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Research Article
Synthesis, Spectral Characterization, and Biological
Evaluation of Transition Metal Complexes of Bidentate N, O
Donor Schiff Bases
Sajjad Hussain Sumrra,
1
Muhammad Ibrahim,
2
Sabahat Ambreen,
3
Muhammad Imran,
4
Muhammad Danish,
1
and Fouzia Sultana Rehmani
3
1
Department of Chemistry, Institute of Chemical and Biological Sciences, University of Gujrat, Gujrat 50700, Pakistan
2
Department of Applied Chemistry, Government College University, Faisalabad 38000, Pakistan
3
Department of Chemistry, University of Karachi, Karachi 75270, Pakistan
4
Department of Chemistry, Government Emerson College, Multan 60700, Pakistan
Correspondence should be addressed to Fouzia Sultana Rehmani; fsrehmani@uok.edu.pk
Received May ; Revised July ; Accepted July ; Published July
Academic Editor: Konstantinos Tsipis
Copyright © Sajjad Hussain Sumrra et al. is is an open access article distributed under the Creative Commons Attribution
License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly
cited.
New series of three bidentate N, O donor type Schi bases (L
)–(L
) were prepared by using ethylene-,-diamine with -methyl
furfural, -anisaldehyde, and -hydroxybenzaldehyde in an equimolar ratio. ese ligands were further complexed with Co(II),
Cu(II), Ni(II), and Zn(II) metals to produce their new metal complexes having an octahedral geometry. ese compounds were
characterized on the basis of their physical, spectral, and analytical data. Elemental analysis and spectral data of the uncomplexed
ligands and their metal(II) complexes were found to be in good agreement with their structures, indicating high purity of all the
compounds. All ligands and their metal complexes were screened for antimicrobial activity. e results of antimicrobial activity
indicated that metal complexes have signicantly higher activity than corresponding ligands. is higher activity might be due to
chelation process which reduces the polarity of metal ion by coordinating with ligands.
1. Introduction
Schi bases played an important role as ligands even a century
aer their discovery in coordination chemistry []. Schi
bases are derived from the condensation reaction of aro-
matic/aliphatic aldehydes and amines. ey are an important
class of organic ligands being extensively studied. Schi base
complexes of transition metals are still relevant to be of great
interest in inorganic chemistry, although this topic has been
extensively studied [–]. e chelating ability and biological
applications of metal complexes have attracted remarkable
attention []. Metal complexes having N, O donor atoms are
very important because of their signicant biological prop-
erties such as antibacterial [, ], antifungal [], anticancer
[], and herbicidal []activity.Inviewofthesignicant
structural and biological applications of ethylenediamine
compounds, we wish to report the synthesis of a new class of
Schi bases (L
)–(L
), derived from the reaction of ethylene-
,-diamine with -methyl furfural, -anisaldehyde, and -
hydroxybenzaldehyde, respectively, and their Co(II), Cu(II),
Ni(II), and Zn(II) metal complexes ()–() (Scheme ). e
compounds were characterized on the basis of physical prop-
erties, elemental analysis, infrared and UV-visible spectra,
and antimicrobial activities. e Schi bases and their metal
chelates were screened for antibacterial activity against six
bacterial strains: Escherichia coli, Streptococcus faecalis, Pseu-
domonas aeruginosa, Klebsiella pneumoniae, Staphylococcus
aureus, and Bacillus subtilis, andalsoscreenedforantifungal
activity against following six fungal strains: Trichophyton
mentogrophytes, Epidermophyton occosum, Aspergillus niger,
Microsporum canis, Fusarium culmorum, and Trichophyton
schoenleinii. e Schi bases showed increased antibacte-
rial activity against certain strains and their activities were
enhanced on chelation (see Figures and ).
Hindawi Publishing Corporation
Bioinorganic Chemistry and Applications
Volume 2014, Article ID 812924, 10 pages
http://dx.doi.org/10.1155/2014/812924
Bioinorganic Chemistry and Applications
O
OH
CH
3
OCH
3
(L
1
)=R =
(L
2
)=R =
(L
3
)=R =
N
R
+
NH
2
NH
2
H
2
N
R–CHO
S
2. Experimental
2.1. Materials and Methods. Chemicals used were of analyt-
ical grade and purchased from commercial sources Sigma
Aldrich and were used without further purication. All
ligand synthesis reactions were carried out in solvents that
were puried and dried before use, using standard literature
methods. e redistilled and deionized water was used in all
experiments. Gallenkamp apparatus was used to determine
melting points of synthesized ligands and decomposition
temperature of the metal complexes. Infrared spectra of solids
(in a KBr matrix) were recorded in the – cm
−1
region
onaNicoletFT-IRImpactDinfraredspectrometer.
1
Hand
13
CNMRspectrawererunonaBrukerAdvance
MHz instrument. Mass spectrometry work was carried
out by Ms. B. Woods N.U.I. Maynooth using an Agilent
Technologies Time-of-Flight LC/MS. UV spectra were
obtained on a Hitachi UV- spectrophotometer. Micro-
analysis(C,H,andN%)ofthesynthesizedcompounds
was carried out using a CHN Analyzer on Perkin Elmer
series II. Molar conductances of the transition metal
complexesweremeasuredin.MinDMFsolutionusingan
Inolab Cond Conductivity Bridge at room temperature.
