Cu(II) complexes of an ionic liquid-based Schiff base [1-{2-((2-hydroxybenzylidene)amino)ethyl}-3-methylimidazolium] PF6: Synthesis, characterization and biological activities

Article abstract of DOI:10.2298/JSC140201078S

Two Cu(II) complexes of an ionic liquid based Schiff base 1-{2-[(2-hydroxybenzylidene)amino]ethyl}-3-methylimidazolium hexafluorophosphate, were prepared and characterized by different analytical and spectroscopic methods such as elemental analysis, magne

Full text of DOI:10.2298/JSC140201078S

J. Serb. Chem. Soc. 80 (1) 35–43 (2015)
JSCS–4694
UDC 546.562+542.9+547.571+547.551+
547.576+547.313.2–304.2
Original scientific paper
Cu(II) complexes of an ionic liquid-based Schiff base
[1-{2-((2-hydroxybenzylidene)amino)ethyl}-3-methyl-
imidazolium]PF6: Synthesis, characterization and
biological activities
1
2
2
*
SANJOY SAHA , DHIRAJ BRAHMAN and BISWAJIT SINHA
1Department of Chemistry, Kalimpong College, Kalimpong-734301, India and 2Department
of Chemistry, University of North Bengal, Darjeeling-734013, India
(Received 1 February, revised 1 June, accepted 21 July 2014)
Abstract: Two Cu(II) complexes of an ionic liquid based Schiff base 1-{2-[(2-
-hydroxybenzylidene)amino]ethyl}-3-methylimidazolium
hexafluorophos-
phate, were prepared and characterized by different analytical and spectro-
scopic methods such as elemental analysis, magnetic susceptibility, UV–Vis,
IR and NMR spectroscopy, and mass spectrometry. The Schiff base ligand was
found to act as a potential bidentate chelating ligand with N, O donor sites and
formed 1:2 metal chelates with Cu(II) salts. The synthesized Cu(II) complexes
were tested for their biological activity.
Keywords: ionic liquid based Schiff base; salicylaldehyde; Cu(II) complexes;
1-(2-aminoethyl)-3-methylimidazolium hexafluorophosphate.
INTRODUCTION
Ionic liquids (ILs) are defined as organic salts formed by the combination of
bulky organic cations with a wide variety of anions, that are generally liquid at
1
room temperature. ILs are made up of large ions that are held together by elec-
trostatic interactions. Due to these interactions, the properties of ILs are consi-
2
derably different from those of molecular liquids. ILs have been widely studied
as alternatives to volatile organic solvents for organic synthesis in homogeneous
3–5
as well as biphasic processes.
Such compounds have received attraction in
synthetic chemistry in recent years due to their excellent characteristics, such as
low vapor pressure, inflammability, high thermal and chemical stability, out-
6,7
standing solubility and the possibility of easy recycling, etc. Based on these
properties, ILs have emerged as a novel class of compounds that have been
employed in many fields, such as electrochemistry, organic synthesis, catalysis,
*Corresponding author. E-mail: biswachem@gmail.com
doi: 10.2298/JSC140201078S
35
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36
SAHA, BRAHMAN and SINHA
gas separation, etc. The use of ionic liquids has also received much attention as
5,8
eco-friendly reaction media in organic synthesis. The hydrophobicity/hydro-
philicity of ionic liquids can be altered by manipulating the structures of the cat-
9
ions and anions. In recent years, a number of ionic liquids have been identified
as solvents for the dissolution of biopolymers such as cellulose, starch, wood,
10–17
lignin, feather, wool, etc.
Recently, many workers have focused on the
preparation and application of functionalized ionic liquids (FILs) for special
tasks, such as those carrying hydroxyl, amino, sulfonic acid or carboxyl groups
18–24
and so on.
The FILs have shown great promise not only as alternative green
25
solvents, but also as reagents or catalysts in many organic transformations.
Among many potential organic compounds, Schiff bases are widely emp-
loyed as ligands in coordination chemistry. These ligands are readily available,
26
versatile and, depending on the nature of the starting materials (primary amines
27
and carbonyl precursors), they exhibit various denticities and functionalities.
