Synthesis , Characterization and Crystal Structure of a New Schiff Base Ligand from a Bis ( Thiazoline ) Template and Hydrolytic Cleavage of the Imine Bond Induced by a Co ( II ) Cation

The reaction of bis-[2-amino-4-pheny1-5-thiazolyl] disulfide with 5-nitro-salicylaldehyde in absolute ethanol resulted in the formation of a new Schiff base ligand H2L (1). Characterization of the ligand was performed by FT-IR, 1H NMR, 13C NMR, UV-Vis, elemental analysis and single crystal X-ray diffraction. The ligand, (1), possesses a disulfide –S–S– bridge of 2.1121 (3) Å length, and the molecule adopts a cis-conformation around this bond. In the crystal structure of (1), an intramolecular O–H···N hydrogen bond with D... A distance of 2.69 (3) Å was present. The reaction of (1) with Co(NO3)2·6H2O and CuCl2·2H2O in methanol afforded the corresponding metal complexes. The obtained solids were further investigated by elemental analysis and UV-Vis titration that confirmed the formation of [CoL] and [ClCuHL] complexes. However, recrystallizaion of the Co(II) complex in dimethylsulfoxide caused the complete hydrolysis of the imine bond and afforded a Co(II) complex in which two 5-nitro-salicylaldehyde and two DMSO molecules were coordinated to the central metal in an octahedral fashion. This structure (2) was also confirmed by single crystal X-ray analysis.


Introduction
Schiff bases are compounds with the structure consisting of a C=N bond, generally formed by condensation of an amine and active carbonyl group, initially developed by Hugo Schiff [1] [2].Schiff base ligands are easily synthesized and form complexes with almost all metal ions in various oxidation states [3] [4].These metal complexes can be used in catalytic reactions and as models for biological systems.This may be related to their biological activity, including antibacterial, antifungal, anticancer, antioxidant, anti-inflammatory, antimalarial, antiviral activity as well as applications in a variety of chemical areas such as oxidation, reduction and hydrolysis catalysis [5]- [18].However, hydrolytic cleavage of Schiff base ligands readily occurs due to the reversible nature of the synthetic procedure.It has been reported that the C=N cleavage occurs on a number of metal sites ranging from simple salts to mixed ligand complexes [19]- [22].However, the solvent system, co-ligands and reaction conditions are also important factors [23]- [25].The hydrolysis of Schiff bases in the presence of a metal ion is observed by Ghosh and coworkers [26].They argue that the hydrolysis is dependent on several factors, and was observed such as the pH of the reaction medium, the size of the chelate rings formed by the diamine fragment of the Schiff base, the coordinating ability of the counter anions, the nature of the metal ions, and the effect of carbonyl compounds [27]- [34].The amine or aldehyde fragment regenerated by the cleavage of the C=N functional group of the Schiff base, can coordinate to the central atom.
Recently, we noticed that a disulfide compound is obtained as a byproduct during the preparation of bis-[2amino-4-pheny1-5-thiazolyl] sulfide by following the procedure reported by Dodson and King [35].These two products may be separated by means of column chromatography.In this regard, the 5-nitro-salicylaldehyde is prepared and then is used in condensation reaction with synthesized bis-[2-amino-4-phenyle-5-thiazolyl] disulfide.Characterization of the prepared ligand are performed by FT-IR, 1 H NMR, 13

Materials and Physical Techniques
All chemicals were purchased from Merck and used without further purification.FT-IR spectra were recorded in the frequency range of 4000 -400 cm −1 with use of a Perkin-Elmer RXI spectrometer using KBr disks at room temperature.Elemental analysis was carried out using a Perkin-Elmer 2400(II) CHN analyzer. 1H NMR and 13 C NMR spectra were recorded with a Bruker Avance 300 spectrometer using DMSO-d 6 as solvent.UV-Vis spectra were recorded with a Perkin-Elmer Lambda 25 spectrophotometer, using two matched 10-mm quartz cells.

Crystal Structure Determination and Refinement
Crystallographic data for (1) were collected with use of a Bruker Apex DUO diffractometer, Cu K α radiation and a Cryosystems low temperature apparatus.The crystal was twinned, and the data reduction was carried out using two components.The twin law was -1 0 0 0 -1 0 -1 0 1.
Data reduction and cell refinement were carried with SAINT [36].An absorption correction was applied with use of the program TWINABS [36].The programs used to solve and refine the structure were SHELXS [37] and SHELXL-2014/7 [38], respectively.The hydrogen atom H4 on the OH group was not located in a difference Fourier map.Its position was estimated based on geometric considerations and its similarity to the position of H1.During refinement, restraints were applied such that O-H was 0.84 Å. and the C-O-H-N grouping was flat.The remaining, C-H hydrogen atoms were included as riding on their parent carbon atoms.All hydrogen U iso values were set to 20% more than the U eq of their parent atoms.
Data for compound (2) were collected at the Advanced Light Source, Lawrence Berkeley National Lab, California using an Oxford Cryostream low temperature device.The radiation employed was 0.77490 Å and the diffraction apparatus was a Bruker D8 with a PHOTON 100 CMOS detector.This structure was also twinned, with a twin law -1 0 -0.292 0 -1 0 0 0 1.Software programs utilized were the same as for (1).Crystal data and refinement details are reported in Table 1.

