Preparation, Characterization and Biological Activities of some Unsymmetrical Schiff Bases Derived from m-phenyelenediamine and their Metal Complexes

Unsymmetrical Schiff bases H2L = [ 1(( E )-(( 3-((( E) -2hydroxybenzylidene) amino)phenyl)imino)methyl) naphthalene-2-ol] and H2L = [1-((E)-((3-(((E)-1-(2-hydroxphenyl) ethylidene ) amino ) phenyl ) imino ) methyl ) naphthalene 2 – ol ] which derived from m phenylenediamine and 2-hydroxynaphthaldehyde and then with salicyaldehyde or 2hydroxyacetophenone, and their complexes of the type [M 2L 2], where n = 1, 2 and M = Mn(II), Co(II), Ni(II) and Cu(II) have been synthesized. Adduct complexes of the type [M2L 2 (py)4] were also prepared in (1 : 4)(complex : py) molar ratio. The complexes and adducts were characterized by elemntal analysis (C. H. N), metal content, (IR, UVv.s, H-NMR) spectroscopy, conductivity and magnetic measurements. The resulted data suggested that the Schiff bases containing ONNO donor atoms acts as dibasic tetradendetate ligands through coordinated with metal ions. Conductivity data in DMSO solution showed that all complexes are nonelectrolyte. Magnetic moment and electronic spectra data indicat that the complexes have either tetrahedral or octahedral geometry while [Ni2 L 2] complexes have square planer geometry. The legends and their complexes were screened for antibacterial activity against Staphylococcusaureus and Escherichia coli. The ligands and their complexes showed some biological activities.

. These complexes have played a major role in the advancement of coordination chemistry whose field of application varies from physicochemical studies (Luo et al., 2003), to biological aspects (El-Motalb et al., 2011).
In biological systems, transition metal ions are usually bound to a macrocycle such as a heme ring or to donor atoms of peptide chains in distorted enviromment (Li et al., 2015), and this unsymmetrical coordination of ligands around central metal ions had lead to a growing interest in the design and synthesis of transition metal complexes of unsymmetrically substituted Schiff base ligands as synthetic models (Hernandez et al., 2004).
Aromatic diamines have the ability to coordinate to a metal directly on their relative ortho, meta or para positions (Hernandez et al., 1997), mphenyelenediamine derived Schiff bases can only coordinate one nitrogen atom to any metal ion. This is due to the big proximity range between the nitrogen atoms of mphenylenediamine and its rigid structure (Torayama et al., 1998).
Generally, m-phenylenediamine Schiff bases have the ability to acquire the formation of dimer complexes, where the Schiff bases act as bridges connecting the two metal cations (Clarke et al., 1998) thus producing dinuclear complexes. The uniqueness of such behaviour however has never been applied and studied in the biological field as compared the Schiff bases derived from the other two aromatic diamines namely its ortho and para -analogues. Such complexes were suggested to have an even better biological activity due to the presence of two metal ions compared to complexes with one metal ion. Therefore synthesis, characterization and the biological activity of two unsymmetrical tetradentate Schiff bases derived from mphenylenediamine and their Mn(II), Co(II), Ni(II) and Cu(II) complexes are described in this work. EXPERIMENTAL All reagents and solvents were of analytical grade used as supplied from fluka or BDH chemical companies, Infrared spectra were recorded on BRUKER mauf. U.K. (400-4000 cm -1 ) using KBr disc. Conductivity measurements were carried out on (10 -3 ) M solution of the complexes in DMSO using Conductivity meter Model PCM3 -JENWAY at ambient temperature. The electronic spectra were recorded in DMSO (10 -3 ) M solution on SPECTRO UV-VIS AUTO, 110 v 60 HZ using 1cm quartz cell (200-1000)nm. Metal content was determined using instrument, AA-7000-UN. BG. IBN-H.C.S.L Atomic Absorption (Flamecont). Elemental analysis were performed on Euro EAE Elemental Analyzer Euro EA 3000 Italy. 1 H-NMR Spectra were recorded in DMSO -d 6 using NMR edy 60 prrouser, Manual Version 1.0 (Nanalysis Crop, 2015). Melting point was recorded on Electro -thermal 9300 Engineering LT D. The magnetic measurements were carried out at 25 0 C on the solid state by (Magnetic Susceptibility Balance Sherwood Scientific Cambridge / U K).

Preparation of the Legends
The unsymmetrical Schiff base ligands were prepared according to the reported procedure (Pethe et al., 2017) by reacting equimolar amounts of m-phenylenediamine (0.01 mole , 1.08 g) in methanol (10 ml) was added slowly to methanolic solution (10 ml) containing 2-hydroxynaphthaldehyde (0.01mole, 1.72g) and refluxed for 1h, then fllowed by addition of salicyaldehyde (0.01 mole, 1.22 g) or 2-hydroxyacetophenone (0.01 mole, 1.36 g), each one dissolved in methanol (10 ml), the resulting coloured mixture was refluxed with stirring for 4 h and cooled, then the precipitate was filtered off and washed with cold methaol and dried under vacuum, (Schem 1 ).

