N/N Bridge Type and Substituent Effects on Chemical and Crystallographic Properties of Schiff-Base ( Salen/Salphen ) Ni ii Complexes

: In total, 13 ligands R- salen (N,N’ -bis(5-R-salicylidene)ethylenediamine (where R = MeO, Me, OH, H, Cl, Br, NO 2 ) and R- salphen (N,N’ -bis(5-R-salicylidene)-1,2-phenylenediamine (where R = MeO, Me, OH, H, Cl, Br) and their 13 nickel complexes NiR salen and NiR salphen were synthesized and characterized using IR (infrared) spectroscopy, mass spectrometry, elemental analysis, magnetic susceptibility, NMR (nuclear magnetic resonance), UV-vis (ultraviolet-visible) spectroscopy, cyclic voltammetry, and X-ray crystal diffraction. Previous studies have shown that all complexes have presented a square planar geometry in a solid state and as a solution (DMSO). In electrochemical studies, it was observed that in N/N aliphatic bridge complexes, the Ni II underwent two redox reactions, which were quasi-reversible process, and the half-wave potential followed a trend depending on the ligand substituent in the 5,5’ -R position. The electron-donor substituent — as -OH, and -CH 3 decreased the E 1/2 potential — favored the reductor ability of nickel. The crystals of the complexes NiMe salen , NiMeO sale n, NiMeOs alphen, and Ni salphen were obtained. It was shown that the crystal packaging corresponded to monoclinic systems in the first three cases, as well as the triclinic for Ni salphen . The Hirshfeld surface analysis showed that the packaging was favored by H∙∙∙H and C∙∙∙H/H∙∙∙C interactions, and C - H∙∙∙O hydrogen bridges when the substituent was - MeO and π -stacking was added to an aromatic bridge. Replacing the N/N bridge with an aromatic ring decreased distortion in square-planar geometry where the angles O-Ni-N formed a perfect square-planar.


Introduction
Salen type ligands, derived from the condensation between salicylaldehyde and a primary diamine, are considered versatile ligands in coordination chemistry because the steric and electronic properties can be modulated by different amine aldehydes precursors. The metal salen complexes have been widely studied in diverse areas of chemistry. The interest lays in its easy synthesis, versatility, and kinetic and thermodynamic stability provided by the chelating capacity of the tetradentate ligand with N2O2 donor atoms. For the aforementioned reasons, such complexes not only play an important role in coordination chemistry, but in various areas such as asymmetric catalysis [1], epoxide formation [2][3][4], olefin hydrogenation [5,6], or in polymerization reactions [7,8]. Moreover, they have been extensively studied in connection with metalloprotein models and, more recently, in bioinorganic chemistry. A complex of Ni II ,Fe II , and Cu II -salen has been synthesized as a biomimetic compound for the study of metalloenzyme active sites and their catalytic mechanism [9][10][11][12].
It is well known that the steric and electronic effect plays an important role in the reactivity of M-salen compounds. The incorporation of electron-donor/withdrawing groups on salen skeleton allows redox potential modulation [9,13] and favors certain geometries [14,15] that impact the interaction with the substrates or specific recognition sites. In recent years, attention has focused on the biological properties of Schiff bases and their metal-compounds [16], showing that ligands by themselves can inhibit a carbonic anhydrase enzyme. The efficiency of these enzymes depend on the properties of the bridge N/N [16]. Several metal-salen complexes have presented different biological activities, such as an antibacterial [17,18] and antiproliferative against different tumor lines [19][20][21][22]. One of the proposed mechanisms of action of these complexes with salen-type ligands is the specific interactions with DNA and RNA. Different types of damage can occur depending on the chemical reactivity of the metal complex. Mn III -salen in the presence of an oxidant induces DNA cleavages [23,24], whereas Co III -salen can cleave DNA under aerobic conditions [25]. Co II -salen and Fe III -salen bind DNA at the minor groove [26], while Ni II -salen causes damage to nucleic acids, specifically causing divisions in guanine residues in the hairpin region of a single chain [27]. On the other hand, it has been reported that square coordination compounds with conjugated systems in their coordination spheres present stacking interactions with DNA [28] and G-quadruplexes. The binding affinity and selectivity of interaction with the latter is modulated by changing the substituents on the salen skeleton and modifying the nature of the N/N-bridge for the Schiff base [29,30]. The correct choice of the N/N bridge plays an important role in the geometry of these compounds, as it lengthens the chain and favors octahedral geometry [31] while adding aromatic rings that favor planar-square geometry. Moreover, it increases π-π interactions [32]. These type of complexes also present a squareplanar geometry [33] and multiple studies have shown how the reactivity of nickel-salen complexes can modulate by the design and control of the nature of the ligands by the choice of the N/N bridge or its substituents. Therefore, in this work, we study the effect the N/N bridge has on geometry and how the half-wave potential (E1/2) changes when a substitute is added. We report the structure of four Nickel complexes with tetradentate N2O2 ligands, analyzing the intermolecular interactions favored by the substituent and the N/N bridge, which modify the distance of Ni-Ni interaction found between dimers.