Schiff bases and their complexes are widely applied in biochemistry, material
science, catalysis, encapsulation, activation, transport and separation phenomena,
28,29
hydrometallurgy, etc.
Schiff bases have been reported to show a variety of
biological actions, such as antibacterial, antifungal, herbicidal, clinical and anal-
30,31
ytical activities by virtue of the azomethine linkage.
Schiff base metal com-
plexes have been the subject intensive study due to their industrial and biological
32–37
applications.
Salicylaldehyde and its derivatives are useful carbonyl
precursors for the synthesis of a large variety of Schiff bases with wide variety of
interesting properties. Hence in this study, an attempt was made to synthesize an
ionic liquid grafted Schiff base 1-{2-[(2-hydroxybenzylidene)amino]ethyl}-3-
-methylimidazolium hexafluorophosphate, and its Cu(II) complexes. The syn-
thesized compounds were characterized by various analytical methods and tested
for their biological activities.
EXPERIMENTAL
Materials and measurements
All the employed reagents were of analytical grade and used as received without further
purification. 1-Methylimidazole, 2-bromoethylamine hydrobromide and potassium hexaflu-
orophosphate were procured from Sigma–Aldrich, Germany. Salicylaldehyde, CuCl2·2H2O,
Cu(NO3)2·3H2O and all other chemicals were purchased from SD Fine Chemicals, India. The
IR spectra were recorded in KBr pellets using a Perkin-Elmer Spectrum RX I FT-IR spec-
trometer, operating in the region 4000 to 400 cm-1. The proton NMR spectra were recorded at
room temperature on an FT-NMR Bruker Avance II 400 MHz spectrometer using DMSO-d6
and D2O as solvents. The chemical shifts are given in ppm downfield of internal standard
tetramethylsilane (TMS). The melting points were recorded using the open capillary method.
Elemental microanalyses (C, H and N) were realized using a Perkin–Elmer (Model 240C)
analyzer. The Cu-content was determined by atomic absorption spectroscopy (AAS) emp-
loying a Varian SpectrAA 50B instrument using a standard Cu-solution from Sigma–Aldrich,
Germany. The mass spectra were recorded on an Agilent 1100 LC equipped with an MSD
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COPPER COMPLEXES OF AN IONIC LIQUID BASED SCHIFF BASE
37
trap. The purity of the Schiff base and its complexes were confirmed by thin layer chromato-
graphy (TLC) on silica gel plates and TLC visualization was realized by UV-light and iodine.
The antibacterial activities (in vitro) of the synthesized ligand and the complexes were studied
by the disc diffusion method against commonly known bacteria, viz., Bacillus subtilis and
Escherichia coli with respect to the standard drug ampicilin.
Physical, analytical and spectral data of the prepared compounds are given in Sup-
plementary material to this paper.
Synthesis of ionic liquid 1-(2-aminoethyl)-3-methylimidazoliumhexafluorophosphate
([2-aemim]PF6)
The amino functionalized ionic liquid was prepared by following literature procedure.38
In a typical experiment, a mixture of 1-methylimidazole (4.10 g, 0.05 mol) and 2-bromo-
ethylamine hydrobromide (10.25 g, 0.05 mol) in 25 mL of acetonitrile was heated with
constant stirring at 80 °C for 4 h. On completion of the reaction, the solvent was removed by
distillation and the residue was recrystallized from ethanol to afford the hydrobromide
[2-aemim]Br as a white solid. Then KPF6 (9.20 g, 0.05 mol) was added to the hydrobromide
[2-aemim]Br in 20 mL of CH3CN/H2O (1:1, V/V). The solution was left for 24 h at room
temperature and then NaOH (2.00 g, 0.05 mol) was added for neutralization. The solvents
were evaporated under vacuum. This was followed by the addition of CH3OH (2 mL) and
CHCl3 (10 mL). The precipitated KBr was filtered off and the solvents were evaporated. The
obtained yellow oil was washed successively with chloroform (3×10 mL) and diethyl ether
(3×10 mL). After drying for 6 h under vacuum at 80 °C, the expected ionic liquid was
obtained.