Preparation Base Adduct Complexes
These complexes were prepared by the addition of pyridine (0.04 mole , 3.16 g ) to (0.01 mole) the prepared complexes in methanol (50 ml) with continuous stirring until complete precipitation. The precipitate was filtered off, washed with cold methanol and dried under vacuum (Osowole, 2008).

Antibacterial Activity
Antibacterial activity was evaluated using agar diffusion method (Kethcum, 1988), Gram -positive bacteria Staphylococcus aureus and Gram -negative bacteria Escherichia coli were cultivated in nutrient agar on petri dishes. The test solution was prepared by dissolving (10 mg) of the tested substances in DMSO (1 mL). A6 mm diameter filter discs were socked in the tested solutions. After 24 h cultivation at 37 o C, diameter of zones of inhibition was determined. DMSO was inactive under applied conditions.

RESULTS AND DISCUSSION
All the metal complexes are quite stable in dry air and insoluble in common organic solvents but soluble in DMF and DMSO. The elemental analysis shows (2 : 2) (M : L) molar ratio for [M 2 L n 2 ] and (2: 2: 4) [M: L n : py] molar ratio for [M 2 L n 2 (PY) 4 ]. Some physical properties of ligands and their complexes are listed (Table 1.). The molar conductance in (10 -3 ) M DMSO solution of complexes are in the range (14.4 -20.0) ohm -1 cm 2 mol -1 , indicating a non electrolytic nature of the complexes (Gerry, 1971).

Infrared Spectra
The IR Spectrum of the free ligands was studied and assigned on the bases of careful comparison of the complex spectra with that of the free legends, ( Table 2). The IR Spectrum of the ligand show characteristic band at (3465 and 3498) cm -1 attributed to the phenolic hydroxyl group in H 2 L 1 and H 2 L 2 respectively (Alias et al., 2014). The absence of this band in the spectra of the complexes indicates the coordination of phenolic oxygen to the metal ion after deprotonation (Pethe et al., 2017). This is further supported by the shifting of ‫ט‬ (C _ O) phenolic band to lowers frequency (1245 -1273) cm -1 in the spectra of all complexes (Nagajothi et al., 2013). The IR spectrum of (H 2 L 1 and H 2 L 2 ) show band at (1612 and 1582 ) cm -1 due to ‫ט‬ (C=N) stretch. On complexation this band shifted to higher frequency in all complexes by (10 -34) cm -1 indicating participation of azomethine nitrogen in complexation (Ramesh et al., 1996). The stretching vibration of ‫ט‬ (C=N) bond of pyridine ring was obsorved at the region (1484 -1506 ) cm -1 in complexes (5,6,7, 8,13,14,15 and 16) which indicates the coordination of the nitrogen atom of pyridine ring with the metal ions (Ceraldo et al., 2011). Some new bands apper in the spectra of the complexes which include of the stretching vibrations of ‫ט‬ (M-N) and ‫ט‬ (M _ O ). These bands apper in the ranges (417-463) cm -1 and (491 -553) cm -1 respectively (Pathan et al., 2012). The I.R Spectra of ligands and Mn(II), Cu(II) have been shown Fig. (1).

Electronic Spectra and Magnetism
The magnetic moments and electronic spectra data at 25 0 C of the ligands and their complexes are listed (Table 3). The electronic spectra of the ligands H 2 L 1 and H 2 L 2 in DMSO solution show intense bands at (37037, 37313) cm -1 and (31446, 31545) cm -1 attributed to benzene π → π* and n → π* transition of the non-bonding electrons present on the nitrogen of the azomethine group respectively, these transitions are also found in the spectra of the complexes but they are shifited to confirmed the coordination of the ligand to the metal ion (Elmali et al., 2005). New bands in spectra of all complexes at range (26993-29411) cm -1 may be associated with the charge transfer transition (Aranha et al., 2016). The dinuclear Mn(II) complexes (1,5,9 and 13) show magnetic moment values at the range (5.75 -5.90) B.M, which are appreciably close to the calculated spin -only value (5.91) B.M, for five unpaired electrons and reveals a high spin state (Cotton et al., 1999). The electeonic sprctra of extremely pale colour high spin Mn (II) complexes don 't show any d-d transitions bands due to spin forbidden as well as parity -forbidden transitions in tetrahedral and octahedral environments (Cotton et al., 1999 ;Al-Kattan and Al-Nidaa, 2013), therefore, the structures of the prepared Mn(II) complexes were proposed depending on the data of other measurements namely metal content and IR spectra.

Antibacterial Activity
The tested compounds were screened in vitro for their antibacterial activity against Gram -negative and Gram -positive (Escherichia coli, Staphylococcus aureus) bacteria, the antibacterial results are given in (Table 4), compared with those of the standard drug (Ciprofloxacin). The results evidently show that the ligands and all tested complexes were active against the Gram -positive and Gram -negative bacteria on the other hand Cu(II) complexes (4, 8, 12 and 16) exhibited higher inhibition activity as compared with their corresponding ligands and other complexes, generally the activity order against Staphylococcus aureus of the synthesized compounds is as follows: CuL > NiL > CoL > H 2 L 2 > H 2 L 1 > MnL, while the activity order against Escherichia coli of the synthesized compounds is as follows :-CuL > NiL > MnL > CoL > H 2 L 2 = H 2 L 1 .