Synthesis of Schiff Base Ligands
Next, 2 mmol of appropriate salicylaldehyde was dissolved in acetonitrile, then 1 mmol of 1,2diaminoethane 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 ( 1 H and 13 C), and mass spectrometry:  [40]. Mesalphen

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 NO2salen 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 NiRsalen complex, a brown-orange precipitate was obtained. For the NiRsalphen complexes, a red solid was precipitated. The solid compound was vacuum filtered and washed with methanol and chloroform. NiNO2salen was washed with cold DMF [43]. The compounds were characterized by elemental analysis, FT-IR spectroscopy, NMR ( 1 H and 13  FT-IR values were comparable to those reported [18]. Insoluble for NRM and mass spectrometry.

X-Ray Crystallography
Suitable single crystals for compounds NiMesalen, NiMeOsalen, NiMeOsalphen, and Nisalphen 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 NiMesalen, NiMeOsalen, NiMeOsalphen, and λCuKα = 1.54184 Å for Nisalphen 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 2 -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) = 1.2Ueq(C) for aromatic and methylene groups, and 1.5Ueq(C) for methyl groups. On the other hand, for the compound NiMesalen, 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 Å 3 . Crystallographic data for all complexes are presented in Table 1 Index ranges

Cyclic Voltammetry
The cyclic voltammetry was carried out with a conventional arrangement 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.

Electronic Spectra
The electronic spectra of the Rsalen 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 Rsalphen were similar to Rsalen spectra. The difference was that Rsalphen spectra showed a small shoulder next to the 260 nm and Rsalphen 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 Rsalphen 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.
NiRsalen showed four characteristics bands (Figures S9-S20, supplementary material): 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 1 A1g → 1 Eg 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 1 A1g → 1 A2g, which is characteristic for a square planar geometry. These electronic transitions were confirmed measuring the magnetic moment (µeff ≈ 0.5, Table S6, supplementary material), meaning that the nickel complexes presented a diamagnetic property, consequences of the eight paired electrons. NiRsalphen 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]. 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].

Figure 2.
Crystal array of compound NiMesalen shows the view along the b-axis and with perspective to plane formed by a-c axes emphasizing the π-stacking and R 1 2 (4) motif. Figure 3 shows the perspective view of the molecular structure of polymorphic compound NiMeOsalen. The NiMeOsalen discrete unit contained one central Ni II ion and one 2,2'-(ethane-1,2-diylbis((nitrilo)methylylidene))bis(4-methoxyphenolato) tetradentate ligand. The Ni II 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 Ni II center had a perfect square-planar geometry, defined by two N and two O atoms with rms of 0.004, 4 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 Ni II 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.   (Table 3), which was slightly higher than that observed in compounds NiMesalen and NiMeOsalen. Nevertheless, the tetracoordinate Ni II in compound NiMeOsalphen had a square plane geometry with a rms of 0.0147 and a 4 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 sixmembered 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)°.  Unlike previously mentioned compounds NiMesalen, NiMeOsalen, and NiMeOsalphen, the single crystal X-ray diffraction analysis revealed that compound Nisalphen crystalized in the triclinic space group P-1. The asymmetric unit of Nisalphen 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 Ni II 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. While investigating the plane formed by three aromatic rings and the square symmetry N2O2 at the metallic center of Ni II , 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 arrangement between them, with an angle of 1.4º.
In molecule A, there was a perfect square planar geometry with a 4 de 0.026 with torsion angles O1A-Ni1A-N1A-C8A (179.72 (15) In the crystalline arrangement of compound Nisalphen, 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 •••. 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.
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-  Despite the small differences on the Ni II -donor atoms length, the nature of the N/N bridge and electron-donor/withdrawn character of the substituents in the 5-and 5'-position of the Schiff base play a key role in packing Ni II 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 Nisalen 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•••π 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•••π 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•••π 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. 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 NiMesalen, 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 NiMeOsalen (3.18 Å) than NiMesalen (3.39 Å). The same was observed for the compounds NiMeOsalphen and NiMesalphen [35].
In the crystalline structure NiOHsalen, 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•••Osalen and Osalen-H•••Osolvent with methanol molecules [44]. The Ni-Ni distance was 3.61 Å, which, when compared to NiMeOsalen (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•••Osalen and Osalen-H•••Osolvent [44]. These solvent interactions also occurred in the Nisalen structure when methanol was the solvate [33].