Synthesis of the ionic liquid-grafted Schiff base
The ionic liquid based Schiff base ligand (L) was synthesized by slight modification of a
literature procedure.39 A mixture of salicylaldehyde (1.22 g, 10 mmol) and [2-aemim]PF6
(2.71 g, 10 mmol) in 10 mL methanol was stirred at room temperature for 1 h. After com-
pletion of the reaction, as indicated by TLC, the reaction mixture was diluted with MeOH (10
mL). The precipitate was then filtered and dried to afford the expected ligand as a pale yellow
solid in good yield.
Synthesis of Cu(II) complexes of the ionic liquid-based Schiff base
To a solution of ligand, LH (0.50 g, 1.30 mmol), in EtOH (20 mL), copper salts, viz.,
CuCl2·2H2O (0.11 g, 0.65 mmol) or Cu(NO3)2·3H2O (0.17 g, 0.65 mmol) were added and the
reaction mixture was refluxed for 8 h at 25 °C until the starting material was completely
consumed as monitored by TLC. On completion of the reaction, the solvent was evaporated
and the reaction mixture was cooled to room temperature. The precipitate was collected by
filtration, washed with C2H5OH (10 mL) and dry ether (10 mL) triply, and further dried in a
desiccator to obtain the complex 2 as a deep green powder and complex 3 as a dark green
powder.
Antibacterial activity
The newly synthesized Cu(II) complexes along with the ligand were tested against the
gram-negative bacterium Escherichia coli ATCC 69905 and the gram-positive bacterium
Bacillus subtilis ATCC 6633. Stock solutions of compounds were prepared by dissolving the
compounds in distilled water and serial dilutions of the compounds were prepared in sterile
distilled water for different concentrations to determine the minimum inhibition concentration
(MIC). The concentrations of the tested compounds were 31.25, 62.5, 125 and 250 µg mL-1 in
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38
SAHA, BRAHMAN and SINHA
comparison to the standard drug ampicillin. The nutrient agar medium was poured into 0.5 mL
culture containing Petri plates. Then the well diffusion technique40,41 was performed. Petri
plates were incubated at 37 °C for 24 h.
RESULTS AND DISCUSSION
All the isolated compounds were found to be air stable and were charac-
terized based on elemental and different spectroscopic analyses. The data are
given in the Supplementary material to this paper.
Infrared spectra of the Schiff base ligand and its complexes
The IR spectra of the free Schiff base and its Cu(II) complexes are presented
in Figs. S-1–S-3 of the Supplementary material. In order to obtain conclusive
insight concerning the coordination mode of the ligand (LH) to the metal ion and
the structure of the metal complexes, the main IR bands were compared with
those of the free ligand. The IR spectrum of the ligand showed a strong broad
–1
band at 3430–3151 cm ; this band was attributed to the hydrogen bonded
…
42,43
–OH of the phenolic group with –N=C group of the ligand (OH N=C).
This band was absent in the spectra of the complexes due to the involvement of
the phenolic –OH group in coordination to the metal ion. A new band in the
–1
range of 3449.07 cm appeared in case of Cu(II) complex 2 and this was
44
assigned to coordinated water molecules in the complex, although in case of
the complex 3 no such band was observed in this region. This indicated the
absence of coordinated water molecules in its structure A weak band at 3101–
–1
2923 cm in the spectrum of the ligand was assigned to H–C(=N) stretching
vibrations. The involvement of deprotonated phenolic moiety in metal complexes
–1
was confirmed by the shift of (–CO) of LH stretching band at 1465.5 cm to
–1
lower frequency region of 1448–1445 cm for the complexes 2 and 3, res-
pectively. In the spectrum of the ligand, a band corresponding to the azomethine
–1
group (–C=N) was found at 1640 cm . On complexation, this band shifted to a
–1
lower wave number range of 1628–1622 cm . This indicated the involvement of
45,46
N-atom of the azomethine (–C=N) group in complex formation
and the band
–1
at 844.5–842.49 cm in the spectra of the complexes 2 and 3 were assigned to
P–F stretching frequency.
