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

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 (L₁) and boron Schiff base (L₂) with their Cu(II) and Pt(IV) complexes.

Experimental

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 L₁ and METTLER TOLEDO for L₂. The spectra of ¹H and ¹³C 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 were 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 were measured in DMSO at room temperature by using Hunts Capacitor Trade Mark British made.  Chloride contents were determined by using Mohr method.

Methods

Synthesis of 2-((1-(4-bromophenyl)ethylidine)amino)phenol (L₁).

This ligand has been prepared in our previous work.^[11]

Synthesis of 2-borane((1-(4-bromophenyl)ethylidene)amino)phenol (L₂).

A hot solution of Schiff base (L₁) (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: The structures of synthesized ligands L1 and L2. Click here to View figure

 

Synthesis of Cu(II) Complexes C₁ and C₃.

To a hot solution of L₁ (0.1 gm, 0.2128 mmol) in a mixture of methanol:aceton (2:1) was added a methanolic solution of CuCl_2.2H₂O (0.0181 gm, 0.1064 mmol) for C₁ 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 (C₃) was similar to that of (C₁), using L₂ (0.0713 gm, 0.2034 mmol) with CuCl₂.2H₂O (0.0173 gm, 0.1017 mmol).

Synthesis of Platinum Complexes C₂ and C₄.

The preparation methods of Platinum complexes were similar to that of (C₁), using L₁ or L₂ 0.1g  (0.2128 and 0.2852 mmol) respectively and K₂PtCl₆(0.0517g, 0.1064 mmol) in 6 ml DMSO for C₂ and (0.0693g, 0.1426 mmol) in 24 ml DMSO for C₄ respectively.

Results and discussion

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

Table 1: The physical properties and analytical data for two ligands and their metal complexes

Symbol	Color	Yield%	m.p °C	CHN analysis Found (calc.)			M%Found (calc.)	Cl%Found (calc.)
L1	Golden-brown	21.9	134-137	56.27(56.18)	5.95(6.80)	4.34(2.97)	_	_
L2	Brown	57.2	>240	48.30(49.29)	5.04(5.76)	4.93(3.99)	_	_
C1	Dark brown	48.3	>360	52.53(51.92)	3.57(3.70)	3.82(4.32)	10.88 (9.87)	2.6 (2.7)
C2	Dark brown	39.2	>360	24.42(23.43)	2.55(3.06)	1.59(1.95)	_	2.8 (4.5)
C3	Dark brown	93.7	>360	32.70(33.79)	4.09(4.94)	1.89(2.81)	4.82 (5.63)	7.4 (7.1)
C4	Dark brown	92.9	>360	21 (21.92)	3.19(4.17)	1.79(1.82)	_	_

 

IR Spectra

The data of IR spectra of studied ligands and their metal complexes are listed in Table (2) and the spectra of L₁ and L₂ 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 L₁, L₂, C₁ and C₂ 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 C₂ complex at 3400 cm^-1 as a result of complexation^{[13,15, 16]} and disappeared in spectra of C₃ and C₄ because of the binding of metal ions with oxygen atom of OH group. The band appeared at  2340 cm^-1 in spectra of L₂, C₃ and C₄ assigned to vibrational mode of B-H.^[12]  The band at 1415 cm^-1 in spectrum of L₂ was attributed to the stretching vibration of B-N, this band not changed in spectra of C₃ and C₄ complexes.^[12] The azomethine group (C=N)  exhibited peak at 1581 cm^-1 for L₁ while this peak shifted to lower frequency at 1562 cm^-1 for L₂ and this due to binding of BH₃ with nitrogen atom of C=N. The spectrum of metal complex C₂ showed shifted to lower frequency in this peak at 1577 cm^-1 comparison with L₁ and the spectrum of C₁ 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.^[13,14,18]

Table 2: IRSpectra of two ligands and their metal complexes

compound	ʋO-H	ʋB-H	ʋB-N	ʋC=N	H2O lattice(coordinate)	ʋM-O	ʋM-N	ʋM-Cl
L1	3448	–	–	1581	–	–	–	–
C1	3440	–	–	1583	–	–	387	312
C2	3400	–	–	1577	3420	500	435	318
L2	3520	2340	1415	1562	3425	–	–	–
C3	–	2340	1413	1566	3452 (3406,756)	441	–	304
C4	–	2340	1415	1565	3448 (3379,651)	491	–	329

 

Figure 2: The IR spectra of Ligands L1 and L2. Click here to View figure

 

¹HNMR Spectra

The data of ¹HNMR are listed in (Table 3) and the NMR spectra in DMSO of  L₁ and L₂ are shown in  (Figure 3). The spectra of two ligands showed chemical shift at 8.9 and 8.5 ppm for L₁ and L₂ 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 L₁ exhibited a peak at δ(1.9 ppm) which assigned to CH₃ protons and acetone (CH₃) .^[14,16] Chemical shifts of methanolic CH₃ 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 L₂ exhibited peak at δ(1 ppm) which was referred to CH₃ 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, H₂O and dissolved water in DMSO. ^{[12,14,16,20 ]} Strong peak appeared at δ(2.5 ppm) was assigned to residual DMSO^[12,14].

