Efficient Synthesis and Characterization of Some Novel Nitro-Schiff Bases and Their Complexes of Nickel ( II ) and Copper ( II )

Synthesis and characterization of some new Schiff base ligands derived from various diamines and nitrosalicylaldehyde and their complexes of Ni(II) and Cu(II) are reported. Several spectral techniques such as UV-Vis, FT-IR, and NMR spectra were used to identify the chemical structures of the reported ligands and their complexes. e ligands are found to be bound to the metal atom through the oxygen atoms of the hydroxyl groups and nitrogen atoms of imine groups, which is also supported by spectroscopic techniques. e results obtained by FT-IR and NMR showed that the Schiff base complexes of transition metal (II) have squareplanar geometry.


Introduction
Schiff bases have played an important role in the development of coordination chemistry as they readily form stable complexes with most of the transition metals.e chemistry of Schiff base ligands species has been gaining considerable interest primarily because of their fascinating structural diversities [1][2][3][4][5][6][7].
2-Hydroxy Schiff base ligands and their complexes derived from the reaction of salicylaldehyde derivatives with diamines have been extensively studied in great details for their various crystallographic, structural, and magnetic features [8][9][10][11][12][13][14].Schiff base ligands are able to coordinate many different metals, and to stabilize them in various oxidation states, enabling the use of Schiff base metal complexes for a large variety of useful catalytic transformations.
Schiff bases and their coordination compounds are well known to be biologically important and of interest for their antibacterial, antitumour, and antitubercular activities [15].Furthermore, it has been used as analytical reagent [16][17][18], polymer-coating, ink, pigment [19], �uorescent materials [20] and catalytic reagents [21].However, changes in the electronic, steric, and geometric properties of the ligand alter the orbitals at the metal center and thus affect its properties.e nitro group is a strong electron withdrawing group and, due to its steric effects, has played an important role in affecting the reactivity and enantioselectivities in synthesis as catalysts [22,23].In continuation of our research on the preparation of Schiff base ligands [24][25][26][27] and their complexes [28][29][30][31], we decided to prepare some new Schiff bases and their complexes, including electron withdrawing substituents.
e present paper describes the synthesis and spectroscopic characterization of several nitro-Schiff base ligands and their complexes with transition metal ions under mild conditions.e corresponding materials were characterized by spectroscopic (IR, UV/Vis, 1 H and 13 C NMR, mass spectra) and physical (melting point) data.

Experimental
2.1.Materials.Chemicals were purchased from the Fluka and Merck Chemical Companies.Solvents were puri�ed by standard methods and dried before use by conventional methods.

Apparatus for Analysis and Physical Measurements.
Melting points were taken on a Gallenkamp melting point apparatus and are uncorrected.UV-Vis spectra were recorded on Beckman DU-64 spectrophotometers. 1 H NMR spectra were obtained at 400 MHz using a Bruker Avance 400 NMR in CDCl 3 and DMSO-d 6 as the solvents. 13C NMR (100 MHz) spectra for compounds were obtained on a 400 MHz Avance Bruker spectrometer.e infrared spectra were determined on a Perkin-Elmer 781 spectrophotometer and an Impact 400 Nicolet Magna series FT-IR spectrophotometer.

Synthesis of Ligands.
In order to prepare of Schiff base ligands (L1-L8), a solution of 5-nitrosalicylaldehyde (4 mmol) in methanol (20 mL) was slowly added over a solution of selected diamine (2 mmol) in the same solvent (20 mL).e mixture was stirred at 30 ∘ C; aer the reaction is completed, the product precipitated as a yellowish orange solid and the crude solid was �ltered off and washed with ethanol twice (2 × 20 mL).

Synthesis of Schiff Base Complexes of Ni(II) and Cu(II).
Metal(II) acetate (1 mmol) were dissolved in 20 mL methanol as stirred for 20 minutes.Also, one mmol of selected Schiff base ligand is added to 20 mL of methanol in a 100 mL two-nec�ed, round-bottomed �as� that was provided with a re�ux condenser and stirred to dissolve.�e metal(II) salt solution was slowly added dropwise to the ligand solution with stirring, the resulting slurry was stirred under N 2 at room temperature.Aer the end of the reaction time, the mixture is cooled until −5 ∘ C over night, the microcrystalline solid was precipitated.�e solution was �ltered to eliminate excess unreacted metal acetate and the crude solids washed with ethanol (3 × 10 mL).

