Syntheses, Characterization, and X-ray Crystal Structure of Heteronuclear Zn/Na Assembled with Salen-type Schiff Base

Herein we reported the crystal structure of a dinuclear complex [{ZnNa(L)(EtOH)(NO3)]}] (2) obtained by reaction with the metalloligand [Zn(L)].(H2O) (1) and sodium nitrate. The metalloligand (1) and the dinuclear (2) complexes are characterized by IR and physical measurement. Spectroscopic evidence indicated that the Schiff base H2L behave an N2O2O’2 coordination tetradentate ligand. Suitable crystals of the compound 2 were grown by slow evaporation of ethanol solution for two weeks. The structure of 2 was elucidated by X-ray diffraction analysis. The compound crystallizes in the monoclinic crystal system with a space group of P21/c. The asymmetric unit of the compound contains one dideprotonated ligand molecule, on tetracoordinated zinc (II) ion, one heptacoordinated sodium(I) ion, one bidentate nitrate group and one coordinated ethanol molecule resulting in a structure formulated as [Zn(μ-L)Na(NO3)(EtOH)]. In the structure the Zn II ion is tetracoordinated and the environment around the zinc (II) is best described as a planar square geometry. The Na ion is sevencoordinated in a severely distorted bipyramid pentagon environment. The interplanar angles between the two phenyl rings with a methoxy substituent is 4.208 (4)°, while the dihedral angle values of these groups with the phenyl carrying amino groups are respectively 7.499 (5)° and 11.594 (5)°. The Zn–Na distance is 3.4244 (9) Å. In the crystal structure numerous intramolecular and intermolecular hydrogen bonds link the molecules in a three-dimensional network.


Introduction
Chemists are increasingly interested in the design and synthesis of heteropolynuclear complexes of d and s-block elements owing to original structures with interesting properties in various fields such as magnetism [1,2], catalysis [3,4] and luminescence [5]. The major procedure used for synthesizing these kings of complexes consist to prepare a metal-ligand with Schiff base containing two inners of different sizes [6,7]. In the metal-ligand, the 3d element occupy the inner with the smaller size [8,9]. The reactions of these metal-ligand complexes with metallic centers of different natures can lead polynuclear compounds with very varied structures. Organic ligands capable of maintaining the M n+ (n = 2 or 3) ion from d-block and the M' n'+ ion (n' = 1 or 2) from s-block by bridging, are widely used to establish M-(di-µ-phenoxo) 2 -M ' bridges in order to improve exchanges between metal centers [10][11][12]. The coordination chemistry of s-block elements has recently developed due to the presence of alkaline elements in biomolecular recognition [13,14]. One of the most studied ions is Na+ and some structures containing this element of the bock s and an element of the block d show that the element s is located in the larger site while the transition ion is in the smaller site [6,7,15,16]. The Schiff base bis (2-hydroxy-3methoxybenzylidene)-1, 2-diaminobenzene, is widely used for synthetizing several 3d/Na heteronuclear complexes whose structures show that the element of s-block is still located in the larger open site, while the d-block element is situated in the smaller closed site [16,17]. Organic molecules with phenolic oxygen atoms are perspective that the work we report focused on the use of the H 2 L Schiff base (H2L = N, N ' -Phenylene-bis-(3-methoxysalicylideneimine)) which has two inners capable to encapsulate two ions of different natures. It is in this context that we report the synthesis of a complex Zn 2+ /Na + formulated as follows [Zn(µ-L)Na(NO 3 )(EtOH)]. The compound is obtained by initially isolating the mononuclear zinc (II) complex before reacting the latter with sodium thiocyanate to obtain a heterotrinuclear complex ( Figure 1). The crystal structure and the physicochemical are reported herein.

General Information
o-vanillin and 1, 3-diaminobenzene, zinc chloride tetrahydrate, and sodium nitrate were purchased from Sigma-Aldrich and used as received without further purification. All solvents used were of reagent grade. The ligand (H 2 L) was synthesized following the reported procedure [18]. Elemental analyses of C, H and N were recorded on a VxRio EL Instrument. Infrared spectra were obtained on an FTIR Spectrum Two of Perkin Elmer spectrometer in the 4000-400 cm -1 region. The molar conductance of 1×10 -3 M in DMSO solutions of the metal complexes was measured at 25°C using a WTW LF-330 conductivity meter with a WTW conductivity cell.

Synthesis and Characterization of (1) and (2)
In a round bottomed flask, the ligand (H 2 L) (1 mmol, 0.374 g) was dissolved in 5 mL of acetonitrile. A solution of ZnCl 2 (1 mmol, 0.1363 g) in 5 mL of methanol was added. After two hours under reflux, the yellowish precipitate was  Table 1. Diffraction data were collected using an ENRAF NONIUS Kappa CCD diffractometer with graphite monochromatized MoKα radiation (λ = 0.71073 Å). All data were corrected for Lorentz and polarization effects. No absorption correction was applied. Complex scattering factors were taken from the program package SHELXTL [19]. The structures were solved by direct methods, which revealed the position of all non-hydrogen atoms. All the structures were refined on F 2 by a full-matrix least-squares procedure using anisotropic displacement parameters for all non-hydrogen atoms [20]. The hydrogen atoms of water molecules and NH groups were located in the Fourier difference maps and refined. Others H atoms (CH and CH 3 groups) were geometrically optimized and refined as riding model by AFIX instructions. Molecular graphics were generated using ORTEP-3 [21]

