Crystal structure of {μ-6,6′-dimethoxy-2,2′-[ethane-1,2-diylbis(nitrilomethanylylidene)]diphenolato}(methanol)(nitrato)nickel(II)sodium

Two phenolate O atoms provided by a Schiff base ligand create a double bridge between Ni2+ and Na+ ions. The coordination environment of the Ni2+ ion is square-planar and it has an unusual seven-coordinated geometry: four atoms from the Schiff base ligand, two from a nitrate anion, which coordinates in a bidentate chelating mode, and one O atom from the coordinated methanol molecule. C—H⋯O weak hydrogen-bond interactions result in the formation of chains along the b-axis direction which are further assembled by bifurcated O—H⋯O hydrogen bonds and π-stacking interactions.


Chemical context
Schiff bases are known to be effective ligands able to coordinate a wide range of different metal ions, and they have been widely utilized in the study of biochemical processes (Lindoy et al., 1976;Correia et al., 2005). Compartmental Schiff base ligands, i.e. tetra-and hexadentate Schiff base ligands with different 'compartments' for different types of metal ions, have been employed extensively as 'blocking ligands'. Typical examples would be e.g. ligands with an N 2 O 4 donor set with two Schiff base N-donor sites, two anionic phenolate donor sites, and two additional ether donor sites. The N 2 O 2 compartment is generally more favorable for 3d metal ions. The additional O-donor atoms provide the opportunity to accommodate a second metal ion, which might be a 3d-, 4f-, sor p-block element, thus allowing the production of di-, tri-or oligonuclear systems (Gheorghe et al., 2006;Costes et al., 2008;Andruh et al., 2009).
Studies on heterometallic complexes began at the end of the 1960s. They are of interest because of their physicochemical properties that arise from the presence of dissimilar metal ions in close proximity. The majority of publications in this field are devoted to the preparation of 3d-4f heterometallic complexes (Costes et al., 1998;Koner et al., 2005;Sakamoto et al., 2001). Metal salicylaldimines, on the other hand, represent a fascinating group of ligands that are not only effective complexing agents for p-and d-block elements, but also for alkali metal ions similar to the more well known ligand systems such as crown ethers, cryptands etc. Much of the interest concerning the coordination chemistry of alkali metal ions originates from the development of molecular systems that can mimic naturally occurring molecules that are responsible for the selective transport of these ions, e.g. through membranes. Some of the alkali-metal-ion adducts behave as precursors for other potentially interesting molecular species that can be used for small-molecule activation (Gambarotta et al., 1982), electron storage (Gallo et al., 1997) and the production of materials with remarkable magnetic properties, the alkali cation being crucial in determining the three-dimensional network in the solid state (Miyasaka et al., 1996).
In the case of compartmental Schiff base ligands such as e.g. N(imine) 2 O(phenoxo) 2 O(methoxy/ethoxy) 2 , the metal ion may be either retained in the plane of the O 4 donor set or sandwiched between two sets of the Schiff base O atoms. The former case is usually characterized by a coordination number of eight from two O(phenoxo) 2 O(methoxy/ethoxy) 2 compartments which belong to different molecules. The latter features a coordination number of six from the O 4 compartment of the Schiff base, and two other donors are provided by coordinating solvent molecules and/or anions. The present paper is devoted to the synthesis and structural analysis of an Ni 2+ -containing complex [NaNi(L)(CH 3 OH)(NO 3 )], (I), in which the Na + ion has a seven-coordination geometry and where H 2 L is the compartmental Schiff base ligand 6,6 0dimethoxy-2,2 0 -(ethane-1,2-diyldiiminodimethylene)diphenol.

Structural commentary
The molecular structure of compound (I) with the atom numbering is shown in Fig. 1. Two phenolate O atoms provided by the Schiff base ligand create a double bridge between the Ni 2+ and Na + ions. The coordination environment of the Ni 2+ ion is square-planar, formed by two imine N atoms and two phenolate O atoms. The Na + ion has an unusual seven-coordinated geometry in which the ion sits in the plane of the Schiff base O atoms. Further significant interactions with two nitrate O atoms and one O atom from the coordinating methanol molecule, which are located above and below the plane formed by L, complete the coordination sphere. Values for the geometric parameters in (I) are in good agreement with those observed for complexes based on similar Schiff base ligands (Allen et al., 1987;Cunningham et al., 2000;Wang & Shen, 2009;Xiao, 2009). The two phenoxo and two methoxy O atoms of the O(phenoxo) 2 O(ethoxy) 2 moiety adopt a planar geometry as evidenced by the small mean deviation of the O atoms (<0.02 Å ), from the O5/O6/O7/O8 least-squares plane. The deviations of the Na + and Ni 2+ ions from the O5/O6/O7/O8 plane [0.166 (1) and 0.008 (2) Å , respectively] indicate that Na and Ni are well incorporated in the O(phenoxo) 2 O(ethoxy) 2 moiety.

Supramolecular features
In the crystal structure, the molecules of the title compound form chains along the b-axis via weak C-HÁ Á ÁO hydrogenbond interactions (Fig. 2, Table 1). The C atom of the ethylene moiety acts as a donor and one O atom of the nitrate anion of the neighboring molecule acts as an acceptor. These chains are further assembled into sheets by a bifurcated O-HÁ Á ÁO hydrogen bond (Steiner, 2002), which involves the coordin- Acta Cryst. (2014). E70, 305-308 research communications Figure 1 The molecular structure of (I), showing 30% probability displacement ellipsoids and the atom-numbering scheme. H atoms have been omitted for clarity. Table 1 Hydrogen-bond geometry (Å , ). Symmetry codes: (i) x À 1; y; z; (ii) x; y À 1; z.

Figure 2
The molecular packing for (I), viewed along the b axis. C-HÁ Á ÁO interactions are shown as dashed lines.
ating methanol molecule and nitrate units (Fig. 3, Table 1) and throughstacking interactions, which exist between phenyl rings of neighbouring molecules, with a separation of 3.5845 (11) Å between the centroids formed by the C atoms of the rings [symmetry code: (iii) Àx + 1, Ày, Àz]. For the O-HÁ Á ÁO hydrogen bond, the O atom of the methanol molecule acts as a donor and the O atoms of the nitrate anion of the neighbouring molecule act as the acceptors.

Synthesis and crystallization
A mixture of 6,6 0 -dimethoxy-2,2 0 -(ethane-1,2-diyldiiminodimethylene)diphenol (1 mmol) and nickel nitrate (1 mmol) in methanol (15 ml) was stirred for 30 min at room temperature. Then, sodium nitrate (1mmol) was added, and the mixture was stirred for another 30 min and filtered. The resulting clear orange filtrate was left at ambient temperature for crystallization in air. The red-orange block-shaped crystals were collected by filtration after 6 d, washed with chilled isopropanol and dried on filter paper (yield 0.28 g, 56%).

Refinement
H atoms were placed in geometrically idealized positions and constrained to ride on their parent atoms, with C-H distances of 0.95 (aromatic) or 0.99 Å (methylene), with U iso (H) = 1.2U eq (C), C-H = 0.98 Å for methyl H atoms, with U iso (H) = 1.5U eq (C), and O-H = 0.82 Å for the hydroxy group of methanol, with U iso (H) = 1.5U eq (O). Crystal data, data collection and structure refinement details are summarized in Table 2.