*3.2. X-Ray*

From the single crystal X-ray di ffraction analysis we found that the compounds NiMe*salen* and NiMeO*salen* of NiII were polymorphs of (2,2'-(ethane-1,2-diylbis((nitrilo)methylylidene)) bis(4-methylphenolato))-nickel(II) methanol solvate [35] and dinuclear bis(2,2'-(ethane-1,2-diylbis ((nitrilo)methylylidene))bis(4-methoxyphenolato))-di-nickel(II) methanol solvate [35], respectively. Polymorphism was found in the crystalline arrangement, since the compound NiMe*salen* (Figure 1) crystallized in the monoclinic crystal system with a space group of C2/c, while in literature it was found that methanol solvated compound crystallized in the triclinic crystal system with a space group of P-1. On the other hand, NiMeO*salen* crystallized in the monoclinic crystal system with a space group of P21/c, while the previously reported methanol solvated compound was space group P21/n and monoclinic crystal system [35].

**Figure 1.** View on the perspective of the compound NiMe*salen,* with a displacement ellipsoid at a 50% probability level for non-H atoms.

Discrete unit NiMe*salen* contained one central NiII ion and one unit of deprotonated 2,2-(ethane-1,2-diylbis((nitrilo)methylylidene))bis(4-methylphenolato) tetradentate ligand. Figure 1 shows the tetracoordinated metal center of NiII despites having a N2O2 coordination environment. Selected bond and angle parameters are given in Table 3.

The NiII center had almost a perfect square-planar geometry, which was defined by two N and two O atoms with τ4 of 0.02 and torsion angles O1-Ni1-N1-C8 (169.73(17)◦), N1'-Ni1-N1-C7 (173.6(2)◦), C2-O1-Ni1-N1 (7.8(2)◦), and C2-O1-Ni1-O1' (171.2(2)◦). In fact, the Ni (II) atom was 0.016 Å out of the plane and formed by O1/O1'/N1'/N1. Each discrete molecule coplanar had an rms (root-mean squeare) of 0.030. There were intermolecular interactions of type C-H···O, a hydrogen bond, and π··· π contacts that stabilized the crystal packing (Figure 2). Intermolecular interactions were established as follows: the hydrogen atom (C) carbon donor atom interacted with the (O) oxygen acceptor atom C8-H8A···O1 (2.45 Å), thus forming a *R1 2(4) motif* along the *c* axes. In this same crystallographic direction, the interaction of type π··· π was represented by the centroid Cg4 and six membered ring C1/C6. The intermolecular contacts of the hydrogen bond and π-stacking array formed a bidimensional complex array along the *a-b* plane.


**Table 3.** Selected bond lengths (Å) and angles (◦) for compounds NiMe*salen,* NiMeO*salen*, NiMeO*salphen*, and Ni*salphen.*

> Operators for generating equivalent atoms: −x + 1, y, −z + 1/2#1.

\*

**Figure 2.** Crystal array of compound NiMe*salen* shows the view along the *b-*axis and with perspective to plane formed by *a-c* axes emphasizing the π-stacking and *R*12 *(4) motif*.

Figure 3 shows the perspective view of the molecular structure of polymorphic compound NiMeO*salen*. The NiMeO*salen* discrete unit contained one central NiII ion and one 2,2-(ethane-1,2-diylbis((nitrilo)methylylidene))bis(4-methoxyphenolato) tetradentate ligand. The NiII ion was tetracoordinated with an N2O2 coordination environment. All bond length and angles corresponded to those reported in the literature [43]. Selected bond and angles parameters are given in Table 3.

The NiII center had a perfect square-planar geometry, defined by two N and two O atoms with rms of 0.004, τ4 of 0.006, and torsion angles O1-Ni1-N1-C8 (176.01(2)◦), N2-Ni1-N1-C7 (176.33(2)◦), C2-O1-Ni1-N1 (10.03 (2)◦), and C2-O1-Ni1-O2 (170.32(2)◦). In fact, the NiII atom was 0.005 Å out of the plane and formed by O1/O2/N2/N1. Nonetheless, it was observed that the six-membered rings deviated slightly from the coplanarity, thus finding an angle of 2.60(1)◦ between the aromatic rings.