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., NiMesalen, NiMeOsalen, NiMeOsalphen, and Nisalphen). 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 •••, 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 Nisalphen stabilized and favored the 3.26 Å distance between the Ni II metal centers, due to the presence of molecules A and B in the asymmetric unit of Nisalphen. 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. Contribution of some intermolecular contacts for HS of the compounds NiMesalen, NiMeOsalen, and NiMeOsalphen, as well as for molecules A and B of compound Nisalphen.

Cyclic Voltammetry
Nickel II -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 NiRsalphen solution bubbled with nitrogen, it started to precipitate. Because the low solubility of the compounds, it was only possible to characterize NiRsalen 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 Nisalen voltammogram under the same condition, caused by the nickel oxidation process ([Ni II L] → [Ni III L ] + 1 e − ) and 2a and 2b due to nickel reduction process ([Ni II L] + 1e − → [Ni I L]) [14,59]. Signal 3a and 3b was also observed to shift lower potential values. The other NiRsalen complexes showed a similar behavior with the signals that shifted to different potentials due to the substituent in the 5,5´position ( Figures S40-S46, supplementary material). 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 12 c,d). The cathodic and anodic peak current were plotted in the square root function of the sweep speed (ʋ 1/2 ). Only the complexes NiMeOHsalen, NiOHsalen, NiClsalen, and NiBrsalen 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 Nisalen and NiMesalen via plotting ipc/ipa vs. logV. The oxidation process for the complexes were irreversible due to the ∆E 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 Ni III -salen complex coordinates solvents, such as DMSO, in their axial position to stabilize the Ni III 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 NiOHsalen, which presented coupled reactions. In comparation with oxidation reactions, the reduction of Ni II was a more quantitative process. ∆E values were close to 59 mV and the ipc/ipa ratio was closer to 1 ( Table 4). 72 For both processes, we found a trend between E1/2 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][62][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.

Conclusions
In this study, various Schiff bases and their Ni II 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 Xray 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 affected 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 electronwithdrawn or electron-donor nature of the R (R') substituent, which followed a correlation with the σp 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 www.mdpi.com/2073-4352/10/7/616/s1, Table   S1. Atomic coordinates (x 10 4 ) and equivalent isotropic displacement parameters (Å 2 10 3 ) for compound NiMesalen. U(eq) is defined as one third of the trace of the orthogonalized U ij tensor. Table S2. Atomic coordinates (×10 4 ) and equivalent isotropic displacement parameters (Å 2 × 10 3 ) for compound NiMeOsalen. U(eq) is defined as one third of the trace of the orthogonalized U ij tensor. Table S3. Atomic coordinates (x 10 4 ) and equivalent isotropic displacement parameters (Å 2 × 10 3 ) for compound NiMeOsalphen. U(eq) is defined as one third of the trace of the orthogonalized U ij tensor. Table S4. Atomic coordinates (×10 4 ) and equivalent isotropic displacement parameters (Å 2 × 10 3 ) for compound Nisalphen. U(eq) is defined as one third of the trace of the orthogonalized U ij tensor.  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 NiMeOsalen, represented with one surrounding moiety to visualize the intermolecular interaction. Figure S2. Hirshfeld surface with dnorm mapped and fingerprint plots for compound NiMesalen, 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 NiMeOsalen, represented with one surrounding moiety to visualize the intermolecular interaction. Figure S4. Hirshfeld surface with dnorm mapped and fingerprint plots for NiMeOsalen, 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 NiMeOsalphen, represented together with one surrounding moiety to visualize the intermolecular interaction. Figure S6. Hirshfeld surface with dnorm mapped and fingerprint plots for compound NiMeOsalphen, 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 Nisalphen, 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 Nisalphen 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 NiMeOsalen in DMSO solution. Figure S10. UV-vis NiMesalen in DMSO