Synthesis and Characterization of a New Unsymmetrical Potentially Pentadentate Schiff Base Ligand and Related Complexes with Manganese ( II ) , Nickel ( II ) , Copper ( II ) , Zinc ( II ) and Cadmium ( II )

New unsymmetric thioether Schiff base amine, 2-(2-aminoethylthio)-N-(thiophene-2ylmethylene)aniline was prepared by reaction of 2-aminothiophenol with N-(2-bromoethyl) phthalimide and then by thiophene-2-carbaldehyde. Then, the phthalimide group converted to the amine by hydrazine hydrate. The new potentially pentadentate N3S2 donor unsymmetrical Schiff base ligand, 2-(2-(pyridine-2-ylmethyleneamino)ethylthio)-N-(thiophene-2-ylmethylene) aniline (L) was prepared via [1 + 1] condensation of pyridine-2-carbaldehyde with this amine. Manganese(II), nickel(II), copper(II), zinc(II) and cadmium(II) complexes of this ligand were prepared by adding metal salts to an ethanolic solution in this ligand. The thioether amine, ligand (L) and unsymmetric Schiff base complexes are characterized by the appropriate spectroscopic methods such as infrared (IR), UV-Vis, H nuclear magnetic resonance (NMR), C NMR studies, X-ray, microanalysis and mass spectrometry.


Introduction
Transition metal complexes with thioether Schiff base ligands have been prepared since the beginning of the development of coordination chemistry and there is continuing interest in these complexes. 1,2Sulfur atom coordinates to the metals through one (terminal) or both (bridging) of the lone pairs, the latter being less common.They usually form strong bonds with soft metals and more rarely with hard ones.They are considered both poor σ-donor and poor π-acceptor ligands, the latter involving the σ* orbitals of the S-X bonds.This π-acceptor character can contribute to the M-S bond strength. 3The synthesis of transition metal complexes of unsymmetrical Schiff bases ligands with the hard and soft atom is an important area of study with implications in bioinorganic chemistry, 4-6 catalysis 7 and medical chemistry. 80][11][12][13] Unsymmetrical linear pentadentate Schiff base ligands that have N 3 S 2 donors or have thiophene and pyridine groups in their structure are very rare.
Here we report the synthesis of new unsymmetric linear pentadentate Schiff base ligand and their chelates with some transition metal ions, e.g., manganese(II), nickel(II), copper(II), zinc(II) and cadmium(II).

Experimental
Chemical and starting materials 2-Aminothiophenol, pyridine-2-carbaldehyde and thiophene-2-carbaldehyde were purchased from Merck and used as received.Other reagents and solvents were of analytical grade and purchased commercially.

Instrumentation
E l e m e n t a l a n a l y s e s w e r e p e r f o r m e d i n a CHNS-O-2400 II PerkinElmer.Infrared spectra were recorded in ATR, using a Bruker FT-IR Equinax-55 spectrophotometer (4000-400 cm -1 ).Mass spectrometry (MS) spectra were obtained using a QP-1100EX Shimadzu GC-MS (EI at 70 eV). 1 H and 13 C nuclear magnetic resonance (NMR) spectra were taken in CDCl 3 or DMSO-d 6 on a Bruker NMR 500 and 400 MHz spectrometer using Si(CH 3 ) 4 as an internal standard.The electronic spectra were taken in GBC UV-Visible Cintra 101 spectrophotometer with 1 cm quartz, in the range of 200-1100 nm at 25 °C.

X-ray crystal structure determination
Single crystal of 2-(2-(2-((thiophene-2-yl) methyleneamino)phenylthio)ethyl) isoindoline-1,3-dione suitable for X-ray analyses was obtained by evaporation of ethanolic solution of the reaction mixture at 298 K.The details of the X-ray crystal data and of the structure solution and refinement are given in Table 1.The X-ray diffraction measurements were made on a STOE IPDS-2T diffractometer with graphite monochromated Mo Kα radiation (λ = 0.71073 Å).Cell constants and an orientation matrix for data collection were obtained by least-squares refinement of diffraction data from 4873 unique reflections.The structure was solved by direct methods and refined on F 2 by a full-matrix least-squares procedure using anisotropic displacement parameters for all non-hydrogen atoms.The hydrogen atoms were located in their calculated positions and refined using a riding model.

