Heteroligand α-Diimine-Zn(II) Complexes with O,N,O′- and O,N,S-Donor Redox-Active Schiff Bases: Synthesis, Structure and Electrochemical Properties

A number of novel heteroligand Zn(II) complexes (1–8) of the general type (Ln)Zn(NN) containing O,N,O′-, O,N,S-donor redox-active Schiff bases and neutral N,N′-chelating ligands (NN) were synthesized. The target Schiff bases LnH2 were obtained as a result of the condensation of 3,5-di-tert-butyl-2-hydroxybenzaldehyde with substituted o-aminophenols or o-aminothiophenol. These ligands with combination with 2,2′-bipyridine, 1,10-phenanthroline, and neocuproine are able to form stable complexes upon coordination with zinc(II) ion. The molecular structures of complexes 4∙H2O, 6, and 8 in crystal state were determined by means of single-crystal X-ray analysis. In the prepared complexes, the redox-active Schiff bases are in the form of doubly deprotonated dianions and act as chelating tridentate ligands. Complexes 6 and 8 possess a strongly distorted pentacoordinate geometry while 4∙H2O is hexacoordinate and contains water molecule coordinated to the central zinc atom. The electrochemical properties of zinc(II) complexes were studied by the cyclic voltammetry. For the studied complexes, O,N,O′- or O,N,S-donor Schiff base ligands are predominantly involved in electrochemical transformations in the anodic region, while the N,N′-coordinated neutral nitrogen donor ligands demonstrate the electrochemical activity in the cathode potential range. A feature of complexes 5 and 8 with sterically hindered tert-butyl groups is the possibility of the formation of relatively stable monocation and monoanion forms under electrochemical conditions. The values of the energy gap between the boundary redox orbitals were determined by electrochemical and spectral methods. The parameters obtained in the first case vary from 1.97 to 2.42 eV, while the optical bang gap reaches 2.87 eV.


Introduction
The formation of organic/organoelemental hybrid molecules containing several types of redox-active ligands requires the development of new approaches characterized by a high efficiency and suitability for preparation of substances with desired properties. One of the promising directions in the synthesis of coordination compounds is the assembly of target complexes via the combination of various redox-active molecular blocks. To solve this problem, it is important to combine several different types of ligands, for example, O,N,O -, O,N,S-tridentate Schiff bases and N,N -chelating ligands.
Zinc compounds are the basis for creating substances with light-emitting properties [1]. Zinc complexes represent one of the unique classes of fluorescent compounds that have

Synthesis and Characterization
The reaction of 3,5-di-tert-butyl- 2-hydroxybenzaldehyde with o-aminophenols with various donor or acceptor substituents as well as with o-aminothiophenol in refluxing methanol leads to the formation of Schiff bases L 1 H2-L 5 H2 (Scheme 1).
The preparative yield of L 1 H2-L 4 H2 was from 55% to 70%. Compounds have been characterized by means of 1 H and 13 C{ 1 H} NMR-, IR-spectroscopy, and mass-spectrometry.
Synthesis of zinc(II) complexes 1-8 was carried out as the exchange reaction between the corresponding Schiff base and ZnCl2 in the presence of α-diimines and triethylamine as deprotonated agent (Scheme 2). The synthesis was performed in anaerobic conditions to prevent side oxidation processes of ligands. Compounds 1-8 were isolated as orange or red-orange crystalline powders with yields up to 68%. Complexes 1-8 are air-stable in the solid state. Compounds The synthesis was performed in anaerobic conditions to prevent side oxidation processes of ligands. Compounds 1-8 were isolated as orange or red-orange crystalline powders with yields up to 68%. Complexes 1-8 are air-stable in the solid state. Compounds have been characterized by means of IR, 1 H and 13 C{ 1 H} NMR spectroscopy (Figures S1-S15), C,H,N-elemental analysis. The spectral and elemental analysis data confirm the composition of complexes. The X-ray suitable crystals of 6, 8·CH 3 CN were grown by the prolonged crystallization of the corresponding powders from acetonitrile solutions under reduced pressure, while the crystals 4·H 2 O·CH 3 CN-by the slow evaporation of acetonitrile on air.
The presence of a large number of the centres of hydrogen binding leads to the fact that the main structural units in crystal 4 are centrosymmetric H-dimers (Table S1) connected by π . . . π interactions into infinite chains that form a three-dimensional grid due to weak CH . . . O and CH . . . π interactions ( Figure S16). There are no classical hydrogen binding donors in the compound 6, therefore, the main structure-forming interaction in these crystals are π . . . π, leading to the formation of ribbons interconnected by weak van der Waals interactions ( Figure S17). Compound 8 not only lacks hydrogen binding donors, but also the formation of π . . . π interactions is hindered by volumetric tert-butyl substituents. The crystals of this compound are formed only due to weak CH . . . O and CH . . . π interactions and consist of corrugated layers parallel to the c0b plane ( Figure S18).

