Formation of Tetranuclear Nickel(II) Complexes with Schi ﬀ -Bases: Crystal Structures and Magnetic Properties

: The cubane-type structure is a typical representative of tetranuclear coordination compounds. In this work, two anionic Schi ﬀ -base ligands, (L 1 ) 2 − and (L 2 ) 2 − , each o ﬀ ering an OˆNˆO coordination pocket, ligate four Ni II ions into a [Ni 4 O 4 ] cubane core. The ligands are H 2 L 1 = 2 − [[(3-ethoxy-2 − hydroxyphenyl) methylene]amino]benzenemethanol and H 2 L 2 = 2 − [[(5-ﬂuoro-2 − hydroxyphenyl)methylene]amino]benzenemethanol. In both compounds, [Ni 4 (L 1 ) 4 (EtOH) 4 ] ( 1 ) and [Ni 4 (L 2 ) 4 (MeOH) 4 ] ( 2 ), alkoxy oxygens of the ligands act in a bridging µ 3 -O binding mode. Magnetic susceptibility and magnetization data for compounds 1 and 2 are presented. The Ni–O–Ni bond angles of the cubane core determined from single crystal X-ray di ﬀ raction data play a key role for a magneto-structural correlation. Dominant intracube ferromagnetic behavior is observed, and the coupling parameters were determined for both compounds, leading to nonzero spin ground states in accordance with the broadly accepted bond angle guideline. ligands bind to the Ni II ions of the cluster cores.

In this paper, we report the synthesis, characterization, and magnetic properties of two cubane-type complexes with stoichiometries [Ni 4 (L 1 ) 4 (EtOH) 4 ] (1) and [Ni 4 (L 2 ) 4 (MeOH) 4 ] (2), both based on anionic Schiff-base ligands (L 1 ) 2− and (L 2 ) 2− , respectively ( Figure 1). The magnetic susceptibility and magnetization data were determined, and the former were fitted based on a Heisenberg Hamiltonian with two different coupling parameters J 1 and J 2 in the case of 1, and with one J parameter for 2. Both compounds are found to be in a ferromagnetic coupling regime, in agreement with their structural parameters.
For angles above 99°, the magnetic coupling between the Ni II ions is antiferromagnetic, but ferromagnetic for smaller angles. To note, structural distortions may affect this kind of guideline.
In this paper, we report the synthesis, characterization, and magnetic properties of two cubanetype complexes with stoichiometries [Ni4(L 1 )4(EtOH)4] (1) and [Ni4(L 2 )4(MeOH)4] (2), both based on anionic Schiff-base ligands (L 1 ) 2− and (L 2 ) 2− , respectively ( Figure 1). The magnetic susceptibility and magnetization data were determined, and the former were fitted based on a Heisenberg Hamiltonian with two different coupling parameters J1 and J2 in the case of 1, and with one J parameter for 2. Both compounds are found to be in a ferromagnetic coupling regime, in agreement with their structural parameters.

Materials
3-Ethoxysalicylaldehyde, 5-fluorosalicylaldehyde, and 2−aminobenzylalcohol were purchased from Sigma Aldrich, USA. Nickel acetate tetrahydrate and solvents employed for the syntheses were of analytical grade and used as received without further purification.

General Methods
Elemental analyses were performed on a 240C elemental analyzer (Perkin-Elmer, USA). IR spectra were recorded on an FT/IR-4000 spectrometer (Jasco, Japan) as KBr pellets in 4000-400 cm -1 region. UV-vis spectra were recorded on a Lambda 35 spectrometer (Perkin-Elmer, USA). 1 H and 13 C NMR spectra were recorded on a 500 MHz spectrometer (Bruker, Germany). Single crystal X-ray diffraction was carried out on an Apex II CCD area diffractometer (Bruker, Germany). Powder X-ray diffraction patterns were measured on a StadiP diffractometer (STOE, Darmstadt, Germany) in Debye Scherrer geometry.

Synthesis of H2L 1
Similar to the previously reported procedure [44,45], 3-ethoxysalicylaldehyde (1.66 g, 0.01 mol) was rected with 2−aminobenzylalcohol (1.23 g, 0.01 mol) in methanol (30 mL). The mixture was stirred at room temperature for 1 h to give a yellow solution, which was evaporated by distillation to give a yellow solid product. The solid was recrystallized from methanol to give the crystalline product H2L 1

Materials
3-Ethoxysalicylaldehyde, 5-fluorosalicylaldehyde, and 2−aminobenzylalcohol were purchased from Sigma Aldrich, USA. Nickel acetate tetrahydrate and solvents employed for the syntheses were of analytical grade and used as received without further purification.