A Stanton SM/S Gouy balance was used to measure the
magnetic susceptibility of the metal complexes at room
temperature by using mercury acetate as a standard.
2.2. Chemistry of Synthesis of Ligands. Dierent aldehydes
such as -methyl furfural, -anisaldehyde, and -hydroxy-
benzaldehyde in methanol ( mL) were added to a reuxed
solution of ethylene-,-diamine in same solvent in an
equimolarratioforminutesfollowedby-dropsof
acetic acid. en the reaction mixture was reuxed for h by
monitoring through TLC. When the reaction was completed,
it was cooled to room temperature, ltered, and volume
reduced to about one-third using rotary evaporator. e solid
product thus obtained was ltered, washed with methanol,
and dried. It was recrystallized in hot methanol/ether ( : ).
e ligands (L
)–(L
) were prepared by following the above
mentioned method.
2.2.1. N-[(E)-(5-Methylfuran-2-yl)methylidene]ethane-1,2-diam-
ine (L
1
). Yield(.g,%),mp
∘
C; color reddish brown.
1
H NMR (ppm d
6
-DMSO) . (s, CH
3
), . (s, H), . (s,
H), . (s, NH
2
), . (d, H), . (d, H), . (s, HC=N);
13
CNMR:(ppmd
6
-DMSO): ., ., ., ., ., .,
., .; IR (KBr, cm
−1
): (NH
2
), (HC=N), ,
0
5
10
15
20
25
30
35
(1)
(2)
(3)
(4)
(5)
(6)
(7)
(8)
(9)
(10)
(11)
(12)
SD
Zone of inhibition
(mm)
Compounds
E. coli
S. faecalis
P. aeruginosa
K. pneumoniae
S. aureus
B. subtilis
(L
1
)
(L
2
)
(L
3
)
F : Comparison of antibacterial activity of Schi bases versus
metal(II) complexes.
0
20
40
60
80
100
120
Inhibition (%)
T. mentogrophytes
E. occosum
A. niger
M. canis
F. culmorum
T. schoenleinii
(1)
(2)
(3)
(4)
(5)
(6)
(7)
(8)
(9)
(10)
(11)
(12)
SD
Compounds
(L
1
)
(L
2
)
(L
3
)
F : Comparison of antifungal activity of Schi bases versus
metal(II) complexes.
(C=C), (C–O); Mass Spectrum (ESI) [M]
+
=..
Anal. calcd. for C
8
H
12
N
2
O (.): C, .; H, .; N, ..
Found: C, .; H, .; N, ..
2.2.2. N-[(E)-(2-Methoxyphenyl)methylidene]ethane-1,2-diam-
ine (L
2
). Yield (. g, %); mp
∘
C; color dark brown.
1
H
NMR (ppm d
6
-DMSO): . (s, OCH
3
), . (s, H), . (s,
H), . (s, NH
2
), . (t, H), . (d, H), . (t, H), .
(d, H), . (s, HC=N);
13
CNMR:(ppmd
6
-DMSO): .,
., ., ., ., ., ., ., ., .; IR (KBr,
cm
−1
): (NH
2
), (OCH
3
), (HC=N), ,
(C=C); Mass Spectrum (ESI): [M]
+
= .. Anal. calcd. for
C
10
H
14
N
2
O (.): C, .; H, .; N, .. Found: C,
.; H, .; N, ..
2.2.3. 2-{(E)-[(2-Aminoethyl)imino]methyl}phenol (L
3
). Yield
(. g, %); mp:
∘
C, color (yellow).
1
H NMR (ppm d
6
-
DMSO): . (s, H), . (s, H), . (s, NH
2
), . (t, H),
. (d, H), . (t, H), . (d, H), . (s, HC=N), .
(s, OH);
13
C NMR (ppm d
6
-DMSO): ., ., ., .,
., ., ., ., .; IR (KBr, cm
−1
): (OH),
(NH
2
), (HC=N); Mass Spectrum (ESI): [M]
+
=..
Anal. calcd. for C
9
H
12
N
2
O (.): C, .; H, .; N, ..
Found:C,.;H,.;N,..
Bioinorganic Chemistry and Applications
N
O
N
O
M
N
N
M
N
O
N
O
M
NH
2
NH
2
NH
2
H
2
O
H
2
O
O
2
H
O
2
H
H
2
O
OH
2
CH
3
OCH
3
OCH
3
H
3
C
H
2
N
H
2
N
H
2
N
Cl
2
Cl
2
M = Co, Ni, Cu, Zn
Metal complexes (1)–(4) of
(L
1
)
M = Co, Ni, Cu, Zn
Metal complexes (5)–(8) of (L
2
)
M = Co, Ni, Cu, Zn
Metal complexes (9)–(12) of
(L
3
)
S
2.3. Chemistry of Synthesis of the Transition Metal(II) Com-
plexes. All complexes were prepared according to the follow-
ing procedure: to a hot magnetically reuxed methanol solu-
tion ( mL) of the respective Schi base ligand ( mmol),
a methanol solution ( mL) of respective metal(II) salt
chloride⋅nH
2
O ( mmol) was added (𝑛=0,or).e
mixture was reuxed for h, during which a precipitated
product was formed. It was then cooled to room temperature,
ltered, and washed with methanol and nally with diethyl
ether. e precipitated product thus obtained was dried and
recrystallized in a mixture of hot aqueous methanol ( : ) to
obtain TLC pure product.
