*2.1. Abbreviation*

## *2.2. Synthesis of Schi*ff *Base Ligands*

Next, 2 mmol of appropriate salicylaldehyde was dissolved in acetonitrile, then 1 mmol of 1,2-diaminoethane or *o*-phenylenediamine was dissolved in acetonitrile and added slowly. The dissolution was stirred for 20 min. The volume was reduced and a solid precipitated (range of yellows in the case of 1,2-diaminoethane bridge and range of orange for *o*-phenylenediamine derivatives), which was vacuum filtered and recrystallized in methanol. The ligands were characterized by elemental analysis, FT-IR spectroscopy, NMR (1H and 13C), and mass spectrometry:

MeO*salen* C18H20N2O4 analysis (%Calculated (Found)): C, 65.86 (66.07); H, 6.09 (6.02); N, 8.54 (8.84). FT-IR (cm−1): <sup>υ</sup>C=N, 1639; υC-O, 1276. FT-IR values were comparable to those reported [34]. NMR 1H (ppm, CDCl3): 8.30 (s, CH=N), 12.63 (s, CAr-O-H); 13C (ppm): 166 (C=N), 155 (CAr-O). M<sup>+</sup> (m/z): 328 (328).

Me*salen* C18H20N2O2 analysis (%C(F)): C, 72.97 (73.10); H, 6.76 (6.38); N, 9.46 (10.05). FT-IR (cm−1): <sup>υ</sup>C=N, 1637; υC-O, 1282. FT-IR values were comparable to those reported [35]. NMR 1H (ppm, CDCl3): 8.29 (s, CH=N), 12.95 (s, CAr-O-H); 13C (ppm): 166 (C=N), 159 (CAr-O). M<sup>+</sup> (m/z): 296 (296).

OH*salen* C16H16N2O4; Analysis (%C(F)): C,63.55 (63.85); H, 5,37 (5.03); N, 9.32 (9.47). FT-IR (cm−1): <sup>υ</sup>C=N, 1640; υC-O, 1258. FT-IR values were slightly lower than those reported [36]. NMR 1H (ppm, Acet-d6): 8.46 (s, CH=N), 12.40 (s, CAr-O-H). Insoluble for NMR and mass spectrometry.

*Salen* C16H16N2O2 analysis (%C(F)): C,71.69 (71.62); H, 5.64 (6.01); N, 10.79 (10.44). FT-IR (cm−1): <sup>υ</sup>C=N, 1636; υC-O, 1284. FT-IR values were comparable to those reported [37]. 1H NMR (ppm, CDCl3): 8.36 (s, CH=N), 13.19 (s, CAr-O-H); 13C (ppm): 166 (C=N), 161 (CAr-O). M<sup>+</sup> (m/z): 268 (268).

Cl*salen* C16H14N2Cl2O2 analysis (%C(F)): C, 56.96 (57.13); H, 4.15 (3.88); N, 8.30 (8.52). FT-IR (cm−1): <sup>υ</sup>C=N, 1631; υC-O, 1274. FT-IR values were slightly lower than those reported [38]. 1H NMR (ppm, CDCl3): 8.29 (s, CH=N), 13.08 (s, CAr-O-H); 13C (ppm): 165 (C=N), 159 (CAr-O). M<sup>+</sup> (m/z): 336 (336).

Br*salen* C16H14Br2N2O2 analysis (%C(F)): C, 45.07 (45.18); H, 3.29 (2.80); N, 6.57(7.22). FT-IR (cm−1): <sup>υ</sup>C=N, 1635; υC-O, 1273. FT-IR values were comparable to those reported [34]. 1H NMR (ppm, CDCl3):8.28(s,CH=N),13.10(s,CAr-O-H);13C(ppm):165(C=N),160(CAr-O).M<sup>+</sup> (m/z):426(426).

NO2*salen* C16H14N4O6 analysis (%C(F)): C, 53.63 (53.69); H, 3.93 (3.59); N, 15.63 (15.76). FT-IR (cm−1): <sup>υ</sup>C=N, 1647; υC-O, 1326. FT-IR values were comparable to those reported [39]. 1H NMR (ppm, DMSO-d6): 8.77 (s, CH=N). M<sup>+</sup> (m/z): 358 (358).