Co(II) Complex
This complex was prepared in a manner as described above with this difference that Co(NO 3 ) 2 To produce single crystals of this complex, the precipitate was dissolved in DMSO and the mixture kept in air to allow the solvent to evaporate.After several weeks the orange plate-like crystals were obtained by slow evaporation of the solvent.However, the X-ray analysis revealed that an unexpected Co(II) complex had been formed in which the imine bond of the Schiff base ligand had been hydrolyzed in DMSO and the product was found to be a Co(II) complex in which 5-nitro-salicylaldehyde and two DMSO molecules were coordinated to the central atom in a slightly distorted octahedral fashion.

Crystal Structure
The condensation reaction between bis-[2-amino-4-pheny1-5-thiazolyl] disulfide and 5-nitro-salicylaldehyde in a 1:2 molar ratio in methanol afforded the desired Schiff base (1), and its molecular structure was further investigated by single crystal X-ray diffraction.The molecular structure, together with the crystallographic numbering scheme of (1) are shown at   As mentioned, we started our investigation to synthesize a complex of Co(II) with the potentially tetradentate N 2 O 2 Schiff base (1).However, recrystallization in DMSO in the presence of trace water from the cobalt salt, caused complete hydrolysis of the Schiff base.Subsequent coordination of the resultant 5-nitro-salicylaldehyde with Co(II) ion occurs with the formation of the [Co(NO 2 -sal) 2 (DMSO) 2 ] complex (Scheme 1).It is well known that the characteristic bond of the Schiff bases, C=N, has reversible nature which allows by hydrolysis, obtaining the initial corresponding aldehyde and amine compounds [48] [49].It seems that the presence of the metal cations accelerates the hydrolysis of the imine bond regenerating the starting materials.X-ray single crystal diffraction study reveals that the crystal structure of the reaction product consists of neutral octahedral units shown in Figure 2. The Co(II) complex lies on center of inversion and the octahedral environment of Co(II) ion is formed by the four oxygen atoms of 5-nitro-salicylaldehyde moiety and two molecules of DMSO in the apical positions.
The 5-nitro-salicylaldehyde ligands coordinate in a typical mode through the phenolate and carbonyl oxygen atoms, forming six-membered chelate rings with the bite angles of 88.40 Numerous complexes are known in which 5-nitro-salicylaldehyde ligand has coordinated to the central atom.Papadopoulos et al., have reported syntheses and X-ray structures of a few Co(II) complexes [50] in which the metal cations were bonded to two deprotonated 5-nitro-salicycaldehyde ligands and one phenanthroline or neocuproine ligand to fulfill its octahedral coordination sphere.In our sample, each substituted salicylaldehyde Figure 2. The molecular structure together with the crystallographic numbering of (2), with thermal ellipsoids drawn at 50% probability level.

FT-IR Study
The main characteristic of the FT-IR spectrum of Schiff base (1), is disappearance of the absorption bands of -NH 2 (3420 cm −1 ) and carbonyl (1665 cm −1 ) fragments from starting materials i.e., bis-[2-amino-4-pheny1-5thiazolyl] disulfide and 5-nitro-salicycaldehyde and in return, appearance of the stretching of azomethine group, υ(C=N), at 1561cm −1 which changes upon coordination of the ligand to the metal atoms.The stretching of OH phenolic group appears at 3429 cm −1 .In addition, the band near 1345 cm -1 is assigned to υ(C-O) stretching of the phenolic group.Although the absorption of υ(C=C) band does not change from the free ligand to the Co(II) and Cu(II) complexes, the downfield shift of the υ(C=N) vibration (1543 cm −1 ) suggests that the nitrogen atom of imine group participates in coordination to the Co(II) and Cu(II) atoms, respectively.

NMR Study
In the 1 H NMR spectra of 2-amino-4-phenylthiazole, the signal at 7.21 ppm is assigned to the amino groups.The protons of the pendant phenyl ring are also observed at 7.23 -7.80 ppm and the hydrogen atom of thiazole ring appears at 6.98 ppm.In the 1 H NMR spectrum of the bis-[2-amino-4-pheny1-5-thiazolyl] disulfide, the signal at 7.69 ppm is assigned to the amino groups and protons of the aromatic ring are also observed at 7.23 -7.80 ppm.
In 5-nitro-salicylaldehyde, the signal at δ 11.96 ppm is assigned to the proton of the OH group and the signal for CHO fragments appears at 10.22 ppm.In (1), the broad signal at δ 12.52 ppm is assigned to the proton of the OH group.It was observed that this signal completely disappears when adding D 2 O.The single proton of -CH=N has chemical shift at δ 9.18 ppm and signals around δ 7.00 -8.80 ppm are attributed to the protons of aromatic rings.In the 13

UV-Vis Studies
The electronic absorption spectral bands of the Schiff base, Cu(II) and Co(II) complexes were recorded over the range 200 -600 nm in DMSO.The electronic spectrum of the Schiff base ligand shows two absorption bands at 372 and 426 nm.The short-wave band may be attributed to electron transitions in the aromatic rings (intra ligand (IL) n-π*), and the longer wavelength band at 426 nm is assigned to the π-π* transition in the azomethine chromospheres in the Schiff base ligand [52].The Cu(II) complex displays absorption bands at 364 nm and 420 nm, respectively.Also, the Co(II) complex displays absorption bands at 365 nm and 429 nm.