General Methods
Elemental analyses were performed on a 240C elemental analyzer (Perkin-Elmer, USA). IR spectra were recorded on an FT/IR-4000 spectrometer (Jasco, Japan) as KBr pellets in 4000-400 cm −1 region. UV-vis spectra were recorded on a Lambda 35 spectrometer (Perkin-Elmer, USA). 1 H and 13 C NMR spectra were recorded on a 500 MHz spectrometer (Bruker, Germany). Single crystal X-ray diffraction was carried out on an Apex II CCD area diffractometer (Bruker, Germany). Powder X-ray diffraction patterns were measured on a StadiP diffractometer (STOE, Darmstadt, Germany) in Debye Scherrer geometry.

Synthesis of H 2 L 1
Similar to the previously reported procedure [44,45], 3-ethoxysalicylaldehyde (1.66 g, 0.01 mol) was rected with 2−aminobenzylalcohol (1.23 g, 0.01 mol) in methanol (30 mL). The mixture was stirred at room temperature for 1 h to give a yellow solution, which was evaporated by distillation to give a yellow solid product. The solid was recrystallized from methanol to give the crystalline product H 2 L 1 . Yield: 92%. Elemental analysis (%) calcd for C 16 5-Fluorosalicylaldehyde (1.40 g, 0.01 mol) was reacted with 2−aminobenzylalcohol (1.23 g, 0.01 mol) in methanol (30 mL). The mixture was stirred at room temperature for 1 h to give a yellow solution, which was evaporated by distillation to give a yellow solid product. The solid was recrystallized from methanol to give the crystalline product H 2 L 2 . Yield: 94%. Elemental analysis (%) calcd for C 14

Synthesis of [Ni 4 (L 2 ) 4 (MeOH) 4 ] (2)
H 2 L 2 (24.5 mg, 0.1 mmol) was reacted with nickel acetate tetrahydrate (24.9 mg, 0.1 mmol) in methanol (20 mL). The mixture was stirred at room temperature for 30 min to give a green solution, which was allowed to stand in air for a few days until three quarter of the solvent was evaporated. Green block-shaped single crystals of the complex were formed at the bottom of the vessel. The crystals were isolated by filtration and dried in air. Yield: 23%. Elemental analysis (%) calcd for C 60

X-ray Crystallography
Diffraction intensities for the complexes were collected at 298(2) K using a Bruker Apex II CCD area-detector diffractometer with MoKα radiation (λ = 0.71073 Å). Collected data were reduced with SAINT [46], and multiscan absorption correction was performed using SADABS [47]. Structures of the complexes were solved by direct methods and refined against F 2 by full-matrix least-squares method using SHELXTL [48]. All nonhydrogen atoms were refined anisotropically. The hydrogen atoms were placed in calculated positions and constrained to ride on their parent atoms. The ethanol ligand C36-C35-O8 of 1 is disordered over two sites, with occupancies of 0.385(2) and 0.615 (2). Crystallographic data for the complexes are summarized in Table 1. Selected Ni-L bond lengths and Ni-O-Ni angles of 1 are given in Table 2. Further L-Ni-L and Ni-O-Ni angles of 2 are reported in Supplementary Materials Table S1.

Synthesis and Characterization
The Schiff-bases H 2 L 1 and H 2 L 2 were readily prepared by the condensation reaction of 2−aminobenzylalcohol with 3-ethoxysalicylaldehyde and 5-fluorosalicylaldehyde, respectively, in methanol. The nickel complexes 1 and 2 were prepared by the reaction of nickel acetate tetrahydrate with H 2 L 1 in ethanol, and with H 2 L 2 in methanol, respectively. Consequently, ethanol and methanol molecules were found as terminal ligands in the coordination environment of 1 and 2, respectively (vide infra).
The infrared spectra of the complexes and the free Schiff-bases were recorded in the region 4000-400 cm −1 . The intense absorption bands at 1606-1608 cm −1 in the spectra of the complexes are assigned to the imine stretching frequency of the Schiff-base ligands [49]. The shift of the bands to lower frequency, compared to the free Schiff-bases (1612-1620 cm −1 ), indicates the coordination of the imine nitrogen atom to the Ni II ion [49,50]. The phenolic ν Ar-O in the free Schiff-bases exhibits strong bands at 1245-1255 cm −1 , whereas these bands are observed in the lower frequency region at 1183-1192 cm −1 in the complexes, indicating the coordination to the nickel atoms through the phenolate oxygen atoms [51]. The Schiff-base coordination to the nickel atoms is substantiated by prominent bands appearing at low wave numbers of 450-620 cm −1 , which can be attributed to ν(Ni-N) and ν(Ni-O) [52].
The UV-vis absorption bands observed around 415-420 nm can be attributed to the transition from the coordinated Schiff-base ligands to the nickel atoms (LMCT) [53]. The bands centered at 241-243 and 300-310 nm may be assigned to the intraligand π-π* and n-π* transitions, respectively [54]. The broad low-intensity absorption bands centered at 570-580 nm are typical d-d bands for the nickel atoms [55].