MeO*salphen* C22H20N2O4 analysis (%C(F)): C, 70.19 (70.30); H, 5.35 (5.02); N, 7.44 (7.95). FT-IR (cm−1): <sup>υ</sup>C=N, 1616; υC-O, 1275. NMR 1H (ppm, Acet-d6): 8.85 (s, CH=N), 12.46 (s, CAr-O-H); 13C NMR (ppm): 166 (C=N), 155 (CAr-O). M + H<sup>+</sup> (m/z): 376 (376). The structure of this ligand was already reported [40].

Me*salphen* C22H20N2O2 analysis (%C(F)): C 76.72 (76.83); H, 5.85 (5.63); N, 8.13 (8.68). FT-IR (cm−1): <sup>υ</sup>C=N, 1618; υC-O, 1283. FT-IR values were comparable to those reported [40] NMR 1H (ppm, CD3CN): 8.70 (s, CH=N), 12.86 (s, CAr-O-H); 13C NMR (ppm): 165 (C=N), 159 (CAr-O). M + H<sup>+</sup> (m/z): 344 (345).

OH*salphen* C20H16N2O4 analysis (%C(F)): C, 68.95 (68.96); H, 4.62 (4.34); N, 8.04 (8.26). FT-IR (cm−1): <sup>υ</sup>C=N, 1614; υC-O, 1277. NMR 1H (ppm, Acet-d6): 8.76 (s, CH=N), 12.30 (s, CAr-O-H); 13C NMR (ppm): 165 (C=N), 155 (CAr-O). M + H<sup>+</sup> (m/z): 348 (349).

*Salphen* C20H16N2O2 analysis (%C(F)): C,75.89 (75.93); H,5.0 (5.1); N,8.91 (8.85). FT-IR (cm−1): <sup>υ</sup>C=N, 1612; υC-O, 1276. IR values were comparable to those reported [41]. M + H<sup>+</sup> (m/z): 316 (317). Insoluble for NMR

Cl*salphen* C20H14N2Cl2O2 analysis (%C(F)): C, 62.45 (62.25); H, 3.66 (3.19); N, 7.27(8.04). FT-IR (cm−1): <sup>υ</sup>C=N, 1614; υC-O, 1273. M + H<sup>+</sup> (m/z): 384 (385). Insoluble for NMR. The structure of this ligand has already been reported [42].

Br*salphen* C20H14N2Br2O2 analysis (%C(F)): C, 50.66 (50.74); H, 2.97 (2.57); N, 5.90 (6.42). FT-IR (cm−1): <sup>υ</sup>C=N, 1612; υC-O, 1373. FT-IR values were comparable to those reported [18]. NMR 1H (ppm, Acet-d6): 8.91 (s, CH=N), 13.09 (s, CAr-O-H); 13C NMR (ppm): 164 (C=N), 160 (CAr-O). M + H<sup>+</sup> (m/z): 474 (474).

## *2.3. Synthesis of Nickel Complexes*

In total, 1 mmol of nickel acetate was dissolved in methanol and the ligand, previously dissolved in methanol/chloroform, was added the dropwise to the nickel acetate solution, in case the ligand NO2*salen* was dissolved in the DMF (dimethylformamide). The mixture of the reaction was stirred for 15 min and a solid compound precipitated. In the case of the NiR*salen* complex, a brown-orange precipitate was obtained. For the NiR*salphen* complexes, a red solid was precipitated. The solid compound was vacuum filtered and washed with methanol and chloroform. NiNO2*salen* was washed with cold DMF [43]. The compounds were characterized by elemental analysis, FT-IR spectroscopy, NMR (1H and 13C), and mass spectrometry:

NiMeO*salen*, N,N'-bis(5-metoxisalicylidene)ethylenediamine, nickel(II), NiC18H18N2O4· H2O analysis (%C(F)): C,53.63 (53.12); H, 5.00 (4.96); N, 6.95 (6.98). FT-IR (cm−1): <sup>υ</sup>C=N, 1626; υC-O, 1328 FT-IR values were slightly lower than those reported [34]. NMR 1H (ppm, DMSO-d6): 7.80 s, CH=N); 13C (ppm): 160 (C=N), 162 (CAr-O). M + H<sup>+</sup> (m/z): 384 (385).

