Synthesis and Characterization of Boron and 2-Aminophenol Schiff Base Ligands with Their Cu ( II ) and Pt ( IV ) Complexes and Evaluation as Antimicrobial Agents

Synthesis, Characterization and biological study of two Schiff base ligands derived from 2-aminophenol and 4bromoacetophenone (1:1) (L1) and boron Schiff base which was derived from L1 and sodium borohydride (1:1) (L2). Cu (II) and Pt (IV) complexes of the Schiff base ligands were synthesized. The prepared compounds have been investigated by molar conductivity, elemental analysis, FTIR, 1 H 13 CNMR, thermal analysis, UV-VIS, as well as atomic absorption and magnetic susceptibility. The biological study of the ligands and their metal complexes were carried out against E-coli, Staphylococcus, as bacteria species, Candida and Mycrosporium Canis as fungi.


INTRODUCTION
The Schiff bases have a wide range of uses and applications because these compounds have excellent properties such as structural similarities with biological molecules, simple preparation methods with flexibility [1], biological activity as antiviral, antileukemic, antibacterial, enzymatic reaction inhibitors and biocidal which is essential for plant growth regulators [2].Boron compounds have been well known since ancient times where used to prepare hard glasses and glazes.Today the uses of boron compounds expended to include semiconductors, hard materials and antitumor medicine [3][4][5].Boron is an electron deficient which possess four valance orbitals and only three electrons, boron tendency to fill the vacant orbital and this promote the electron acceptor behavior and lead to increase the stability of imine group (C=N) of boron Schiff base, therefore the organoborane compounds are more stable than organometallic compounds [6].Boron Schiff bases have many applications such as catalyst [7], determination of enantiopurity of chiral amino alcohols [8], photoelectronic application [6,9] and nutraceutical industry [10].In this paper we report the synthesis, structural characterization and biological evaluation of two Schiff base ligands, 2-aminophenol Schiff base (L 1 ) and boron Schiff base (L 2 ) with their Cu (II) and Pt (IV) complexes.

Material and Instruments
All chemicals used were of analytical reagent grade.Melting points were determined by using Gallenkamp melting point apparatus.Thermal analysis (TG and DTG) were performed by using METTLER TA4000 SYSTEM for L 1 and METTLER TOLEDO for L 2 .The spectra of 1 H and 13 C NMR were recorded on (Bruker NMR spectrometer 400 MHz Avance III 400).The electronic spectra of all prepared compounds were recorded on (SHIMADZU 1800-UV spectrophotometer) in DMSO.FTIR were carried out in the range (400-4000) cm -1 using KBr disk for the ligands and CsI disk in the range (250-4000) cm -1 for metal complexes by using SHIMADZU 8400s spectrophotometer.Elemental microanalysis was carried out by (CHNS Elemental Analyzer CHNS-932).Metal contents were determined by using (Nov AA 350 spectrophotometer).Magnetic susceptibility measurements were performed at room temperature by using (Auto Magnetic Susceptibility Balance Model Sherwood Scientific).Molar conductivity was measured in DMSO at room temperature by using Hunts Capacitor Trade Mark British made.Chloride contents were determined by using Mohr method.

Synthesis of 2-borane((1-(4-bromophenyl)ethylidene)amino)phenol (L 2 ):
A hot solution of Schiff base (L 1 ) (0.173 gm, 0.59 mmol) in 60 ml tetrahydrofurane (THF) was added gradually to a boiled suspension of sodium borohydride (0.045 gm, 1.19 mmol) in 10 ml THF.The mixture was heated under reflux for 16 hr with stirring and the color of solution changed from green to brown.The brown product was obtained by concentrated the reaction mixture and then a little amount of n-hexane was added, washed several times with n-hexane and dried in air (Figure 1).

Synthesis of Cu (II) complexes C 1 and C 3 :
To a hot solution of L 1 (0.1 gm, 0.2128 mmol) in a mixture of methanol:aceton (2:1) was added a methanolic solution of CuCl 2. 2H 2 O (0.0181 gm, 0.1064 mmol) for C 1 complex.The mixture was heated under reflux for four hours.Dark brown precipitate was obtained after evaporating part of the solvent and adding ether.Washed several times with ether and dried.The preparation method of copper complex (C 3 ) was similar to that of (C 1 ), using L 2 (0.0713 gm, 0.2034 mmol) with CuCl 2 .2H 2 O (0.0173 gm, 0.1017 mmol).

Synthesis of platinum complexes C 2 and C 4 :
The preparation methods of platinum complexes were similar to that of (C 1 ), using L 1 or L 2 0.1g (0.2128 and 0.2852 mmol) respectively and K 2 PtCl 6 (0.0517g, 0.1064 mmol) in 6 ml DMSO for C 2 and (0.0693 g, 0.1426 mmol) in 24 ml DMSO for C 4 respectively.

RESULTS AND DISCUSSION
The physical and analytical data (Table 1) are agreement with suggested structures of studied compounds.