Table 3: Chemical shifts for ¹HNMR of L₁ and L₂.

L1			L2
Chemical shifts δ(ppm)	Mark	Assignments in DMSO	Chemical shifts δ(ppm)	Mark	Assignments in DMSO
1.9,9H,s	8,16,18	CH3 protons and CH3 in acetone protons	1,3H,s	8	CH3 protons
3.3,3H,s	15	Methanolic CH3 protons  and dissolved water in DMSO	1.65,4H,s	16	2(CH2) of THF
4.3,1H,s	15	Methanolic OH proton	3.4,9H,s	15,17,18	BH,H2O,2(CH2) of THF and dissolved water in DMSO
6.3-6.6,8H,m	3-6 10,11,13,14	Aromatic protons	6.3-6.8,8H,m	3-6 10,11,13,14	Aromatic protons
8.9,1H,s	2	O-H proton	8.5,1H,s	2	O-H proton

 

Figure 3: 1HNMR spectra of L1 and L2.     Click here to View figure

 

¹³CNMR Spectra

The data of ¹³CNMR are listed in (Table 4). The ligands spectra in DMSO exhibited multiple peaks at range δ(114-125.2) ppm which due to aromatic carbon.^[14] The single peaks observed at δ(130.4, 125 ppm) for L₁ and L₂ respectively attributed to chemical shift of C-N (mark:1).^[21] The peak at δ(132.6 and 132.5 ppm) for L₁ and L₂ respectively due to carbon of C-Br.^[16] Chemical shift of phenolic carbon (mark:2) appeared at δ(144 and 144.1 ppm) for L₁ and L₂ respectively.^[16] The azomethine carbon showed chemical shift at δ(172 and 166.1 ppm) for L₁ and L₂ respectively. ^[14] The spectrum of L₁ exhibited peak at  δ(20 ppm) which due to CH₃ carbon,^[14] while the peak at   δ(34 ppm) was attributed to CH₃ of acetone.^[16] The methanolic carbon exhibited chemical shift at  δ(49 ppm) and the carbon of acetone (mark:17) showed chemical shift at δ(204 ppm).^[16] The spectrum of L₂ showed another peaks, the peak appeared at  δ(18 ppm) which assigned to methyl group carbon.^[14] The carbons of THF (mark:16) showed a peak at δ(24.5 ppm) , while the carbons (mark:17) exhibited peak at δ(68.3 ppm).^[16] The peak appeared at δ(40 ppm) was attributed to chemical shift of DMSO.^[16]

Table 4: Chemical shifts (ppm) for ¹³CNMR of L₁ and L_2.

L1			L2
Chemical shifts δ(ppm)	Mark	Assignment in DMSO	Chemical shifts δ(ppm)	Mark	Assignment in DMSO
20	8	Carbon of methyl group	18	8	Carbon of methyl group
34	16,18	Carbon of acetone	24.5	16	2(CH2) of THF
49	15	Methanolic carbon	68.3	17	2(CH2) of THF
114-12 5.2	3-6 9, 10, 11, 13, 14	Aromatic carbons	114.5-12 0.5	3-6 9, 10, 11, 13, 14	Aromatic carbon
130.4	1	Carbon of amine	125	1	Carbon of amine
132.6	12	Carbon of brom	132.5	12	Carbon of brom
144	2	Phenolic carbon	144.1	2	Phenolic carbon
172	7	Carbon of azomethine	166.1	7	Carbon of azomethine
204	17	Carbon of acetone

 

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 L₁ and L₂ respectively. The thermogram of L₁ showed the decomposition of L₁ through four steps at temperature range (33.7-601.3°C) without residue, while the decomposition of L₂ carried out through four steps at temperature range (20.6-650°C) with residue and this means the L₂ is more stable than L₁. ^[22,23,24]

Table 5: Thermal decomposition of L₁ and L₂.

Compound	Steps	Temp. range of Decomposition°C	Peak temp at DTG°C	% weight loss Found (calc.)
L1C6H12NOBr.2CH3OH.2CH3COCH3M. wt= 469.9	2 CH3OH 2 CH3COCH3 8 H	33.7-172.3	146.3	38.83 (40.00)
	C6Br + C6	172.3-280.7	207.0	46.48 (47.64)
	C	280.7-354.3	324.0	3.17 (2.55)
	CH3 + N + OH	354.3-601.3	458.3	11.53 (9.78)
L2C14H15NOBBr.0.1C4H8O.2.2H2OM. wt= 350.52	0.1C4H4O 2.2H2O	20.6-85	88.0	12.30(13.35)
	Br	85-170	178.0	21.53(22.79)
	BH3+CH3+OH	170-320	323.0	13.73(13.07)
	C6H4	320-446	480.0	22.13(21.68)
	C6H4CN(residue)	446-650	–	30.30(29.09)