Solution Studies
In this study, we also report further investigation of the Cu(II) and Co(II) complexes.It is important to note that the method used in this research has some advantages over the Jobs analysis.In fact, a limited number of samples (10 samples for instance) with different volume ratios should be prepared in Jobs analysis.However, in the UV-Vis titration the desired amount of metal (5 × 10 −3 M) is added to the ligand solution (5.0 × 10 −5 M) in one cell and then this addition continues up to 30 times.The absorbance spectrum of the mixture is recorded after each addition.the numbers of obtained points are higher in this method and the accuracy would definitely be better.In a typical procedure, 2.0 mL of ligand solution in methanol is placed in the spectrophotometer cell and the absorbance of the solution is measured.Then an appropriate amount of CuCl 2 or Co(NO 3 ) 2 in methanol is added in a stepwise manner using a 2-µl Hamilton syringe.The absorbance spectrum of the solution is recorded after each addition.The Cu(II) and Co(II) solutions were continually added until the desired metal to ligand mole ratio was achieved.The electronic absorption spectra of the Schiff base ligand (1) in the presence of increasing concentration of CuCl 2 and Co(NO 3 ) 2 in MeOH at room temperature is shown in Figure 3 and

Conclusion
In continuation of recently published papers regarding bithiazole-containing materials [53] [54], in this study, we synthesized and characterized a novel Schiff base ligand from a bis-[2-amino-4-phenyl-5-thiazolyl] disulfide template (1).The [CoL] and [ClCuHL] complexes of the ligand were prepared and the products were further investigated by UV-Vis that confirmed the formation of desired complex in a 1:1 molar ratio.Using DMSO as recrystallization solvent caused the complete hydrolysis of the imine bond and afforded a Co(II) (2) complex in which two 5-nitro-salicylaldehyde and two solvent molecules were coordinated to the Co(II) atom in a octahedral fashion.The solid state structures of both (1) and (2) were further substantiated by single crystal X-ray diffraction.
Scheme 1.A drawing of the principle products characterized in this report, [ClCuHL] and the hydrolysis product(2)

Figure 1 ,
with thermal ellipsoids drawn at the 50% probability level.As shown, the molecule possesses a disulfide (-S-S-) bond and adopts a cis configuration with respect to this bond.The C10-S2-S3 and C11-S3-S2 bond angles are 101.4(3)˚ and 100.4 (3)˚ which indicate the tetrahedral environments around each sulfur atom with slightly deviations.This small deviation from ideal tetrahedral angle results from the repulsion between lone pairs on the sulfur atoms.The dihedral angle of C10-S2-S3-C11 is −58.5 (4)˚ which indicates that it is twisted around the disulfide bond.In comparison to the average S−S bond length reported for similar structures (2.02 ± 0.03 Å)[41]-[44], the S−S bond in this present Schiff base is somewhat elongated at 2.112 (3) Å (see

Figure 4 ,
respectively.The resulting absorbance against [L]/[M 2+ ] (M 2+ : Cu(II), Co(II)) mole ratio plot is shown in the inset of each figure.As mentioned, the injection point at ligand-to-metal molar ratio of about1 indicates the formation of ML compound with mole ratio of 1:1.

Figure 3 .
Figure 3. Electronic absorption spectra of the ligand (1) in MeOH in the presence of increasing concentration of CuCl 2 at room temperature.(b) Corresponding mole ratio plot of L/Cu(II).

Figure 4 .
Figure 4. (a) Electronic absorption spectra of the ligand (1) in MeOH in the presence of increasing concentration of Co(NO 3 ) 2 at room temperature.(b) Corresponding mole ratio plot of L/Co(II).

Table 2
).For example, Schroth et al., have synthesized two compounds with S−S [51]coordinated through the deprotonated phenolic oxygen atom and through the carbonyl oxygen.The reported Co-O bond distances range from 2.0202 (2) Å to 2.140 (1) Å and are in good agreement with those found in our structure.In addition, Tangoulis et al., has recently published a work in which single chain magnets of Co(II) are formed by using 5-nitro-salicycaldehyde as ligand[51]. ligand C NMR spectrum, the carbon atoms of -CH=N and C-O groups show resonances at 170.7 and 157.9 ppm, respectively.The remaining carbon atoms of the aromatic rings have signals that range from 107.3 to 139.8 ppm.As both complexes are paramagnetic and exhibit broad 1 H NMR signals, no further analysis can be performed.