NiMe*salen*, N,N'-bis(5-metylsalicylidene)ethylenediamine, nickel(II), NiC18H18N2O2 analysis (%C(F)): C,61.19 (61.23); H, 5.10 (5.13); N, 7.93 (7.93). FT-IR (cm−1): <sup>υ</sup>C=N, 1624; υC-O, 1316 FT-IR values were comparable to those reported [35]. NMR 1H (ppm, CDCl3): 7.25 (s, CH=N); 13C (ppm): 161 (C=N), 163 (CAr-O). M + H<sup>+</sup> (m/z): 352 (353).

NiOH*salen*, N,N'-bis(5-hidroxisalicylidene)ethylenediamine, nickel(II), NiC16H14N2O4· 2H2O analysis (%C(F)): C, 48.94 (48.49); H, 4.15 (4.61); N, 7.39 (7.12). FT-IR (cm−1): <sup>υ</sup>C=N, 1614; υC-O, 1301. NMR 1H (ppm, DMSO-d6): 7.75 (s, CH=N); 13C (ppm): 159 (C=N), 162 (CAr-O). M + H<sup>+</sup> (m/z): 356 (357). The structure of this complex was already reported [44].

Ni*salen*, N,N'-bis(salicylaldehyde)ethylenediamine, nickel(II), NiC16H14N2O2 analysis (%C(F)): C, 58.77 (59.51); H, 3.89 (3.92); N, 8.57 (8.92). FT-IR (cm−1): <sup>υ</sup>C=N, 1624; υC-O, 1320. NMR 1H (ppm, CDCl3): 7.38 (s, CH=N); 13C (ppm): 162 (C=N), 165 (CAr-O). M + H<sup>+</sup> (m/z): 324 (325). The structure of this complex was already reported [33].

NiCl*salen*, N,N'-bis(5-chlorosalicylidene)ethylenediamine, nickel(II), NiC16H12N2Cl2O2· H2O analysis (%C(F)): C, 46.22 (46.22); H, 3.42 (2.75); N, 6.80 (6.89). FT-IR (cm−1): <sup>υ</sup>C=N, 1624; υC-O, 1312. FT-IR values were comparable to those reported [45]. NMR 1H (ppm, CDCl3): 7.44 (s, CH=N). M+H<sup>+</sup> (m/z): 394 (394).

NiBr*salen*, N,N'-bis(5-bromosalicylidene)ethylenediamine, nickel(II) NiC16H12Br2N2O2· 2H2O analysis (%C(F)): C, 37.04 (36.85); H, 3.10 (3.05); N, 5.39 (5.46). FT-IR (cm−1): <sup>υ</sup>C=N, 1626; υC-O, 1309 FT-IR values are comparable to those reported [34]. NMR 1H (ppm, CDCl3): 7.45 (s, CH=N). M + H<sup>+</sup> (m/z): 482 (483).

NiNO2*salen*, N,N'-bis(5-nitrosalicylidene)ethylenediamine), nickel(II), NiC16H12N4O6· 1.6H2O analysis (%C(F)): C, 42.98 (43.18); H, 3.11 (3.46); N, 12.74 (12.59). FT-IR (cm−1): <sup>υ</sup>C=N, 1639; υC-O, 1321. FT-IR values were comparable to those reported [39]. NMR 1H (ppm, DMSO-d6): 7.95 (s, CH=N). Insoluble for mass spectrometry.

NiMeO*salphen* (N,N'-bis(5-metoxisalicylidene)-1,2-phenylenediamine, nickel(II) NiC22H18N2O4 analysis (%C(F)): C, 61.01 (61.50); H, 4.18 (3.71); N, 6.46 (7.24). FT-IR (cm−1): <sup>υ</sup>C=N, 1616; υC-O, 1213. NMR 1H (ppm, CDCl3): 8.23 (s, CH=N). M + H<sup>+</sup> (m/z): 432 (433).