 

Figure 4: Thermograms of Ligands L1 and L2. Click here to View figure

 

Electronic spectra, Magnetic Moments and Molar Conductivity

The data of UV-Vis spectra of L₁, L₂ 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 L₁ and L₂ respectively due to π→ π^* transition and weak intensity band at 23980 and 19758 cm^-1 of L₁ and L₂ respectively which referred to n→π^* transition. The copper and platinum complexes of L₁ and L₂ exhibited blue shift of ligand bands (π→ π^*). The spectrum of copper complex (C₁) (Figure 5) showed three bands at 15329, 19794 and 20976 cm^-1 which were assigned to ²B₁g→²A₁g, ²B₁g→²B₂g and ²B₁g→²Eg transitions respectively of tetragonally distorted octahedral Cu(II) complexes.^[14] A three absorption bands were observed in the spectrum of copper complex (C₃) at 11030, 15480 and 19671 cm^-1 which attributed to ²B₁g→²A₁g, ²B₁g→²B₂g and ²B₁g→²Eg 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 C₁ and C₃ were µ_eff.=2.4 and 2.6 B.M of C₁ and C₂ respectively, these values due to octahedral geometry.^[26] The spectrum of diamagnetic platinum complex C₂ showed bands at 12195, 22935 and 29614 cm^-1 which assigned to ¹A₁g→³T₁g(H), ¹A₁g→³T₂g and ¹A₁g→¹T₁g(C.T) transition respectively of octahedral platinum (IV) complexes. ^[13,27,28] Two absorption bands were observed in spectrum of Pt(IV) complex (C₄) (Figure 5) at 11035 and 21643 cm^-1 which were referred to ¹A₁g→³T₁g(H) and ¹A₁g→³T₂g 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^-1 for C₁, C₂, C₃ and C₄ complexes respectively indicating the nonelectrolytic properties of complexes.^[29] The suggested structures of complexes were shown in (Figure 6).

Table 6: Electronic spectra, magnetic moment and molar conductance with suggested structures of L₁and L₂ complexes.

symbol	Band position cm-1	Assignments	µeff. B.M	Molar Conductivity S.mol-1.cm-1	Suggested structures
C1 Cu(II)	ʋ1  15329 ʋ2  19794 ʋ3  20976	2B1g→2A1g 2B1g→2B2g 2B1g→2Eg(C.T)	2.4	0.0337	octahedral
C2 Pt(IV)	ʋ1  12195 ʋ2  22935 ʋ3  29614	1A1g→3T1g(H) 1A1g→3T2g 1A1g→1T1g(C.T)	diamagnetic	0.0591	octahedral
C3 Cu(II)	ʋ1 11030 ʋ2 15480 ʋ3 19671 ʋ4 24154	2B1g→2A1g 2B1g→2B2g 2B1g→2Eg L→M(C.T)	2.6	0.121	octahedral
C4 Pt(IV)	ʋ1 11035 ʋ2 21643	1A1g→3T1g(H) 1A1g→3T2g	diamagnetic	0.163	octahedral

 

Figure 5: UV-VIS spectra of L1, L2, C1 and C4 complexes. Click here to View figure

 

Figure 6: The suggested structures of synthesized complexes. Click here to View figure

 

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.

Table 7: Antibacterial and antifungal activity of two ligands and  their metal complexes.

Compounds	Staphylococcus aureus diameter mm	E-Coli diameter mm	Candida diameter mm	Mycrosporium Canis diameter mm
DMSO	zero	zero	zero	zero
2-aminophenol	36	zero	18	zero
4-bromoacetophenone	zero	zero	zero	zero
Sodium borohydride	zero	zero	zero	zero
L1	20	zero	12	15
L2	23	zero	12	zero
C1	32	zero	34	23
C2	20	zero	10	zero
C3	20	zero	15	13
C4	zero	zero	zero	zero
Cephalexim	25	zero	–	–
Cefatoxim	48	45	–	–
Ketoconazole	–	–	zero	20

 

Conclusions

The new Schiff base ligand derived from 2-aminophenole with 4-bromoacetophenone (L1) and the Schiff base of boron (L2) with their metal complexes have been synthesized and characterized. The results showed that the coordination of L1 with Cu(II) and Pt(IV) through N atom as donor atom while the coordination of L2 with metal ions through O atom. The results of electronic spectra and magnetic susceptibitity exhibited the octahedral geometry for Pt complexes and destorted octahedral for Cu complexes. The thermal analysis give a good characterization of L1 and L2 and showed that L2 is more stable than L1. The synthesized compounds were studied as antibacterial and antifungal , the results showed a different activities against Staphylococcus aureus, Candida and Mycrosporium Canis , while inactive against E-Coli.

Acknowledgment

The authors gratefully acknowledgment chemistry department laboratories in college of science / Baghdad University and college of science / Al-Mustansiriya University for supporting in the measurements of FTIR ,UV-Vis spectra, melting point, conductivity and magnetic susceptibility

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