Transition metal complexes containing a tridentate cefadroxil-based Schiff base: An effective paramagnetic semiquinone Ag (I) complex against Staphylococcus aureus and Pseudomonas aeruginosa

Article abstract of DOI:10.1002/aoc.4562

The aim of this study was to prepare transition metal complexes containing a cefadroxil-based Schiff base ligand (H₂L, 3) obtained from the condensation of cefadroxil 1 with salicylaldehyde 2, and to study their bactericidal activity against G(

Full text of DOI:10.1002/aoc.4562

Received: 8 March 2018
DOI: 10.1002/aoc.4562
Revised: 13 July 2018
Accepted: 20 July 2018
F U L L P A P E R
Transition metal complexes containing a tridentate
cefadroxil‐based Schiff base: An effective paramagnetic
semiquinone Ag (I) complex against Staphylococcus aureus
and Pseudomonas aeruginosa
J.R. Anacona1
| Virginia Barrios2 | Alina Bravo2 | Freddy Celis3
1 Department of Chemistry, Universidad
de Oriente, Cumana, Venezuela
2 Department of Bioanalysis, Universidad
de Oriente, Cumana, Venezuela
3 Facultad de Ciencias Naturales y Exactas,
Universidad de Playa Ancha, Valparaíso,
Chile
The aim of this study was to prepare transition metal complexes containing a
cefadroxil‐based Schiff base ligand (H2L, 3) obtained from the condensation
of cefadroxil 1 with salicylaldehyde 2, and to study their bactericidal activity
against G(+) and G(–) strains. Spectroscopic and physicochemical techniques,
namely, transmission electron microscopy, UV–Vis, Fourier transform‐
infrared, electron paramagnetic resonance, 1H NMR, mass spectrometry,
magnetic susceptibility, molar conductance, molecular modelling, together
with elemental and thermal analyses were used to find out the binding mode
and composition of these complexes. Based on the characterization studies,
the general formulae [ML (H2O)3], where M = Mn 4, Co 5, Ni 6, Zn 7 and
Ag 8, were proposed for the complexes. The Schiff base 3 behaved as a
dianionic tridentate OOO chelating agent. An octahedral geometry for all the
complexes was suggested on the basis of magnetic and spectral data. Free 1,
3 and its metal complexes 4–8 were tested for bactericidal activity by using both
agar disc diffusion method and the minimal inhibitory concentration. In vitro
bacterial viability revealed that 3 and complexes 4–7 had similar bactericidal
activity than 1 against Staphylococcus epidermidis showing that they have good
activity as bactericides. Based on magnetic data, the paramagnetic complex 8
was shown to be a semiquinone complex stabilized by the metal, and exhibited
much better bactericidal activity than 1 against all bacterial strains tested.
Correspondence
J. R. Anacona, Department of Chemistry,
Universidad de Oriente, Cumana 6101.
Venezuela.
Email: juananacona@hotmail.com
Highlights
• A new Schiff base using cefadroxil and salicylaldehyde was prepared.
• Cefadroxil‐based Schiff base transition metal complexes were prepared.
• Magnetic, spectroscopic properties and antibacterial activity were studied.
KEYWORDS
antibacterial activity, cefadroxil‐based Schiff base, salicylaldehyde, transition metal complexes
Appl Organometal Chem. 2018;e4562.
wileyonlinelibrary.com/journal/aoc
© 2018 John Wiley & Sons, Ltd.
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https://doi.org/10.1002/aoc.4562

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ANACONA ET AL.
spectrophotometer. Elemental analyses of C, H, N and S
were performed on a LECO CHNS 932 model microana-
lytical instrument. Elemental analyses gave agreement
better than 0.3% with the calculated values. A Perkin–
Elmer spectrophotometer was used to determine, after
treatment with mineral acids, the metal contents of
the metal complexes and the metal oxides. Water
and the metal content were also obtained from the
thermigravimetric analysis curves. Electron impact‐mass
spectra were recorded on JEOL MS Route Instrument.
Magnetic susceptibility measurements at low tempera-
tures were performed with a Quantum Design SQUID
MPMS XL‐5 magnetometer between 5 and 300 K using
a magnetic field of 100 Oe. Magnetic susceptibilities at
room temperature were also measured on a calibrated
Johnson Matthey Magnetic Susceptibility balance using
HgCo (NCS)4. A JEOL model 1200 EX instrument was
used to perform transmission electron microscopy
(TEM). Proton NMR spectra were recorded in DMSO‐d6
solvent on a Varian spectrometer. Low‐temperature elec-
tron paramagnetic resonance (EPR) spectrum at 77 K was
recorded in the X‐band on a Bruker ECS 106 spectrome-
ter. Thermal analysis measurements (TG and DSC) of
metal complexes were performed on a simultaneous
thermal analyzer, STA‐6000 (Perkin Elmer) instrument
at a heating rate of 4 °C min–1 up to 600°C. X‐ray diffrac-
tion (XRD) measurements of metal complexes and the
final residues of thermal decomposition were carried
out in powder form on a HZG 4 diffractometer over the
range of 2θ = 2–70° using Ni‐filtered CuK α radiation
(λ = 0.1541 nm).
1 | INTRODUCTION
Several pathogenic microbes (bacteria, viruses, fungi, par-
asites) have emerged as major causes of human disease,
especially among the immunocompromised and those
hospitalized with serious underlying disease.[1] The mor-
bidity and mortality associated with these infections are
substantial, and it is clear that infectious diseases have
emerged as important public health problems.[2] Numer-
ous factors have contributed to the increase in microbial
infections. The increasing use of broad‐spectrum antibi-
otics, cytotoxic chemotherapies and transplantation
further increases the risk for both common and uncom-
mon opportunistic bacterial infections.[3]
The treatment of infectious diseases still remains an
important and challenging problem because of a combi-
nation of factors, including emerging infectious diseases
and the increasing number of multi‐drug‐resistant micro-
bial pathogens.[1–3] Therefore, there is a pressing need to
search for a new generation of antimicrobial agents that
are safe and can be used for the cure of multidrug‐
resistant microbial infections.
