A tetranuclear cubane-like nickel(II) complex with a tridentate salicylideneimine Schiff base ligand: tetrakis[μ3-4-methyl-N-(2-oxidophenyl)salicylideneiminato]tetrakis[methanolnickel(II)] methanol 0.8-solvate

In the continuation of our research on Ni4 L 4 cubane-like clusters with salicylideneimine Schiff base type of ligands, the crystal and molecular structure of an analogous complex of formula [Ni4 L 4(CH3OH)4]·0.8CH3OH [L = N-(2-hydroxy-4-methylphenyl)salicylideneimine] is reported in order to investigate the influence of the methyl-group position of the ligand on the geometry of the cubane core, as well as on the supramolecular assembling of the cluster units and consequently on the magnetostructural properties of this class of compounds.

The tetranuclear title complex, [Ni 4 (C 14 H 11 NO 2 ) 4 (CH 3 OH) 4 ]Á0.8CH 3 OH, has a distorted cubane topology shaped by four Schiff base ligands. The cubane  )] core is formed via the O atoms from the Schiff base ligands. The octahedrally coordinated Ni II ions occupy alternating vertices of the cube. Each Ni II ion is coordinated by one O,N,O 0 -tridentate dianionic ligand, two O atoms of oxidophenyl groups from adjacent ligands and the O atom of a coordinating methanol molecule. The cubane core is stabilized via an intramolecular O-HÁ Á ÁO hydrogen bond between the hydroxy group of the coordinating methanol molecules and the phenolate O atom of the aldehyde Schiff base fragment. Additional stabilization is obtained via intramolecular C-HÁ Á ÁO hydrogen bonds involving aromatic C-H groups and the oxygen atoms of adjacent methanol molecules. In the crystal, complex molecules are linked into chains parallel to the c axis via weak C-HÁ Á ÁO hydrogen bonds. The partial-occupancy disordered methanol solvent molecule has a site occupancy of 0.8 and is linked to the tetranuclear unit via an intermolecular C-HÁ Á ÁO hydrogen bond involving a phenolate O atom.

Chemical context
Octahedrally coordinated Ni II atoms are paramagnetic and spanned by an appropriate bridging ligand. They can be organized into polynuclear units of different nuclearity with potential practical applications as nanomagnetic devices, switches and sensors or single-molecule magnets (Ji et al., 2009;Karmakar & Khanra, 2014;Kou et al., 2010;Osa et al., 2004;Perlepe et al., 2014;Pardo et al., 2008;Papatriantafyllopoulou et al., 2008;Polyakov et al., 2012). One of the major requirements in designing single-molecule magnets (SMM) is to obtain slight structural changes in enduring metal-organic frameworks. The important subject in this field is the relationship between the magnetic behaviour of the molecule and its microenvironment. It is known that any symmetry decrease manifested as reduced symmetry of the arrangement of ligands around metal atoms (no imposed crystallographic symmetry within complex molecule), crystallographic disorders of terminal groups of the ligand molecules, existence of two or more crystallographically independent complex molecules in one asymmetric unit or weakly interacting solvent molecules (Lawrence et al., 2008;Cotton et al., 2007) influences the magnetic properties strongly. Although it has been ISSN 2056-9890 shown that Ni 4 O 4 cubane-like Ni units have a rather robust structure with persistent geometrical parameters, even weak interactions influence their magnetic behaviour, causing almost indiscernible distortions of the cubane core. The particular importance of the Ni-3 -O-Ni bond angles is emphasized in the modelling of the intramolecular magnetic interactions. Previous investigations showed that ferromagnetic interactions are associated with angles close to 90 and antiferromagnetic interactions with larger angles (Ballester et al., 1992;Bertrand et al., 1978;Gladfelter et al., 1981;Halcrow et al., 1995;Petit et al., 2012;Zhang et al., 2012). Therefore, the cubane Ni 4 L 4 topology represents a plethora of possibilities in the design of single-molecule magnets.

Structural commentary
In the title compound, each Ni II ion ( Fig. 1)

Supramolecular features
The coordinating methanol molecules participate in the formation of intramolecular hydrogen bonds with the phenolate O atoms of the Schiff base ligand (O11, O21, O31 and O41). These intramolecular hydrogen bonds (Table 1 The molecular structure of the title compound, with displacement ellipsoids drawn at the 50% probability level. The edges of the Ni 4 O 4 cubane are denoted in violet.

Table 1
Hydrogen-bond geometry (Å , ). The methanol molecule of crystallization interacts with the complex units via an intermolecular hydrogen bond with the phenolate O31 atom. In the crystal, the Ni 4 L 4 complex molecules are linked into chains running parallel to the c axis by weak C-HÁ Á ÁO hydrogen bonds between the C46 aromatic carbon atom and the O11 phenolate oxygen atom (Table 1). In the framework of our research on this type of Ni 4 L 4 units, we have published analogous Ni 4 L 4 cubane-like units with the N-(2-hydroxy-5-methylphenyl)salicylideneimine ligand (Cindrić et al., 2016). In these compounds, similar C-HÁ Á ÁO hydrogen bonds involving an aromatic C-H group and one phenolate oxygen atom result in the formation of discrete centrosymmetric dimers.

Synthesis and crystallization
The title compound was prepared by mixing a methanolic solution of Ni(O 2 CMe) 2 Á4H 2 O (1 mmol in 10 ml) and a methanolic solution of the Schiff base ligand N-(2-hydroxy-4methylphenyl)salicylideneimine (1 mmol in 10 ml) at room temperature. After two days, green prismatic single crystals suitable for X-ray analysis were obtained on slow evaporation of the solvent.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. The methanol molecule is disordered and was refined with a site-occupancy factor of 0.80. The C-bound hydrogen atoms were placed in geometrically idealized positions and constrained to ride on their parent atoms, with C-H = 0.93-0.96 Å , and with U iso (H) = 1.2U eq (C) or 1.5U eq (C) for methyl H atoms. A rotating model was used for the methyl groups. The hydroxy H atoms of the coordinating methanol molecules were firstly found in a difference Fourier map and then refined by constraining the C-H bond length to be 0.84 (2) Å and the isotropic displacement parameters to be 1.2 times the equivalent isotropic displacement parameters of the parent oxygen atoms. The hydroxy H atom of the disordered methanol molecule was located in a difference Fourier map and refined with fixed coordinates and U iso (H) = 1.5U eq (O). Displacement restraints (SIMU and DELU; Sheldrick, 2015) were applied to the disordered partial methanol molecule.

Tetrakis[µ 3 -4-methyl-N-(2-oxidophenyl)salicylideneiminato]tetrakis[methanolnickel(II)] methanol 0.8-solvate
Crystal data where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 0.94 e Å −3 Δρ min = −0.52 e Å −3 Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.