Similarly with NiMe*salen,* in the crystalline arrangemen<sup>t</sup> for the compound NiMeO*salen*, there were intermolecular interactions for the C-H···O hydrogen bonding and intermolecular contacts of type π···π (Figure 4). For the no classical hydrogen bond, these interactions were formed between the (C) carbon donor atom and two (O) oxygen acceptor atoms (C8-H8B···O1 (2.61 Å) and C8-H8B···O1(2.45 Å)), thus forming an *R12(4) motif* along the *b axes.* Additionally, there were C17-H17C···O4 (2.75 Å) and C18-H18C···O3 (2.82 Å), which formed an *R22(6) motif* along the *b-c plane*. The weak interaction π···π had a distance of 3.95(8) Å between Cg4 and Cg5. Cg4 represent the six membered ring C1/C6 and Cg5 correspond to the C11/C16 ring. Finally, the intermolecular contacts of no classical hydrogen bond and π-stacking formed a tridimensional supramolecular array.

**Figure 3.** Perspective of the compound NiMeO*salen* with a displacement ellipsoid at a 50% probability level for non-H atoms.

**Figure 4.** Crystal array of compound NiMeO*salen*. View along the *c axes* and with a perspective of a plane formed by the *a-b* axes, which emphasizes the *R12(4)* and *R22(6) motifs*, as well as π-stacking.

The compound NiMeO*salphen* crystallized in a monoclinic crystal system with the space group P21/n. The asymmetric unit consisted of one nickel(II) center and one 2,2-{1,2-phenylenebis[(azanylylidene)methylylidene]}bis(4-methoxyphenolato) ligand. The ORTEP diagram is shown in Figure 5. The squared plane of NiII center was chelated by two oxygen and two nitrogen atoms that derived from a *salen* ligand, with Ni-O and Ni-N bond distances ranging from 1.8394 to 1.8643 (13) Å. O-Ni-O, N-Ni-N, and O-Ni-N bond angles of 84.12 to 179.34◦. The length distance of Ni-N was, on average, 1.8634(12) Å (Table 3), which was slightly higher than that observed in compounds NiMe*salen* and NiMeO*salen*. Nevertheless, the tetracoordinate NiII in compound NiMeO*salphen* had a square plane geometry with a rms of 0.0147 and a τ4 of 0.020. An analysis of the coplanarity shows that there were angles of 3.07 (7) and 5.06 (7)◦ between the square plane N2O2

at the metal center. The planes formed by the six-membered rings C1/C6 and C11/C16, respectively. Additionally, there was a perfect coplanarity between the square plane N2O2 at the metal center and the ring formed by the C8-C9-C1 /C22 atoms with an angle of 0.52 (7)◦.

**Figure 5.** Perspective of the compound NiMeO*salphen* with a displacement ellipsoids at a 65% probability level for non-H atoms.

In the crystal packing, there were C-H···O no classic hydrogen bonds and π···π intermolecular contacts (Figure 6).

**Figure 6.** Crystal array of compound NiMeO*salphen*. View is a tridimensional perspective emphasizing *<sup>R</sup>*2<sup>2</sup>*(30)* and *<sup>R</sup>*2<sup>2</sup>*(11) motifs* and π-stacking.

The interactions of type hydrogen bond were observed between C17-H17C···O2 (2.43 Å), C18-H18C···O3 (2.50 Å), and C21-H21···O3 (2.42 Å). These intermolecular contacts formed R22(30) and *<sup>R</sup>*2<sup>2</sup>*(11) motifs* along the *a*-*c* plane. Moreover, there were weak π-π interactions with a distance of centroids Cg4-Cg6 (3.84 Å) and Cg5-Cg6 (3.56 Å). Cg4 represented the six membered ring C1/C6, while Cg5 corresponded to the C8-C9-C19/C22. Cg6 was formed by the C11/C16 ring. Finally, all intermolecular contacts formed a tridimensional supramolecular array.

Unlike previously mentioned compounds NiMe*salen*, NiMeO*salen*, and NiMeO*salphen*, the single crystal X-ray diffraction analysis revealed that compound Ni*salphen* crystalized in the triclinic space group P-1. The asymmetric unit of Ni*salphen* contained two molecules of the nickel coordination compound and three molecules of the chloroform, which was then used as a solvent (Figure 7). Each metal central of NiII ion was tetracoordinated with one unit of a deprotonated *salphen* ligand with *salphen* = N,N-o-phenylenebis(salicylideneimine). Selected bond and angles parameters are given in Table 3.

**Figure 7.** Perspective of the compound Ni*salphen* with a displacement ellipsoids at a 55% probability level for non-H atoms.