Schiff bases and their metal complexes exhibit a vast
rangeofbiologicalactivities,[4–6] includinggenotoxicity,[7,8]
antimicrobial,[9–13] antifungal,[14] insecticide, herbicide
and antiprotozoal activity. Light metal complexes contain-
ing antibiotic‐based Schiff bases as ligands may be particu-
larly useful in this matter, extending the landscape
of drug design and enabling novel mechanisms of
action.[15,16] Taking into account these antecedents and
as a part of our work in the research of new antimicrobial
metal‐based antibiotics,[17–20] we report here the prepara-
tion and characterization of metal complexes containing a
cefadroxil‐based Schiff base 3 derived from the condensa-
tion of free cefadroxil 1 with salicylaldehyde 2. The
chemical structure of 1 is shown in Figure 1.
3 | SYNTHESIS
3.1 | Synthesis of cefadroxil‐based Schiff
base (H2L, 3)
To 1 mmol of 1 in 25 ml of hot ethanol was added 1 mmol
of 2, and the mixture was refluxed under inert atmo-
sphere for 3 h and then left to stand overnight. The yel-
low precipitate formed from the Schiff base 3 was
separated by filtration and washed with water, ethanol
and ether, and dried under reduced pressure at room
temperature. Recrystallization from the hot ethanol gave
purified Schiff base 3. Yield 75%; m.p. 194–196°C. Molec-
ular parent peak at m/z 468 (calc. 467.5). Elemental anal-
ysis. Calcd (%): C, 59.1; H, 4.5; N, 9.0; S, 6.9. Found: C,
2 | EXPERIMENTAL
2.1 | Materials and methods
An Electro‐thermal 9100 apparatus was used to measure
the melting points. Fourier transform‐infrared (FTIR)
spectra were recorded as KBr pellets on a Perkin‐Elmer
157 instrument. Measurements in the UV–Vis region
were recorded in DMSO solution using a Perkin–Elmer
1
58.8; H, 4.3; N. 9.2; S, 7.2 for C23H21N3O6S. H NMR
(80 MHz, DMSO‐d6): δ = 11.20 (s, 1H, salicyl –OH), 9.22
(s, 1H, COOH), 9.02 (s, 1H, HC=N), 8.46 (d, 1H, NH),
6.38–7.22 (m, 8H, benzene), 5.52 (s, 1H, phenyl –OH),
4.67 (s, 1H, NCH), 4.45 (d, 1H, O=C –CH), 3.18 (q, 2H,
S –CH2), 2.26 (s, 1H, phenyl –CH), 1.22–1.30 (s, 3H,
FIGURE 1 Chemical structure of cefadroxil

ANACONA ET AL.
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CH3) ppm; IR (KBr, cm−1): 3430 (OH), 3290 (NH), 3080
and 3100 (phenyl CH), 2940 (aliph CH), 1770 (C=O,
lactam), 1700 (C=O, COOH), 1690 (C=O amide), 1630
(C=N imine), 1270 phenolic (C –OH), 1090 and 740
(benzene ring).
1610 (COO asymm), 1290 phenolic (C –O), 1380 (COO
sym), 1100 and 710 (benzene ring), 800 and 530 (H2O),
510 (Ni –O).
3.2.4 | ZnL (H2O)3] 7
Yield 85%; m.p.(dec.) > 230°C. Molecular ion peak of m/z
586.4 (calcd 584.9). Elemental analysis. Calcd (%): C, 47.2;
H, 4.3; N, 7.2; S, 5,5; Zn, 11.2. Found: C, 47.3; H, 4.4; N.
3.2 | General method for the synthesis of
Schiff base metal complexes
1
The complexes 4–8 were prepared by the same general
procedure. To a solution of 1 mmol of the metal acetate
salt in 20 ml of methanol, a solution of 1 mmol of 3 in
10 ml of methanol was added slowly with mechanical
stirring. To this solution NaOH (0.1% in methanol) was
added to adjust the pH to between 7 and 8, and the mix-
ture was refluxed under inert atmosphere for 4 h where a
precipitate formed. The solid was filtered, washed with
water, methanol and ether, and finally dried under
reduced pressure at room temperature. Recrystallization
from hot ethanol gave the pure metal complex.
7.5; S, 5.7; Zn, 11.4 for ZnC23H25N3O9S. H‐NMR (80
MHz, DMSO‐d6): δ = 9.02 (s, 1H, HC=N), 8.46 (d, 1H,
NH), 6.38–7.22 (m, 8H, benzene), 5.52 (s, 1H, phenyl –
OH), 4.67 (d, 1H, N –CH), 4.45 (d, 1H, O=C –CH), 3.18
(q, 2H, S –CH2), 2.26 (s, 1H, phenyl –CH), 1.22–1.30
(s, 3H, CH3) ppm; IR (KBr, cm−1): 3430 (OH), 3290
(NH), 3020 and 3070 (phenyl CH), 2980 (aliph CH),
1690 (C=O amide), 1625 (C=N imine), 1615 (COO
asymm), 1280 phenolic (C –O), 1390 (COO sym), 1090
and 740 (benzene ring), 820 and 540 (H2O), 520 (Zn –O).