Structural Description of the Complexes
Compounds 1 and 2 crystallize in the monoclinic space groups C2/c and P2 1 /n, respectively, and their molecular structures are shown in Figure 2. Both clusters present analogous stoichiometries, namely, [Ni 4 (L 1 ) 4 (EtOH) 4 ] for 1 and [Ni 4 (L 2 ) 4 (MeOH) 4 ] for 2, and they reveal the same structural architecture and binding configuration of their ligands. Therefore, the structural description is focused on compound 1. Its main element consists in a [Ni 4 O 4 ] cubane core, which resides on a crystallographic two-fold rotation axis. The Ni II ions are connected by alkoxy oxygens from four anionic Schiff-base ligands (L 1 ) 2− exhibiting four µ 3 -O binding modes. It is worth noting the bridging Ni-O-Ni angles within the cubane core, which amount to 94.  Figure 3 illustrates the cubane core of 1 and the binding mode of the deprotonated ligand (L 1 ) 2− with the Ni II ion. The ligand chelates in a nearly coplanar fashion the metal ion in an OˆNˆO coordination pocket. The alkoxy oxygen connects three adjacent Ni II ions in a bridging µ 3 -O binding mode, whereas the phenolate oxygen binds monodentate to a Ni II ion only. In the cluster, the metal ion resides in a slightly distorted octahedral NO 5 coordination geometry, comprising oxygens from three alkoxy, one phenolate, and one ethoxy group from a ligated solvent molecule, besides one imino nitrogen. We note also that the hydroxyl hydrogen atoms of the solvent ligands form intracluster hydrogen bonds with adjacent phenolate oxygen atoms ( Figure 2 and Table 3). Table 3. Hydrogen bond distances (Å) and bond angles ( • ) for complexes 1 and 2. molecule, besides one imino nitrogen. We note also that the hydroxyl hydrogen atoms of the solvent ligands form intracluster hydrogen bonds with adjacent phenolate oxygen atoms ( Figure 2 and Table  3).   In contrast to compound 1, there is no crystallographic two-fold rotation axis imposed on cluster 2, which results in an additional scant variation of the structural parameters. The binding mode of (L 2 ) 2− with the Ni II ion for 2 is shown in Supplementary Materials Figure S1. The mean bridging Ni-O-Ni angles within the cubane core of 2 taken from all four μ3-O atoms amount to 95.4°, 98.3°, and 100.3°, and are thus quite similar to those for 1. The crystal packing of both compounds shows no special feature, and due to the bulky ligand shell around the cubane core, the metal centers on neighbouring molecules are distant from each other (> 8.5 Å), which minimizes any intercluster magnetic coupling.