Cyclic Voltammetry
The electrochemical behaviour of zinc complexes 1-8 was studied by cyclic voltammetry in dichloromethane solutions using a GC working electrode ( Table 2).

Cyclic Voltammetry
The electrochemical behaviour of zinc complexes 1-8 was studied by cyclic voltammetry in dichloromethane solutions using a GC working electrode (Table 2).

Oxidation
The electrochemical oxidation of complexes 1-8 proceeds in two or three successive anodic stages ( Figure 5). For complexes 1-3, 5, and 8, the first anodic peak is quasi-reversible and one-electron, indicating the formation of monocationic particles relatively stable over the CV experiment time ( Figure 6, Figures S19, S21 and S27).  Based on the values of current ratios, monocationic complexes formed during the electrooxidation of compounds 3 and 8, containing substituents in the positions of 3,5aromatic rings in Schiff bases, have the highest stability. The second oxidation stage for complexes 1-3, 5, and 8 is irreversible, which suggests the formation of an unstable dica-   Based on the values of current ratios, monocationic complexes formed during the electrooxidation of compounds 3 and 8, containing substituents in the positions of 3,5-aromatic rings in Schiff bases, have the highest stability. The second oxidation stage for complexes 1-3, 5, and 8 is irreversible, which suggests the formation of an unstable dication intermediate. An increase in the potential sweep range with the capture of the second anodic peak leads to a decrease or complete loss of the reversibility of the first oxidation stage ( Figure 5 and Figure S21), which indicates the occurrence of a chemical stage-partial decoordination of the doubly oxidized form of the ligand (Scheme 3).      Stabilization of this type of intermediate can be achieved due to the presence of a neocuproine ligand in 7, since complex 6 with the Bipy ligand does not exhibit such behaviour. For Schiff bases acting as ligands, electrically induced cyclization is a typical reaction [44,45]. The ratio of currents for the second stage is less than 1, which implies the occurrence of the subsequent chemical stage, i.e., deprotonation of the tertiary carbon atom with the formation of benzothiazole. Complexes 4 and 5 are characterized by additional anodic steps in the potential range from 1.55 to 1.62 V, which are comparable with the second oxidation potentials (1. .66 V) of free ligands, which may indicate the decoordination of Schiff bases and subsequent reactions deeper oxidation.

