol molecule. In [MoO2(L 1 )] n , the octahedral environment of each Mo atom is realized through the Mo=O···Mo interaction with the neighboring [MoO2(L 1 )] unit ( Figure 1b). This is consistent with the tendency of cis-dioxomolybdenum(VI) complexes to form polynuclear assemblies in the absence of a suitable donor for the sixth coordination site [30]. The longest distances within the Mo coordination sphere are those positioned trans to the oxo ligands, i.e., the Mo1-N2 and Mo1-O6 bonds (Mo1A-N2A, Mo1A-O6A, and Mo1B-N2B; Mo1B-O6B in [MoO 2 (L 1 )(2-PrOH)]·(2-PrOH). The main geometrical features of the Mo(VI) complexes investigated here are comparable with those of Mo(VI) complexes with similar ligands reported in the literature [6][7][8]31,32]. The largest differences in the molecular geometries between the complexes can be observed in the relative position of the phenyl rings of the aldehyde and hydrazone residues ( Figure 1b and Table S5, Figure 2). It is also interesting to observe that, in all of the inspected complexes, the Mo atom is shifted from the plane defined by the coordinating O1, N2, O4, and O7 atoms toward the oxo O6 atom (Table S5).   (Table S6 and Figure S13). In the structures of Mo(VI) complexes with alcohols as ancillary ligands, which crystalize as solvates, the presence of additional solvent molecules leads to an exceedingly rich pattern of intermolecular interactions (Table S6 and Figures S14 and S15).

Impedance Spectroscopy Investigation of the Chosen Molybdenum Complexes
For the impedance spectroscopy study, we selected four samples prepared from methanol and acetonitrile. The complexes prepared from methanol provided mononu-    (Table S6 and Figure S13). In the structures of Mo(VI) complexes with alcohols as ancillary ligands, which crystalize as solvates, the presence of additional solvent molecules leads to an exceedingly rich pattern of intermolecular interactions (Table S6 and Figures S14 and S15).

Impedance Spectroscopy Investigation of the Chosen Molybdenum Complexes
For the impedance spectroscopy study, we selected four samples prepared from methanol and acetonitrile. The complexes prepared from methanol provided mononu-   (Table S6 and Figures S11 and S12). Crystal packing in the [MoO2(L 1 )] n relies exclusively on C-H···O interactions since the only good hydrogen-bond donor, O5-H5 is functionally engaged in the intramolecular hydrogen bonding (Table S6 and Figure S13). In the structures of Mo(VI) complexes with alcohols as ancillary ligands, which crystalize as solvates, the presence of additional solvent molecules leads to an exceedingly rich pattern of intermolecular interactions (Table S6 and Figures S14 and S15).

Impedance Spectroscopy Investigation of the Chosen Molybdenum Complexes
For the impedance spectroscopy study, we selected four samples prepared from methanol and acetonitrile. The complexes prepared from methanol provided mononuclear compounds  The frequency dependence of the real part of the electrical conductivity, σ , or the conductivity spectra at different temperatures for the sample [MoO 2 (L 1 )] n , is presented in Figure 3a. It is evident that the electrical conductivity, σ , increases with increasing temperature. As for all other samples, the conductivity is thermally activated and shows semiconducting behavior. Moreover, at high temperatures and low frequencies, the conductivity shows no frequency-dependence (constant value), with the so-called plateau corresponding to the DC conductivity (see Figure 3a). In the high-frequency region, the conductivity depends on the frequency and is related to the AC conductivity (dispersion part). Moving from DC to the dispersion region, the crossover shifts to higher frequencies as the temperature increases. Similar behavior in the conductivity spectra can be observed via IS in a wide range of semiconductive materials, amorphous and/or crystalline [33][34][35][36][37][38][39][40][41][42]. molecule left in the crystal structure above this temperature and the obtained compounds are stable up to ~300 °C. The frequency dependence of the real part of the electrical conductivity, σ′, or the conductivity spectra at different temperatures for the sample [MoO2(L 1 )]n, is presented in Figure 3a. It is evident that the electrical conductivity, σ′, increases with increasing temperature. As for all other samples, the conductivity is thermally activated and shows semiconducting behavior. Moreover, at high temperatures and low frequencies, the conductivity shows no frequency-dependence (constant value), with the so-called plateau corresponding to the DC conductivity (see Figure 3a). In the high-frequency region, the conductivity depends on the frequency and is related to the AC conductivity (dispersion part). Moving from DC to the dispersion region, the crossover shifts to higher frequencies as the temperature increases. Similar behavior in the conductivity spectra can be observed via IS in a wide range of semiconductive materials, amorphous and/or crystalline [33][34][35][36][37][38][39][40][41][42]. Due to the low conductivity of the samples studied, it is obvious that, at lower temperatures, it is not possible to determine the DC value from the graph visually. In this case, the DC conductivity value was obtained by analyzing the experimental complex impedance spectra via EEC modeling using the CNLL S method. (See Figure 3b and the Materials and Methods section for details. The experimental complex impedance spectrum of [MoO2(L 1 )]n was plotted in the complex plane, i.e., with the imaginary part as a function of the real part, Z″ vs. Z′ (a so-called Nyquist diagram). The impedance spectrum with the shape of a semicircle was analyzed by the EEC model, and a good overlap was observed, indicating a good agreement between the experimental data (red curve) and the theoretical model. The EEC model used is based on the R-CPE parallel circuit, where R denotes the resistor corresponding to the sample resistance, and the constant phase element, CPE, determines the sample capacitance. From the intersection with the X-axis, resistance is calculated @ each temperature, and for the [MoO2(L 1 )]n compound @ 200 °C, R is equal to 1.06  10 9 Ω. DC conductivity was calculated to be 2.34  10 −10 (Ωcm) −1 @ 200 °C. Considering that the sample [MoO2(L 1 )]n is a polynuclear compound, the in situ IS method confirmed that there was no change during the heating/cooling run, which is consistent with the TG analysis and the observed one-step decomposition.
For the studied compound [MoO2(L 1 )]n (see Figure 3a,b), the DC conductivity exhibited an Arrhenius temperature dependence and hence had characteristic activation energy (Figure 3c). The activation energy for DC conductivity, EDC, was determined from the slope of log(σDC) vs. 1000/T using the following equation: σDC = σ * 0 e [−EDC/kBT] , where σDC is the DC conductivity, σ * 0 is the pre-exponential factor, EDC is the activation energy, kB is the Due to the low conductivity of the samples studied, it is obvious that, at lower temperatures, it is not possible to determine the DC value from the graph visually. In this case, the DC conductivity value was obtained by analyzing the experimental complex impedance spectra via EEC modeling using the CNLL S method. (See Figure 3b and the Section 2 for details. The experimental complex impedance spectrum of [MoO 2 (L 1 )] n was plotted in the complex plane, i.e., with the imaginary part as a function of the real part, Z" vs. Z (a so-called Nyquist diagram). The impedance spectrum with the shape of a semicircle was analyzed by the EEC model, and a good overlap was observed, indicating a good agreement between the experimental data (red curve) and the theoretical model. The EEC model used is based on the R-CPE parallel circuit, where R denotes the resistor corresponding to the sample resistance, and the constant phase element, CPE, determines the sample capacitance. From the intersection with the X-axis, resistance is calculated @ each temperature, and for the [MoO 2 (L 1 )] n compound @ 200 • C, R is equal to 1.06 × 10 9 Ω. DC conductivity was calculated to be 2.34 × 10 −10 (Ωcm) −1 @ 200 • C. Considering that the sample [MoO 2 (L 1 )] n is a polynuclear compound, the in situ IS method confirmed that there was no change during the heating/cooling run, which is consistent with the TG analysis and the observed one-step decomposition.