2.3.1. Co(II) Metal Complex of (L
1
)(1). Yield (. g, %), mp
–
∘
C, IR (KBr) (H
2
O), (HC=N), (C–O),
(M–N), (M–O); UV (DMSO) 𝜆
max
(cm
−1
) ,
and ; conductance (Ω
−1
cm
2
mol
−1
) .; B.M. (𝜇
e
):
.. Anal. calcd. for C
16
H
28
N
4
O
4
CoCl
2
(.); C, .;
H, .; N, .; Co, .. Found: C, .; H, .; N, .;
Co, ..
2.3.2. Ni(II) Metal Complex of (L
1
)(2). Yield (. g, %),
mp –
∘
C; IR (KBr): (H
2
O), (HC=N), (C–
O), (M–N), (M–O); UV (DMSO) 𝜆
max
(cm
−1
): ,
, and ; conductance (Ω
−1
cm
2
mol
−1
): .;
B.M. (𝜇
e
): .. Anal. calcd. for C
16
H
28
N
4
O
4
NiCl
2
(.);
C, .; H, .; N, ., Ni, .. Found: C, .; H, .;
N, .; Ni, ..
2.3.3. Cu(II) Metal Complex of (L
1
)(3). Yield (. g, %);
mp –
∘
C; IR (KBr): (H
2
O), (HC=N),
(C–O), (M–N), (M–O); UV (DMSO) 𝜆
max
(cm
−1
):
, and ; conductance (Ω
−1
cm
2
mol
−1
): .;
B.M. (𝜇
e
): .. Anal. calcd. for C
16
H
28
N
4
O
4
CuCl
2
(.);
C, .; H, .; N, .; Cu, .. Found: C, .; H, .;
N, .; Cu, ..
2.3.4. Zn(II) Metal Complex of (L
1
)(4). Yield (. g, %);
mp –
∘
C;
1
HNMR:(ppmd
6
-DMSO): . (s, CH
3
),
. (s, H), . (s, H), . (s, NH
2
), . (d, H), .
(d, H), . (s, HC=N), . (s, H, H
2
O);
13
CNMR(ppm
d
6
-DMSO): ., ., ., ., ., ., ., .; IR
(KBr): (H
2
O), (HC=N), (C–O), (M–N),
(M–O);UV(DMSO)𝜆
max
(cm
−1
): ; conductance
(Ω
−1
cm
2
mol
−1
): .; B.M. (𝜇
e
): diamagnetic. Anal. calcd.
for C
16
H
28
N
4
O
4
ZnCl
2
(.); C, .; H, .; N, .;
Zn, .. Found: C, .; H, .; N, .; Zn, ..
2.3.5. Co(II) Metal Complex of (L
2
)(5). Yield (. g, %),
mp –
∘
C, IR (KBr) (H
2
O), (HC=N), (C–
O), (M–N), (M–O); UV (DMSO) 𝜆
max
(cm
−1
),
and ; conductance (Ω
−1
cm
2
mol
−1
) .; B.M.
(𝜇
e
): .. Anal. calcd. for C
20
H
32
N
4
O
4
CoCl
2
(.); C,
.;H,.;N,.;Co,..Found:C,.;H,.;
N, .; Co, ..
2.3.6. Ni(II) Metal Complex of (L
2
)(6). Yield (. g, %),
mp –
∘
C; IR (KBr): (H
2
O), (HC=N), (C–
O), (M–N), (M–O); UV (DMSO) 𝜆
max
(cm
−1
): ,
, and ; conductance (Ω
−1
cm
2
mol
−1
): .;
B.M. (𝜇
e
): .. Anal. calcd. for C
20
H
32
N
4
O
4
NiCl
2
(.);
C,.;H,.;N,.,Ni,..Found:C,.;H,.;
N, .; Ni, ..
Bioinorganic Chemistry and Applications
T : Antibacterial bioassay of ligands and their metal(II) complexes (zone of inhibition in mm).
Compounds (a) (b) (c) (d) (e) (f) SA
(L
1
) .
(L
2
) .
(L
3
) .
(1) .
(2) .
(3) .
(4) .
(5) .
(6) .
(7) .
(8) .
(9) .
(10) .
(11) .
(12) .
SD .
(a) E. coli; (b) S. faecalis; (c) P. aer u g i n o s a ; (d) K. pneumoniae; (e) S. aureus; (f) B. subtilis; SD: standard drug; weaker = – mm, moderate = – mm, above
mm = signicant, and SA = statistical analysis.
2.3.7. Cu(II) Metal Complex of (L
2
)(7). Yield (. g, %);
mp –
∘
C; IR (KBr): (H
2
O), (HC=N),
(C–O), (M–N), (M–O); UV (DMSO) 𝜆
max
(cm
−1
):
,and;conductance(Ω
−1
cm
2
mol
−1
): .;
B.M. (𝜇
e
): .. Anal. calcd. for C
20
H
32
N
4
O
4
CuCl
2
(.);
C, .; H, .; N, .; Cu, .. Found: C, .; H, .;
N, .; Cu, ..