While investigating the plane formed by three aromatic rings and the square symmetry N2O2 at the metallic center of NiII, we found a coplanarity in each molecule with rms of 0.060 and 0.019 Å for molecules A and B, respectively. Furthermore, these molecules had a parallel arrangemen<sup>t</sup> between them, with an angle of 1.4◦.

In molecule A, there was a perfect square planar geometry with a τ4 de 0.026 with torsion angles O1A-Ni1A-N1A-C8A (179.72(15)◦), N2A-Ni1A-N1A-C7A (177.8(2)◦), C2A-O1A-Ni1A-N1A (0.90 (2)◦), and C2A-O1A-Ni1A-O2A (179.6(2)◦). Similarly, molecule B had a perfect square planar geometry (<sup>τ</sup>4 de 0.024) with torsion angles O1B-Ni1B-N1B-C8B (179.26(15)◦), N2B-Ni1B-N1B-C7B (178.8(2)◦), C2B-O1B-Ni1B-N1B (6.0(2)◦), and C2B-O1B-Ni1B-O2B (173.7(2)◦).

In the crystalline arrangemen<sup>t</sup> of compound Ni*salphen*, a Ni-Ni distance of 3.26 Å was observed. The short distance found between both metal centers was favored by the interaction of the π···π. There were electronic densities in the coplanar and parllel A-A and A-B molecules.

This system obtained dinuclear structural arrangements with possible applications in molecular modeling and bioinorganic systems. Additionally, there were intermolecular interactions for C-H···O hydrogen bonding. Figure 8 shows the crystalline array with intermolecular contacts.

**Figure 8.** Crystal array of compound Ni*salphen*, view a tridimensional perspective emphasizing the π-stacking.

For the no classical hydrogen bond, interactions were formed between the carbon donor atom of the chloroform molecule solvent and the two oxygen acceptor atoms of the *salphen* ligand: C41-H41···O1A (2.26 Å), C41-H41···O2A (2.26 Å), C51-H51···O1B (2.29 Å), and C51-H51···O2B (2.19 Å). These show a linear, bifurcated, and trifurcated form for two, three, and four centers, respectively, in the intermolecular interaction. Additionally, there were C51···H16A (2.99 Å) and C52···H14B (3.01 Å) intermolecular contacts.

Despite the small differences on the NiII-donor atoms length, the nature of the N/N bridge and electron-donor/withdrawn character of the substituents in the 5- and <sup>5</sup>-position of the Schiff base play a key role in packing NiII coordination compounds. This can be observed in the Ni-Ni distance found in the different crystal structures obtained here and those previously reported.

Another important factor that influenced crystalline packing was the solvent. The Ni*salen* solvate reported by Siegler et al. showed a crystalline arrangement; the interactions stabilizing the crystal depended on it. It favored dimers when the Ni-Ni distances were modified according to acetone, 3.16 Å, CHCl3, 3.13 Å (system monoclinic); CHCl3, 3.19 Å (system orthorhombic); CH2Cl2, 3.28 Å; C2H4O2, 3.37 Å; DMF, 3.3901 Å; or the favor 1-D chain, as was the case for the methanol solvate Ni-Ni 3.44 Å, wherein the solvent joined the monomers through C-H···O interactions in one direction [33].

Comparing the intermolecular interactions found on *Nisalen* and *Nisalphen*, the incorporation of an extra aromatic ring in the ligand structure increased the number on the π···π and C-H···<sup>π</sup> interactions. In the two *Nisalen* structures reported with the same crystalline system (triclinic), the π···π interaction found a length of 3.63 [55] and 4.43 Å [33]. Meanwhile, the two C-H···<sup>π</sup> were observed. On the other hand, the *Nisalphen* structure presented two π···π interactions with lengths of 3.89 and 4.55 and four C-H···<sup>π</sup> interactions of 3.22, 3.39, 3.65, and 3.68 Å. The sum of all interactions led to a Ni-Ni distance of 3.26 Å, which was slightly smaller than the length found in both *Nisalen* with 3.63 and 3.36 Å, respectively.

Substituents also played an important role in crystal packing. *NiMeOsalphen* presented three π··· π interactions (i.e., 3.56, 3.65, and 3.84 Å), three C-H··· π interactions (3.24, 3.33 and 3.38 Å), and a C-H··· O interaction with a length of 2.50 Å. For *NiMeOsalen*, only one π··· π interaction of 3.95 Å was observed. There were two C-H··· π interactions with lengths of 3.61 and 3.77 Å and two C-H···O interactions with lengths of 2.75 and 2.82 Å. The Ni-Ni distance observed in these examples could be closely related with the C-H···O and π··· π interactions from the methoxy groups and the extra aromatic ring for *NiMeOsalphen*. The Ni-Ni distance on *NiMeOsalphen* could be longer than *NiMeOsalen* due to the π··· π interaction found between the two dimeric units.

