Effect of Diamine Bridge on Reactivity of Tetradentate ONNO Nickel(II) Complexes

Two new square planar ONNO nickel(II) complexes C2_core and C3_core have been synthesized and characterized by single crystal X-ray diffraction, NMR spectroscopy, thermogravimetry, and DFT calculations. The experimental results revealed the effect of the length of diamine bridge in the ligand on the behavior of the studied complexes in the reaction with N -heterocyclic aromatic amines, while DFT calculations provided a basis for the rationalization of this observation. The complex with propylenediamine bridge ( C3_core ) readily reacts with pyridine and its derivatives with the formation of high-spin (paramagnetic) complexes with octahedral geometry characterized by X-ray diffraction; electron-donating substituents on the pyridine ring facilitate the coordination of axial ligands. On the contrary, the complex with ethylenediamine bridge ( C2_core) does not undergo such a reaction because of the high deformation energy of the core required for the formation of C2_Py complex.


INTRODUCTION
Nickel(II) complexes (d 8 -configuration) can exist in singlet or triplet spin state. Square planar Ni (II) complexes are diamagnetic, with a singlet ground state (S = 0). However, when their geometry changes to square pyramidal or octahedral (with coordination number of five and six, respectively), these complexes become paramagnetic, with a triplet ground state (S = 1). Such transition between two spin states was denoted Coordination-Induced Spin-State Switching (CISSS). This effect was first discovered for Ni(II) by Herges and co-workers, [1][2][3] and later was observed for other transition metal complexes, e.g., Fe(II), Fe(III), Mn(II), Mn(III), and Co(II). Spin-state switching phenomena in transition metal complexes is currently an intensively studied area in coordination chemistry, because such systems have a potential for application in memory devices, chemical sensors, and contrast agents for magnetic resonance imaging. [4][5][6][7][8] A number of Ni(II) complexes, in which the metal center is located in ONNO environment, have been synthesized previously. [9][10][11][12][13] The most common representatives of tetradentate ONNO ligands are found in the salen series, [salen = N,N'-bis(salicylidene)ethylenediamine]. [13,14] However, their acacen analogues [acacen = N,N'-bis(acetylacetone)ethylenediamine] are much less frequently used as ligands in coordination chemistry and the library of their complexes with transition metals is significantly smaller. [9,15,16] The principal examples of such compounds are [Co(acacen)L2] + complexes, which have been studied extensively as potential protein inhibitors (e.g., for selective inhibition of Human α-Thrombin). [17,18] Another prominent representative of such complexes is Fe(II) spin-crossover complex with the wide thermal hysteresis loop. [19,20] Square planar nickel(II) complexes can change their coordination sphere by addition of donor ligands, this transformation can be detected by shifts in the UV-vis spectra. [1,3,13] Propensity of Ni(II) complexes towards formation of octahedral complexes depends on the nature of equatorial ligands and basicity of the axial ligand. The first structurally characterized octahedral nickel(II) complexes with acacen equatorial ligand were reported in 2010. [11] Recrystallization of Ni{N,N-ethylenebis(1,1,1-

