**4. Conclusions**

In this study, various Schi ff bases and their NiII complexes were synthesized. All the prepared ligand and complexes were analyzed via C, H, and N analyses. They were assigned molecular structures and geometries using information obtained from UV-Vis, magnetic susceptibility, and X-ray crystallography, all of which corresponded to square-planar geometry in the solid state. The Hirshfeld surface analysis was used to study intermolecular interactions. This analysis revealed that the O···H, H···H and π··· π contacts were the most significant in the crystal array of the compounds *NiMesalen, NiMeOsalen*, and *NiMeOsalphen*, and O···H, H···H, Cl···H and π··· π contacts in the crystal array of the compound *Nisalphen*. The no classical hydrogen bonding and π··· π stacking information conveyed by Hirshfeld surface analysis were consistent with the crystal structure analysis. The substituents and the N/N bridge a ffected the crystal packing and electronic properties of nickel. According to the structures obtained for *Nisalphen, NiMeOsalen,* and *NiMeOsalphen*, it was possible to observe that the addition of an aromatic ring in the N/N bridge increased the number on π··· π and C-H··· π interactions and decreased their length. Substituents also played an important role in crystal packing for *NiMeOsalen* and *NiMeOsalphen.* i.e., a higher contribution for the O···H interaction. Due to this contribution, the length of π··· π interactions were minor in both complexes. In N/N aliphatic bridge complexes, the substituents also had an important role. The E1/2 depended on the electron-withdrawn or electron-donor nature of the R (R') substituent, which followed a correlation with the <sup>σ</sup>p of Hammet in such a way that, for the electron-donor substituent, -OH and -CH3 decreased the half-wave potential, instead favoring nickel's reductor ability.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2073-4352/10/7/616/s1, Table S1. Atomic coordinates (x 104) and equivalent isotropic displacement parameters (Å2 103) for compound NiMe*salen.* U(eq) is defined as one third of the trace of the orthogonalized Uij tensor. Table S2. Atomic coordinates (×104) and equivalent isotropic displacement parameters (Å2 × 103) for compound NiMeO*salen*. U(eq) is defined as one third of the trace of the orthogonalized Uij tensor. Table S3. Atomic coordinates (x 104) and equivalent isotropic displacement parameters (Å2 × 103) for compound NiMeO*salphen*. U(eq) is defined as one third of the trace of the orthogonalized Uij tensor. Table S4. Atomic coordinates (×104) and equivalent isotropic displacement parameters (Å2 × 103) for compound Ni*salphen*. U(eq) is defined as one third of the trace of the orthogonalized Uij tensor. Table S5. Bond lengths [Å] and angles [◦] for compound Ni*salphen*. Table S6. E ffective magnetic moment and number of unpaired electrons of NiII complexes. Table S7. 1H-NMR values for the ligands and nickel complexes. Table S8. 13C-NMR values for the ligands and nickel complexes. Figure S1. Normalized contact distance (dnorm, defined in terms of de, di, and the van der Waals radii of the atoms) mapped on the Hirshfeld surface of the compound NiMeO*salen*, represented with one surrounding moiety to visualize the intermolecular interaction. Figure S2. Hirshfeld surface with dnorm mapped and fingerprint plots for compound NiMe*salen*, with C···H interaction (first row) and H···H, O···H interactions (row 2–3). The color ranges from dark blue to red with increasing frequency (relative area of the surface), corresponding to each kind of interaction. Figure S3. Normalized contact distance (dnorm, defined in terms of de, di, and the van der Waals radii of the atoms) mapped on the Hirshfeld surface of the compound NiMeO*salen*, represented with one surrounding moiety to visualize the intermolecular interaction. Figure S4. Hirshfeld surface with dnorm mapped and fingerprint plots for NiMeO*salen*, with C···H interaction (first row) and H···H, O···H interactions (row 2–3). The color ranges