Synthesis, Characterization and Crystal Structure of Cobalt(II) Complex of a Schiff Base Derived from Isoniazid and Pyridine-4-Carboxaldehyde

A new hydrogen bonded Cobalt(II) Schiff base complex, N’-(pyridine-4-carboxaldehyde) isonicotinoylhydrazone Cobalt(II), has been synthesized from isoniazid and pyridine-4-carboxaldehyde and characterized by IR spectroscopy, H-NMR, elemental analysis, TGA and single crystal X-ray structure determination. X-ray crystal structure analysis shows an octahedral complex with a metal centre coordinated to two ligand molecules through the pyridine nitrogen atoms and four water molecules and containing two nitrate groups as counter ions. The complex crystallizes in the monoclinic crystal system and P2(1)/n space group. The unit cell dimensions are: a = 7.2108(4) Å, b = 16.6020(9) Å, c = 13.0389(6) Å, α = 90 ̊, β = 103.972(4) ̊, γ = 90 ̊. The molecule is symmetrical about the cobalt centre as observed from the H-NMR and C-NMR and confirmed by the single crystal X-ray structure of the complex. Thermogravimetric analysis shows two steps decomposition of the complex to leave a metal oxide residue. The title compound is expected to be biologically active as one of the precursors (isoniazid) is a therapeutic agent with well-established clinical applications.


Introduction
Schiff bases display a variety of coordination modes, and have therefore attracted a lot of attention in the development of coordination Chemistry [1] [2]. The most commonly used organic connecting ligands are rigid linear bridging heterocyclic compounds such as pyrazine and 4,4'-bipyridine. However, there has been renewed interest in Schiff bases because of their varied coordination modes. The Schiff bases display varying degrees of flexibility and hence adopt different coordination modes in different complexes [3] [4] [5], the most common being the bidentate modes. Owing to their structural flexibility, tridentate [6], tetradentate [7] [8], pentadentate [9] and even hexadentate [10] coordination modes have been reported, and the flexible ligands have been employed in order to gain access to topologies not available from logical combination of rigid building blocks.
The coordination chemistry of cobalt is of considerable interest since Cobalt(II) and Cobalt(III) complexes derived from Schiff bases are reported to be biologically active [11]. The cobalt Schiff base complexes are also an important class of coordination compounds, not only because of their involvement in Vitamin B12 models and oxygen carrier properties, but also due to their interesting magnetic and spectroscopic as well as diverse metal-ligand interactions [5] [12].
Heterocyclic Schiff base ligands containing O-or N-donors and their metal complexes have been shown to exhibit interesting properties [11]. The study of the structural chemistry of Schiff base ligands and their complexes is important in understanding the complex biological properties of these systems.
Our group recently embarked on studies on the biological activities of complexes of heterocyclic Schiff base ligands, exploring the presence of the characteristic azomethine (-N=CH-) functionality of the Schiff base.
Intermolecular forces have played a very important role in crystal engineering especially the strong hydrogen bonds. Hydrogen bonds can effectively bind counter ions and even solvent molecules in a structure thereby contributing to the shape and supramolecular assembly of the compound. Such interactions have been found to stabilize the crystal lattice [11] [13] [14] [15] [16]. Intermolecular interactions are therefore of fundamental significance to hydrogen-bonded organic and metal-organic supramolecular structures [16]. Recently, the water content in self-assembly processes has been found to remarkably influence the structural topologies of compounds [15]. In continuation of our work on the coordination chemistry of Schiff bases derived from isoniazid, we here report on the synthesis, characterization and crystal structure of a new Cobalt(II) complex of N'-(pyridine-4-carboxaldehyde) isonicotinoylhydrazone in which bonding is through the N-atom of pyridine, with the imine group remaining uncoordinated.

Materials and Method
All chemicals and solvents used were of reagent grade. Pyridine-4-carboxaldehyde,

X-Ray Crystallographic Study
The single crystals used in the study were selected from those obtained by slow  [17]. All non-Hydrogen atoms were refined anisotropically. Hydrogen atoms were included in calculated positions, assigned isotropic displacement parameters and allowed to ride on their parent carbon atoms. All calculations were carried out using the SHELXTL package [18]. Table 1 presents the crystal data and refinement parameters; Table 2 lists the selected bond lengths and bond angles while Table 3 gives the hydrogen bond parameters. CCDC 1419814

Results and Discussion
The Schiff base ligand was prepared by reacting one mole of isoniazid with one mole of the pyridine-4-carboxaldehyde in methanol as solvent. The air stable ligand, N'-(pyridine-4-carboxaldehyde) isonicotinoylhydrazone was obtained as a cream coloured powder. The complex which was precipitated from a 2:1 ligand to metal ratio in a water-methanol mixture is stable in air. The elemental analysis data show close agreement between the experimental and calculated values. The close agreement of the experimental and calculated values is indicative of the purity of the compounds as well as agreement with the proposed structure.

Spectroscopic Analyses
The IR spectrum of the ligand shows a strong absorption band at 3191 cm −1 , at-

Thermal Analysis
The   corresponding to the decomposition of the two nitrate groups and the coordinated water molecules. This leaves a CoO residue which represents 13.14% (calc. 12.88%) of the mass.

Crystal and Molecular Structure
The complex crystallizes in the monoclinic crystal system and P2(1)/n space group and contains two molecules in the unit cell. The significant bond lengths and bond angles are given in Table 2. The ORTEP drawing of the complex showing the coordination geometry is shown in Figure 3 while the packing diagram is presented in Figure 4.   Figure   4(b). The monomer of one complex therefore is linked to the monomer of another complex through intermolecular hydrogen bonding, involving coordinated water ligands and uncoordinated nitrates as shown in Figure 5. This kind of interaction is found to stabilize the structure [16].

Conclusion
We have reported here, the isolation of a Cobalt(II) complex of a pyridine-containing heterocyclic Schiff base ligand. The complex shows negligible distortion of the octahedral geometry. Unlike in many Schiff base complexes which coordinate to the metal through the azomethine nitrogen and carbonyl oxygen atoms, the Schiff base in this case is found to coordinate to the metal through the pyridine nitrogen, forming a perfectly linear "head to head" N-Co-N system, with the imine (-N=CH-) groups remaining unusually uncoordinated. The molecule is symmetrical about the cobalt centre as observed from the 1 H-and 13 C-NMR and confirmed by the X-ray structure of the complex.