trifluoroacetylacetoneiminato)} [NiL] from pyridine resulted in brown crystals of [NiL(Py)2]
characterized by X-ray diffraction. Moreover, the reaction with the bidentate ligand 1,4diazabicyclo[2.2.2]octane (dabco) led to the formation of one-dimensional coordination polymer, in which dabco axially bridges [NiL] units in a linear fashion. It was noted that no corresponding solid products were formed in the absence of CF3 groups in the equatorial ligand. [11] Thus, the incorporation of highly polar functional group (such as CF3) near coordination sphere of transition metal plays the important role in the formation of octahedral complexes with N-donor ligands. Very recently, a family of planar nickel(II) complexes with four phenazine-based acacen-type ligands has been reported. [9] The complexes demonstrate CISSS behavior with fluorescence detection and can find application as molecular sensors. The addition of pyridine to them in the excited state is more favorable than in the ground state. The electron-withdrawing CF3 substituents in the ligand strongly affect the acidity of the nickel center and facilitate coordination of axial ligands, increasing the CISSS sensitivity.
Here we report the synthesis and characterization of two new nickel(II) complexes based on acacen framework ( Figure 1). We focused on such type of Ni(II) complexes because the acacen/acacpn ligands shown in Figure 1 are promising ligand platform, where phenyl rings can be easily functionalized with various substituents to tune the electronic properties of the resulting complex. The acacen type of ligands is based on N,N'-bis(acetylacetone)ethylenediamine, while acacpn ligands -on N,N'bis(acetylacetone)propylenediamine. Studying the reactivity of these complexes towards pyridines, we found that only one of them is capable of forming octahedral complexes and changing its spin state upon coordination of axial ligands. In contrast to the above-mentioned research, [9,11] where electronic effect of substituents [21] plays the crucial role in the formation of octahedral Ni(II) complex, the results presented in this paper are strongly connected with geometric effect.

X-ray crystallographic analysis
Single crystal X-ray diffraction data for C2_core, C3_core, C3_Py, C3_DMAP and C3_MOPy compounds were collected on D8 Venture single crystal diffractometer using MoKα radiation. The data treatment and reduction were done using the Bruker software suite. [22][23][24][25] Structures were solved and refined using SHELXT [26] and SHELXL [27] software with the atomic scattering factors taken from the International Tables. [28] The C3_DMAP structure was solved with the SHELXS [29] software using Direct Methods and refined with the olex2.refine [30] refinement package employing the Gauss-Newton minimization in Olex software. [31] The crystals of C2_core were twinned, which was taken into account in data reduction and structure refinement. A detailed description of data collection and structures refinement are presented in Supporting Information. Mercury CSD 4.1.0 software was used for the preparations of crystal structure figures. [32] Crystallographic data for the crystal structures described in this publication have been deposited with the Cambridge Crystallographic Data Centre with 2106933 -2106937 deposition numbers.

Hirshfeld surface analysis
The Hirshfeld surfaces and two-dimensional fingerprint plots were generated using Crystal Explorer 17.5 to provide the information about intermolecular interactions in the crystal structures of C2_core, C3_core, C3_Py and C3_MOPy. [33][34][35] Due to significant disorder in the crystal structure in the case of C3_DMAP, the Hirshfeld surface analysis has not been made.

Theoretical methods
Geometry optimization of square planar Ni(II) complexes (C2_core and C3_core) and octahedral Ni(II) complexes (C3_Py, C3_MOPy and C3_DMAP) was performed in different spin states using ADF package. [36,37] BP86 functional [38,39] was used with TDZP basis set, which consists of triple-ζ quality basis set on the Ni and double-ζ quality on the other atoms, including one polarization function. Scalar relativistic correction ZORA, [40] D3 dispersion correction by Grimme, [41] and COSMO solvation model (pyridine with ε=12.4) [42] were included. To ensure that the obtained geometries are in energy minima, the vibrational analysis was performed and no imaginary frequencies were found. Single-point energy calculations with BP86 and S12g [43] functionals and TZ2P basis set were subsequently performed. The S12g functional is especially well-suited for studies of CISSS. [43,44]

Nuclear magnetic resonance
The 1 H NMR and 13 C NMR spectra were recorded on the Agilent (400 MHz) spectrometer. Proton chemical shifts were reported in ppm (δ) relative to the internal standardtetramethylsilane (TMS: δ 0.00 ppm). Carbon chemical shifts were reported in ppm (δ) relative to the signal corresponding to the residual non-deuterated solvent (CDCl3: δ 77.0). Data are presented as follows: chemical shift, integration, multiplicity (s = singlet, d = doublet, t = triplet, dd = doublet of doublets, br = broad, m = multiplet), and coupling constant (Hz), see Supporting information (SI).

