Ligand-Modulated Nuclearity and Geometry in Nickel(II) Hydrazone Complexes: From Mononuclear Complexes to Acetato- and/or Phenoxido-Bridged Clusters

The propensity of 4-hydroxybenzhydrazone-related ligands derived from 3-methoxysalicylaldehyde (H2L3OMe), 4-methoxysalicylaldehyde (H2L4OMe), and salicylaldehyde (H2LH) to act as chelating and/or bridging ligands in Ni(II) complexes was investigated. Three clusters of different nuclearities, [Ni3(L3OMe)2(OAc)2(MeOH)2]∙2MeOH∙MeCN (1∙2MeOH∙MeCN), [Ni2(HL4OMe)(L4OMe)(OAc)(MeOH)2]∙4.7MeOH (2∙4.7MeOH), and [Ni4(HLH)2(LH)2(OAc)2]∙4MeOH·0.63H2O·0.5MeCN·HOAc (3∙4MeOH·0.63H2O·0.5MeCN·HOAc), were prepared from Ni(OAc)2∙4H2O and the corresponding ligand in the presence of Et3N. The hydrazones in these acetato- and phenoxido-bridged clusters acted as singly or doubly deprotonated ligands. When pyridine was used, mononuclear complexes with the square-planar geometry seemed to be favoured, as found for complexes [Ni(L3OMe)(py)] (4), [Ni(L4Ome)(py)] (5) and [Ni(LH)(py)] (6). Ligand substituent effects and the stability of square-planar complexes were investigated and quantified by extensive quantum chemical analysis. Obtained results showed that standard Gibbs energies of binding were lower for square-planar than for octahedral complexes. Starting from [MoO2(L)(EtOH)] complexes as precursors and applying the metal-exchange procedure, the mononuclear complexes [Ni(HL3OMe)2]∙MeOH (7∙MeOH) and [Ni(HLH)]∙2MeOH (9∙2MeOH) and hybrid organic–inorganic compound [Ni2(HL4OMe)2(CH3OH)4][Mo4O10(OCH3)6] (10) were achieved. The octahedral complexes [Ni(HL)2] (7–9) can also be obtained by the direct synthesis from Ni(Oac)2∙4H2O and the appropriate ligand under specific reaction conditions. Crystal and molecular structures of 1∙2MeOH∙MeCN, 2∙4.7MeOH, 3∙4MeOH∙0.63H2O∙0.5MeCN∙HOAc, 4, 5, 9∙2MeOH, and 10 were determined by the single-crystal X-ray diffraction method.


Introduction
The chemistry of multinuclear coordination compounds has been receiving ongoing interest because of their unique and attractive structures, interesting properties, and potential applications such as magnetic materials, solar energy conversion systems, and electroluminescent and photovoltaic devices [1][2][3][4][5][6][7][8][9]. Investigations span the whole range of research fields, from theoretical to biomimetic [10,11]. To date, many research groups are attempting to synthesise Ni(II) complexes bridged by carboxylate and hydroxido groups to mimic the structure and catalytic function of the active site of the native urease enzymes [12][13][14][15][16].
The formation of a specific multinuclear architecture largely relies on the choice of ligand(s) and/or reaction conditions. Thus, bridging ligands containing at least one deprotonated O-donor atom (hydroxo, alkoxo, oximato, etc.) have been successfully used for their synthesis [17,18]. Aroylhydrazone ligands ArC=N-NH-(C=O)-R with O-donor atoms have the ability to bridge metallic ions and to build a range of polymetallic

Results and Discussion
The appropriate synthetic approach, i.e., the stoichiometric ratio of Ni(II) salt and 4-hydroxybenzhydrazone-related starting compounds, the addition of triethylamine or pyridine, adequate reaction temperature, and other conditions, allowed the formation of three acetato-and phenoxido-bridged di-, tri-, and tetranuclear clusters, six mononuclear complexes, and one hybrid organic-inorganic compound (Scheme S2). It is important to mention that multinuclear clusters bearing tridentate ONO hydrazone ligands and acetatobridging ligands have not been structurally characterised previously. Only examples of trinuclear acetato-bridged nickel clusters containing tridentate Schiff base ligands have been reported so far [44][45][46][47].