Results and Discussion
The IR spectrum for 2-(2-(2-((thiophene-2-yl) methyleneamino)phenylthio)ethyl) isoindoline-1,3dione shows bands at 1769 and 1707 cm -1 assignable to the carbonyl groups (C=O) in phthalimide, band at 1610 cm -1 assignable to the imine stretch and a band at 751 cm -1 due to the thioethers stretch.The IR spectrum for 2-(2-aminoethylthio)-N-(thiophene-2-ylmethylene) aniline shows bands at 3340 and 3172 cm -1 assignable to the aliphatic primary amine vibrations (NH 2 ), band at 1607 cm -1 assignable to the imine stretch and a band at 750 cm -1 due to the thioethers stretch.The IR spectrum for the ligand (L) confirms the presence of two different imines (1647 and 1608 cm -1 ) and the absence of carbonyl and amine functional groups of the starting materials.In these compounds, the imine stretch band shifts to lower wave number (1607-1610 cm -1 ) which supports overlapped resonances of the imine bond with the phenyl and thiophene rings.The vibration peak of C-S is observed in 748 cm -1 .In the IR spectra of manganese(II), nickel(II) and copper(II) complexes, the band at about 1608 or 1609 cm -1 and in the IR spectra of zinc(II) and cadmium(II) complexes, the bands in the range of 1605-650 cm -1 are associated with ν(C=N) vibration and the vibration peaks of C-S are observed in the range of 751-778 cm -1 .16] 1 H and 13 C NMR results obtained for prepared compounds at ambient temperature in CDCl 3 and DMSO-d 6 , respectively, are presented in Experimental section.The 1 H and 13 C NMR spectra of the 2-(2-aminoethylthio)-N-(thiophene-2-ylmethylene)aniline indicate that this compound has synthesized.The slightly broad signal at 2.9-3.2ppm in the 1 H NMR spectrum of this Schiff base amine is assigned to the NH 2 protons and the ν(CH=N) proton exhibits a singlet resonance at 8.79 ppm.
The 1 H NMR spectrum exhibits a singlet at 2.86 ppm corresponding to the methylen protons and the resonances for the aromatic rings (phenyl and thiophene) protons are observed in the region of 6.68-7.82ppm.In the 13  For the cadmium(II) complex, we propose two conformation isomer Z and E for this complex (Figure 1).Z and E configurations would be expected to lead to two separate resonances in the 1 H NMR spectrum for the thiophene ring and imine group attached to it.The 1 H NMR spectrum (Figure 2) of the cadmium(II) complex confirm this two isomer in E/Z ratio approximately 70/30.The 1 H NMR spectrum of this complex shows two signals at 7.71 and 7.81 ppm corresponding to the imine proton (H l in Z and E).The thiophene protons appear as two triplets at 7.25, 7.58 ppm (H n in Z and E) and four doublets at 7.71, 7.87 ppm (H m in Z and E) and 8.07, 8.76 ppm (H o in Z and E).It was not possible to assign these resonances unambiguously to individual two-conformation isomer Z and E for this complex directly from the spectrum.However, it should be noted, that the sulfur atom of thiophene in conformation isomer E can be coordinated to the Cd and the resulted signals (thiophene ring and imine group attached to it) moved downfield.Also, the 1 H NMR spectrum of this complex shows two triplet signals due to the methylene protons (H f and H g ) at 3.15 and 3.79 ppm, and also shows a signal at 5.32 ppm, corresponding to coordinated H 2 O. 17 The pyridine protons appear as two triplets at 7.81, 8.22 ppm (H b , H c ) and two doublets at 7.99, 8.80 ppm (H d , H a ), and the benzene ring protons appear as two triplets and two doublets at 6.55, 7.04 ppm (H j , H i ) and 6.74, 7.29 ppm (H k , H h ), respectively.The singlet at 8.57 ppm (which is shifted upfield) corresponds to the imine proton (H e ). 18In the 13    resonances for the aromatic rings carbons are observed in the region 115.4-150.2ppm.
The EI mass spectra of the 2-(2-aminoethylthio)-N-(thiophene-2-ylmethylene)aniline and the unsymmetrical Schiff bases ligand (L), provide strong evidence for the formation of these compounds and exhibit peaks at higher molecular weights.The peak in the spectrum of 2-(2-aminoethylthio)-N-(thiophene-2-ylmethylene)aniline is observed at m/z 262 and the peak in the spectrum of the ligand is observed at m/z 351 corresponding to [L] + .The absorption spectrum for 0.001 mol L -1 solutions of the ligand (L) in DMF shows two absorption bands at 314 nm (log ε = 4.05) and 327 nm (log ε = 4.02), that these bands can be attributed to π→π* and n→π* transitions. 19The absorption spectra for 0.001 mol L -1 solutions of complexes in DMF show bands at 308 to 325 nm corresponding to π→π* transitions.In general, the electronic transitions for zinc(II), cadmium(II) (d 10 ) and manganese(II) (d 5 high spin) complexes are spin-forbidden and hence cannot be observed.Complex copper(II) shows only one band at 550 nm (log ε = 2.77) corresponding to d→d transition.][22] The elemental analyses for these complexes are in good agreement with the above molecular formula.In absence of crystal structures, we must be careful in any attempt to elucidate the coordinative environment of the metal ions on these systems.Taking into account that there are five donor atoms in the ligand, it could be suggested a distorted tetrahedral or square planar environment for each metal or five coordinated geometry for manganese(II) and cadmium(II), which would be in agreement with the UV-Vis data.