Ultraviolet-visible spectroscopy (UV-Vis)
The UV-Vis absorption spectra were obtained on Shimadzu UV-2401PC UV spectrophotometer in chloroform and pyridine.

Thermogravimetric analysis
Thermogravimetric analysis (TGA) was carried out with the TA Instruments Q50 Thermal Gravimetric Analyzer. The sample was measured in 20-700° range with 10K/min heating rate and the nitrogen as a purge gas.

Square planar Ni(II) complexes
Ni(II) complexes with tetradentate ligand containing two (C2_core) and three (C3_core) carbon atoms in the bridge were synthesized according to the general procedure shown in Scheme 1. Details for each step of synthesis can be found in SI.  Table 1 and Table S1 (SI), respectively. Crystal structures of C2_core and C3_core are presented in Figure 2 and Figure S5 in SI.
Both studied compounds adopt almost perfect square planar geometry, which can be quantitatively characterized by τ4 parameter [45] proposed for tetracoordinate complexes (Table 1 and Eq. S1 in SI). The value of this parameter can range from 0 to 1 when passing from a perfect square planar to a perfect tetrahedral geometry. For the studied structures, the values are close to zero that well compliments the visual inspection of the structures. Further comparison of the structures shows that C3_core molecule is more bent than both C2_core molecules. Dihedral angles between average ONNO plane and corresponding two parts of the acacen ligand (φ1, φ2) are more than twice larger for C3_core than for both C2_core molecules (Table 1). A detailed description of the τ4, φ1 and φ2 parameters is given in SI. In the crystal structure of C2_core, A and B molecules create separate layers stabilized by C-H···O, C-H···C and π-π interactions, which form a herringbone packing motif (  Figure S11 in SI shows the 3D Hirshfeld surfaces and 2D fingerprint plots for C2_core and C3_core.
The quantitative decomposition of all intermolecular interactions shows that the C-H···H, C-H···C and C-H···O interactions are responsible for the crystal packing and formation of the three-dimensional network structure of C2_core and C3_core.

