A tetranuclear nickel(II) complex, [Ni4(L)4](ClO4)4·C2H3N·2H2O, with an asymmetric Ni4O4 open-cubane-like core

A tetranuclear complex with an open-cubane like structure was synthesized from 2-methoxy-6-(pyridin-2-yl-hydrazonomethyl)-phenol and characterized using micro-analytical and spectroscopic techniques, and single-crystal X-ray diffraction analysis.


Chemical context
Polynuclear metal(II) complexes have attracted much attention owing to their structural variety and significant applications in biology, catalysis, molecular recognition and magnetism (Alcantara et al., 2006;Powell, 2003). As such, complexes containing a tetranuclear cubane-like core have been an important class of compounds (Yang et al., 2005). The synthesis of such polynuclear metal complexes can often be promoted with the use of polydentate Schiff base ligands possessing nitrogen and oxygen donor atoms. Such Schiff bases are known to form high nuclearity complexes with interesting architectures, and the hydroxyl groups and other donor atoms are often suitable for the synthesis of polynuclear complexes (Gungor & Kara, 2015;Dutta et al., 2020;Shit et al., 2013). Several tetranuclear nickel(II) complexes have also been synthesized and their different electronic properties explored (Lin et al., 2011;Nihei et al., 2003;Zhang et al., 2012;Liu et al., 2012;Shit et al., 2013). As part of our study of polynuclear complexes, we have been interested in cubanelike structures to build complexes with high nuclearity (Ray et al., 2009;Chakraborty et al., 2009;Sagar et al., 2017;Pouralimardan et al., 2007;Patel et al., 2019). In this article, the results were obtained with the Schiff base ligand (HL) 2-methoxy-6-(pyridin-2-yl-hydrazonomethyl)-phenol, which can bind one or two metal ions, simultaneously. The stoichiometric reaction of nickel(II) perchlorate hexahydrate with this ligand resulted the formation of Ni 4 O 4 distorted cubanelike structure described herein.
geometries around each Ni centre are slightly distorted from a perfect square-pyramidal environment. The hydroxyl group of each HL phenol is deprotonated and the oxygen atoms bridge two nickel centres. Similarly, the oxygen atom of the methoxy group coordinates to a second nickel centre in a 2 -mode. Each nickel centre is connected to the 2 -oxygen atoms, resulting in the construction of an Ni 4 O 4 cubane-like core (Fig. 2). The basal plane of each nickel centre is constituted by one phenoxy oxygen, one methoxy oxygen, one azomethine nitrogen and one pyridine nitrogen atom. As a result of its weakly coordinating nature, each methoxy oxygen remains in an axial position. The Ni-N/O bond lengths are in the range 1.932 (7)-1.988 (5) Å and are very close to these reported for similar tetranuclear cubane-coretype complexes (Zhang et al., 2011(Zhang et al., , 2013Yu et al., 2011;Tong et al., 2002;Mandal et al., 2008;Clemente-Juan et al., 2000;Li et al., 2006;Sun et al., 2011;Saha et al., 2014;Yang et al., 2006).

Supramolecular features
In the polynuclear crystal, intermolecular hydrogen-bonding interactions are detected involving C-H and N-H donors from the hydrazone Schiff base and acceptor oxygen atoms of perchlorate counter-ions and solvate water molecules (Fig. 3). The important hydrogen-bonding parameters are collected in Table 2. The two tetranuclear complexes are interconnected through intermolecular hydrogen bonding between C-HÁ Á ÁO and N-HÁ Á ÁO hydrogen bonds with the perchlorate ions, forming heterosynthons (Fig. 3). Additionally, oxygen atoms of solvate water molecules also act as acceptor atoms for intermolecular hydrogen bonds. Furthermore, stabilization of the tetranuclear crystal lattice is facilitated by the presence of various weak (aryl-aryl, aryl-chelate and chelate-chelate) intramolecular stacking interactions (Fig. 4). The orthorhombic cell contains four formula units, and the packing is shown in Fig. 5. The entire stacking pattern reveals that the intermolecular hydrogen bonds remain between perchlorate counter-ions and C-H/N-H moieties of the same molecule or adjacent molecules. Similarly, solvate water molecules also exert cooperative intermolecular hydrogen bonds from C-H/ N-H moieties of the complex, and the crystal lattice is also stabilized viastacking interactions [centroid-centroid distances = 3.343 (3)-3.668 (3) Å ].

Refinement
Crystallographic data and refinement details are presented in Table 3. H atoms were located in difference-Fourier maps and constrained to ride on their parent atoms with with C-H bond distances of 0.95 Å (aromatic H), 0.98 Å (methyl H) and 0.88 Å (N-H) and were refined as riding with isotropic displacement parameters 1.2 and 1.5 times those of the parent C/N atoms. Water H atoms were refined isotropically with Uĩso(H) = 1.5Ueq(O). Three of the four perchlorate anions are disordered over two orientations and were restrained to have tetrahedral geometries with occupancies of 0.57 (6) Data collection: APEX2 (Bruker, 2005); cell refinement: SAINT (Bruker, 2002); data reduction: SAINT (Bruker, 2002); program(s) used to solve structure: SHELXT (Sheldrick 2015a); program(s) used to refine structure: SHELXL2018/3 (Sheldrick, 2015b); molecular graphics: SHELXTL (Sheldrick 2008); software used to prepare material for publication: SHELXTL (Sheldrick 2008).

cyclo-Tetrakis(µ-2-methoxy-6-{[2-(pyridin-2-yl)hydrazin-1-ylidene]methyl}phenolato)tetranickel(II)
tetrakis ( (5) 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.