General Study
The ligand phenylimino) methyl)-6methoxyphenol was prepared by mixing under reflux ovanillin and 1, 2-diamino benzene in molar ratio 2:1 in ethanol, following a reported method [18]. The H 2 L react with ZnCl 2 in methanol to yield [Zn(L)] . H 2 O (1). This metalloligand react with sodium nitrate in molar ratio 1:3 and the complex formulated as [{Zn II Na I (L)(NO 3 )}] (2) was afforded after slow evaporation of the filtrate. The IR spectrum of the metalloligand (1) shows an absorption band at 1625 cm -1 (1632 cm -1 for the free H 2 L) which is indicative of a coordinated azomethine (C=N). Additional band ν(C-OPh) is pointed at 1233 cm -1 correspond to the deprotonated phenoxy group. The corresponding band in the free ligand appears at 1254 cm -1 band. For the complex (2) the ν(C=N) band is pointed at 1617 cm -1 and ν(C-OPh) is shifted to high frequency and pointed at 1241 cm -1 . The bands at 1488 (ν 1 ), 1313 (ν 5 ), 1041 (ν 2 ) and ν 1 -ν 5 = 175 cm -1 are indicative of the presence of a bidentate chelating nitrate group. The molar conductance Ʌ measured in DMF solution are 5 and 7 S cm 2 mol -1 respectively for (1) and (2) which are indicative of a neutral electrolytes [22].

Crystal Structure
A novel hetero neutral dinuclear (Zn II Na I ) compound has been obtained by step-by-step method and its structure is depicted. The compound crystallizes in the monoclinic crystal system with a space group of P2 1 /c. The asymmetric unit of the structure of [Zn(µ-L)Na(NO 3 )(EtOH)] contains one dideprotonated L 2ligand molecule H 2 L = N, N ' -Phenylene-bis-(3-methoxysalicylideneimine)), one Zn II cation, one Na I cation, one bidentate nitrate anion and one coordinated ethanol molecule. The molecular structure of the title compound is shown in Figure 2, while selected bond lengths and angles of the coordination environment of the metal centers are listed in Table 2. In the structure of the dinuclear complex, the Zn atom and the Na atom are bridged by two phenoxo oxygen atoms. The four-coordinated Zn II ion, which occupy the smaller inner of the ligand, is coordinated in a planar square geometry with the basal plane formed by two nitrogen atoms and two phenoxo oxygen atoms. The cissoid angles around the zinc atom which are in the range [84.77 (7) [25]. The sodium cation is situated in the planar cavity defined by the two phenoxo oxygen atoms and the two methoxy oxygen atoms with maximum displacement of 0.131 (1) Å. The interplanar angles between the two phenyl groups with a methoxy substituent is 4.208 (4)°, while the dihedral angle values of these groups with the phenyl carrying amino groups are respectively 7.499 (5)° and 11.594 (5)°. The Zn-Na distance is 3.4244 (9) Å. The positive charge of the Zn(II) and Na(I) ions are balanced by the ligand L 2and the nitrate anion.
The environment around the 7-fold sodium atom is best described as a severely distorted bipyramid pentagon ( Figure  3). The basal angles are in the range [60.57 (5)-96.31 (7)]. These values deviate from the value of the ideal angle for a regular pentagon (72°) and the sum of the equatorial angles around the sodium atom is 365.53°. The apical position for the pentagon pyramid around Na is occupied by O5 and O8 with the angle value of 136.37 (7)°.
The dinuclear units are connected by hydrogen bonds of type EtO-H···O i -NO 2 and, weak intermolecular hydrogen bonds of type C-H···O-NO 2 (C13-H···O7 ii and C15-H···O5 ii ) (Figure 4, Table 3). The combined hydrogen bonds links give rise to a three-dimensional network architecture. It should be noted that the methoxy and phenoxo oxygen atoms are by no means engaged in any interaction in the structure.
A survey of the Cambridge Structural Database (CSD) (Version 5.39, last update November 2017; Groom et al., 2016) [26] shows that dinuclear complexes of the ligand bis(2-hydroxy-3-methoxybenzylidene)-1, 2-diaminobenzene where the smaller N 2 O 2 cage is occupied by a 3d metal and the larger, open O 2 O 2 cage is occupied by one s-block metal are well documented. Complexes formed by transition metal and s-block atom, with the above organic ligand in which the d-block metal atom is situated in the smaller N 2 O 2 cage and the s-block metal atom is situated in the larger O 2 O 2 cage have been reported: twelve entries corresponding to d-s complexes [1,10,11,16,[27][28][29] were found in the CSD. One trinuclear Ni-K-Ni complex has been reported recently [30].

Conclusion
In summary, we have successfully synthesized and studied the hetero dinuclear complex containing one zinc (II) and one natrium (I) ions. A suitable crystal of the complex was obtained by slow evaporation of ethanol solution of the compound for two weeks. Infrared spectra of the compound and the metalloligand were discussed in this paper. Crystal data, X-ray data collection, data reduction and structure refinement of complex 2 are studied. The heteronuclear complex crystallizes in the monoclinic system with a space group P2 1 /c. The environment around the zinc (II) is square planar while the environment around the 7-fold sodium atom can be described as a severely distorted bipyramidal pentagon.

Conflicts of Interest
The authors declare that they no competing interest.