3.2.5 | [AgL (H2O)3] 8
3.2.1 | [MnL (H2O)3] 4
Yield 73%; m.p.(dec.) > 230°C. Molecular ion peak of m/z
628.9 (calcd 627.4). Elemental analysis. Calcd (%): C, 44.0;
H, 4.0; N, 6.7; S, 5,1; Ag, 17.2. Found: C, 44.2; H, 4.3; N.
6.4; S, 5.4; Ag, 17.4 for AgC23H25N3O9S. IR (KBr, cm−1):
3430 (OH), 3300 (NH), 3085 and 3100 (phenyl CH),
2980 (aliph CH), 1690 (C=O amide), 1625 (C=N imine),
1600 (COO asymm), 1295 phenolic (C –O), 1390 (COO
sym), 1060 and 750 (benzene ring), 830 and 560 (H2O),
530 (Ag –O).
Yield 76%; m.p.(dec.) > 230°C. Molecular ion peak of m/z
575.6 (calcd 574.5). Elemental analysis. Calcd (%): C, 48.1;
H, 4.4; N, 7.3; S, 5,6; Mn, 9.6. Found: C, 48.3; H, 4.1; N.
7.5; S, 5.9; Mn, 9.8 for MnC23H25N3O9S. IR (KBr, cm−1):
3430 (OH), 3300 (NH), 3000 and 3040 (phenyl CH),
2980 (aliph CH), 1690 (C=O amide), 1625 (C=N imine),
1600 (COO asymm), 1285 phenolic (C –O), 1380 (COO
sym), 1080 and 730 (benzene ring), 800 and 560 (H2O),
540 (Mn –O).
3.3 | Molecular modelling study
3.2.2 | [CoL (H2O)3] 5
Calculations were carried out to optimize molecular
geometries for zinc (II) complex as the representative
compound. The total computations were accomplished
using the DFT (B3LYP) method at the STO‐3G/
LANL2DZ basis set level by using Gaussian 09 program
package with the aid of the Gauss View visualization
program.[21]
Yield 77%; m.p.(dec.) > 230°C. Molecular ion peak of m/z
579.4 (calcd 578.5). Elemental analysis. Calcd (%): C, 47.8;
H, 4.4; N, 7.3; S, 5.5; Co, 10.2. Found: C, 47.9; H, 4.7; N.
7.1; S, 5.2; Co, 10.4 for CoC23H25N3O9S. IR (KBr, cm−1):
3430 (OH), 3300 (NH), 3040 and 3060 (phenyl CH),
3000 (aliph CH), 1690 (C=O amide), 1630 (C=N imine),
1610 (COO asymm), 1290 phenolic (C –O), 1390 (COO
sym), 1070 and 720 (benzene ring), 830 and 530 (H2O),
510 (Co –O).
3.4 | Antibacterial activity
Cefadroxil Schiff base and its complexes were tested for
antibacterial activity against pathogenic bacteria G(+)
Staphylococcus aureus ATCC 25923 and G(–) Escherichia
coli ATCC 25922, Klebsiella pneumoniae ATCC 700603,
Pseudomonas aeruginosa ATCC 27853, Staphylococcus
epidermidis ATCC 14990, by using both the paper disc
diffusion method, measuring the size of inhibition zone
diameter,[22] and the minimal inhibition concentration
(MIC) through serial tube dilution technique.[23]
3.2.3 | [NiL (H2O)3] 6
Yield 70%; m.p.(dec.) > 230°C. Molecular ion peak of m/z
579.7 (calcd 578.2). Elemental analysis. Calcd (%): C, 47.8;
H, 4.4; N, 7.3; S, 5,6; Ni, 10.2. Found: C, 47.6; H, 4.2; N.
7.5; S, 5.5; Ni, 10.4 for NiC23H25N3O9S. IR (KBr, cm−1):
3430 (OH), 3300 (NH), 3060 and 3080 (phenyl CH),
2990 (aliph CH), 1690 (C=O amide), 1630 (C=N imine),

4 of 10
ANACONA ET AL.
Appropriate culture samples for antibacterial studies
were collected from Microbiology Laboratory of the
Department of Biological Sciences, UDO University. The
growth inhibition diameter around the disc was mea-
sured after 24 h incubation. Three replicates were used
for each treatment and the results were expressed as the
mean SD (standard deviation). All MIC values are
DMSO. Molar conductance values measured in DMSO
revealed the non‐electrolytic nature of the metal com-
plexes (10.0–20.0 Ω–1 cm2 mol–1) following published
values.[24,25]
4.1 | Thermal analysis
given in μmol ml−1
.
The thermal behaviours of the complexes are summa-
rized in Table 1. The results showed good agreement with
the formulae suggested from the elemental analyses. The
complexes 4–8 did not undergo any change in weight loss
due to lattice water or solvent molecules when heated
between 30°C and 145°C. The loss of three coordinated
water molecules was indicated by endothermic decompo-
sitions in the 145–215°C temperature range (obs = 8.4–
9.5%, calcd = 8.6–9.4). When complexes were heated
above 210°C, gradual thermal decompositions were
observed and it was not possible to distinguish intermedi-
ate solid products.[26] The final decomposition step
included complete evaporation of (3) as well as formation
of metal oxide: MnO, CoO, NiO, ZnO and Ag2O as final
product,[27] from which the metal content can be calcu-
lated and compared with that obtained from analytical
determination.[28] Their compositions were also con-
firmed observing weak diffraction lines on X‐ray patterns
of the metal oxides.