For NiMe*salen*, the π··· π interaction was retained but the main contribution for the crystal stabilization relied on the C-H··· π interaction with distance values of 3.64 and 3.66 Å. These interactions kept the two units close enough to establish a Ni-Ni distance of 3.39 Å. The C-H···O interactions elicited by the methoxy groups contributed a shorter Ni-Ni distance for NiMeO*salen* (3.18 Å) than NiMe*salen* (3.39 Å). The same was observed for the compounds NiMeO*salphen* and *NiMesalphen* [35].

In the crystalline structure NiOH*salen*, two interactions of π-π were shown. However, the -OH groups in the structure stabilized the crystalline packing mainly by the interaction of the hydrogen bridges for the Osolvent-H··· O*salen* and O*salen*-H··· Osolvent with methanol molecules [44]. The Ni-Ni distance was 3.61 Å, which, when compared to NiMeO*salen* (3.18 Å), increased because of the sovlent's role in the packing. One methanol molecule formed a hydrogen bridge interaction with two neighboring molecules, Osolvent-H··· O*salen* and O*salen*-H··· Osolvent [44]. These solvent interactions also occurred in the Ni*salen* structure when methanol was the solvate [33].

## *3.3. Hirshfeld Surface Analysis*

Hirshfeld's surface (HS) analysis provided detailed information regarding intermolecular interactions. A better understanding of the problem may help address the challenge of quantitatively understanding intermolecular contacts using visual information on color and shadow on surfaces [56].

The Crystal Explorer 17 program [57] was used to generate the HS and 2D fingerprint plots of the complexes (i.e., NiMe*salen*, NiMeO*salen*, NiMeO*salphen*, and Ni*salphen*). The *dnorm* HS was obtained, which combined the normalized distances from the closer atom inside the surface (*di*) and outside the surface (*de*) to the HS, showing all contacts of the crystal structure. The red regions indicate the contacts were shorter than the sum of the van der Waals radii of the involved atoms. The blue and white regions indicated that the contacts were longer and closer to the van der Waals limit. Figure 9 shows the HS and all compound interactions.

The *dnorm* HS of the compounds showed red spots, which indicated close-contacts in the crystal structure, i.e., non-classical hydrogen bonds C-H···O and π··· π, as well as intermolecular interactions between centroids of six-membered rings in phenyl groups. The shape index was a function of HS and very helpful when investigating the π··· π stacking interaction. The blue and red zone indicated a region with a stacking arrangement. Figure 10 presents the shape index mapped on the compounds' HS. The blue zone indicated the presence of π··· π stacking interactions in the crystal structure. The π··· π interaction in compound Ni*salphen* stabilized and favored the 3.26 Å distance between the NiII metal centers, due to the presence of molecules A and B in the asymmetric unit of Ni*salphen*. Figures S1–S8 present the details of the fingerprint plots for each compound. In them, they describe the intermolecular interactions around the HS.

Figure 11 shows the contributions of contacts obtained from the decomposition of the fingerprint plots. The fingerprint plots of NiMe*salen*, NiMeO*salen*, and NiMeO*salphen* were similar, indicating that the H···H and C··· H/H···C were the most important contributors for crystal packing. H···H contacts contributed 64.4% (NiMe*salen*), 46.4% (NiMeO*salen*), and 32.4% (NiMeO*salphen*), while C··· H/H··· C contacts contributed 16% (NiMe*salen)*, 21.2% (NiMeO*salen*), and 20% (NiMeO*salphen*). A similar trend was observed in the fingerprint plot for Ni*salphen,* where the H···H and X··· H/H···X contacts had greater contributors for stabilizing interactions, with H···H contacts contributing 32.5% and 27.5% in molecules A and B, respectively. The contributions for C··· H/H···C, O···H, and C···C contacts were approximately of 20%, 5%, and 8% for molecules A and B, while the Cl···H/H···Cl contact contributed 19.9% and 23.8% in molecules A and B, respectively.

**Figure 9.** Hirshfeld surface (HS) with *dnorm* mapped and fingerprinted plots of the compounds NiMe*salen* (**a**), NiMeO*salen* (**b**), NiMeO*salphen* (**c**), and Ni*salphen* (**d**) for all interactions.