Reduction
For complexes 1-3, several peaks are recorded in the cathodic region in the range from −1.30 to −1.74 V (Figure 8, Figures S20 and S21). In view of the presence of an electroactive NO 2 group, the reduction mechanism of these compounds becomes much more complicated.  For compounds 1 and 2, a weakly resolvable pre-peak at −1.30 V is observed presumably corresponds to the reduction of the nitro group. For a free ligand, a redox process is also irreversible, but it is observed at a potential of −0.70 V. Tak account the presence of the dianionic form of the ligand in 1 and 2 and the expect of reduction potentials to the cathodic region, it can be assumed that the nitro g involved in this electrode process. The additional cathode peaks observed in the r −1.40 to −1.50 V are in good agreement with the previously obtained results for zi plexes with coordinated bipyridyl or phenanthroline and an additional S-dono [46,47]. The coordination of the azomethine group at the zinc atom also allows us sider the possibility of its participation in the cathode stage, however, then one expect fairly close values of the reduction potentials in a series of complexes 1-3 ever, a rather large scatter of potential values is observed. For most other comple first reduction step is quasi-reversible and one-electron (e.g., complexes 8, 4 and 6 9, Figures S23 and S26, respectively). For compounds 1 and 2, a weakly resolvable pre-peak at −1.30 V is observed, which presumably corresponds to the reduction of the nitro group. For a free ligand, a similar redox process is also irreversible, but it is observed at a potential of −0.70 V. Taking into account the presence of the dianionic form of the ligand in 1 and 2 and the expected shift of reduction potentials to the cathodic region, it can be assumed that the nitro group is involved in this electrode process. The additional cathode peaks observed in the range of −1.40 to −1.50 V are in good agreement with the previously obtained results for zinc complexes with coordinated bipyridyl or phenanthroline and an additional S-donor ligand [46,47]. The coordination of the azomethine group at the zinc atom also allows us to consider the possibility of its participation in the cathode stage, however, then one would expect fairly close values of the reduction potentials in a series of complexes 1-3. However, a rather large scatter of potential values is observed. For most other complexes, the first reduction step is quasi-reversible and one-electron (e.g., complexes 8, 4 and 6, Figure 9, Figures S23 and S26, respectively). plexes with coordinated bipyridyl or phenanthroline and an additional S-donor ligand [46,47]. The coordination of the azomethine group at the zinc atom also allows us to consider the possibility of its participation in the cathode stage, however, then one would expect fairly close values of the reduction potentials in a series of complexes 1-3. However, a rather large scatter of potential values is observed. For most other complexes, the first reduction step is quasi-reversible and one-electron (e.g., complexes 8, 4 and 6, Figure  9, Figures S23 and S26, respectively). As a result of the electrochemical reduction stage, a relatively stable monoanionic complex is formed (Scheme 5). Based on the ratios of the currents, the most stable monoanions are generated during the electroreduction of complexes 6-8. The nature of the substituents in the redox-active Schiff base influences the values of the oxidation potentials. In the case of 1 and 4, the replacement of the electron-withdrawing nitro group by a chlorine atom with the simultaneous introduction of a methyl substituent leads to a decrease in the oxidation potential by 0.17 V, as well as the presence of two donor tert-butyl groups in 8. A more pronounced effect is observed for pairs 2 and 5, or 1 and 8, for which there is a synchronous shift of E ox1 1/2 and E ox2 to the cathode region. For complexes 1 and 2, the transition from bipyridyl to phenanthroline ligand does not affect the value of the oxidation potential. At the same time, for complex 3, this value shifts to the anode region, as well as the reduction potential. This fact indicates the presence of a weak electronic interaction between two redox-active ligands. A distinctive feature of compounds 5 and 8 containing donor tert-butyl groups in the O,N,O′-ligand is the stabilization of the monocationic and monoanionic forms of the complexes. The Schiff base takes part in the electrooxidation, while the neutral nitrogen-containing ligand is involved to a greater extent in the cathodic process. An analysis of the potential values suggests that, on the one hand, these complexes act as electron donors and are easily oxidized, and on the other hand, they are weak electron acceptors. This dual behaviour allows us to consider them as electronic reservoirs, which makes it possible to use this type of complexes to initiate photoinduced redox transformations or as electrocatalysts.
One of the main indicators used to assess the possibility of using substances in photovoltaic devices is the energy gap (ΔE), which is the difference in the energies of the boundary orbitals. The value of ΔE can be determined theoretically using quantum chemical calculations or measured experimentally using electrochemistry or UV-visible spectroscopy. Elec- The nature of the substituents in the redox-active Schiff base influences the values of the oxidation potentials. In the case of 1 and 4, the replacement of the electron-withdrawing nitro group by a chlorine atom with the simultaneous introduction of a methyl substituent leads to a decrease in the oxidation potential by 0.17 V, as well as the presence of two donor tert-butyl groups in 8. A more pronounced effect is observed for pairs 2 and 5, or 1 and 8, for which there is a synchronous shift of E ox1 1/2 and E ox2 to the cathode region. For complexes 1 and 2, the transition from bipyridyl to phenanthroline ligand does not affect the value of the oxidation potential. At the same time, for complex 3, this value shifts to the anode region, as well as the reduction potential. This fact indicates the presence of a weak electronic interaction between two redox-active ligands. A distinctive feature of compounds 5 and 8 containing donor tert-butyl groups in the O,N,O -ligand is the stabilization of the monocationic and monoanionic forms of the complexes. The Schiff base takes part in the electrooxidation, while the neutral nitrogen-containing ligand is involved to a greater extent in the cathodic process. An analysis of the potential values suggests that, on the one hand, these complexes act as electron donors and are easily oxidized, and on the other hand, they are weak electron acceptors. This dual behaviour allows us to consider them as electronic reservoirs, which makes it possible to use this type of complexes to initiate photoinduced redox transformations or as electrocatalysts.
One of the main indicators used to assess the possibility of using substances in photovoltaic devices is the energy gap (∆E), which is the difference in the energies of the boundary orbitals. The value of ∆E can be determined theoretically using quantum chemical calculations or measured experimentally using electrochemistry or UV-visible spectroscopy. Electrochemical methods are widely used to determine the electrochemical gap ∆E el since the values of the standard reduction and oxidation potentials correlate with the LUMO and HOMO energies. In the case of most of the studied complexes, this value was determined as the difference in the potentials of the peaks ( Table 2). The values of ∆E el range from 1.97 to 2.42 eV, while the minimum values were obtained for complexes 5 and 8. The sequential replacement of electron-withdrawing groups in the Schiff base with donor ones leads to a decrease in the value of ∆E el in a row of 1, 4, 8, or for 2 and 5. This effect is observed mainly due to a decrease in the value of the first oxidation potential due to the involvement of the coordinated Schiff base in the redox processes. In the series of complexes 1-3, the ∆E el parameter is not significantly affected by the auxiliary neutral ligand.