4 | RESULTS AND DISCUSSION
The synthetic route of 3 is illustrated in Scheme 1. The
carbonyl C=O group available in 2 was allowed to react
with the amino group of free 1 to obtain 3. This Schiff
base and complexes 4–8 were isolated as coloured amor-
phous solids in very good yields. The Schiff base 3, 4, 7
and 8 solids were yellow, while 5 and 6 complexes were
beige in colour. Elemental analyses for the metal com-
plexes agreed well with a 1:1:3 metal:Schiff base:coordi-
nated water composition. Thus, the general formulae
[ML (H2O)3], where M (II) = Mn 4, Co 5, Ni 6, Zn 7
and Ag 8, were assigned to the products. The metal com-
plexes were very air stable solids at room temperature,
decomposed above 230°C, and they are insoluble in water
and other common organic solvents but soluble in
SCHEME 1 Synthetic route of Schiff
base ligand 3
TABLE 1 Thermal analyses results of the metal complexes
Weight loss %
Found
Temperature
range (°C)
Residue %
(Calcd)
Assignment
Found
(Calcd)
4
5
6
7
8
9.1
9.4
9.5
9.5
8.4
(9.4)
150−210
210–570
> 570
Loss of 3H2O
Removal of L
MnO residue
12.8
(12.4)
(9.3)
(9.3)
(9.2)
(8.6)
145–205
205–580
> 580
Loss of 3H2O
Removal of L
CoO residue
12.7
12.6
14.2
18.1
(12.9)
(12.9)
(13.9)
(18.5)
145–215
215–575
> 575
Loss of 3H2O
Removal of L
NiO residue
145–205
205–580
> 580
Loss of 3H2O
Removal of L
ZnO residue
150–210
210–575
> 590
Loss of 3H2O
Removal of L
Ag2O residue

ANACONA ET AL.
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absorption band at 1770 cm–1 that was assigned to lactam
ν(C=O) stretching mode coming from 1, which in
complexes 4–8 was displaced and overlapped to lower
frequencies suggesting coordination of this group to
metal ions.
The presence in complexes 4–8 of new νasym and νsym
bands of the (−COO–) group at 1615–1600 cm−1 and
1390–1380 cm−1 ranges, respectively, and the absence of
the stretching frequency at 1700 cm−1 assigned to
COOH‐group in the Schiff base 3 indicated that the
4.2 | 1H NMR spectra
The obtained chemical shift values were found to be sim-
ilar to those of Schiff base ligands previously reported in
the literature.[29,30] The metal complexes, except 7, were
paramagnetic, therefore the 1H‐NMR spectra of them
were not obtained. 1H‐NMR spectra of 3 and 7 were
1
recorded in DMSO‐d6. Comparison of the H‐NMR spec-
trum of complex 7 with that of 3 showed the absence of
the signals assigned to the salicyl –OH and COOH pro-
tons of 3, suggesting deprotonation and formation of
salicyl –O –Zn and COO –Zn bonds.[30] Because of the
low solubility of the diamagnetic complex 7 in most
organic solvents, it was not possible to record satisfactory
13C‐NMR spectrum.
reaction occurred. Furthermore, the Δν values (νasym
ν
–
sym) > 200 cm−1 were consistent with monodentate car-
boxylate coordination. The absence of ν(M–N) stretching
vibration bands in the 400–490 cm–1 range in the spectra
of the complexes could be further evidence that the nitro-
gen atom of the imine moiety was not bonded to the
metal ion. The IR bands observed in complexes 4–8 in
the 510–540 cm–1 region, and absent in (3), were tenta-
tively assigned to ν(M–O) vibrations. In addition to the
O –H stretching vibration at 3430 cm–1, the spectra of
the aquo complexes 4–8 showed bands of moderate inten-
sity in the region 800–830 cm−1 and 530–560 cm–1, which
are usually assigned to rocking and wagging modes,
respectively, of coordinated water.[31,32] Schiff base (3)
and complexes 4–8 also showed bands in the 1050–1100
cm−1 and 700–750 cm–1 ranges belonging to benzene ring
vibrations.[31] Based on all these data, we conclude that
the β‐lactam –O, ortho –salicyl –O and carboxylate –O
atoms were involved in coordination, and that 3 behaves
as a dianionic tridentate OOO chelating agent.
4.3 | Infrared spectra
In Table 2, functional groups and characteristic IR
absorptions of 3 and metal complexes 4–8 can be seen.
The IR spectrum of 3 exhibited a characteristic medium
broad band centred at 2730 cm−1 attributed to the
intramolecular H‐bonded salicyl –OH···N, which is com-
mon in aromatic azomethine compounds containing
ortho‐OH groups.[31] Upon complexation, the band at
2730 cm−1 disappeared strongly, suggesting the coordina-
tion of the deprotonated salicyl –O to the central metal
atom. In the Schiff base 3, the band at 1270 cm−1 attrib-
uted to the salicyl (C–OH) stretching vibration is also
displaced to 1280–1295 cm–1 range in metal complexes,
suggesting deprotonation and coordination to the metal.
The presence of such absorptions in addition to the
observed IR band at 1630 cm–1 attributed to ν(C=N)
vibration suggest that the Schiff base 3 was formed.[31,32]
The IR spectra of the metal complexes 4–8 showed
similarities to one another and exhibited absorption
bands in the 1625–1630 cm–1 range, which can be
assigned to the C=N stretching frequencies of 3, strongly
suggesting non‐involvement of this moiety in coordina-
tion. The IR spectrum of the Schiff base 3 presented an
4.4 | Magnetic properties
Pascal's constants were used to calculate corrected mag-
netic moments.[33] Complex 4 had a magnetic moment
μ
eff = 6.04 μB, which is typical of high spin d5 systems,
S = 5/2 ground state. Complex 5 had a magnetic moment
μ
eff = 4.43 μB, which is a typical value of a d7 system,
S = 3/2 ground state, in an octahedral arrangement
around the metal.[34] Complex 6 had a magnetic moment
TABLE 2 Main vibrational wavenumbers of the metal complexes (cm–1
)
Compound
νC=O lactam
νC=O amide
νC=N imino
νCOO asym
νCOO sym
Δν
νM–O
Cefadroxil 1
1770
1770
1690
1690
1690
1690
1690
1690
1690
Schiff base 3
1630
1625
1630
1630
1625
1625
4
5
6
7
8
1600
1610
1610
1615
1600
1380
1390
1380
1390
1390
220
220
230
225
210
540
510
510
520
530
Δν = (νasym – νsym).