Therefore, the IR spectral data indicated that the coordination of ligand to
metal ion occurred through the N-atom of the azomethine (–C=N) group and the
O-atom of phenolic (O–Ar) group. Assignment of the proposed coordination sites
–1
was further supported by the appearance of medium bands at 634–620 cm and
–1
47,48
548–558 cm due to M–O and M–N stretching frequencies, respectively.
Mass spectra
The mass spectra of the compound [2-aemim]PF and the ligand, LH,
6
+
showed molecular ion peaks (m/z) at 126.20 and 231, which correspond to M ,
[C H N ] and M+1, [C H N O +1] peaks, respectively. The observed mass
+
+
6
12
3
13 16 3
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COPPER COMPLEXES OF AN IONIC LIQUID BASED SCHIFF BASE
39
spectra confirmed the formation of the proposed ligand. Again the mass spectra
of the Cu(II) complexes 2 and 3 showed molecular ion peaks (m/z) at 557 and
523, respectively. These molecular ion peaks were assigned to
2+
2+
Cu(C H N O ) and M″+2 (where M″ = Cu(C H N O ) ) peaks, respect-
26 34
6
4
26 30 6 2
ively. These peaks support the proposed structure of the complexes and con-
firmed the ML stoichiometry for the complexes. It is assumed that the Cu(II)
2
complex 2 had an octahedral coordination site with two water molecules occupy-
ing two axial positions; however, the Cu(II) complex 3 has square planar geo-
metry. The different molecular ion peaks, appeared in the mass spectra of the
complexes, were attributed to different fragmentations of the metal complexes by
successive rupture of different bonds in order to form stable ions. The fragment-
ation patterns of the complexes 2 and 3 are shown in Figs. 1 and 2, respectively.
The mass spectra of the ligand and complexes were in good agreement with the
structure as revealed by elemental analyses and spectral analyses.
Fig. 1. The mass fragmentation pattern of the Cu(II) complex 2. m/z: 1, 557; 2, 439; 3, 427; 4,
397; 5, 304; 6, 290 and 7, 250.
1
13
H-/ C-NMR spectroscopy
1
The H-NMR spectra of ligand and complexes were recorded in DMSO-d
6
1
and the spectra showed well-resolved signals as expected. The H-NMR spec-
trum of the ligand showed a singlet at δ 3.82 ppm (3H, s, CH ), a triplet at δ 3.99
3
ppm (2H, t, CH ), a triplet at δ 4.52 ppm (2H, t, CH ), a multiplet at δ 6.85–7.42
2
2
ppm (4H, m, Ar-H), a singlet at δ 7.67 ppm (1H, s, NCH) and a singlet at δ 7.33
ppm (1H, s, NCH). The spectrum of the ligand showed a sharp singlet at δ 8.56
ppm assignable to the proton of the azomethine group (–CH=N–), presumably
due to the effect of the ortho-hydroxyl group in the aromatic ring. A sharp singlet
13
in the downfield region at 12.56 ppm is attributed to the hydroxyl proton. The C-
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40
SAHA, BRAHMAN and SINHA
Fig. 2. The mass fragmentation pattern of the Cu(II) complex 3. m/z: 1, 523; 2, 507; 3, 491; 4,
427; 5, 413; 6, 333; 7, 333; 8, 231 and 9, 231.
-NMR spectra of the ligand and complexes were recorded in DMSO-d as
6
solvent. The number of signals of sharp peaks represents the number of carbons
of the compound that are chemically non-equivalent. The spectra exhibited the
azomethine, C=N carbon at δ 159.91 ppm and the phenolic, C–OH carbon at δ
161.29 ppm. The chemical shifts of the aromatic carbons appeared at 137.31,
135.59, 123.75 and 122.41 ppm. The magnetic moment measurement of both the
9
Cu(II) complexes, in which the electronic configuration is d , showed that these
complexes are paramagnetic (the observed magnetic moments of Cu(II) complex
1
2 and complex 3 were 1.89 and 1.72 μ , respectively) and their H-NMR spectra
B
displayed only the broad signals assignable to the alkyl group as all of these
proton signals are in close proximity. However, a signal for the Cu(II) center was
49
not detectable.