2.3.8. Zn(II) Metal Complex of (L
2
)(8). Yield (. g, %);
mp –
∘
C;
1
HNMR:(ppmd
6
-DMSO): . (s, OCH
3
),
. (s, H), . (s, H), . (s, NH
2
), . (t, H),
. (d, H), . (t, H), . (d, H), . (s, HC=N),
. (s, H, H
2
O);
13
C NMR (ppm d
6
-DMSO): ., .,
., ., ., ., ., ., ., .; IR (KBr):
(H
2
O), (HC=N), (C–O), (M–N),
(M–O); UV (DMSO) 𝜆
max
(cm
−1
): ; conductance
(Ω
−1
cm
2
mol
−1
): .; B.M. (𝜇
e
): diamagnetic. Anal. calcd.
for C
20
H
32
N
4
O
4
ZnCl
2
(.); C, .; H, .; N, .;
Zn,..Found:C,.;H,.;N,.;Zn,..
2.3.9. Co(II) Metal Complex of (L
3
)(9). Yield (. g, %),
mp –
∘
C, IR (KBr) (H
2
O), (HC=N), (C–
O), (M–N), (M–O); UV (DMSO) 𝜆
max
(cm
−1
),
and ; conductance (Ω
−1
cm
2
mol
−1
) .; B.M.
(𝜇
e
): .. Anal. calcd. for C
18
H
26
N
4
O
4
Co(.);C,.;
H, .; N, .; Co, .. Found: C, .; H, .; N, .;
Co, ..
2.3.10. Ni(II) Metal Complex of (L
3
)(10). Yield (. g, %),
mp –
∘
C; IR (KBr): (H
2
O), (HC=N), (C–
O), (M–N), (M–O); UV (DMSO) 𝜆
max
(cm
−1
): ,
, and ; conductance (Ω
−1
cm
2
mol
−1
): .;
B.M. (𝜇
e
): .. Anal. calcd. for C
18
H
26
N
4
O
4
Ni (.); C,
.; H, .; N, ., Ni, .. Found: C, .; H, .; N,
.; Ni, ..
2.3.11. Cu(II) Metal Complex of (L
3
)(11). Yield (. g, %);
mp –
∘
C; IR (KBr): (H
2
O), (HC=N),
(C–O), (M–N), (M–O); UV (DMSO) 𝜆
max
(cm
−1
):
,and;conductance(Ω
−1
cm
2
mol
−1
): .;
B.M. (𝜇
e
): .. Anal. calcd. for C
18
H
26
N
4
O
4
Cu (.); C,
.;H,.;N,.;Cu,..Found:C,.;H,.;N,
.; Cu, ..
2.3.12. Zn(II) Metal Complex of (L
3
)(12). Yield (. g, %);
mp –
∘
C;
1
HNMR:(ppmd
6
-DMSO): . (s, H), .
(s, H), . (s, NH
2
), . (t, H), . (d, H), . (t, H), .
(d,H).(s,HC=N),.(s,H,H
2
O);
13
C NMR (ppm d
6
-
DMSO): ., ., ., ., ., ., ., ., .; IR
(KBr): (H
2
O), (HC=N), (C–O), (M–N),
(M–O); UV (DMSO) 𝜆
max
(cm
−1
): ; conductance
(Ω
−1
cm
2
mol
−1
): .; B.M. (𝜇
e
): diamagnetic. Anal. calcd.
for C
18
H
26
N
4
O
4
Zn (.); C, .; H, .; N, .; Zn,
.. Found: C, .; H, .; N, .; Zn, ..
2.4. Biological Activity
2.4.1. In Vitro Antibacterial Activity. All newly synthesized
Schi bases (L
)–(L
) and their transition metal(II) com-
plexes ()–() were screened for their in vitro antibacterial
activity against (Escherichia coli, Streptococcus faecalis, Pseu-
domonas aeruginosa, Klebsiella pneumoniae, Staphylococcus
aureus, and Bacillus subtilis) bacterial strains by the agar-
well diusion method [] and recorded in Table .Small
portion ( mL) of nutrient broth was inoculated with the
test organisms and incubated at
∘
Cforh.Usingasterile
Bioinorganic Chemistry and Applications
T : Antifungal bioassay of ligands and their metal(II) complexes (% inhibition).
Compounds (a) (b) (c) (d) (e) (f) SA
(L
1
) .
(L
2
) .
(L
3
) .
(1) .
(2) .
(3) .
(4) .
(5) .
(6) .
(7) .
(8) .
(9) .
(10) .
(11) .
(12) .
(a) T. mentogrophytes; (b) E. occosum; (c) A. niger; (d) M. canis; (e) F.c ulmorum; (f) T. schoenleinii; weaker = –%, moderate = –%, –% = signicant,
and SA = statistical analysis.
pipette, . mL of the broth culture of the test organism
wasaddedtomLofmoltenagarwhichhadbeencooled
to
∘
C, mixed well, and poured into a sterile petri dish.
Duplicate plates of each organism were prepared. e agar
was allowed to set and harden and the required numbers
of holes were cut using a sterile cork borer ensuring proper
distribution of holes on the border and one in the center.