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ANACONA ET AL.
μ
eff = 3.25 μB characteristic of two unpaired electrons and
On the other hand, an EPR spectrum with no hyper-
fine splitting and a g‐value similar to the free electron
value (2.0023) should be expected in the case of a radical
stabilized through the metal in which the unpaired elec-
tron is delocalized over the ligand. In the absence of the
silver hyperfine coupling constant it is not possible to
estimate the extent of delocalization of the unpaired elec-
tron onto the Schiff base ligand. Because the EPR spectral
data for complex 8 do not provide firm evidence that it is
an Ag (II) species, the reaction should be regarded as a
ligand‐ rather than a metal‐based radical process.
greater than the spin‐only value, presumably due to the
orbital contribution. Complex 6 probably has distorted
octahedral geometry. Complex 7 was diamagnetic as
expected for the d10 configuration.
As the reactions to prepare complex 8 were carried
out under ambient atmosphere, presumably air oxidation
occurred and ligand 3 having a delocalized π‐orbital sys-
tem is able to coordinate to the central silver ion under
the form of semiquinone, allowing to obtain paramag-
netic complex (8).
When silver (I) acetate or nitrate was added to an
aqueous suspension or to a 50% MeOH solution of 3, a
light yellow precipitate was produced, which soon
became dark red‐brown. The starting Ag (I) d10 material
is EPR silent, while the EPR spectrum of the dark orange
product showed the signal assigned to the paramagnetic
complex 8. Magnetic susceptibility measurements in the
temperature range between 5 and 300 K showed that
Schiff bases resulting from aromatic aldehydes ortho‐
substituted with a hydroxyl group have small homolytic
bond dissociation energy of (ortho) O –H bond, because
its radical (ortho)O● is conjugatively stabilized by the
benzene ring or the electron pair of the α‐oxygen. In the
present case, the salicyl Schiff base radical is stable and
easily obtained by hydrogen abstraction in the presence
of Ag (I) to give a semiquinone complex stabilized by
the silver centre and by conjugation with the aromatic
ring. The following equations would be applied:
complex 8 obeyed a Curie–Weiss law of the type χ–1
M
= –15 + 2.28 T in the full temperature range. From the
χ−1M vs. T plot the μeff value = 1.88 μB was obtained. This
value agreed with the value expected for a system with
one unpaired electron, S = 1/2 ground state, without cou-
plings confirming the presence of monomeric species.[34]
The X‐band EPR spectrum of the dark red‐brown
product 8 as a DMSO glass at 77 K is shown in Figure 2.
The observed values g║ = 2.12 and g⊥ = 2.07 are similar
with other reported values for silver (II) complexes.[35–37]
However, the EPR spectrum did not contain any structural
information that was related to nuclear spin interactions.
Both the hyperfine doublet structure characteristic of the
nuclear spins [107Ag (I = 1/2), 109Ag (I = ½)] and the
superhyperfine couplings due to 14N nuclei were not
observed.[38]
Â
Ã
Η2L þ Agþ þ 3Η2O þ O2→ AgL ðΗ2OÞ3
(1)
●
þΗþ þ ΗO2
●
ΗO2 →O2 þ Ηþ
(2)
●
●–
4.5 | UV–Vis spectra
The UV−Vis spectra in DMSO solution of 3 and com-
plexes 4–8 showed two broad bands at 300 and 350 nm.
The former band was assigned to the absorption of (1Lb
→ 1A1) of the benzene rings, while the second band was
assigned to n–π* (C=N–) transition of the azomethine
group. Both bands remained almost unchanged in the
spectra of metal complexes, reflecting that azomethine
nitrogen was not involved in coordination.[39]
The electronic spectra of 4, 7 and 8 did not show any
d–d transitions, but displayed charge transfer bands at 23
800, 26 666 and 23 256 cm–1, respectively. The UV–Vis
spectrum of 5 displayed two d–d bands at 17 857 and
4
21 739 cm–1 attributed to the T1g
→
4A2g (ν2) and
4T1g(F) → 4T1g(P) (ν3) transitions, respectively, in an octa-
hedral symmetry around the cobalt (II) ion.[40] By using
ν2, ν3 and the procedure of Lever,[41] the ligand field
parameters (10Dq, B', β) and ν1 were calculated for com-
plex 5. The result of the ratio ν2/ν1 is found to be 1.82 as
required for the octahedral cobalt (II) complexes.[42,43]
The values of ν1 = 9812 cm–1, the magnetic moment and
the calculated ligand field parameters of Dq = 1335 cm–
FIGURE 2 X‐band electron paramagnetic resonance (EPR)
spectrum of 8 as a DMSO glass at 77 K

ANACONA ET AL.
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1, B' = 976 cm–1 and β = 0.924 were also consistent with
the octahedral geometry.
Bond angles are close to an octahedral geometry
confirming the proposed structure of the complex. All
our attempts to grow single crystals of the metal com-
plexes were not successful, because of their insolubility
in most common organic solvents. XRD revealed that
complexes 4−8 only formed amorphous materials. No
crystal structures of complexes containing different
cefadroxil‐based Schiff bases have been reported so far.