NiMe*salphen*, N,N'-bis(5-metylsalicylidene)-1,2-phenylenediamine, nickel(II) NiC22H18N2O2, analysis (%C(F)): C, 65.88 (66.30); H, 4.52 (4.02); N, 6.98 (7.31). FT-IR (cm−1): <sup>υ</sup>C=N, 1624; υC-O, 1213. M + H<sup>+</sup> (m/z): 400 (401). Insoluble for NRM. The structure of this complex was already reported [35].

NiOH*salphen*, N,N'-bis(5-hidroxisalicylidene)-1,2-phenylenediamine, nickel(II) NiC20H14N2O4 · CH3OH analysis (%C(F)): C, 57.70 (57.63); H, 4.15 (3.66); N, 6.40 (6.62). FT-IR (cm−1): <sup>υ</sup>C=N, 1610; υC-O, 1220. NMR 1H (ppm, DMF-7): 8.00 (s, CH=N); 13C (ppm): 162 (C=N), 155 (CAr-O). M+H<sup>+</sup> (m/z): 404 (405).

Ni*salphen*, N,N'-bis(salicylaldehyde) -1,2-phenylenediamine, nickel(II) NiC20H14N2O2 analysis (%C(F)): C, 64.8 (64.4); H, 3.70(3.78); N, 7,32 (7.51). FT-IR (cm−1): <sup>υ</sup>C=N, 1604; υC-O, 1295 FT-IR values were comparable to those reported [41]. M + H<sup>+</sup> (m/z):372 (373). Insoluble for NRM.

NiCl*salphen*, N,N'-bis(5-chlorosalicylidene)-1,2-phenylenediamine, nickel(II), NiC20H12N2Cl2O2; analysis (%C(F)): C, 54.35 (55.59); H, 2.73 (2.28); N, 6.33 (7.16). FT-IR (cm−1): <sup>υ</sup>C=N, 1608; υC-O, 1290. FT-IR values were comparable to those reported [38]. Insoluble for NRM and mass spectrometry.

NiBr*salphen*, N,N'-bis(5-bromosalicylidene)-1,2-phenylenediamine, nickel(II), NiC20H12N2Br2O2; analysis (%C(F)): C, 45.25 (45.77); H, 2.27 (2.16); N, 5.27 (5.87). FT-IR (cm−1): <sup>υ</sup>C=N, 1606; υC-O, 1328. FT-IR values were comparable to those reported [18]. Insoluble for NRM and mass spectrometry.

## *2.4. X-Ray Crystallography*

Suitable single crystals for compounds NiMe*salen*, NiMeO*salen*, NiMeO*salphen*, and Ni*salphen* were mounted on a glass fiber. Crystallographic data were collected with an Oxford Diffraction Gemini "A" diffractometer with a CCD area detector, with λMoKα = 0.71073 Å for NiMe*salen*, NiMeO*salen*, NiMeO*salphen,* and λCuKα = 1.54184 Å for Ni*salphen* at 130 K. Unit cell parameters were determined with a set of three runs of 15 frames (1◦ in ω). The double pass method of scanning was used