Agar plugs were removed. Dierent cork borers were used for
dierent test organisms. Using a . mL pipette, 𝜇Lofthe
test sample dissolved in an appropriate solvent was poured
into appropriately labelled cups. e same concentrations of
the standard antibacterial agent (streptomycin in mg/mL)
andthesolvent(ascontrol)wereused.eplateswereleat
room temperature for h to allow diusion of the sample and
incubated face upwards at
∘
C for h. e diameter of the
zones of inhibition was measured to the nearest mm.
2.4.2. In Vitro Antifungal Activity. Antifungal activities of all
compounds were studied against six fungal strains Trichophy-
ton mentogrophytes, Epidermophyton occosum, Aspergillus
niger, Microsporum canis, Fusarium culmorum, and Tri-
chophyton schoenleinii according to recommended procedure
[] and recorded in Table . Test sample was dissolved in
sterile DMSO to serve as stock solution. Sabouraud dextrose
agar was prepared by mixing Sabouraud % glucose agar and
agar in distilled water. It was then stirred with a magnetic
stirrertodissolveitandaknownamountwasdispensedinto
screw capped test tubes. Test tubes containing media were
autoclaved at
∘
C for min. Tubes were allowed to cool to
∘
C and the test sample of desired concentrations pipetted
from the stock solution into the nonsolidied Sabouraud
agar media. Tubes were then allowed to solidify in a slanting
position at room temperature. Each tube was inoculated with
a mm diameter piece of inoculum removed from a seven-
day-old culture of fungi.
2.4.3. Minimum Inhibitory Concentration (MIC). Com-
pounds containing promising antibacterial activity were
selected for minimum inhibitory concentration (MIC) stud-
ies []. e minimum inhibitory concentration was deter-
mined using the disc diusion technique by preparing discs
containing , , , and 𝜇gmL
−1
concentrations of the
compounds along with standards at the same concentrations.
3. Results and Discussion
e condensation of ethylene-,-diamine and -methyl
furfural, -anisaldehyde, and -hydroxybenzaldehyde in :
molar ratio aorded three Schi base ligands (L
)–(L
)
(Scheme ). ese ligands were air and moisture stable com-
pounds. All of them were colored compounds. ese were
microcrystalline solids which melted at –
∘
C. All were
soluble in DMSO and DMF at room temperature and soluble
on heating in methanol and ethanol.
ese bidentate ligands reacted readily with Co(II),
Cu(II), Ni(II), and Zn(II) metals as their chlorides
[CoCl
2
⋅H
2
O, NiCl
2
⋅H
2
O, CuCl
2
⋅H
2
O, and ZnCl
2
]
in methanol to form their metal(II) complexes (Scheme ).
All the synthesized metal(II) complexes were intensely
colored except Zn(II) complexes which were white and all
complexes were microcrystalline in nature. e metal(II)
complexes decomposed without melting. ey were all
insoluble in common organic solvents such as ethanol,
methanol, dichloromethane, and acetone but soluble in
DMSO and DMF.
e spectral data and elemental analysis of the prepared
ligands and their metal(II) complexes were in good agree-
ment with their structure, indicating the high purity of all the
compounds. e analytical data of the complexes indicated a
: metal : ligand stoichiometry.
Bioinorganic Chemistry and Applications
3.1. IR Spectra. ese ligands can coordinate through the
azomethine-N, furanyl-O, methoxy-O, and oxygen atom
from the deprotonation of the phenolic group. Some of the
characteristic IR spectral data were reported in experimental
part. e ligands (L
)–(L
) displayed band at – cm
−1
resulting from NH
2
vibrations []. e ligand (L
) showed
band resulting from OH vibrations []atcm
−1
.How-
ever, the IR spectra of the ligand (L
) demonstrated vibrations
at cm
−1
due to OCH
3
stretching []. e Schi bases
(L
)–(L
) possessed the characteristic azomethine (HC=N)
stretching []at–cm
−1
, hence giving clue of con-
densation product. e ligand (L
) showed the bands at
cm
−1
due to (C–O) vibrations []. e comparison
of the IR spectra of the Schi bases (L
)–(L
) with their
metal(II) complexes ()–() indicated that the Schi bases
were principally coordinated to the metal(II) ions bidentately.
e IR bands of azomethine group appearing in Schi bases
complexes shied to lower frequency (– cm
−1
)at–
cm
−1
conrming the coordination of the azomethine
nitrogen [] with the metal(II) atoms. IR bands at –
cm
−1
resulting from NH
2
vibrations of ligands (L
)–
(L
) remained unchanged in all the complexes showing their
no involvement in the coordination. e following evidences
further support the mode of chelation.
(i) Appearance of the new bands in their metal com-
plexes at – and – cm
−1
which were
assigned to v(M–N) []andv(M–O) vibrations,
respectively, and these bands were absent in their
uncomplexed ligands.
(ii) e (C–O) vibrations of ligand (L
) at cm
−1
were shied to lower frequency – cm
−1
in the
metal(II) complexes ()–().is,inturn,supported
the evidence of the participation of heteroatom-O in
the coordination.
(iii) Appearance of the new bands at – cm
−1
due
to v(C–O) vibrations in the metal(II) complexes ()–
() indicated the coordination of OCH
3
group with
the metal atoms [].