Complex 6 presented two major absorptions maxima,
at 16 666 and 23 530 cm–1 due to d–d bands
assignable, considering Oh symmetry, to the transitions
3
3
3A2g(F) → 3T1g(F) (ν2) and A2g(F) → T1g(P) (ν3), respec-
tively.[43] Ligand field parameters (10Dq, B', β) and ν1
were also calculated for complex 6.[38] The obtained
values of ν1 = 9800 cm–1, ν2/ν1 = 1.70, the magnetic
moment (μspin only < μobs ~ μS + L), Dq = 980 cm–1, B' =
720 cm–1 and β = 0.682 were also consistent with Oh
geometry.[43]
4.7 | In vitro antibacterial activity
Cefadroxil‐based Schiff base 3 and metal complexes 4−8
were tested for antibacterial activity against G(+) and
G(−) microorganisms and compared with free 1 used as
reference. The compounds were tested at a concentration
of 400 μg ml−1 in DMSO using the paper disc diffusion
method.[22] The growth inhibition zones were measured
in diameter (mm) and the results are reproduced in
Table 3. Cefadroxil 1 and Schiff base 3 were effective
against three bacterial strains, with inhibitory zones of
17–30 mm and 18–32 mm, respectively. Complexes 4
and 5 were effective against four organisms, with inhibi-
tory zones of 9–30 mm and 9−20 mm, respectively,
whereas complexes 6 and 7 were effective against two
organisms with inhibitory zones of 7–19 mm and 15−25
mm, respectively. Complex 8 was highly effective against
all the bacterial strains with zones of inhibition of 10–33
mm. The metal (II) acetates used for the synthesis of the
complexes were effective against five organisms, with
inhibitory zones of 3–5 mm. Consequently, the enhanced
activity of the metal complexes arises from coordination.
The MICs of the ligand and metal complexes that
showed antibacterial activities were determined using
the serial tube dilution technique.[23] The serial tube
concentrations were 400, 200, 100, 50, 25, 12.5 and
6.25 μg ml−1 for the compounds. Standard antibiotic
cefadroxil 1 was used as positive control. MIC values of
the five bacteria strains used in this study are presented
in Table 4. It was found that the synthesized compounds
3, 4, 6 and 7 exhibited promising bactericidal activity
especially against S. epidermidis, with MIC values of
0.043–0.053 μmol ml−1, which are lower than that of the
reference drug 1 (MIC = 0.069 μmol ml−1). All com-
pounds tested showed no activity at all against P.
aeruginosa with the exception of 8, which showed a very
Complexes 5 and 6 have β values less than unity, sug-
gesting the metal–ligand bonds have covalent character.
The β value for the nickel (II) complex 6 was less than
that for the cobalt (II) complex 5, suggesting the greater
covalent nature of the former.
4.6 | Molecular modelling
GAUSSIAN 09 program package with the aid of the
GaussView visualization program[21] was used to carry
out the calculations to get the most stable geometry of
Schiff base complexes. The optimized stable molecular
structure of complex 7, as an example compound, is
shown in Figure 3.
The Schiff base 3 was found to act as a tridentate
ligand as steric restraints will not permit the imine nitro-
gen to coordinate to a metal contained in the ligand cav-
ity. The geometry about Zn (II) ion showed to be distorted
octahedral, as expected, with the Schiff base 3 coordi-
nated through salicyl −O (1.907 Ǻ), β‐lactam −O (1.916
Ǻ) and carboxylate −O atoms (1.912 Ǻ). Three water mol-
ecules, two of them in trans positions, equidistant from
Zn (II) (1.912 Ǻ) complete the metal coordination sphere.
significant activity with a MIC value of 0.040 μmol ml−1
Complex 8 showed the lowest MIC value of 0.040 μmol
.
ml−1 against five bacterial strains used in this study and
was more active than all the compounds tested when
their MIC values are compared. Compounds 3, 4, 5 and
7 showed intermediate bactericidal activity against S.
aureus, with MIC values of 0.086–0.214 μmol ml−1, while
complex 6 showed no significant antibacterial activity,
FIGURE 3 Proposed structure of complex 7 excluding hydrogen
atoms

8 of 10
ANACONA ET AL.
TABLE 3 Antibacterial activity of test compounds (zone of inhibition in mm)
Compound
E.c
S.e
S.a
K.p
P.a
Cefadroxil 1
17 1.0
18 1.5
17 1.0 15
1.0
30 1.5
32 1.0
30 1.0
20 1.0
19 1.0
25 1.0
33 1.5
20 1.0
24 1.0
20 1.0
20 1.5
0
0
0
Schiff base 3
0
4
9
9
0
0
1.0
1.0
0
5
0
6
0
7
1.0
0
7
0
15 1.0
25 1.5
0
8
18 1.0
12 1.0
1.0
10 1.0
Control
4
1.0
3
1.0
3
1.0
3
3
1.0
E.c, Escherichia coli ATCC 25922.
S.e, Staphylococcus epidermidis ATCC 14990.
S.a, Staphylococcus aureus ATCC 25923.
K.p, Klebsiella pneumoniae ATCC 700603.
P.a, Pseudomonas aeruginosa ATCC 27853.
Values are the mean standard deviation of the mean.
TABLE 4 Antibacterial activity of test compounds (MIC μmol
ionic silver Ag (I) or metallic silver Ag (0) have been
developed. Previously reported MIC values of AgNPs
against P. aeruginosa and S. aureus were found to be
0.6–3 μg ml−1 and 0.8–7 μg ml−1, respectively, using
AgNPs in the size range between 5 and 30 nm.[46–48]
Because of higher surface area and more effective binding
to the bacterial membrane, smaller AgNPs are considered
to be more effective than larger particles.