UV-Vis Spectroscopy and DFT Calculations
According to the literature data, an overestimation of ∆E el compared to the spectral value (∆E) is typical for polymers and crystalline materials, while the opposite effect is recorded for low molecular weight compounds [48]. As a result, we have studied the activity of the complexes in the UV-visible range of the spectrum (280-600 nm) in chloroform (Table 3). For the investigated zinc(II) complexes in this spectral range, two or three absorption bands are observed with an intensity maximum in the region of 430-500 nm (Table 3, Figure 10). recorded for low molecular weight compounds [48]. As a result, we have studied the ac tivity of the complexes in the UV-visible range of the spectrum (280-600 nm) in chloro form (Table 3). For the investigated zinc(II) complexes in this spectral range, two or three absorption bands are observed with an intensity maximum in the region of 430-500 nm (Table 3, Figure 10).    4, 6, and 8, respectively). A closer look at these transitions revealed that neither of them involves the metal atom region. In 4, the transition corresponds to the intraligand excitation from the π-bonding (HOMO) to the π* orbital (LUMO+1) both centered on the Schiff ligand ( Figure 11). Note that the HOMO orbital also covers regions of the lone electronic pairs on chlorine atoms while the LUMO+1 orbital does not; hence, the corresponding transition can be described as the combination of n-π and π-π intraligand charge transfers. For 6 and 8, the interligand charge transfer is observed that corresponds to the excitations from the same HOMO π-bonding orbital of the Schiff base to the LUMO+1 and LUMO+2 π-antibonding orbitals centered on both ligands ( Figure 12). The additional calculations for the other complexes under study demonstrated a similar order of molecular orbitals ( Figures S29  and S30): for instance, the HOMO is always a π-bonding orbital located on the Schiff base, and the LUMO is located on a neutral NN ligand. This allows to assume that the discussed excitation mechanisms could be a common feature for this class of compounds. A closer look at these transitions revealed that neither of them involves the metal atom region. In 4, the transition corresponds to the intraligand excitation from the π-bonding (HOMO) to the π* orbital (LUMO+1) both centered on the Schiff ligand ( Figure 11). Note that the HOMO orbital also covers regions of the lone electronic pairs on chlorine atoms while the LUMO+1 orbital does not; hence, the corresponding transition can be described as the combination of n-π and π-π intraligand charge transfers. For 6 and 8, the interligand charge transfer is observed that corresponds to the excitations from the same HOMO πbonding orbital of the Schiff base to the LUMO+1 and LUMO+2 π-antibonding orbitals centered on both ligands ( Figure 12). The additional calculations for the other complexes under study demonstrated a similar order of molecular orbitals ( Figures S29 and S30): for instance, the HOMO is always a π-bonding orbital located on the Schiff base, and the LUMO is located on a neutral NN ligand. This allows to assume that the discussed excitation mechanisms could be a common feature for this class of compounds.
The IR spectra were recorded on an FSM-1201 FT-IR spectrometer (LLC "Monitoring", Saint Petersburg, Russia) in KBr pellets. The NMR spectra were measured in CDCl 3 on Bruker Avance HD 400 spectrometers (Bruker Biospin AG, Faellanden, Switzerland) with a frequency of 400 MHz (1H) and 100 MHz ( 13 C) using Me 4 Si as an internal standard. The chemical shift values are given in ppm with reference to the solvent, and the coupling constants (J) are given in Hz. The elemental analysis was carried out on a Euro EA 3000 (C,H,N) elemental analyzer (EuroVector Srl, Redavalle, Italy). Mass spectra (HRMS) were recorded on a Bruker UHR-TOF Maxis™ Impact mass spectrometer (Bruker Daltonik GmbH, Bremen, Germany). The UV-VIS spectra were recorded with a SF-104 spectrophotometer (AKVILON, Podol'sk, Russia) in a range of 300-600 nm.