Description of the crystal structure
Single crystal of 2-(2-(2-((thiophene-2-yl) methyleneamino)phenylthio)ethyl)isoindoline-1,3-dione were obtained by slow evaporation of an ethanol solution of this compound.Crystal and structure refinement data are given in Table 1, and the molecular structure is shown in Figure 3.This compound crystallizes in monoclinic system in the P2 1 /c space group with the disordered thiophene ring (with occupancies of 0.64 and 0.36). 23The bond lengths and angles are within the expected range of values.Selected bond lengths, bond angles and hydrogen bonds are given in Tables 2 and 3

Conclusion
In this work, we designed and prepared the unsymmetrical amine and then unsymmetrical potentially pentadentate Schiff base ligand (L) containing the pyridine and thiophene moieties and/or hard-soft donors (N 3 S 2 ).Then five complexes of manganese(II), nickel(II), copper(II), zinc(II) and cadmium(II) from ligand were synthesized and characterized by various spectroscopic methods.Taking into account that there are five donor atoms in the ligand, it was found that only N3S donor set has bonding interaction (in agreement with the UV-Vis data).
In the absence of crystal structures, it could be suggested a distorted tetrahedral or square planar environment for each metal or five coordinated geometry for manganese(II) and cadmium(II).

[Scheme 2 .Scheme 1 .Scheme 3 .
Scheme 2. Synthesis of the ligand (L). d C NMR spectrum of this Schiff base amine, azomethine carbon resonance is observed in the 155.8 ppm and the resonances for the aliphatic C-N and C-S carbon atoms are observed in the 38.1 and 40.8 ppm, respectively.The aromatic carbons of the benzyl and thiophene rings are observed in the 115.1-148.5 ppm range.The 1 H NMR spectrum of ligand (L) ( 1 H and 13 C NMR spectra of ligand show amount of impurity due to its instability), shows the aliphatic protons in the region of 3.12-3.81ppm, the aromatic ring protons in the region of 6.53-8.39ppm and the ν(CH=N) protons in the 8.63 and 8.74 ppm.The 13 C NMR spectrum of ligand features 2 signals for the aliphatic carbon atoms and 15 signals for the aromatic carbon atoms in the region of 35.4-149.7 ppm.In this spectrum, azomethine carbons are observed in the 154.2 and 163.2 ppm.
C NMR spectrum of [CdL] 2+ , azomethine carbon resonances are observed at 150.2 and 162.5 ppm.Two methylene carbon atoms present in complex exhibited signals at 34.0 and 58.3 ppm and the

Figure 1 .
Figure 1.Two conformation isomer Z and E for the cadmium(II) complex.