Octahedral Ni(II) complexes
Taking into account that this type of square planar Ni(II) complexes can exhibit the phenomenon of coordination-induced spin-state switching, we studied their behavior in reaction with N-heterocyclic aromatic amines. Pyridine as a reference, and its different derivatives, such as 3-methylpyridine, 4methylpyridine, 3,4-dimethylpyridine, 3,5-dimethylpyridine, 3-methoxypyridine, 4-methoxypyridine (MOPy), and 4-dimethylaminopyridine (DMAP), were tested as potential axial ligands. In the case of methoxypyridine ligands, it is known that the oxygen atom of the methoxy group does not act as Odonor atom, and thus the methoxypyridine only behaves via coordination by pyridine N-donor atom.
One of the first observations is that C2_core is particularly stable and disfavors the coordination of any axial ligands. In contrast, its structural analog with three carbon atoms in the bridge (C3_core) is less stable (as can be seen from comparison of Ni-O and Ni-N distances for C2_core and C3_core in Table   1) and creates octahedral complexes in reaction with pyridine (Py) and its derivatives. C3_core. In C3_Py system, the core part is an umbrella conformation similar to C3_core (Figure 3).
However, despite the similarities in the bend of the acacpn part (φ1 and φ2 angles) in square planar C3_core and octahedral C3_Py complexes, there is a significant difference between the relative positions of phenyl substituents (larger tilt angles in C3_Py). The bent structure of C3_Py is caused by mutual orientation of axially coordinated Py ligands, which are almost perpendicular in the complex; the twist angle between the planes passing through top and bottom pyridine rings is 88.4°. The preference of perpendicular conformation for axial pyridine ligands has been observed earlier for Ni(II) complexes. [12] Selected bond distances and other parameters for the studied octahedral complexes are reported in Table 2. In the crystal of C3_Py, the molecules are stabilized mainly by C-H···H and C-H···π interactions. The  The C3_MOPy and C3_DMAP complexes crystallize in the monoclinic P21/c space group. In C3_MOPy, the independent part of the unit cell consists of one molecule, while in C3_DMAP it contains three molecules (A, B, and C), see Figure 3 and Figures S9, S10 in SI. The preparation of C3_DMAP crystals of sufficient quality for the single crystal X-ray diffraction measurements required use of a mixture of solvents (hexane:CH2Cl2), which were also observed in the crystal structure.
In octahedral C3_MOPy Ni(II) system, geometry of the acacpn part is more planar than in C3_Py complex, and mutual orientation of the axial ligands is also significantly different, which is clearly seen in Figure 3. In C3_MOPy, the axial ligands are in trans configuration and torsionally twisted with respect to each other by 29.2°. Similar orientation of axial MOPy ligands were previously observed in other crystal structures of octahedral complexes of transition metals. [46,47] The bond lengths between the axial nitrogen atoms and nickel are nearly identical (2.193 ( Table 2). As in the other investigated structures, there are no strong hydrogen bonds in the C3_MOPy crystal. Two 4-methoxypyridine ligands interact with the phenyl ring and the axial ligand from the neighboring molecule, while the bottom ligand additionally interacts with one of methylene groups from the propylenediamine bridge. The Hirshfeld surface analysis is shown in Fig. S12.
The C3_DMAP crystal structure has disordered parts, especially in the propylenediamine bridge ( Fig.   S9 in SI). The distances between the nickel atom and equatorial donor atoms are quite similar to those found in the C3_Py and C3_MOPy crystals ( Table 2). Axial ligands in the molecules B and C have perpendicular orientation relative to each other and core is bent, in contrast to the molecule A, with axial ligands oriented almost coplanar, characterized by planar core geometry (φ1=0.59 and φ2=0.14). The twist angles for the axial ligands are 19.8°, 82.0°, and 75.8° for A, B, and C molecules, respectively, indicating conformational flexibility of the molecule. The different orientation of axial ligands is most likely caused by intermolecular interactions in crystal packing. Similar to the previously mentioned complexes, the structure of C3_DMAP crystal is mainly stabilized by C-H···H and C-H···π interactions.