For the studied compound [MoO 2 (L 1 )] n (see Figure 3a,b), the DC conductivity exhibited an Arrhenius temperature dependence and hence had characteristic activation energy (Figure 3c). The activation energy for DC conductivity, E DC , was determined from the slope of log(σ DC ) vs. 1000/T using the following equation: where σ DC is the DC conductivity, σ * 0 is the pre-exponential factor, E DC is the activation energy, k B is the Boltzmann constant, and T is the temperature (K). It can be seen that there are insignificant differences between the heating and the cooling runs, which is another indicator that, in the specified temperature range, there was no change and/or possible transformation of the compound [MoO 2 (L 1 )] n . This is to be expected since this compound is stable and does not contain molecules of coordinated or crystalline solvents that would be lost during heating. The low activation energy (67.2 kJ mol −1 ) of this type of compound indicates that the polynuclear complex, in which the polymerization was achieved through the terminal oxygen atom, enabled a significant and uninterrupted transfer of charge carriers through this structure.
On the other hand, for samples [MoO 2 (L 1 )(MeOH)], [MoO 2 (L 2 )(MeOH)], and [MoO 2 (L 2 )(H 2 O)], there was a certain change. Figure 4 shows the conductivity spectra during the heating and cooling cycles for the sample [MoO 2 (L 1 )(MeOH)], the thermogram for which shows a twostep decomposition, as discussed above. As previously stated, the first step corresponded to the release of the coordinated methanol molecule which, according to the TG curve, took place in the range of 100-126 • C.
Boltzmann constant, and T is the temperature (K). It can be seen that there are insignificant differences between the heating and the cooling runs, which is another indicator that, in the specified temperature range, there was no change and/or possible transformation of the compound [MoO2(L 1 )]n. This is to be expected since this compound is stable and does not contain molecules of coordinated or crystalline solvents that would be lost during heating. The low activation energy (67.2 kJ mol −1 ) of this type of compound indicates that the polynuclear complex, in which the polymerization was achieved through the terminal oxygen atom, enabled a significant and uninterrupted transfer of charge carriers through this structure.
On the other hand, for samples [MoO2(L 1 )(MeOH)], [MoO2(L 2 )(MeOH)], and [MoO2(L 2 )(H2O)], there was a certain change. Figure 4 shows the conductivity spectra during the heating and cooling cycles for the sample [MoO2(L 1 )(MeOH)], the thermogram for which shows a two-step decomposition, as discussed above. As previously stated, the first step corresponded to the release of the coordinated methanol molecule which, according to the TG curve, took place in the range of 100-126 °C. Structural change associated with the release of a coordinated methanol molecule can be seen in situ in the conductivity spectra, as shown in Figure 4. During the heating run, a non-monotonic change with the increase in temperature was visible, i.e., a sudden jump in conductivity above 120 °C, with emphasis on the temperature range between 120-180 °C (see Figure 4a), corresponding to the first stage of decay visible in the TG curve. In contrast, during the cooling run, a monotonic change of conductivity with temperature was found (see Figure 4b). It can be assumed that the compound of the mononuclear complex polymerized after heating to 230 °C as it lost solvent (methanol), which was also confirmed by IR spectroscopy. The compounds [MoO2(L 2 )(MeOH)] and [MoO2(L 2 )(H2O)] showed similar behavior along with non-monotonic changes in conductivity with increase in temperature in the heating cycle, which could be related to structural change/transformation (see Figure S17). These three mononuclear compounds, in contrast to [MoO2(L 1 )]n, show a linear temperature dependence (1000/T) of DC conductivity only in the cooling run. An example of a [MoO2(L 1 )(MeOH)] compound in a heating/cooling cycle is shown in Figure 4c, from which the activation energy was determined for the cooling cycle. The calculated values are listed in Table 2. Structural change associated with the release of a coordinated methanol molecule can be seen in situ in the conductivity spectra, as shown in Figure 4. During the heating run, a non-monotonic change with the increase in temperature was visible, i.e., a sudden jump in conductivity above 120 • C, with emphasis on the temperature range between 120-180 • C (see Figure 4a), corresponding to the first stage of decay visible in the TG curve. In contrast, during the cooling run, a monotonic change of conductivity with temperature was found (see Figure 4b). It can be assumed that the compound of the mononuclear complex polymerized after heating to 230 • C as it lost solvent (methanol), which was also confirmed by IR spectroscopy. The compounds [MoO 2 (L 2 )(MeOH)] and [MoO 2 (L 2 )(H 2 O)] showed similar behavior along with non-monotonic changes in conductivity with increase in temperature in the heating cycle, which could be related to structural change/transformation (see Figure S17). These three mononuclear compounds, in contrast to [MoO 2 (L 1 )] n , show a linear temperature dependence (1000/T) of DC conductivity only in the cooling run. An example of a [MoO 2 (L 1 )(MeOH)] compound in a heating/cooling cycle is shown in Figure 4c, from which the activation energy was determined for the cooling cycle. The calculated values are listed in Table 2. According to the results shown in Table 2, it can be assumed that the [MoO 2 (L 1 )(MeOH)] and [MoO 2 (L 2 )(MeOH)] compounds polymerized after heating, which is indicated by a similar value of calculated activation energy, as it was for the [MoO 2 (L 1 )] n (67.2 vs 65-66 kJ mol −1 ). Although the polymerization of the compound [MoO 2 (L 2 )(H 2 O)] was proven, there was a difference in the activation energy from the methanol-coordinated complexes. This observation can be correlated to the stability of such a compound. Based on the TG study, it is obvious that a longer period of time is needed for polymerization in the case of the water-coordinated compound, and as result, this reflects upon the electrical properties and the activation energy as well. Moreover, we should emphasize here the interesting behavior observed for the sample [MoO 2 (L 2 )(MeOH)], which showed low activation energy. However, its conductivity was still several orders of magnitude lower compared to similar compounds, where polymerization is expected upon heating, including [MoO 2 (L 1 )(MeOH)] and the polymer [MoO 2 (L 1 )] n . This effect should be further investigated.
Furthermore, the conductivity isotherms @200 • C in the cooling run for all of the studied compounds are shown in Figure 5, and the corresponding DC conductivity values are shown in Table 3.
[MoO2(L 2 )(H2O)] 105.1 According to the results shown in Table 2, it can be assumed that the [MoO2(L 1 )(MeOH)] and [MoO2(L 2 )(MeOH)] compounds polymerized after heating, which is indicated by a similar value of calculated activation energy, as it was for the [MoO2(L 1 )]n (67.2 vs 65-66 kJ mol −1 ). Although the polymerization of the compound [MoO2(L2)(H2O)] was proven, there was a difference in the activation energy from the methanol-coordinated complexes. This observation can be correlated to the stability of such a compound. Based on the TG study, it is obvious that a longer period of time is needed for polymerization in the case of the water-coordinated compound, and as result, this reflects upon the electrical properties and the activation energy as well. Moreover, we should emphasize here the interesting behavior observed for the sample [MoO2(L 2 )(MeOH)], which showed low activation energy. However, its conductivity was still several orders of magnitude lower compared to similar compounds, where polymerization is expected upon heating, including [MoO2(L 1 )(MeOH)] and the polymer [MoO2(L 1 )]n. This effect should be further investigated.