(iv) e disappearance of ](OH) band at cm
−1
in
()–() complexes and appearance of new bands at
– cm
−1
due to the ](C–O) stretching mode
in the complexes revealed the deprotonation of the
hydroxyl OH group found in the ligand (L
). It, in
turn, indicated that the proton of the OH group
was replaced by the metal ions in the formation of
complexes.
(v) All the metal(II) complexes displayed new broad
peaks at – cm
−1
which were assigned to
water molecules.
ese new bands were only observed in the spectra of the
complexes but absent in the spectra of the Schi bases.
erefore, these clues supported the evidence of the par-
ticipation of heteroatom-O, deprotonation of benzilidene-O,
and azomethine-N in the coordination. All these evidences
compromise with the complexation of the metal(II) ions to
the prepared Schi bases.
3.2.
1
𝐻
NMR Spectra.
1
HNMRspectraoftheSchibases
and their diamagnetic Zn(II) complexes were recorded in
DMSO-d
6
.
1
H NMR spectral data of the Schi bases (L
)–
(L
) and their diamagnetic Zn(II) complexes are provided in
the experimental section. e
1
HNMRspectraoftheSchi
base ligands (L
)–(L
) demonstrated characteristic amino
(NH
) and azomethine (CH=N) protons at .–. and
.–. ppm as a singlet, respectively. e (CH
3
)protonsof
the ligands (L
) were observed at . ppm as a singlet. e
(OCH
) proton present in the ligand (L
) was observed at
. ppm as a singlet. e (CH
) protons present in all the
ligands (L
)–(L
) were observed at .–. ppm as a singlet.
In case of the ligand (L
) the O–H proton was observed at
. ppm as a singlet. e furan protons of ligand (L
) were
foundat.–.ppmasadoublet.ephenylprotons
found in ligands (L
) and (L
) were found at .–. ppm
as a doublet, double doublet, and triplet.
e coordination of the azomethine (HC=N) nitrogen
was assigned by the downeld shiing of the azomethine
protonsignalfrom.–.infeeligandsto.–.ppm
in their Zn(II) complexes, respectively. is downeld shi-
ing of azomethine proton in Zn(II) complexes was attributed
to the discharging of electronic cloud towards the Zn(II)
ion. e hydroxyl (OH) proton at . ppm in the ligand
(L
) disappeared in the spectra of its Zn(II) complexes,
indicating deprotonation and coordination of the oxygen
with the metal ion. All other protons underwent downeld
shi by .–. ppm owing to the increased conjugation on
complexation with the zinc metal atom. us, the number of
protons calculated from the integration curves [, ]and
obtained values of the expected CHN analysis agreed well
with each other.
3.3.
13
𝐶
NMR Spectra.
13
CNMRspectraoftheSchibases
and their diamagnetic Zn(II) complexes were recorded in
DMSO-d
6
.e
13
CNMRspectraldataarereportedalong
with their possible assignments in the Experimental section
and all the carbons were found in the expected regions.
e
13
CNMR spectra of the Schi base ligands (L
)–(L
)
showed characteristic azomethine (CH=N) carbons at .–
. ppm. e (CH
3
), (CH
2
), and (OCH
3
)carbonsofthe
ligands were observed at ., .–., and . ppm,
respectively. All the furanyl and phenyl carbons were found
at .–. ppm.
Downeld shiing of the azomethine carbons from 𝛿
.–. ppm in the free ligands to .–. ppm in its
Zn(II) complexes was due to shiing of electronic density
towards the Zn(II) ion. Similarly, all carbons of hetero-
aromatic and phenyl rings being near to the coordination
sites also showed downeld shiing by .–. ppm due
to the increased conjugation and coordination with the
metal atoms. e downeld shiing also conrmed the
coordination of the azomethine to the zinc metal atom.
Moreover, the presence of the number of carbons is well in
agreement with the expected values [, ]. Furthermore,
the conclusions drawn from these studies present further
support to the modes of bonding discussed in their IR and
1
HNMRspectra.
Bioinorganic Chemistry and Applications
3.4. Mass Spectra. e mass fragmentation pattern of the
ligands (L
)–(L
) followed the cleavage of C=N (exocyclic),
C=C, and C–O bonds. e mass spectral data and the most
stable fragmentation values of the ligands were depicted in
Experimental section. All the ligands showed pronounced
molecular ion peaks. e data of the Schi bases shown by
mass spectra strongly conrmed the formation of the ligands
possessing proposed structures and also their bonding pat-
tern.
3.5. Molar Conductances. Molar conductance studies of the
complexeswerecarriedoutinDMF.edataofmolarcon-
ductances (.–. ohm
−1
cm
2
mol
−1
) of metal(II) com-
plexes ()–() showed that these complexes were electrolytic
[] in nature. e metal(II) complexes ()–() exhibited
conductances in the range .–., thus indicating their
nonelectrolytic [, ]nature.