General Method for Preparation of Zinc Complexes 1-8
Synthesis of zinc complexes was carried out as follows: 1 equiv. neutral nitrogencontaining ligand (NN) (0.35 mmol) dissolved in acetonitrile (2 mL) was added to a solution of ZnCl 2 (0.35 mmol) in the same solvent (5 mL) under argon. As a result of mixing, a suspension of the (NN)ZnCl 2 complex is formed, then 0.35 mmol of Schiff base and 2 equiv. triethylamine (0.7 mmol) were added under extensive stirring. During the reaction, the initial precipitate of (NN)ZnCl 2 gradually dissolved and the color of the solution changed to intense orange or brick red. The resulting solution was left for 2 days at 0-4 • C. The resulting colored precipitates of the complexes were filtered off, washed with n-hexane, and dried under reduced pressure. Crystals suitable for X-ray diffraction analysis were obtained by recrystallization of the compounds in acetonitrile. The yield of 5 in the form of orange powder was 68% (0.165 g). IR (KBr, cm −1 ): 3050,2951,2902,2866,1609,1593,1539,1520,1486,1458,1428,1382,1362,1300,1273,1253,1230,1196,1160,1141,1127,1102

X-ray Diffraction
The X-ray diffraction data were collected on a D8 Venture automatic diffractometer using graphite monochromated MoKα (λ 0.71073Å) radiation. SADABS program [52] was used to perform absorption correction. All structures were solved by direct methods and refined by full-matrix least-squares using the SHELXL2018/3 [53] with OLEX2 [54]. All the non-hydrogen atoms were refined with anisotropic atomic displacement parameters. The positions of hydrogen atoms were located from the Fourier electron density synthesis and were included in the refinement in the isotropic riding model approximation.The main crystallographic data and structure refinement details for all complexes are presented in Table S2. CCDC 2209787 (4·H 2 O·CH 3 CN), 2209788 (6), 2212742 (8·CH 3 CN) contain the supplementary crystallographic data. These data can also be obtained free of charge at ccdc.cam.ac.uk/structures/ from the Cambridge Crystallographic Data Centre.

Cyclic Voltammetry
Electrochemical studies were carried out using VERSASTAT-3 potentiostat (PAR) in three-electrode mode. The stationary glassy carbon (d = 2 mm) disk was used as the working electrode; the auxiliary electrode was the platinum-flag electrode. The reference electrode was Ag/AgCl/KCl (sat.) with a watertight diaphragm. All measurements were carried out under argon. The samples were dissolved in the pre-deaerated solvent. The scan rate (ν) was 200 mV·s −1 . The supporting electrolyte 0.1 M Bu 4 NClO 4 was dried under reduced pressure (48 h) at 50 • C. The concentration of compounds was 1-3 mmol.