Spectroscopic and thermogravimetric studies
Square planar Ni(II) complexes (C2_core and C3_core) were studied by 1 H NMR spectroscopy.
Additionally, the experiment with deuterated pyridine was performed for both complexes. No differences between 1 H NMR spectrum of C2_core and spectrum recorded after Py-d5 addition have been observed (see Fig. S13 in SI). Despite the pyridine addition, the complex is still unsubstituted, diamagnetic (S = 0), and has square planar geometry. In contrast, there are significant differences in spectra of C3_core upon addition of Py-d5shifts and broadening of the signals (Figure 4). The most noticeable changes were observed for olefinic (H8, H9, H8ʹ and H9ʹ) and aliphatic (H10A, H10B and H10ʹA, H10ʹB) protons -their signals changed position and shape. UV-Vis spectra also confirm that addition of pyridine to C3_core changes the geometry of the complex from square planar to octahedral -the maximum of absorption is shifted from 416 nm to 360 nm, respectively ( Figure 5). In case of C2_core, no changes in UV-Vis spectrum were observed.  description of the spin-state energetics, single point calculations at the S12g/TZ2P level were additionally performed. As anticipated, the ground state of C2_core and C3_core is a singlet state (S = 0), as for other square planar complexes of Ni(II). [12,13,48,49] DFT calculations revealed large singlettriplet (S-T) energy gaps for both complexes. ΔE(S-T) for C2_core is 20.9 kcal/mol, whereas for C3_core this gap is 15.2 kcal/mol at the S12g/TZ2P level of theory. Additionally, we compared the calculated geometries with the crystal structures. The most suitable indicator of geometrical similarity is the root-mean-square deviation (RMSD) of atomic positions in the optimized structure from their positions in the crystal. Figure 6 shows the RMSD values calculated for both complexes studied in singlet and triplet states. Obviously, the deviations are large for the triplet, while the structures in the singlet state are very similar to those found in the crystals. As mentioned before, C2_core does not react with pyridine to form an octahedral complex. To understand its behavior, DFT calculations were performed for the initial cores (C2_core and C3_core) with two pyridine ligands. The geometries of the complexes were optimized taking into account both spin states, although initial geometries for singlet and triplet states were identical. The calculation results show that, in the singlet state, C2_Py and C3_Py Ni(II) complexes have a sandwich-type geometry ( Figure 7). The pyridine ligands are not directly bound to the nickel atom but both are located parallel to the Ni(II) core, providing π-π interactions between N-heterocyclic aromatic amine and tetradentate ONNO part of the core system. Alternatively, the complexes in the triplet state have an octahedral geometry with direct interactions of the pyridine ligands with the nickel atom. Thus, spin state switching occurs when going from the square planar to octahedral complex. In the case of C3_Py, the optimized geometry is quite similar to that in the crystal structurethe axial ligands are perpendicular to each other (twist angle is 89.0° calc. vs. 88.4° exp.) and the main core is symmetrically bent. The overlay of the crystal and optimized structures is shown in Figure 7. In turn, optimization of C2_Py complex leads to the structure with asymmetrically bent core as a result of π-π interactions between the bottom pyridine ligand and one of the phenyl rings of the core (3.62 Å). To find a difference between octahedral C3_Py and C2_Py complexes, the bonding energy, ΔEbond, of two pyridine ligands to the Ni(II) core was calculated (Table 3) at BP86-D3/TZ2P level using a Morokuma-type energy decomposition method. [50] The overall bonding energy is made up of two major components: deformation energy (ΔEdef) and interaction energy (ΔEint). ΔEdef is the amount of energy required to deform the separate fragments from their equilibrium structures to their geometries in the final complex. ΔEint corresponds to the actual energy change when the prepared fragments are combined to form the total system. The results show that the deformation energy of ligands is very small, ΔEdef (lig) < 1 kcal/mol, and the interactions between them are repulsive in both complexes, ΔEint (lig-lig) is positive. The deformation energy of the core in C2_Py complex is significantly higher than in C3_Py due to its large distortion in the complex from the equilibrium geometry. In turn, the interaction energy between core and two axial ligands is greater in C3_Py complex. This is in agreement with Ni-N bond lengths, which are shorter in C3_Py than in C2_Py complex. As a result, this leads to a significant difference in the total bonding energy, -5.5 kcal/mol for C2_Py and -17.5 kcal/mol for C3_Py. This clearly demonstrates that the binding of pyridine ligands to the Ni(II) core is substantially more preferable in the case of C3_core. It can be assumed that interactions with neighboring C2_core molecules in the crystal are stronger than the weak coordination of axial pyridine ligands and, therefore, only square planar C2_core Ni(II) system crystallizes from pyridine solution, in contrast to the complex with C3 bridge. Table 3. Interaction energies (in kcal/mol) between core and ligands, ΔEint (core-lig), and between two ligands, ΔEint (lig-lig), deformation energies (in kcal/mol) of core, ΔEdef (core), and ligands, ΔEdef (lig), and total bonding energies (in kcal/mol), ΔEbond, for C2_Py and C3_Py complexes in triplet spin state.
* ΔEbond = ΔEint(core-lig) + ΔEint(lig-lig) + ΔEdef(core) + ΔEdef(lig) To estimate the effect of the electron-donating groups (-OCH3 and -NMe2) in para-position of pyridine on the binding of axial ligands, the interaction energies in C3_MOPy and C3_DMAP Ni(II) complexes were also calculated. The values of ΔEint are larger for C3_MOPy (-43.7 kcal/mol) and C3_DMAP (-44.3 kcal/mol) than for C3_Py (-39.2 kcal/mol). This trend is in line with the effect of substituents on the energy of σ-donating and π-accepting orbitals of pyridine, which participate in bonding interactions with transition metal (Fig. S15, SI). The methoxy and dimethylamino substituents increase the energy of the pyridine orbitals, thus providing better interactions with the nickel orbitals. All this confirms that the electron-donating substituents in pyridine facilitate the coordination of axial ligands to C3_core.
In addition, modelling of octahedral complexes with different mutual orientation of axial ligands found in crystals was carried out. When the ligands are perpendicular to each other, the acacpn Ni(II) core is strongly bent, whereas the coplanar arrangement of the ligands causes the geometry of the Ni(II) core to flatten due to steric effect (see Fig. S16 in SI). In similar crystal structures of ONNO Ni(II) complexes, the twist angle for the axial N-heterocyclic ligands in most cases exceeds 60°. [12,51,52] This is quite opposite to ONON Ni(II) type complexes, where the ligands are mostly in coplanar position to each other, and the Ni(II) core is more flattened. [53][54][55] As expected, the studied systems with mutual perpendicular orientation of the ligands are more stable than their conformers with the coplanar one (see Table S3 in SI). The bent Ni(II) core in the former case provides larger dispersion interactions between phenyl rings of the acacpn ligand and one of the axial N-heterocyclic ligands, which are responsible for the greater stability of such systems. However, according to the results of calculation, the conformers with the coplanar ligand arrangement are only slightly less preferable, ΔErel < 2 kcal/mol, and therefore were also found in the crystal structures of C3_MOPy and C3_DMAP. An interesting case is the crystal of C3_DMAP, where three molecules with slightly different conformations were observed due to molecular flexibility. The global minimum is the structure with perpendicular orientation of the axial ligands resembling the molecule B in the crystal. Two other structures obtained during the geometry optimization of the molecules A and C are the local minima, which are 1.28 and 1.42 kcal/mol higher in energy than the global minimum. Thus, the appearance of one or another conformer in the crystal structure is determined not only by their relative stability but also by intermolecular interactions in the solid state, which ultimately can stabilize the higher-energy conformer.