Furthermore, the conductivity isotherms @200 °C in the cooling run for all of the studied compounds are shown in Figure 5, and the corresponding DC conductivity values are shown in Table 3.  Based on Table 3, the following conductivity trend can be seen in the cooling cycle @200 °C:  Based on Table 3, the following conductivity trend can be seen in the cooling cycle @200 • C: Here, a few comments are in order. First, the compound [MoO 2 (L 2 )(H 2 O)], obtained from the H 2 L 2 ligand, shows significantly lower conductivity in comparison to [MoO 2 (L 1 )(MeOH)] and ([MoO 2 (L 1 )] n ). This fact could be attributed to the type of ligand and the slow release of the coordinated solvent during heating. Accordingly, its activation energy was significantly higher, in the range of 100 kJ mol -1 . Second, the structural change (polymerization) after the solvent release during heating for compounds [MoO 2 (L 1 )(MeOH)], ([MoO 2 (L 2 )(MeOH)], and [MoO 2 (L 2 )(H 2 O)] was indicated by the observed differences in the conductivity spectra in the heating vs. the cooling runs (see Figure 4). It can be observed that, at lower temperatures for all compounds, the conductivity was almost temperatureindependent, and the increase in conductivity was visible only above 120 • C. Furthermore, the [MoO 2 (L 1 )(MeOH)] sample polymerized and had higher conductivity in the cooling run as compared to the heating run (see Figure 5). On the other hand, in the [MoO 2 (L 2 )(MeOH)] and [MoO 2 (L 2 )(H 2 O)] compounds (see Figure S17), the release of the solvent molecule (crystalline, coordinated) was observed at higher temperatures, and the conductivity in the cooling run was somewhat lower, with a difference of almost an order of magnitude in the case of the complex [MoO 2 (L 2 )(H 2 O)]. Finally, an interesting, but still unexpected, behavior for the compound [MoO 2 (L 2 )(MeOH)] should be emphasized. Although the complex showed a low activation energy, its DC conductivity was still several orders of magnitude lower compared to the compounds prepared by reactions with the H 2 L 1 ligand, namely: [MoO 2 (L 1 )(MeOH)] and [MoO 2 (L 1 )] n . However, the activation energy value (66.0 kJ/mol) for the sample [MoO 2 (L 1 )(MeOH)] indicated polymerization, while differences in the conductivity values did not. It should certainly be taken into account the type of H 2 L 2 ligand, and the observed steric influence of the ligand on conductivity transport should not be ruled out in compounds [MoO 2 (L 2 )(H 2 O) and [MoO 2 (L 2 )(MeOH)]. As mentioned previously, this behavior needs to be investigated in more detail.

Conclusions
New molybdenum complexes containing hydrazine ligands obtained from 5-nitrobenzaldehyde and 2-hydroxy or 4-hydroxybenzhydrazide were prepared and characterized. Chosen Mo(VI) compounds were investigated by the impedance spectroscopy method. For the first time, impedance spectroscopy was used to follow in situ transformations of mononuclear to polynuclear complexes, supporting the results obtained by heat treatment and thermogravimetric analysis for mononuclear compounds. All of the studied complexes showed an increase in conductivity with temperature; in other words, they behaved as semiconductors. Samples with high conductivity certainly open the possibility for the application of such complexes as semiconductors in various devices. For instance, the sample [MoO 2 (L 1 )] n , along with [MoO 2 (L 1 )(MeOH)], the conductivity of which increased by four orders of magnitude after the heating cycle and structural transformation, turned out to be potential candidates for target application due to the considerably high conductivity that they exhibited for this class of materials. Future research will be directed in that direction, along with the preparation of analogous complexes with different transition metal centers (i.e., Cu or V) in order to correlate the obtained electric results.
Supplementary Materials: The following supporting information can be downloaded at: https://www. mdpi.com/article/10.3390/ma16031064/s1, Figure S1. DSC curve for the H 2 L 1 ligand; Figure S2. TG-DSC curve for the H 2 L 1 ligand; Figure S3 Table  S2. Selected geometrical parameters (bond lengths and angles) for H 2 L 2 ; Figure S9. Crystal packing in H 2 L 2 is shown down the: (a) a-axis and (b) b-axis; Table S3. The geometry of hydrogen bonds and C (Å, o) for H 2 L 2 ; Figure