Antibacterial activity
The newly synthesized Cu(II) complexes along with the ligand were tested
against the gram negative bacterium E. coli and the gram positive bacterium B.
40,41
subtilis using the well diffusion technique.
No clear inhibition zone sur-
rounding the well were formed against the ligand and its Cu(II) complexes (inhi-
bition zones against E. coli are shown in Fig. 3), whereas CuCl ·2H O and
2
2
Cu(NO ) ·3H O showed antibacterial activities with clearly defined well diame-
3 2
2
–1
ters at a concentration of 250 µg mL against the bacteria selected for this study.
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COPPER COMPLEXES OF AN IONIC LIQUID BASED SCHIFF BASE
41
Fig. 3. Inhibition zones for anti-bacterial activities:
A: for 1-{2-[(2-hydroxybenzylidene)amino]ethyl}-
-3-methyimidazolium hexaflurophosphate (1); B: for
the Cu(II) complex 2; C: for CuCl2·2H2O; D: for
Cu(NO3)2·3H2O against E. Coli.
CONCLUSION
In this paper, the synthesis, characterization and biological activities of the
synthesized ionic liquid based Schiff base and its two Cu(II) complexes were
reported. The complexes were formed in 1:2 (metal:ligand) ratio, as confirmed by
the spectral analysis. The results of different analytical and spectroscopic anal-
yses revealed that the complexes have different coordination geometries. The
Schiff base ligand acts as a bidentate ligand and binds to metal ions through the
phenolic oxygen and the azomethine nitrogen Again the synthesized ligand and
the complexes showed no antibacterial activities against two commonly known
bacteria, viz., B. subtilis and E. coli.
SUPPLEMENTARY MATERIAL
Physical, analytic and spectral data of the prepared compounds and their IR spectra are
available electronically from http://www.shd.org.rs/JSCS/, or from the corresponding author
on request.
Acknowledgement. The authors are grateful to the Departmental Special Assistance
Scheme under the University Grants Commission, New Delhi (SAP-DRS-III, NO.540/12/
/DRS/2013) for financial support.
И З В О Д
КОМПЛЕКСИ БАКРА(II) СА ШИФОВОМ БАЗОМ ДОБИЈЕНОМ ИЗ ЈОНСКЕ ТЕЧНОСТИ
[1-{2-((2-ХИДРОКСИБЕНЗИЛИДЕН)АМИНО)ЕТИЛ}-3-МЕТИЛИМИДАЗОЛИЈУМ]PF6:
СИНТЕЗА, КАРАКТЕРИЗАЦИЈА И БИОЛОШКА АКТИВНОСТ
1
2
2
SANJOY SAHA , DHIRAJ BRAHMAN AND BISWAJIT SINHA
1Department of Chemistry, Kalimpong College, Kalimpong-734301, India и 2Department of Chemistry,
University of North Bengal, Darjeeling-734013, India
Синтетизована су два комплекса бакра(II) са Шифовом базом добијеном из јонске
течности, 1-{2-[(2-хидроксибензилиден)амино]етил}-3-метилимидазолијум-хексафлуо-
рофосфатa. Добијени комплекси су окарактерисани применом различитих аналитичких
и спектроскопских метода, као што су елементарна микроанализа, магнетна сусцеп-
тибилност, UV–Vis, IR и NMR спектроскопија и масена спектрометрија. Применом ових
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42
SAHA, BRAHMAN and SINHA
метода утврђено је да се Шифова база бидентатно координује преко атома азота и атома
кисеоника, при чему настају комплекси у којима су два молекула лиганда координована
за Cu(II) јон. Поред тога, испитивана је биолошка активност комплекса бакра(II).
(Примљено 1. фебруара, ревидирано 1. јуна, прихваћено 21. јула 2014)
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