3.6. Magnetic Measurements. e magnetic moment (B.M)
values of all the metal(II) complexes, ()–(), were obtained
at room temperature. e observed magnetic moment
values of Co(II) complexes were found in the range of
.–. B.M indicating the Co(II) complexes as high-
spin suggesting three unpaired electrons in an octahedral
environment []. e Ni(II) complexes showed magnetic
moment values in the range of .–. B.M indicative
of two unpaired electrons per Ni(II) ion suggesting these
complexestohaveanoctahedral[] geometry. e mea-
sured magnetic moment values .–. B.M for Cu(II)
complexes are indicative of one unpaired electron per Cu(II)
ion for d
9
-system suggesting octahedral []geometry.All
the Zn(II) complexes were found to be diamagnetic []as
expected.
3.7. Electronic Spectra. e electronic spectra of Co(II) com-
plexes generally exhibited []threeabsorptionbandsin
the regions –, –, and – cm
−1
which may be assigned to T
1
g →T
2
g(F), T
1
g →A
2
g(F),
and T
1
g →Tg(P) transitions, respectively, and are sug-
gestive of octahedral geometry around the Co(II) ion. e
electronic spectral data of Ni(II) complexes showed []the
bandsintheregions–,–,and–
cm
−1
assigned, respectively, to the d-d transitions of
A
2
g(F) →T
2
g(F) and A
2
g(F) →T
1
g(F). Also a strong
band due to metal to ligand charge transfer appeared at
– cm
−1
. e electronic spectra of all the Cu(II)
complexes exhibited [] absorption bands in the region
at – and – cm
−1
which may be assigned
to the transitions Eg →T
2
g. e high energy band at
– cm
−1
was due to forbidden ligand to metal
charge transfer. On the basis of electronic spectra, octahedral
geometry around the Cu(II) ion was suggested. e Zn(II)
complexes did not show any d-d transition thus showing
diamagnetic nature and their spectra were dominated only
by a charge transfer band []at–cm
−1
.
3.8. Biological Evaluation
3.8.1. Antibacterial Bioassay (In Vitro). e newly synthe-
sized Schi bases (L
)–(L
) and their metal(II) complexes
()–() have been subjected for the screening of their in
vitro antibacterial activity against Escherichia coli, Strepto-
coccus faecalis, Pseudomonas aeruginosa, Klebsiella pneumo-
niae, Staphylococcus aureus, and Bacillus subtilis bacterial
strains according to standard procedure []andresultswere
reported in Table . e obtained results were compared
with those of the standard drug streptomycin. e synthe-
sized ligand (L
) exhibited a signicant (– mm) activ-
ity against Streptococcus faecalis, Pseudomonas aeruginosa,
Klebsiella pneumoniae,andBacillus subtilis bacterial strains
and moderate (- mm) activity against Escherichia coli and
Staphylococcus aureus.eligand(L
) showed a signicant
(- mm) activity against Pseudomonas aeruginosa and
Staphylococcus aureus, moderate (- mm) activity against
Escherichia coli, Streptococcus faecalis, and Bacillus subtilis,
and weaker ( mm) against Klebsiella pneumoniae.e
ligand (L
) demonstrated a signicant (– mm) activity
against Escherichia coli and Streptococcus faecalis, moderate
(– mm) against Pseudomonas aeruginosa, Klebsiella pneu-
moniae,andBacillus subtilis, and weaker ( mm) activity by
Staphylococcus aureus. e metal complexes (), (), and ()–
() displayed overall signicant (≥ mm) activity against
all the bacterial strains. Compounds ()–() exhibited overall
a signicant (– mm) activity against all bacterial strains
except Streptococcus faecalis and Staphylococcus aureus of
(), Escherichia coli and Klebsiella pneumoniae of (), and
Staphylococcus aureus of () which possessed moderate (–
mm) activity. Beside this, the compounds (), (), and
() exhibited overall a signicant (– mm) activity against
all bacterial strains except Streptococcus faecalis of () and
Streptococcus faecalis and Klebsiella pneumoniae of () which
possessed moderate (- mm) activity. Also, compound ()
showed signicant (– mm) activity against Escherichia
coli, Streptococcus faecalis, Pseudomonas aeruginosa, Kleb-
siella pneumoniae, and Staphylococcus aureus, and moderate
( mm) activity was shown against Klebsiella pneumoniae.
Compound () exhibited signicant (– mm) activity
against Escherichia coli, Streptococcus faecalis, Klebsiella pneu-
moniae, Staphylococcus aureus, and Bacillus subtilis,except
Pseudomonas aeruginosa
which possessed moderate (–
mm) activity.
3.8.2. Antifungal Bioassay (In Vitro). e antifungal screen-
ing of all compounds was carried out against Trichophyton
mentogrophytes, Epidermophyton occosum, Aspergillus niger,
Microsporum canis, Fusarium culmorum, and Trichophyton
schoenleinii fungal strains (Table ) according to the litera-
ture protocol []. e results of inhibition were compared
with the results of standard drugs, miconazole and ampho-
tericinB.eligand(L
) possessed signicant (%) activity
against Epidermophyton occosum fungal strain, moderate
(–%) against Trichophyton mentogrophytes, Microsporum
canis, Fusarium culmorum, and Trichophyton schoenleinii,
but no activity against Aspergillus niger.eligand(L
)
Bioinorganic Chemistry and Applications
T : Minimum inhibitory concentration (𝜇g/mL) of the selected compounds (3)–(5) and (9)–(12) against selected bacteria.