The bridging ligands, such as acetate, are crucial for the construction of the clusters. However, the hydrazone ligand with phenoxido-bridges also plays an important role in maintaining the cluster structure. The assembly process is additionally sensitive to the position of the methoxy group of the ligand. Namely, in cluster 1·2MeOH·MeCN derived from L 3OMe , it is sterically in an almost perfect position to participate in complexation. The results also indicate a marked relationship between the nuclearity of Ni(II) clusters and the tautomeric form of the deprotonated ligand (Scheme S1). Thus, H 2 L 3OMe undergoes complexation via tautomerisation of the keto-form, double-deprotonation, and formation of the trinuclear cluster 1·2MeOH·MeCN. In contrast, ligands H 2 L 4OMe and H 2 L H participate in complexation via differently deprotonated forms, HL − and L 2− forms, thus yielding dinuclear and tetranuclear complexes, 2·4.7MeOH and 3·4MeOH·0.63H 2 O·0.5MeCN·HOAc, respectively. The presence of hydrazidato and hydrazonato tautomeric forms of the ligand (having =N-NH-(C=O)-and =N-N=(C-O − )-moiety, respectively) is supported by IR spectroscopy as well as by the single-crystal X-ray diffraction method. The occurrence of the different forms can be explained by the electron donor effect of the methoxy group at the ortho position, which may promote ligand tautomerisation and deprotonation during the complexation.
The molecular structure of the trinuclear complex 1·2MeOH·MeCN (Figure 1) reveals three octahedrally coordinated Ni(II) ions. The Ni1 and Ni3 atoms are coordinated by NO5 donor atoms set in an analogous manner, while the central Ni2 atom is coordinated by six oxygen atoms. The coordination sphere of Ni1 and Ni3 is built up of two hydrazonato L 2− ligands, one oxygen atom from the acetate ion, and two methanol molecules in apical positions of an octahedron. The hydrazonato ligand surrounds peripheral Ni1 and Ni3 ions, forming two chelate rings fused along Ni1-N11 and Ni3-N21 bonds. The central Ni2 atom is surrounded by two bridging phenoxido atoms O12 and O22, of two hydrazone ligands, two hydrazone methoxy O15 and O25 atoms, and the O2 and O3 atoms from two acetate ions. The methoxy and bridging phenoxido donor atoms form a five-membered chelate ring at Ni2. The bond distances values within coordination spheres (Table S1) are between 1.990(3) Å (for Ni3-N21 bond distance) and 2.169 (3) Å (for Ni3-O23 bond distance with coordinated methanol molecule), being slightly longer than in mononuclear octahedral Ni complexes 4 and 5. These coordination modes govern Ni . . . Ni separations of 3.486(1) Å and 3.468(1) Å for Ni1 . . . Ni2 and Ni2 . . . Ni3 separations, respectively, spanning nickel centres in the bent fashion (Ni1-Ni2-Ni3 angle amounts 144.76(2) • ).
The hydrazonato L 2− ligand form is evidenced by the C-O single-bond length values of the five-membered chelate ring: C11-O11 and C21-O21 of 1.296(4) and 1.289(4) Å, respectively, which are most susceptible on ligand tautomerisation. Although the methoxy group does not exhibit remarkable donor capabilities, especially in comparison with the imino nitrogen or phenoxido oxygen atoms for the metal coordination sphere, its position on the aldehyde residue (ring) allows it to participate in coordination.