Transmission electron microscopy analysis on bulk
amorphous samples of complex 8 revealed that the parti-
cles are polydispersed, hexagonal in shape, with size >
250 nm, and there is no agglomeration observed between
them. These findings suggested that complex 8 MIC
values against P. aeruginosa and S. aureus should be
improved preparing compound 8 at the nano‐scale level
(< 50 nm).
ml−1
)
E.c
S.e
S.a
K.p
P.a
1
3
4
5
6
7
8
0.275
R
0.069
0.053
0.044
0.173
0.043
0.043
0.040
0.275
0.214
0.087
0.086
0.692
0.086
0.040
R
R
R
R
0.087
0.173
R
0.696
0.691
R
R
R
R
R
R
R
0.040
0.040
0.040
E.c, Escherichia coli ATCC 25922.
S.e, Staphylococcus epidermidis ATCC 14990.
S.a, Staphylococcus aureus ATCC 25923.
K.p, Klebsiella pneumoniae ATCC 700603.
P.a, Pseudomonas aeruginosa ATCC 27853.
R = resistant.
5 | CONCLUSIONS
which may be assigned to its low lipophilicity. In a metal
complex, coordination may augment the lipophilic char-
acter of the central metal atom, which favours further
its permeability through the lipid layers of the cell mem-
brane.[44,45] The obtained results shown in Tables 3 and 4
may highlight that the antibacterial activity of complexes
4–8 compared with free 1 probably reflected a different
mechanism of action.
Silver nanoparticles (AgNPs) have been widely used
as antimicrobial agents for the treatment of bacterial
infections in humans. Because of this application, many
methods and approaches concerning the synthesis of
AgNPs, as well as silver‐based compounds containing
Transition metal complexes 4–8 containing a tridentate
cefadroxil‐based Schiff base (H2L, 3) derived from the
condensation of cefadroxil 1 with salicylaldehyde 2 were
synthesized, characterized and screened for antibacterial
activity. The Schiff base coordination to metal occurs
through the carboxylate –O, β‐lactam –O and salicyl –O
atoms. The Schiff base 3 and its metal complexes 4 and
7 presented lower bactericidal activity than the free 1
against S. epidermidis and S. aureus, showing that they
have good activity as bactericides. Complex 8 exhibited
much better bactericidal activity than the free 1 against
all bacterial strains tested.

ANACONA ET AL.
9 of 10
[19] J. R. Anacona, J. J. Santaella, Spectrochimica Acta Part A 2013,
115, 800.
ACKNOWLEDGEMENTS
The authors gratefully acknowledge the Universidad de
Oriente and Universidad de Playa Ancha for support.
[20] J. R. Anacona, J. L. Rodriguez, J. Camus, Spectrochimica Acta
Part A 2014, 129, 96.
[21] M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A.
Robb, J. R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci,
G. A. Peterson, H. Nakatsuji, M. Caricato, X. Li, H. P.
Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, J. L.
Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J.
Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H.
Nakai, T. Vreven, J. A. Jr Montgomery, E. Peralta, F. Ogliaro,
M. Bearpark, J. J. Heyd, E. Brothers, K. N. Kudin, V. N.
Staroverov, R. Kobayashi, J. Normand, K. Raghavachari, A.
Rendell, J. C. Burant, S. S. Iyengar, J. Tomasi, M. Cossi, N.
Rega, J. M. Millam, M. Klene, J. E. Knox, J. B. Cross, V.
Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann,
O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski,
R. L. Martin, K. Morokuma, V. G. Zakrzewski, G. A. Voth, P.
Salvador, J. J. Dannenberg, S. Dapprich, A. D. Daniels, O.
Farkas, J. B. Foresman, J. V. Ortiz, J. Cioslowski, D. J. Fox,
Gaussian 09, Gaussian, Wallingford CT 2009.
CONFLICT OF INTEREST
The authors of the above manuscript report no conflict of
interest in this work.
ORCID
J.R. Anacona http://orcid.org/0000-0002-3222-2674
REFERENCES
[1] S. B. Levy, B. Marshall, Nat. Med. 2004, 10(12 suppl), S122.
[2] C. Llor, L. Bjerrum, Ther. Adv. Drug Saf. 2014, 5(6), 229.
[3] W. L. Hamilton, R. Wenlock, Camb. Med. J. 2016. https://doi.
org/10.7244/cmj.2016.01.001
[22] H. F. Abd ElHalim, M. M. Omar, G. G. Mohamed, M. A. Sayed,
[4] M. N. Patel, C. R. Patel, H. N. Joshi, Appl. Biochem. Biotechnol.
2013, 169, 1329.
Eur. J. Chem. 2011, 2, 178.
[23] Clinical and Laboratory Standards Institute, Performance Stan-
dards for Antimicrobial Susceptibility Testing. Approved
Standard M2A6, CLSI, Wayne, PA USA 1997.
[5] A. G. Pershina, A. E. Sazonov, D. V. Novikov, A. S. Knvazev, T.
I. Izaak, V. I. Itin, E. P. Naiden, A. A. Magaeva, O. G.
Terechova, J. Nanosci. Nanotechnol. 2011, 11, 2673.
[24] W. J. Geary, Coord. Chem. Rev. 1971, 7, 81.
[6] T. J. Hubin, P. N. A. Amoyaw, K. D. Roewe, N. C. Simpson, R.
D. Maples, T. N. Carder Freeman, A. N. Cain, J. G. Le, S. J.
Archibald, S. I. Khan, B. L. Tekwani, M. O. Faruk Khan,
Bioorg. Med. Chem. 2014, 22, 3239.
[25] I. Ali, W. A. Wani, K. Saleem, Synth. React. Inorg. Met. Org.