D-H···A d(D-H) d(H···A) d(D···A) Angle (D-H···A)
In the crystal structure of 1 (Figure 4a), the molecules are stacked along the b-axis direction. In the crystal structure of 2 (Figure 4b), two adjacent molecules are linked through intermolecular In contrast to compound 1, there is no crystallographic two-fold rotation axis imposed on cluster 2, which results in an additional scant variation of the structural parameters. The binding mode of (L 2 ) 2− with the Ni II ion for 2 is shown in Supplementary Materials Figure S1. The mean bridging Ni-O-Ni angles within the cubane core of 2 taken from all four µ 3 -O atoms amount to 95.4 • , 98.3 • , and 100.3 • , and are thus quite similar to those for 1. The crystal packing of both compounds shows no special feature, and due to the bulky ligand shell around the cubane core, the metal centers on neighbouring molecules are distant from each other (>8.5 Å), which minimizes any intercluster magnetic coupling.
In the crystal structure of 1 (Figure 4a), the molecules are stacked along the b-axis direction. In the crystal structure of 2 (Figure 4b), two adjacent molecules are linked through intermolecular C−H···F interactions, forming dimers.
In contrast to compound 1, there is no crystallographic two-fold rotation axis imposed on cluster 2, which results in an additional scant variation of the structural parameters. The binding mode of (L 2 ) 2− with the Ni II ion for 2 is shown in Supplementary Materials Figure S1. The mean bridging Ni-O-Ni angles within the cubane core of 2 taken from all four μ3-O atoms amount to 95.4°, 98.3°, and 100.3°, and are thus quite similar to those for 1. The crystal packing of both compounds shows no special feature, and due to the bulky ligand shell around the cubane core, the metal centers on neighbouring molecules are distant from each other (> 8.5 Å), which minimizes any intercluster magnetic coupling.
In the crystal structure of 1 (Figure 4a), the molecules are stacked along the b-axis direction. In the crystal structure of 2 (Figure 4b), two adjacent molecules are linked through intermolecular C−H···F interactions, forming dimers.