Intramolecular hydrogen bonds between coordinated methanol molecules and the acetate O3 and O2 atoms, O14-H14O···O3 and O23-H23O···O2, span three nickel centres. The complex molecules are assembled into infinite chains of R 4 4 (20) centrosymmetric rings via O16-H16O···O2ME and O2ME-H2ME···O11 hydrogen bonds (O2ME-H2ME denotes O-H functionality of the non-coordinated methanol molecule) (Figure 2A). Many C-H···Otype hydrogen bonds snap complex molecules, forming crystal structures. The acetonitrile molecules are loosely bound to a trinuclear cluster by means of the C-H···N weak hydrogen bonds acting as a proton acceptor. The proton donors are the aryl -CH group and less acidic methyl C217-H217B group (Table S2). The solvent molecules are accommodated within channels formed between complex molecules in an AB alternating fashion along the b axis ( Figure 2B). The asymmetric unit of 2•4.7MeOH (Figure 3a) contains two Ni ions, two hydrazidato ligands, one acetate ion, two coordinated methanol molecules, and 4.7 methanol molecules of crystallisation. Each Ni ion is octahedrally coordinated by a NO5 donor atoms set formed by a tridentate ONO hydrazidato ligand, bridging acetate ion, and the methanol molecule. The phenoxido O12 and O22 donor atoms act as bridging ones, thus forming a Ni2O2 non-centrosymmetric moiety with a Ni1…Ni2 separation of 3.004(1) Å. The shortest bond distances are Ni-N in relation to Ni-O, which are supposed to be longer than 2.00 Å for an octahedral Ni coordination environment (Table S3). Despite the bridging function, the Ni-O bond distances are not longer than in mononuclear octahedral Ni complexes. The deviation from the regular nickel octahedral surrounding is described by trans bond angle values formed by atoms in apical positions: O1-Ni1-O13 174.27(7)° and O2-Ni2-O23 174.33(7)°. The asymmetric unit of 2·4.7MeOH (Figure 3a) contains two Ni ions, two hydrazidato ligands, one acetate ion, two coordinated methanol molecules, and 4.7 methanol molecules of crystallisation. Each Ni ion is octahedrally coordinated by a NO5 donor atoms set formed by a tridentate ONO hydrazidato ligand, bridging acetate ion, and the methanol molecule. The phenoxido O12 and O22 donor atoms act as bridging ones, thus forming a Ni2O2 non-centrosymmetric moiety with a Ni1 . . . Ni2 separation of 3.004(1) Å. The shortest bond distances are Ni-N in relation to Ni-O, which are supposed to be longer than 2.00 Å for an octahedral Ni coordination environment (Table S3). Despite the bridging function, the Ni-O bond distances are not longer than in mononuclear octahedral Ni complexes. The deviation from the regular nickel octahedral surrounding is described by trans bond angle values formed by atoms in apical positions: O1-Ni1-O13 174.27(7) • and O2-Ni2-O23 174.33 (7) • .
The C-O bond distances of the five-membered chelate ring amounting to 1.260(3) and 1.257(3) Å for O11-C11 and O21-C21, respectively, assume the keto form of the ligand. These distances are significantly shorter than C11-O11 and C21-O21 of 1.296(4) and 1.289(4) Å in 1·2MeOH·MeCN. This is additionally sustained by the crystallographically proven existence of the intermolecular hydrogen bonds via the -NH group of the central hydrazone linkage, =N-NH-(C=O)-. The -OH group of the first ligand (labelled as O14) is deprotonated, proving the electroneutrality of the complex molecule. This is confirmed by the closeness of the possible hydrogen position of that -OH group with the neighbouring hydrogen atoms positions: H14 . . . H24 of approximately 1.2 Å.
The crystal structure of 2·4.7MeOH reveals a rich variety of hydrogen bonds (Figure 4), which are established both between the complex molecules and the solvent ones ( Figure S1). This leads to the formation of a complex network of O-H···O-type hydrogen bonds (being in the O···O distance range of 2.559(2)-2.797(4) Å) and the N-H···O ones, which are additionally supported by the weaker C-H···O (range 3.206(3)-3.696(5) Å) and C-H···N (C4ME-H4MB···N12 of 3.376(3) Å) interactions. A close inspection of supramolecular architecture reveals that solvent molecules are accommodated within channels, spreading parallel to the c axis and alternating with complex molecules in an AB manner ( Figure 4b).