**Figure 10.** HS of the compounds NiMe*salen* (**a**), NiMeO*salen* (**b**), NiMeO*salphen* (**c**), and Ni*salphen* (**d**), mapped with shape index.

**Figure 11.** Contribution of some intermolecular contacts for HS of the compounds NiMe*salen*, NiMeO*salen*, and NiMeO*salphen,* as well as for molecules A and B of compound Ni*salphen.*

## *3.4. Cyclic Voltammetry*

NickelII-*salen* compounds have a neutral charge and show low solubility. Adding an extra aromatic ring in the ligand structure (i.e., *salphen*-type ligands) causes the solubility to decrease even more. When the NiR*salphen* solution bubbled with nitrogen, it started to precipitate. Because the low solubility of the compounds, it was only possible to characterize NiR*salen* compounds in the electrochemical study.

We performed voltammetry of the ligands (Figure 12b). The *salen* ligand ran in the direction of the positive potential in an interval of −3.2 to 1.0 V. In an inversion study, reduction signals 3a and 3b were associated with C=N reduction and an irreversible oxidation signal, 4a [58]. Figure 12a shows the Ni*salen* voltammogram under the same condition, caused by the nickel oxidation process ([NiIIL] → [NiIIIL ] + 1 e) and 2a and 2b due to nickel reduction process ([Ni ¯ IIL] + 1 e¯ → [NiIL]) [14,59]. Signal 3a and 3b was also observed to shift lower potential values. The other NiR*salen* complexes showed a similar behavior with the signals that shifted to different potentials due to the substituent in the 5,5position (Figures S40–S46, Supplementary Materials). In this work, only the processes associated with the reduction and oxidation of nickel were reported. The voltammograms were run in an interval of −2.4 to 1.0 V (Figure 12c,d).

The cathodic and anodic peak current were plotted in the square root function of the sweep speed (ƙ1/2). Only the complexes NiMeOH*salen*, NiOH*salen*, NiCl*salen,* and NiBr*salen* presented a linear dependence, which means that the oxidation of nickel was a diffusion-controlled process. A coupled reaction was suggested to impact the reversible process, thus confirming that Ni*salen* and NiMe*salen* via plotting *<sup>i</sup>*pc/*i*pa vs. logV. The oxidation process for the complexes were irreversible due to the ΔE being too big. The electron transference was a slow process, as is shown in Table 4. The oxidation process involved an EC mechanism and the NiIII-*salen* complex coordinates solvents, such as DMSO, in their axial position to stabilize the NiIII oxidations for electronic density [14,60].

With regard to the oxidation process, the reductions were a quasi-reversible process and we found that all nickel complex reductions were diffusion-controlled processes, except for NiOH*salen*, which presented coupled reactions. In comparation with oxidation reactions, the reduction of NiII was a more quantitative process. ΔE values were close to 59 mV and the *<sup>i</sup>*pc/*i*pa ratio was closer to 1 (Table 4).

For both processes, we found a trend between *<sup>E</sup>*1/2 and the effect of the substituent. Correlations were made with the Hammett sigma in the para-position. The metal center's acidity was influenced by the effect of the substituent. Therefore, the oxidative and reductive capacity of nickel modulated with the correct use of these substituents [13,61–63]. Electron-donor substituents shifted the E1/2 to a lower potential value and the electron-withdrawn groups shifted toward a more positive potential value. Thus, an electron-donor group improved the reductive capacity and electron-withdrawn groups improved the oxidative capacity of nickel, as shown in Figure 13.

**Figure 12.** Voltammogram of Ni*salen* 1 mM (DMSO). (**a**) Ni*salen* voltammogram from −3.2 to 1.0 V; (**b**) *salen* voltammogram from −3.2 to 1.0 V; (**c**) Ni*salen* voltammogram from negative potential to 2.4 to 1.0 V; (**d**) Ni*salen* voltammogram from positive potential to −2.4 to 1.0 V. All the experiments were referenced to the pair Fc+/Fc.


**Table 4.** Cyclic voltammetric parameter for NiR*salen* complexes, referenced to the pair Fc+/Fc.

**Figure 13.** Left: correlation between E1/2 of NiII/Ni<sup>I</sup> and the <sup>σ</sup>p constant for the NiR*salen* complexes. Right: correlation between E1/2 of NiIII/NiII and the <sup>σ</sup>p constant for the NiR*salen* complexes.