Computational Details
The density functional theory calculations were performed using the Gaussian09 program [55]. The PBE0 functional [56,57] and the standard 6-311++G(d,p) basis set was utilized to perform all the calculations. The non-specific solvation effects which can affect molecular properties in a solution were accounted for by means of the SCRF-PCM model [58] (ε = 4.806 for CHCl 3 ). The empirical dispersion correction by Grimme [59] with the Becke-Johnson damping scheme [60] were employed for the geometry optimization procedures. The TD-DFT calculations were performed using the first ten singlet states and including SCRF equilibrium correction.

Conclusions
The interaction of O,N,O-, O,N,S-donor Schiff bases derived from 3,5-di-tert-butyl-2-hydroxybenzaldehyde with ZnCl 2 and N,N -diimine ligands leads to the formation of heteroligand zinc(II) complexes with a preparative yield of up to 68%. The molecular structures of compounds 4·H 2 O, 6, and 8 in crystal were studied by X-ray diffraction. The data of X-ray diffraction analysis confirm the fact that the Schiff bases in zinc complexes are in the dianionic imino-bis-phenolate or thiophenolate-iminophenolate forms. It is worth noting that the tridentate ligands in the pentacoordinate complexes 6 and 8 are distorted more significantly as compared with the hexacoordinate complex 4·H 2 O.
Due to the redox inertness of the zinc(II) ion, the synthesized complexes are convenient objects for studying the electrochemical behavior, since the contribution of different types of ligands to redox transformations can be estimated. Based on the results of CV experiments, complexes 1-3 with electron-withdrawing groups in the O,N,O -donor ligand tend to form relatively stable monocationic complexes containing oxidized forms of Schiff bases. In the cathodic area for these compounds, the reduction proceeds in irreversible manner and involves both the redox-active nitro-group and the neutral N,N -donor ligand. The replacement of an oxygen atom by sulfur in complexes 6 and 7 or the introduction of chlorine substituents into the aromatic ring of O,N,O -redox-active ligand in the case of compound 4 contributes to the destabilization of the electrogenerated oxidized forms. The reversibility of the second oxidation stage for complex 7 can be explained by the intramolecular cyclization reaction with the formation of a heterocyclic radical coordinated at the zinc atom.
Compounds 4-8 are characterized by the participation of neutral N,N -diimine ligands in redox transformations with the formation of stable monoanion species upon electroreduction. A general tendency for the heteroligand zinc complexes is the separation of ligand functions: in the anodic region, coordinated Schiff bases are predominantly involved in redox processes, while in the cathodic region, neutral N,N -diimine ligands are participated. A specific feature of compounds 5 and 8 is the possibility to form the relatively stable monocations and monoanions, which makes it possible to consider these objects as potential reservoirs for electron storage and transfer. Variation in the substituents nature in the Schiff bases affects the oxidation potentials and, accordingly, the reducing activity of this type of complexes. On the one hand, the above complexes are electron donors and are easily oxidized, and on the other hand, they act as weak electron acceptors. This dual behavior causes the use of this type complex to initiate photoinduced redox transformations or as electrocatalysts.
It was found that the variation of the substituents nature in the Schiff base has the greatest effect on the value of ∆E el than the size of the N,N -diimine aromatic system. The ∆E el values range from 1.97 to 2.42 eV. The minimum values were obtained for complexes 5 and 8 capable of forming stable oxidized/reduced forms. The substitution of electronwithdrawing groups by donor ones in the redox-active ligand promotes a decrease in the first oxidation potential for compounds 4, 5, and 8 and, consequently ∆E el is lowered to 1.97 eV. The calculated values of the energy gap ∆E, determined on the basis of spectral data, are characterized by slightly overestimated values (2. 49-2.87 eV) compared to the values of ∆E el , which is typical of low molecular weight compounds.
Supplementary Materials: The following supporting information can be downloaded at: https: //www.mdpi.com/article/10.3390/molecules27238216/s1, Figures S1-S15 with NMR data; Figures S16-S18 with crystal packing of 4·H 2 O·CH 3 CN, 6, and 8·CH 3 CN; Figures S19-S27 with CV curves for complexes; Figures S28-S30 with DFT results (General view of the optimized molecular structure of 4·H 2 O; The isosurfaces of HOMO and LUIMO orbitals in the selected complexes); Table S1 with H-bonds in crystals of investigated compounds; Table S2 with the main crystallographic data and structure refinement details.

Data Availability Statement:
The data presented in this study are available in this article.