The AB linkage based on hydrogen bonds occurs along the b axis.  The tetranuclear core of the cubane topology of 3·4MeOH·0.63H 2 O·0.5MeCN·HOAc ( Figure 3b) is built up of four nickel centres and four phenoxido oxygen atoms situated at the apexes of a non-symmetrical cube in a manner of two penetrated Ni4 and O4 tetrahedra. In that way, the phenoxido oxygen atoms of six-membered chelate rings span three nickel centres. The cubane-like topology results in Ni1···Ni2 and Ni3···Ni4 separations of 2.957(1) Å and 2.960(1) Å, respectively. Each nickel atom is coordinated octahedrally by a NO5 donor atoms set derived from four ONO tridentate hydrazone ligands and two bridging acetate ions, each of them spanning two nickel centres.
The three solvent methanol molecules are situated in general positions, while the atoms of two methanol molecules are refined with the 0.5 occupancy factor. The unit cell also contains one half acetonitrile molecule per complex unit, which is not included in hydrogen bond formation. The oxygen atom of one of the methanol molecules is disordered over two positions within the unit cell. Accordingly, the water molecule O1W is refined with the 0.63 occupancy factor (see Experimental Section).
The presence of acetic acid instead of the acetate ion is concluded based on C-O bond geometry (1.34 and 1.29 Å) as well as clear and reasonable geometry of the HL − and L 2− ligands form within the Ni 4 L 4 core, which compensates the electroneutrality of the complex molecule. Moreover, the -OH group of the acid participates in hydrogen bond formation (O6-H6···O2ME of 2.721(14) Å; Table S2). The crystal structure is dominated, except the two abovementioned N-H···O bonds, by plenty of O-H···O ( Figure 5A) and C-H···O-type hydrogen bonds (for a detailed description, see the Supplementary Materials, Figure S2, Table S2).

Mononuclear Nickel(II) Complexes
The synthesis conditions were further modified by performing the reaction of Ni(OAc)2•4H2O and the corresponding hydrazone ligand either with the addition of pyridine (instead of Et3N) or without the addition of base. Good-quality red crystals of the mononuclear complexes [Ni(L 3OMe )(py)] (4), [Ni(L 4OMe )(py)] (5), and [Ni(L H )(py)] (6) were obtained under solvothermal conditions upon addition of pyridine to the reaction mixture in methanol. Otherwise, under reflux conditions, powdered solids deposited. A systematic variation in the pyridine amount did not affect the product or the number of coordinated pyridine ligands. Additionally, in pyridine, clusters 1-3 transform into the mononuclear complexes 4-6, respectively. Again, one hydrazone ligand is bound to Ni(II) in the tridentate fashion, and the remaining coordination sites are occupied by one pyridine molecule.
Reported structurally characterised nickel(II) complexes with a tridentate hydrazone ligand and pyridine are either in a distorted octahedral or a square planar environment ([Ni(L)(py)3] [48,49] or [Ni(L)(py)] [50,51], respectively. To investigate the possibility of the existence of octahedral complexes with three pyridine molecules, quantum chemical calculations of those theoretically built complexes were performed and standard Gibbs energies of binding were compared to the standard Gibbs energies of binding calculated for experimentally obtained square planar complexes [52]. In each case, the difference is negative (Table 1), meaning that square planar complexes with one pyridine molecule are more stable than corresponding octahedral complexes with three pyridine molecules.  The crystal structure of 3·4MeOH·0.63H 2 O·0.5MeCN·HOAc reveals two types of supramolecular channels along the a axis ( Figure 5B). One type of channel accommodates solvent molecules whose atoms are not disordered but hydrogen-bonded with complex molecules, and the other one is fulfilled with the disordered O1ME methanol molecule, which is not linked by hydrogen bonds with complex molecules (see text above and Table S2). It seems that non-hydrogen bond formation with the O1ME methanol molecule enables its spatial flexibility over two positions within channels.