Magnetic Properties
The temperature dependence of the magnetic susceptibilities of the complexes 1 and 2 were each measured on powder samples over the temperature range 1.9-300 K in a 1 kOe magnetic field. Transmission powder X-ray analysis was utilized to ensure that the single-crystal data were representative of the bulk material (Supplementary Materials Figure S2). At room temperature, the χ M T product of 1 and 2 amounts to 5.9 and 5.8 cm 3 K mol −1 , respectively, and is thus greater than the spin-only value of 4.0 cm 3 K mol −1 for four noninteracting Ni II ions with S = 1 and g = 2. Between 300 and 100 K, the χ M T product for 1 and 2 increases slowly; when below 100 K the values increase more rapidly reaching a value of 17 and 24 cm 3 K mol −1 , respectively (Figures 5a and 6a). This increase in the χ M T product is indicative of dominant intracube ferromagnetic interactions between the paramagnetic centers. This goes in line with the 1/χ M vs. T plot, where the data above 200 K follow the Curie-Weiss law with a positive Weiss constant θ of 7.0 and 6.8 K, respectively ( Figure S3).
The field dependence of the magnetization at 1.9 K for 1 and 2 is shown in Figures 5b and 6b. The magnetization shows a rapid increase up to a field of 10 kOe, after which it increases only gradually reaching at 50 kOe values of 8.8 and 7.7 µ B , respectively. The initial steep increase of the magnetization points as well to intramolecular ferromagnetic interactions.
spin-only value of 4.0 cm 3 K mol -1 for four noninteracting Ni II ions with S = 1 and g = 2. Between 300 and 100 K, the χMT product for 1 and 2 increases slowly; when below 100 K the values increase more rapidly reaching a value of 17 and 24 cm 3 K mol -1 , respectively (Figures 5a and 6a). This increase in the χMT product is indicative of dominant intracube ferromagnetic interactions between the paramagnetic centers. This goes in line with the 1/χM vs T plot, where the data above 200 K follow the Curie-Weiss law with a positive Weiss constant θ of 7.0 and 6.8 K, respectively ( Figure S3). The field dependence of the magnetization at 1.9 K for 1 and 2 is shown in Figures 5b and 6b. The magnetization shows a rapid increase up to a field of 10 kOe, after which it increases only gradually reaching at 50 kOe values of 8.8 and 7.7 μB, respectively. The initial steep increase of the magnetization points as well to intramolecular ferromagnetic interactions.
Given that out of the twelve Ni-O-Ni angles per cluster, eight lie in the ferromagnetic and only four in the antiferromagnetic regime, it seems useful to apply a Heisenberg Hamiltonian containing two coupling parameters, J1 and J2, rapidly reaching a value of 17 and 24 cm 3 K mol -1 , respectively (Figures 5a and 6a). This increase in the χMT product is indicative of dominant intracube ferromagnetic interactions between the paramagnetic centers. This goes in line with the 1/χM vs T plot, where the data above 200 K follow the Curie-Weiss law with a positive Weiss constant θ of 7.0 and 6.8 K, respectively ( Figure S3). The field dependence of the magnetization at 1.9 K for 1 and 2 is shown in Figures 5b and 6b. The magnetization shows a rapid increase up to a field of 10 kOe, after which it increases only gradually reaching at 50 kOe values of 8.8 and 7.7 μB, respectively. The initial steep increase of the magnetization points as well to intramolecular ferromagnetic interactions.
Given that out of the twelve Ni-O-Ni angles per cluster, eight lie in the ferromagnetic and only four in the antiferromagnetic regime, it seems useful to apply a Heisenberg Hamiltonian containing two coupling parameters, J1 and J2, Given that out of the twelve Ni-O-Ni angles per cluster, eight lie in the ferromagnetic and only four in the antiferromagnetic regime, it seems useful to apply a Heisenberg Hamiltonian containing two coupling parameters, J 1 and J 2 , which is based on the coupling pattern as shown in Scheme 1. The Zero Field Splitting for the ground state is taken into account with the Hamiltonian Crystals 2020, 10, x FOR PEER REVIEW 9 of 13 H = -2J1(S1S2 + S3S4) -2J2(S1S3 + S1S4 + S2S3 + S2S4) which is based on the coupling pattern as shown in Scheme 1. The Zero Field Splitting for the ground state is taken into account with the Hamiltonian H = D[Sz 2 -S(S + 1)/3] Scheme 1. Coupling scheme for 2; for 1, the cluster lies on a two-fold rotation axis and thus Ni2 is equivalent to Ni1 and labeled Ni1'; correspondingly, Ni3 and Ni4 are equivalent and labeled as Ni2, with Ni2' in the structural data, see Figure 3b. This coupling scheme with the corresponding energy levels, including Van Vlecks' equation for the magnetic susceptibility and the zero field splitting Hamiltonian, has already been described by Escuer et al [56]. For 1, the best fit to the χMT product was achieved with parameter values for the gfactor g = 2.28, the antiferromagnetic exchange parameter J1 = −7.6 K, and the ferromagnetic exchange parameter J2 = 15.1 K. The resulting spin ground state, ST = 4, shows a small zero field splitting D = 0.12 K, which is responsible for the continuous increase of the χMT values towards lowest by Escuer et al. [56]. For 1, the best fit to the χ M T product was achieved with parameter values for the g-factor g = 2.28, the antiferromagnetic exchange parameter J 1 = −7.6 K, and the ferromagnetic exchange parameter J 2 = 15.1 K. The resulting spin ground state, S T = 4, shows a small zero field splitting D = 0.12 K, which is responsible for the continuous increase of the χ M T values towards lowest temperatures, see Figure 5. The fit returns well-defined values for g and D; the parameters J 1 and J 2 are counteracting and exhibit larger error bars. The antiferromagnetic exchange J 1 is attributed to the Ni1-O2-Ni1 and Ni2-O5-Ni2 paths with Ni-O-Ni angles close to 101.5 • , see Figure 3a and Table 2. The ferromagnetic exchange J 2 represents an average over the remaining four paths, with Ni-O-Ni angles ranging from 94 • to 98 • .
For the cubane core of 2, the χ M T fit results in g = 2.33, J = 5.1 K, and D = 0.33 K, see Figure 6a. As for 1, the fit yields the spin ground state S T = 4 and returns well-defined g and D parameters. However, an individual fit of J 1 and J 2 parameters was impossible. These parameters are strongly correlated, and only a single average J could be obtained. The cubane core of 2 exhibits no twofold rotation axis, as present in 1. The reduced symmetry of 2 results in four individual Ni II ions, see Scheme 1, and twelve different exchange paths, see Supplementary Materials Table S1. The Ni-O-Ni angles aggregate in three groups around values of 95.4 • , 98.3 • , and 100.3 • , which is similar to 1. Interestingly, a single ferromagnetic J parameter well describes the data. This J parameter matches well the values determined for a Ni II cubane cluster with a similar Schiff-base ligand. 57 Overall, all the obtained parameters compare well with the range of parameters given in the literature for other Ni II cubane clusters [56][57][58][59][60][61][62][63].

Conclusions
In summary, two cubane-type compounds comprising a [Ni 4 O 4 ] core and two different chelating Schiff-base ligands that offer OˆNˆO coordination pockets have been synthesized and structurally as well as magnetically characterized. Both compounds show dominant intracube ferromagnetic interactions leading to a nonzero spin ground state. It has been demonstrated that even slight structural rearrangements of the cubane core due to a different substitution pattern of the ligands lead to a noticeable variation in strengths of the magnetic coupling between Ni(II) ions. The coupling parameters, however, correlate well with the Ni-O-Ni angles determined from single crystal structure analyses.