Number E. coli S. faecalis P. aeruginosa K. pneumoniae S. aureus B. subtilis
(3) —. — — — —
(4) . — — — — —
(5) . . . — — —
(9) —— — . — —
(10) . . . . — —
(11) —— — — —.
(12) . . . . . .
showed signicant (–%) activity against Trichophyton
mentogrophytes and Fusarium culmorum andmoderate(–
%) activity against Epidermophyton occosum, Aspergillus
niger, and Trichophyton schoenleinii,anditwasinactive
against Microsporum canis.However,(L
) exhibited signif-
icant (–%) activity against Fusarium culmorum and
Aspergillus niger but showed moderate (–%) activity
against Trichophyton mentogrophytes, Epidermophyton oc-
cosum, Microsporum canis, and Trichophyton schoenleinii.
e compound () showed signicant (–%) activity
against all fungal strains except Aspergillus niger strain which
had weaker (%) activity. Similarly, compound () also
possessed signicant (–%) activity against Trichophy-
ton mentogrophytes, Epidermophyton occosum, Aspergillus
niger, and Fusarium culmorum andmoderate(%)activity
against Trichophyton schoenleinii but weaker (%) activ-
ity against Microsporum canis.Aswell,thecompound()
displayed signicant (–%) activity against Epidermo-
phyton occosum and Aspergillus niger,moderate(–%)
against Trichophyton mentogrophytes, Microsporum canis,
and Fusarium culmorum, and also weaker (%) activity
against Trichophyton schoenleinii.ecompounds() and ()
similarly possessed signicant (–%) activity against all
fungal strains except Aspergillus niger strain of compound ()
which observed moderate (%) activity. e compound ()
exhibited signicant (–%) activity against Trichophyton
mentogrophytes, Aspergillus niger, Microsporum canis,and
Fusarium culmorum fungal strains, but strain Trichophy-
ton schoenleinii showed moderate (%) activity and was
inactive against Epidermophyton occosum. Besides this, the
compound () demonstrated signicant (–%) activity
against all strains except Microsporum canis which had
weaker (%) activity. e compound () showed signicant
(–%) activity against Trichophyton mentogrophytes, Epi-
dermophyton occosum, Microsporum canis
,andTrichophy-
ton schoenleinii, and also moderate (–%) activity was
observed against Aspergillus niger and Fusarium culmorum,
respectively. e compound () showed signicant (–
%) activity against Trichophyton mentogrophytes, Microspo-
rum canis, Aspergillus niger, and Fusarium culmorum and
moderate (%) activity against Epidermophyton occosum
and it was inactive against Trichophyton schoenleinii.On
the contrary, the compound () exhibited signicant (–
%) activity against all fungal strains. e compound
() presented signicant (–%) activity against Tri-
chophyton mentogrophytes, Epidermophyton occosum,and
Fusarium culmorum fungal strains, and other le behind
strains Aspergillus niger, Microsporum canis, and Trichophy-
ton schoenleinii showed moderate (–%) activity. Sim-
ilarly, the compound () showed signicant activity (–
%) against Epidermophyton occosum, Microsporum canis,
and Trichophyton schoenleinii although le behind strains
Trichophyton mentogrophytes, Aspergillus niger, and Fusarium
culmorum displayed moderate (–%) activity. It is obvious
from the data reported in Table that (L
) showed overall
good fungal activity as compared to other two ligands. e
Ni(II) complex () of (L
) was found to be the most active
complex. e metal(II) complexes showed enhanced activity
results rather than their uncomplexed Schi bases due to
complexation.
3.8.3. Minimum Inhibitory Concentration (MIC). e syn-
thesized ligands and their transition metal(II) complexes
showing promising antibacterial activity (above %) were
selected for MIC studies and obtained results are reported
in Table . e antibacterial results indicated that all the
metal(II) complexes ()–() and ()–() were found to
display activity more than %; therefore, these complexes
were selected for their MIC screening. e MIC values of
these compounds fall in the range . to . 𝜇g/mL.
Amongst these, the compound () was found to be the most
active possessing maximum inhibition . 𝜇g/mL against
bacterial strain K. pneumoniae.
4. Conclusions
ree bidentate N, O donor type Schi bases were prepared
by using ethylene-,-diamine with -methyl--furaldehyde,
-anisaldehyde, and -hydroxybenzaldehyde in an equimolar
ratio. ese ligands were further complexed with transition
metals to produce their new metal complexes. Elemental
analysis and spectral data of the uncomplexed ligands and
their metal(II) complexes were found to be in good agree-
ment with their structures, indicating high purity of all the
compounds. All ligands and their metal complexes were
screened for antimicrobial activity. e results of antimi-
crobial activity indicated that metal complexes have signi-
cantly higher activity than corresponding ligands. is higher
activity might be due to chelation process which reduces the
polarity of metal ion by coordinating with ligands.
Bioinorganic Chemistry and Applications
Conflict of Interests
e authors declare that there is no conict of interests
regarding the publication of this paper and are responsible
for the contents and writing of the paper.
Acknowledgments
e authors are thankful to HEJ Research Institute of
Chemistry, International Center for Chemical and Biological
Sciences, University of Karachi, Pakistan, for providing their
help in taking NMR and mass spectra and, for the help in
carrying out antibacterial and antifungal bioassay.
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