Mononuclear Nickel(II) Complexes
The synthesis conditions were further modified by performing the reaction of Ni(OAc) 2 · Otherwise, under reflux conditions, powdered solids deposited. A systematic variation in the pyridine amount did not affect the product or the number of coordinated pyridine ligands. Additionally, in pyridine, clusters 1-3 transform into the mononuclear complexes 4-6, respectively. Again, one hydrazone ligand is bound to Ni(II) in the tridentate fashion, and the remaining coordination sites are occupied by one pyridine molecule.
Reported structurally characterised nickel(II) complexes with a tridentate hydrazone ligand and pyridine are either in a distorted octahedral or a square planar environment ([Ni(L)(py) 3 ] [48,49] or [Ni(L)(py)] [50,51], respectively. To investigate the possibility of the existence of octahedral complexes with three pyridine molecules, quantum chemical calculations of those theoretically built complexes were performed and standard Gibbs energies of binding were compared to the standard Gibbs energies of binding calculated for experimentally obtained square planar complexes [52]. In each case, the difference is negative (Table 1), meaning that square planar complexes with one pyridine molecule are more stable than corresponding octahedral complexes with three pyridine molecules.  2 ]·MeOH (7·MeOH) was also obtained as a solvate by this procedure, but despite many attempts, good-quality crystals suitable for single-crystal X-ray diffraction experiments were not obtained. Each time, the crystals isolated were extremely thin green plates that lose solvent molecules at room temperature. Solvent removal in 9·2MeOH and 7·MeOH resulted in the destruction of the original crystalline order resulting in either a product of decreased crystallinity or an amorphous material. The resulting materials obtained upon desolvation of 9·2MeOH and 7·MeOH gave the same IR spectra as 9 and 7, respectively, obtained upon the reaction of Ni(OAc) 2 and the corresponding ligand.
While  The molecules of complexes 4 and 5 are joined via O4-H4O···N2 intermolecular hydrogen bonds into zig-zag chains spreading along the b axis. The other H-bonds are of the C-H···O or C-H···N type, but all of them are formed with the N2 and O4 atoms as proton acceptors ( Figures S3 and S4, Table S2). The complex molecules in 9·2MeOH are mutually linked via O3-H3O···O2 intermolecular hydrogen bonds into 2D chains of hydrogen-bonded rings spreading along the b axis ( Figures S5 and S6). One methanol molecule is attached to a complex molecule via an N2-H2···O4 intermolecular hydrogen bond and with another methanol molecule via an O4-H4O···O5 intermolecular hydrogen bond with the O4 methanol oxygen atom simultaneously acting as a proton donor and proton acceptor.  (Figure 7). The complex cation formed by the crystallographically imposed inversion centre is built up of dinuclear nickel units (Ni . . . Ni separation of 3.092(1) Å) with each nickel centre octahedrally surrounded by NO5 donor atoms from one monoanionic tridentate ONO hydrazidato HL − ligand form, two coordinated methanol molecules, and the phenoxido oxygen atom from another hydrazone ligand. Bond distances are as expected for octahedral nickel dinuclear bridging complexes and are found to be similar to these in complex 2·4.7MeOH. The Ni-O(CH 3 OH) bond distances with methanol molecules are among the longest ones in the coordination sphere (Table S6)  The cations and anions are linked into a 1D zig-zag chain via the N2-H2N···O12 intermolecular hydrogen bond formed between the -NH ligand part and the terminal oxo O12 atom of the anion (Table S2; Figure 8A). The chains spread along the c axis. Another Hbond O5-H5O···O8 formed between the hydroxyl O5 group of the ligand and the terminal O8 atom of the anion joins cations and anions along the a axis into 1D chains.