to exclude any noise [46]. The collected frames were integrated by using an orientation matrix determined from the narrow frame scans. Final cell constants were determined by a global refinement. Collected data were corrected for absorbance by using an analytical numeric absorption correction with a multifaceted crystal model based on expressions upon the Laue symmetry with equivalent reflections [47]. Structure solutions and refinement were carried out with the SHELXS-2014 [48] and SHELXL-2014 [49] packages. WinGX v2018.3 [50] software was used to prepare material for publication. Full-matrix least-squares refinement was carried out by minimizing *(Fo*<sup>2</sup> – *Fc*2) 2. All non-hydrogen atoms were refined anisotropically. H atoms attached to C atoms were placed in geometrically idealized positions and refined as riding on their parent atoms, with C-H = 0.95 – 0.99 Å and with *Uiso*(H) = 1.2 *Ueq*(C) for aromatic and methylene groups, and 1.5 *Ueq*(C) for methyl groups. On the other hand, for the compound NiMe*salen*, the solvent molecules were significantly disordered and could not be modeled properly (i.e., SQUEEZE [51]). Part of the PLATON package of crystallographic software was used to calculate the solvent disorder area and remove contributions to the overall intensity data. The disordered solvents area was centered around the 0.500–0.034 position and showed an estimated total of 60 electrons and a void volume of 180 Å3. Crystallographic data for all complexes are presented in Table 1. The crystallographic data for the structures reported in this paper was deposited with the Cambridge Crystallographic Data Centre as supplementary publication no. CCDC 2006691–2006694. Copies of the data can be obtained free of charge on application to CCDC, 12 Union Road, Cambridge, CB2 1EZ, UK. (fax: (+44) 1223-336-033, e-mail: deposit@ccdc.cam.ac.uk).


**Table 1.** X-ray diffraction data collection and refinement parameters for the compounds NiMe*salen,* NiMeO*salen,* NiMeO*salphen,* and Ni*salphen.*

## *2.5. Cyclic Voltammetry*

The cyclic voltammetry was carried out with a conventional arrangemen<sup>t</sup> of three electrodes: a vitreous carbon working electrode, a platinum counter electrode, and a silver pseud-electrode. The potentials were referenced to the saturated calomel electrode (SCE) with ferrocene as an internal standard (E◦Fc+/Fc = +0.46 V vs. SCE). The experiments were collected in 0.001 M DMSO solutions under nitrogen atmosphere. The supporting electrolyte was 0.1 M of tetrabutylammonium hexafluorophosphate.

## **3. Results and Discussion**

## *3.1. Electronic Spectra*

The electronic spectra of the R*salen* ligand showed three absorption maxima. The first was the 255–270 nm region and the second was the 315–350 nm region. These two bands were attributed to π → π\* transitions, with a high molar absorptivity coefficient. They also had a third band between 420–430 nm due to n → π\* of the group C=N. Electronic spectra of R*salphen* were similar to Rs*alen* spectra. The difference was that R*salphen* spectra showed a small shoulder next to the 260 nm and R*salphen* ligands had another band in the 270–400 region due to the π → π\* transition for the third aromatic ring in N/N bridge. All R*salphen* transitions were shifted to a major wavelength values and had bigger values of molar extinction coefficients, because the higher aromaticity of the ligands favored the delocalization of electron density.

NiR*salen* showed four characteristics bands (Figures S9–S20, Supplementary Materials): the first two were in the 260–268 nm and 320–380 nm regions, with a high molar absorptivity coefficient, both due to π → π\* transitions of the ligand. The third ws the 405–518 nm due to a ligand-metal charge transfer (LMCT) transition, from the phenolate to M due to 1A1g → 1Eg transition [52,53]. The last band in the 500–680 nm region was owed to the d-d transition [52,54]. These bands could not be characterized with precision because of the low solubility of the compounds, since they presented a very low molar absorptivity coefficient. These bands were attributed to 1A1g → 1A2g, which is characteristic for a square planar geometry. These electronic transitions were confirmed measuring the magnetic moment (μeff ≈ 0.5, Table S6, Supplementary Materials), meaning that the nickel complexes presented a diamagnetic property, consequences of the eight paired electrons. NiR*salphen* had the same trend that their ligands and had one more π → π\* transition due to the third aromatic ring and higher molar extinction coefficients (Data Table 2). All maxima shifted to major wavelength values [52,54].


**Table 2.** Electronic spectral data of the Schiff bases and their complexes.

The last band involved with the d-d transition provided an approximation of the intensity of the complex field, since the energy of this electronic transition was associated with 10 Dq. This band could not be observed for complexes with the imine aliphatic bridge since it was masked by high intensity transitions. On the other hand, in the compounds with the aromatic bridge, we observed that those with a substituted electron-withdrawn (-Br, -Cl) had a greater wavelength value, thus decreasing the energy necessary to carry out this transition, especially when compared to the substituted electron-donor (-MeO, -OH) [43].