from dark blue to red with increasing frequency (relative area of the surface) corresponding to each kind of interaction. Figure S5. Normalized contact distance (dnorm, defined in terms of de, di, and the van der Waals radii of the atoms) mapped on the Hirshfeld surface of the compound NiMeO*salphen*, represented together with one surrounding moiety to visualize the intermolecular interaction. Figure S6. Hirshfeld surface with dnorm mapped and fingerprint plots for compound NiMeO*salphen*, with C···H interaction (first row) and H···H, O···H interactions (row 2–3). The color ranges from dark blue to red with increasing frequency (relative area of the surface) corresponding to each kind of interaction. Figure S7. Normalized contact distance (dnorm, defined in terms of de, di, and the van der Waals radii of the atoms) mapped on the Hirshfeld surface of the compound Ni*salphen*, represented together with one surrounding moiety to visualize the intermolecular interaction. Figure S8. Hirshfeld surface with dnorm mapped and fingerprint plots of the two molecules name A and B in compound Ni*salphen* for C···H interaction (first row) and H···H, Cl···H interactions (row 2–3). The color ranges from dark blue to red with increasing frequency (relative area of the surface) corresponding to each kind of interaction. Figure S9. UV-vis of NiMeO*salen* in DMSO solution. Figure S10. UV-vis NiMe*salen* in DMSO solution. Figure S11. UV-vis NiOH*salen* in DMSO solution. Figure S12. UV-vis of Ni*salen* in DMSO solution. Figure S13. UV-vis of NiCl*salen* in DMSO solution. Figure S14. UV-vis of NiBr*salen* in DMSO solution. Figure S16. UV-vis NiMeO*salphen* in DMSO solution. Figure S17. UV-vis NiOH*salphen* in DMSO solution. Figure S18. UV-vis Ni*salphen* in DMSO solution. Figure S19. UV-vis NiCl*salphen* in DMSO solution. Figure S20. UV-vis NiBr*salphen* in DMSO solution. Figure S21. 1H-NMR of NiMeO*salen* in DMSO-d6. Figure S22. 13C-NMR of NiMeO*salen* in DMSO-d6. Figure S23. 1H-NMR of NiMe*salen* in chloroform. Figure S24. 13C-NMR of NiMe*salen* in chloroform. Figure S25. 1H-NMR of NiOH*salen* in DMSO-d6. Figure S26. 13C-NMR of NiOH*salen* in DMSO-d6. Figure S27. 1H-NMR of Ni*salen* in chloroform. Figure S28. 13C-NMR of Ni*salen* in chloroform. Figure S29. COSY spectrum of Ni*salen* in chloroform. Figure S30. HSQC spectrum of Ni*salen* in choloform. Figure S31. 1H-NMR of NaCl*salen* in chloroform. Figure S32. 1H-NMR of NiBr*salen* in chloroform. Figure S33. 1H-NMR of NiNO2*salen* in DMSO-d6. Figure S34. 1H-NMR of NiOH*salphen* in DMF-d7. Figure S35. 13C-NMR of NiOH*salphen* in DMF-d7. Figure S36. COSY spectrum of NiOH*salphen* in DMF-d7. Figure S37. HSQC spectrum of NiOH*salphen* in DMF-d7. Figure S38. 1H-NMR of NiMeO*salphen* in CDCl3. Figure S39. COSY spectrum of NiMeO*salphen* in CDCl3. Figure S40. Voltammogram of NiMeO*salen* 1 mM in DMSO. Figure S41. Voltammogram of NiMe*salen* 1 mM in DMSO. Figure S42. Voltammogram of NiOH*salen* 1 mM in DMSO. Figure S43. Voltammogram of Ni*salen* 1 mM in DMSO. Figure S44. Voltammogram of NiCl*salen* 1 mM in DMSO. Figure S45. Voltammogram of NiBr*salen* 1 mM in DMSO. Figure S46. Voltammogram of NiNO2*salen* 1 mM in DMSO.

**Author Contributions:** Conceptualization, C.S.N.-R. and A.S.-B.; methodology, C.S.N.-R., A.S.-B., and F.L.O.-V.; software, M.F.-A.; formal analysis, C.S.N.-R., A.S.-B., J.C.G.-R., and M.F.-A.; investigation, L.R.-A.; direction and resources, L.R.-A.; data curation, M.F.-A.; writing—original draft preparation, C.S.N.-R., A.S.-B., and M.F.-A.; writing—review and editing, L.R.-A. and J.C.G.-R., supervision,. L.R.-A.; project administration, L.R.-A.; funding acquisition, L.R.-A. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Acknowledgments:** We thank USAII for elemental analysis, mass spectrometry, nuclear magnetic resonance tests and chromatography coupled to mass spectrometry tests; PAPIIT IN230020 for financial support and PAPIIT IN217613 for scholarships (CSNR, and ASB).

**Conflicts of Interest:** The authors declare no conflict of interest.