Thermal Analysis
Crystals of 1·2MeOH·MeCN, 2·4.7MeOH, and 3·MeOH·0.63H 2 O·0.5MeCN·HOAc are unstable and easily lose coordinated and uncoordinated solvent molecules even at −15 • C. This can be associated with their position within the channels of their crystal structures. However, at room temperature, further destruction of the structure occurred. The mass loss corresponding to the first step in the TG curves of the residual [Ni 3 (L 3OMe ) 2 (OAc) 2 ], [Ni 2 (HL 4OMe )(L 4OMe )(OAc) 2 ], and [Ni 4 (HL H ) 2 (L H ) 2 (OAc) 2 ] was related to the decomposition of the coordinated acetato ligand. This process occurred in the range of 25-111 • C, 25-116 • C, and 25-131 • C, respectively. On further heating, the decomposition of the hydrazone ligand started at 262 • C for 1, 208 • C for 2, and 204 • C for 3. The final residue was identified as NiO.
Compounds [Ni(L)(py)] (4-6) with coordinated pyridine showed considerable thermal stability. Furthermore, grinding of samples does not change the crystal structure as seen from PXRD patterns ( Figure S7). Release of a pyridine molecule in 4 and 5 upon heating was accompanied by complex decomposition (in the range of 248-365 • C and 264-369 • C, respectively). The first step in the thermogravimetric curve of 6 was related to the loss of pyridine (243-291 • C), followed by a significant weight loss (397-401 • C) due to ligand decomposition.
The corresponding thermograms obtained for all compounds are given in Figures S9-S18.

Spectroscopic Characterisation
The spectra of complexes 7-10 showed a set of bands related to a singly deprotonated ligand, while those of 1, 4-6 displayed bands due to a doubly deprotonated ligand. However, the spectra of 2 and 3 showed two sets of bands belonging to singly and doubly deprotonated ligands [43]. IR spectra of the oligonuclear and mononuclear compounds are given in Figures S19-S22.
The band characteristic for the C=O group at, ca., 1645 cm −1 (seen in the IR spectrum of H 2 L) shifted to 1557-1540 cm −1 in the spectra of the complexes, suggesting coordination of the hydrazidato ligand through the carbonyl oxygen atom [43,53]. On the other hand, the presence of a new band in the range of 1297-1273 cm −1 , due to stretching vibrations of the C-O bond, suggested tautomerism (=N-NH-(C=O)-=N-N=(C-OH)-), deprotonation, and coordination of hydrazonato form through the oxygen atom.
In the IR spectra of the ligands, vibration bands belonging to C=N imine and C-O phenolic groups were found at, ca., 1630 cm −1 and 1355 cm −1 , respectively. In the case of the HL − ligand, these bands shifted to 1615-1604 cm −1 and 1379-1328 cm −1 . respectively. For L 2ligand, they were found at lower wave numbers (in the range 1610-1598 cm −1 and 1285-1236 cm −1 , respectively). This finding indicates the coordination of the ligands to the metal centre through the nitrogen and oxygen atoms of these two groups.

Preparative Part
Ligands H 2 L H , H 2 L 3OMe ·H 2 O, and H 2 L 4OMe ·H 2 O were prepared by the reaction of 4-hydroxybenzhydrazide with an appropriate aldehyde under mechanochemical conditions [40]. NMR spectral data are given in Table S7 together with the NMR numbering scheme (Scheme S3). Complexes [MoO 2 (L)(EtOH)] were obtained according to the procedure described in the literature [58]. Commercially available solvents and chemicals purchased from Aldrich (Amsterdam, Netherlands) were used without further purification. and assigned isotropic displacement parameters being 1.2 times larger than the equivalent isotropic displacement parameters of the parent atoms. Complex 2 contained 4.7 molecules of methanol crystallisation molecules per complex molecule. The 0.7 methanol molecule was accomplished by occupancy refinement as a free variable for atoms O1ME and C1ME. The O5M atom of one of the methanol molecules was disordered over two positions A and B, whose occupancies were refined in the ratio 0.7:0.3, which was determined due to the presence of the 0.7MeOH molecule and the short distance between the water oxygen atom and the minor position of the methanol oxygen atom, i.e., both the water and the methanol oxygen atom minor positions alternate in the 0.7:0.3 ratio within the unit cell. The hydrogen atom at O5MB was not found.