CONCLUSIONS
Two square planar ONNO Ni(II) complexes with two and three carbon atoms in the diamine bridge of the ligand (C2_core and C3_core) were prepared and characterized through single crystal X-ray diffraction. Using DFT calculations, it was confirmed that in the ground state both tetradentate Ni(II) complexes are low-spin (S = 0) with large singlet-triplet energy gaps.
Unexpectedly, the length of the diamine bridge in the equatorial ligand plays a crucial role in the reaction of Ni(II) complexes with N-heterocyclic aromatic amines. Only C3_core can attach substituted pyridines switching from square planar low-spin to octahedral high-spin structure. This was first indicated by a color change and then confirmed by X-ray diffraction. DFT calculations revealed that C2_core does not form an octahedral complex with pyridine due to high deformation energy of the core required for its formation.
The electron-donating substituents on the pyridine ring facilitate the coordination of axial ligands to C3_core. Moreover, complexes with mutual perpendicular orientation of the axial ligands are more stable than their conformers with coplanar orientation because of larger dispersion interactions between core and one of the axial N-heterocyclic ligands.
Our findings show that not only nature of the equatorial ligand but also its small geometric changes are important in the design of new transition metal-based complexes with coordination-induced spin-state switching behavior.

Table of Contents
Comparison of two synthesized and characterized square planar Ni(II) complexes showed that the length of the diamine bridge in the equatorial ligand plays a crucial role in the reaction with N-heterocyclic aromatic amines. Only complex with propylenediamine bridge attaches pyridine derivatives switching from square planar low-spin to octahedral high-spin structure.