Nano Met. Chem. 2013, 43, 1162.
[26] R. M. Issa, S. A. Amer, I. A. Mansour, A. L. AbdelMonsef, J.
Therm. Anal. Cal. 2007, 90, 261.
[7] A. Chakraborty, P. Kumar, K. Ghosh, P. Roy, Eur. J.
Pharmacol. 2010, 647, 1.
[27] R. MrozekLyszczek, J. Therm. Anal. Cal. 2004, 78, 473.
[28] D. C. Harris, Quantitative Chemical Analysis, 7th ed., W. H.
Freeman, New York 2007.
[8] M. Tümer, E. Akgun, S. Toroglu, A. Kayraldiz, L. Donbak,
J. Coord. Chem. 2008, 61, 2935.
[29] Z. H. Chohan, Appl. Organomet. Chem. 2006, 20, 112.
[9] A. Gölcü, M. Tumer, H. Demirelli, R. A. Wheatley, Inorg. Chim.
Acta 2005, 358, 1785.
[30] P. E. Aranha, M. P. Do Santo, S. Romera, E. R. Dockal, Polyhe-
dron 2007, 26, 1373.
[10] H. L. Singh, Spectrochim. Acta Part A 2010, 76, 253.
[31] G. Socrates, Infrared and Raman Characteristic Group
Frequencies: Tables and Charts, 3rd ed., Wiley, New York 2004.
[11] D. Y. Ma, L. X. Zhang, X. Y. Rao, T. L. Wu, D. H. Li, X. Q. Xie,
H. F. Guo, L. Qin, J. Coord. Chem. 2013, 66, 3261.
[32] K. Nakamoto, Infrared and Raman Spectra of Inorganic and
Coordination Compounds: Part A: Theory and Applications in
Inorganic Chemistry, 6th ed., Wiley, New York 2008.
[12] H. Keypour, M. Shayesteh, M. Rezaeivala, F. Chalabian, Y.
Elerman, O. Buyukgungor, J. Molec. Struc. 2013, 1032, 62.
[13] C. J. Dhanaraj, J. Johnson, J. Joseph, R. S. Joseyphus, J. Coord.
Chem. 2013, 66, 1416.
[33] G. A. Bain, J. F. Barry, J. Chem. Ed. 2008, 85, 532.
[34] Z. H. Chohan, A. Munawar, C. T. Supuran, Met. Based Drugs
2001, 8, 137.
[14] A. N. Kursunlu, E. Guler, F. Sevgi, B. Ozkalp, J. Molec. Struc.
2013, 1048, 476.
[35] W. Grochala, R. Hoffmann, Angew. Chem. Int. Ed. 2001, 40,
[15] N. Dharmaraj, P. Viswanathamurthi, K. Natarajan, Transition
Met. Chem. 2001, 26, 105.
2742.
[36] S. E. McLain, M. R. Dolgos, D. A. Tennant, J. C. Turner, T.
Barnes, T. Proffen, B. C. Sales, R. I. Bewley, Nat. Mater. 2006,
5, 561.
[16] S. A. Patil, S. N. Unki, A. D. Kulkarni, V. H. Naik, U. Kamble,
P. S. Badami, J. Coord. Chem. 2011, 64, 323.
[17] J. R. Anacona, M. Rincones, Spectrochimica Acta Part A 2015,
141, 169.
[37] S. Kamdaiah, E. M. Peters, M. Jansen, Z. Anorg. Allg. Chem.
2008, 634, 2483. https://doi.org/10.1002/zaac.200800299
[18] J. R. Anacona, G. Calvo, J. Camus, Monatsh. Fur Chemie 2016,
147, 725.
[38] O. A. ElGammal, G. M. Abu ElReash, T. A. Yousef, M. Mefreh,
Spectrochim. Acta Part A 2015, 146, 163.

10 of 10
ANACONA ET AL.
[39] M. Sonmez, A. Levent, M. Sekerci, Russ. J. Coord. Chem. 2004,
30, 655.
[46] S. Gurunathan, J. W. Han, D. N. Kwon, J. H. Kim, Nanoscale
Res. Lett. 2014, 9, 373.
[40] D. N. Sathyanarayana, Electronic Absorption Spectroscopy and
Related Techniques, Universities Press, India 2001.
[47] Y. Yuan, Q. Peng, S. Gurunathan, Int. J. Mol. Sci. 2017, 18, 569.
[48] L. Yang, M. Nilsson, M. Gjermansen, M. Givskov, T.
[41] M. M. Aboaly, M. M. H. Khalil, Spect. Lett. 2001, 34, 495.
TolkerNielsen, Mol. Microbiol. 2009, 74, 1380.
[42] V. Jena, Electronic Spectra of Transition Metal Complexes, Lulu
Press, USA 2015.
How to cite this article: Anacona JR, Barrios V,
Bravo A, Celis F. Transition metal complexes
containing a tridentate cefadroxil‐based Schiff base:
An effective paramagnetic semiquinone Ag (I)
complex against Staphylococcus aureus and
Pseudomonas aeruginosa. Appl Organometal Chem.
2018;e4562. https://doi.org/10.1002/aoc.4562
[43] M. Manjunatha, V. H. Naikz, A. D. Kulkarni, S. A. Patil, J.
Coord. Chem. 2011, 64, 4264.
[44] G. B. Bagihalli, P. G. Avaji, S. A. Patil, P. S. Badami, Eur. J.
Med. Chem. 2008, 43, 2639.
[45] Z. H. Chohan, M. Arif, M. A. Akhtar, C. T. Supuran, Bioinorg.
Chem. Appl. 2006, 2006, 1. https://doi.org/10.1155/BCA/2006/
83131