Complex 3·4MeOH·0.63H 2 O·0.5MeCN·HOAc contained five methanol molecules of crystallisation, with three of them situated in general positions and two of them refined with the atom's occupancy factor of 0.5, with half an acetonitrile molecule per complex molecule and 0.63 water molecules and one acetate acid molecule situated at general positions. The O1ME atom of one of the methanol molecules was disordered over two positions, and occupancies were refined in the ratio 63:37. In accordance with the disorder of the O1Me atom, the water molecule O1W atom occupancy was set to 0.63. The O1ME and O1W hydrogen atoms were not found in difference Fourier maps in the final stages of refinement. The geometrical calculations and structural analyses were performed using PARST [64] and MERCURY [65] programs. Drawings were made by MERCURY. Main geometrical features (selected bond distances and angles) along with hydrogen bond geometry for the structures are given in Tables S2-S7. 3.2.3. Thermal, Spectroscopic, and Magnetic Measurements Thermogravimetric (TG) analysis was carried out with a Mettler-Toledo TGA/SDTA851e thermobalance (Mettler Toledo, Columbus, OH, USA) using aluminium crucibles. All experiments were recorded in a dynamic atmosphere with a flow rate of 200 cm 3 min −1 . Heating rates of 5 K min −1 were used for all investigations. Elemental analyses were provided by the Analytical Services Laboratory of the Ruder Bošković Institute, Zagreb.
Fourier Transform Infrared spectra (FT-IR) were recorded in KBr pellets with a Perkin-Elmer Spectrum Two spectrophotometer (Waltham, MA, USA). Spectra were recorded in the spectral range between 4500 and 450 cm −1 . NMR spectra of hydrazones were recorded on a Bruker Avance III HD 400 spectrometer operating at 400 MHz equipped with a broadband observed (BBO) Prodigy cryoprobe and z-gradient accessory. Compounds were dissolved in DMSO-d 6 and measured in 5 mm NMR tubes at 298 K with TMS as an internal standard. The sample concentration was 10 mg/mL (Table S8, Scheme S3).
The magnetic measurements were carried out by a Sherwood Scientific magnetic susceptibility balance.

Quantum Chemical Calculation
Calculations of geometry optimisation and harmonic vibrational frequencies were carried out using the Gaussian 16 program package [66]. Geometry optimisation for ground states were performed using hybrid functionals B3LYP [67,68] with the D3 version of Grimme's dispersion [69] in combination with the 6-31G(d) basis set. For all optimised structures, harmonic frequencies were calculated to ensure that obtained geometries correspond to the minimum on the potential energy surface. The standard Gibbs energies were calculated at T = 298.15 K and p = 101325 Pa. In all nickel(II) complexes, nickel was in the low-spin (singlet) state.
Absence of the bridging ligands results in the formation of the mononuclear complexes [Ni(HL) 2 ] (7-9). When pyridine is used, the ligands coordinate only one nickel atom and the square-planar geometry is favoured as found for complexes [Ni(L)(py)] (4-6). Quantum chemical calculations reveal the stability of square planar complexes in comparison to the corresponding octahedral complexes. In each case, the standard Gibbs energies of binding are lower for square planar than for octahedral complex in a range of −70 to −80 kJ mol −1 . The mononuclear complexes 7·MeOH, 9·2MeOH, and hybrid organicinorganic compound [Ni 2 (HL 4OMe ) 2