Results and Discussion
In this research, �rstly, 2 moles of 5-nitro-salicylaldehyde and 1 mole diamine were reacted together and the corresponding products L1-L8 were obtained under mild and easy conditions (Scheme 1).e con�rmation of these products was demonstrated by spectroscopic and physical data.e results of these reactions were shown in Table 1.As indicated in this table, a lot of useful and convenient Schiff bases were afforded in high yields and appropriate reaction times.
In continuation of this research, in order to prepare all complexes, we applied Schiff base ligands in the reaction with equal moles amounts of nickel(II) and copper(II) acetate salts in methanol solution (Scheme 2).
e corresponding results of these reactions were summarized in Table 2.Because of high reactivity regarding ligands in complex formation, the reactions have been proceeded under mild conditions at room temperature.is factor was restrained from oxidation of metal(II) to metal(III) in products.
e para-nitro-substitution of phenyl ring causes that the phenolic OH becomes more acidic; this affected on the conditions of direction synthesis of ligands and complexes.Basically, the aldehyde group becomes more active and increases the yield of the Schiff base formation.In addition to the formation of complexes, the phenolic hydrogen releases S 2 easier and thus the complex formation occurs in low reaction time at room temperature.
3.1.Electronic Spectra.e electronic spectra of the ligands and their metal complexes were recorded in 1,2dichloroethane.e electronic spectral data of the H2L ligands and their metal complexes are summarized in Table 3. e H2L shows two bands at 380-410 nm ( = 8.9 × 10 3 mol −1 cm −1 ) and 275-315 nm (   × 1 3 mol −1 cm −1 ), which may be assigned to the    * and    * transitions, respectively; the complexes show two bands in the 295-335 and 335-360 nm ranges which are assigned to intraligand transition [32][33][34].In the complexes, the    * transitions due to the azomethine group are shied to the lower energy.From these results, the nitrogen atom of the imines group appears to be coordinated to the metal ion [35].e remainder of the observed bands at about 290-320 nm are assigned as    * type transitions involving molecular orbital located on the phenolic chromophore.e nickel(II) complexes show two bands at 430 and 581 nm and 430 and 570 nm are due to 3 T 1  3 T 2 transitions, indicating square planar environment around the nickel(II) ion [36][37][38].For the nitro-substituted ligands, a considerable overlap between    * 2 and    * 3 is observed.e    * 1 transition has been assumed to be localized mainly on the azomethine chromophore [39].Instead, the    * 2 band has been assigned to a transition involving mainly  molecular orbitals of the aromatic ring of the salicylidenenimine moiety [40].In the ligands, these bonds were observed at about 280-320 nm.is blue shi in the complexes may be due to the electron's donation of a lone pair by the oxygen of the phenoxy group to the central metal atom.

IR Spectra.
Important spectral bands of the H2L and its metal complexes are presented in Table 3. Signi�cant frequencies were selected by comparing the IR spectra of the ligands with those of the metal complexes.e IR spectrum of the H2L shows broad medium bands in the 3450-3200 and 3100-2600 cm −1 ranges, which attributed to intramolecular hydrogen bonding [41].e spectra are showed broad strong bands at 3396 and 3250 and strong bands at 1662 and 1600 cm −1 are assigned to the (NH), (C-O), and (C=N), respectively [42,43].Also, the strong and medium bands appear at 1570, 1508, and 1326 cm −1 , correspond to (C-C)Ar., (CH-C)AL, and (C-O), respectively [44].
e ligands and complexes were characterized mainly using the imine and phenolic bands.e main infrared bands and their assignments are listed in Table 3. e IR spectra of the complexes in comparison with the free ligands to determine the changes that might have taken place during the complexation.e band at 1610-1640 cm −1 is characteristic of the azomethine nitrogen atom in the free ligand.e observed lowering in this frequency to region 1590-1615 cm −1 in all the complexes and indicates the involvement of the azomethine nitrogen atom in coordination with metalation [45,46].e spectra of the ligands shows broad bands in the rang 3200-3500 cm −1 assignable to intramolecular H-bonded of phenolic groups [47], which are absent in the spectra of their complexes, indicating that the oxygen of the -OH groups is coordinated to the metal ion [48].us, the entire ligands act as tetradentate chelating compound coordinated to metal ion through two oxygen and two nitrogen atoms [49].ese data are well in accordance with those of reported complexes [50,51].
Upon coordination bands at 1512 and 1341 cm −1 which are typical of nitro group, nitro ligands undergo minor changes in complexes.It may therefore be that the nitro group is not coordinated to the metal ions.In all the complexes, the bands at 617-461 cm −1 and 461-420 cm −1 rang can be attributed to the  - and  - modes, respectively.

Magnetic Properties of Complexes.
Molecular paramagnetism is a characteristic property of unpaired electron systems.In most coordination compounds of the transition metals and some organometallic compounds as well, paramagnetic behavior is encountered due to the incompletely �lled 3d, 4d, and 5d electron shell.It should be noted that simple paramagnetism will be found only if there is sufficient magnetic dilution.is is the case if, due to the presence of large organic ligands, the paramagnetic centers are well separated, thus avoiding cooperative interactions of the ferro and antiferromagnetic type.Magnetic susceptibility amounts of the complexes were summarized in Table 4.
T 3: Uv-Vis spectral data (DMF, nm) and IR bands (cm −1 ) of ligands and their complexes.Ar-H and 6.9-7.1 ppm a doublet peak for Ar-H.e absence of peak in 14.0 ppm, noted in the metal(II) complex, indicates the loss of the -OH proton due to complexation [52].Intermolecular hydrogen bonding also accounts for the high frequency of the signals for the orthophenolic hydrogens in all the Schiff bases.By comparing the 1 H NMR spectra of all the Schiff bases with those of their corresponding metal(II) complexes, it is noted that there is a down�eld shi in the frequency of azomethine protons of the aromatic bridge and up to �eld shi in the aliphatic bridge con�rming coordination of the metal ion to both groups.

Conclusion
In this study, we have reported a mild and convenient method for synthesis of some Schiff base complexes of metal(Π) at room temperature.Also the desired Schiff bases for preparation of these complexes have been obtained through easy, simple, and efficient reaction of nitrosalicylaldehyde with various diamines.e resulting products have been afforded in excellent yields and efficient reaction times.e ligands are found to be bound to the metal atom through the oxygen atoms of the hydroxyl groups and nitrogen atoms of imine chromophore which is also supported by spectroscopic techniques.
T 1: e results of the reaction for preparation of nitro-Schiff bases.
a Isolated yields.
Spectra of all the complexes were recorded in DMSO-d 6 solution at 400 MHz and chemical shis are in units of ppm relative to TMS as internal standard on the delta () scale.e general 1 H NMR spectrum of the Schiff bases in DMSO shows the following signals: 14.0-14.5  a singlet and broad band for phenolic (O-H) group, 8.9  a singlet band for azomethine hydrogen (HC=N), 8.5-8.6  a doublet band for Ar-H, 8.1-8.2  as dd for 1H NMR and 13 C NMR Spectra.