IR Spectra
The data of IR spectra of studied ligands and their metal complexes are listed in Table 2 and the spectra of L 1 and L 2 are shown in Figure 2. The peaks appeared at (3420-3452) cm -1 were attributed to lattice water while the coordinated water was exhibited peaks at observed at (3379, 651) and (3406, 756) cm -1 [12][13][14].The spectra of L 1 , L 2 , C 1 and C 2 were exhibited strong peaks at (3400-3520) cm -1 which were assigned to stretching vibration of OH group, this band was shifted to lower frequency in the spectrum of C 2 complex at 3400 cm -1 as a result of complexation [15,16] and disappeared in spectra of C 3 and C 4 because of the binding of metal ions with oxygen atom of OH group.The band appeared at 2340 cm -1 in spectra of L 2 , C 3 and C 4 assigned to vibrational mode of B-H [12].The band at 1415 cm -1 in spectrum of L 2 was attributed to the stretching vibration of B-N, this band not changed in spectra of C 3 and C 4 complexes [12].The azomethine group (C=N) exhibited peak at 1581 cm -1 for L 1 while this peak shifted to lower frequency at 1562 cm -1 for L 2 and this due to binding of BH 3 with nitrogen atom of C=N.The spectrum of metal complex C 2 showed shifted to lower frequency in this peak at 1577 cm -1 comparison with L 1 and the spectrum of C 1 exhibited change in profile of C=N peak, this due to the binding between N atom and metal ions [16,17].Peaks that noticed at range 304-491 cm -1 were assigned to stretching vibration of M-Cl, M-O and M-N [13,15,18].  1 HNMR Spectra The data of 1 HNMR are listed in Table 3 and the NMR spectra in DMSO of L 1 and L 2 are shown in (Figure 3).The spectra of two ligands showed chemical shift at 8.9 and 8.5 ppm for L 1 and L 2 respectively and this due to OH proton [14].The multiple peaks observed at the range δ(6.3-6.8)ppm referred to aromatic protons [19].The spectrum of L 1 exhibited a peak at δ (1.9 ppm) which assigned to CH 3 protons and acetone (CH 3 ) [14,16].Chemical shifts of methanolic CH 3 protons and the dissolved water in DMSO appeared at δ (3.3 ppm) [16].The weak band appeared at δ (4.3 ppm) is due to methanolic OH proton [16].The spectrum of L 2 exhibited peak at δ (1 ppm) which was referred to CH 3 protons [14,16].The peak observed at (δ 1.65 ppm) attributed to THF protons (mark: 16), while the broad peak appeared at δ (3.4 ppm) due to protons of THF (mark: 17), BH, H 2 O and dissolved water in DMSO [12][13][14][15][16][17][18][19][20].Strong peak appeared at δ (2.5 ppm) was assigned to residual DMSO [12,14].
Thermal Analysis TG and DTG TG and DTG data of studied ligands are listed in Table 5 and their thermograms are showed in (Figure 4).The first step for each ligand involved loss of solvents and water molecules because of their low boiling points at temperature range (33.7-172.3°C)and (20.6-85°C) for L 1 and L 2 respectively.The thermogram of L 1 showed the decomposition of L 1 through four steps at temperature range (33.7-601.3°C)without residue, while the decomposition of L 2 carried out through four steps at temperature range (20.6-650°C) with residue and this means the L 2 is more stable than L 1 [22][23][24].

Electronic Spectra, Magnetic Moments and Molar Conductivity
The data of UV-Vis spectra of L 1 , L 2 and their metal complexes in DMSO are listed in Table 6.The spectra of ligands (Figure 5) showed the intense band at 40650 and 41493 cm -1 for L 1 and L 2 respectively due to π→ π * transition and weak intensity band at 23980 and 19758 cm -1 of L 1 and L 2 respectively which referred to n→π * transition.The copper and platinum complexes of L 1 and L 2 exhibited blue shift of ligand bands (π→ π * ).The spectrum of copper complex (C 1 ) (Figure 5) showed three bands at 15329, 19794 and 20976 cm -1 which were assigned to 2 B 1 g→ 2 A 1 g, 2 B 1 g→ 2 B 2 g and 2 B 1 g→ 2 Eg transitions respectively of tetragonally distorted octahedral Cu(II) complexes [14].A three absorption bands were observed in the spectrum of copper complex (C 3 ) at 11030, 15480 and 19671 cm -1 which attributed to 2 B 1 g→ 2 A 1 g, 2 B 1 g→ 2 B 2 g and 2 B 1 g→ 2 Eg transitions respectively, as well as the absorption band at 24154 cm -1 which were belong to charge transfer transition (C.T) [13,14,25].The magnetic moments of copper complexes C 1 and C 3 were µ eff =2.4 and 2.6 B.M of C 1 and C 2 respectively, these values due to octahedral geometry [26].The spectrum of diamagnetic platinum complex C 2 showed bands at 12195, 22935 and 29614 cm -1 which assigned to 1 A 1 g→ 3 T 1 g (H), 1 A 1 g→ 3 T 2 g and 1 A 1 g→ 1 T 1 g(C.T) transition respectively of octahedral platinum (IV) complexes [13,27,28].Two absorption bands were observed in spectrum of Pt(IV) complex (C 4 ) (Figure 5) at 11035 and 21643 cm -1 which were referred to 1 A 1 g→ 3 T 1 g(H) and 1 A 1 g→ 3 T 2 g transitions respectively of octahedral platinum (IV) complexes.[13,27,28].The molar conductivity data in DMSO were 0.033, 0.059, 0.121 and 0.163 S.mol -1 .cm - for C 1 , C 2 , C 3 and C 4 complexes respectively indicating the nonelectrolytic properties of complexes [29].The suggested structures of complexes were shown in (Figure 6).

Biological Screening
The data of biological activity were listed in Table 7.The studied compounds and starting materials were tested as antibacterial and antifungal agents against gram negative bacteria (E.coli), gram positive bacteria (Staphylococcus aureus), (Candida) and (Mycrosporium Canis).The test was carried out by diffusion method for studied compounds in DMSO and concentration 10 -2 M. The tested compounds exhibited different activities against Staphylococcus aureus, Candida and Mycrosporium canis, while these compounds were inactive against E. coli.The results have been compared with three kinds of antibiotics cephalexim, cefatoxim and ketoconazole.

Figure 1 :
Figure 1: The structures of synthesized ligands L1 and L2

Figure 2 :
Figure 2: The IR spectra of ligands L1 and L2