Antibacterial activity of Pd(II) complexes with salicylaldehyde-amino acids Schiff bases ligands

Article abstract of DOI:10.1248/cpb.c12-01087

Palladium(II) complexes with Schiff bases ligands derived from salicylaldehyde and amino acids (Ala, Gly, Met, Ser, Val) have been synthesized and characterized by Fourier transform (FT)-IR, UV-Vis and ¹H-NMR spectroscopy. The electrospray mass

Full text of DOI:10.1248/cpb.c12-01087

12
Regular Article
Chem. Pharm. Bull. 62(1) 12–15 (2014)
Vol. 62, No. 1
Antibacterial Activity of Pd(II) Complexes with Salicylaldehyde-Amino
Acids Schiff Bases Ligands
,b
Cristina Rîmbu,a Ramona Danac,b and Aurel Pui*
aꢀFacultyꢀofꢀVeterinaryꢀMedicine,ꢀMicrobiology-ImmunologyꢀDepartmentꢀIaşi,ꢀUniversityꢀofꢀAgriculturalꢀSciencesꢀandꢀ
VeterinaryꢀMedicineꢀIaşi;ꢀIaşiꢀ700490,ꢀRomania:ꢀandꢀbꢀFacultyꢀofꢀChemistry,ꢀAlexandruꢀIoanꢀCuzaꢀfromꢀIaşiꢀ
University;ꢀIaşiꢀ700506,ꢀRomania.
Received December 19, 2012; accepted October 14, 2013
Palladium(II) complexes with Schiff bases ligands derived from salicylaldehyde and amino acids (Ala,
Gly, Met, Ser, Val) have been synthesized and characterized by Fourier transform (FT)-IR, UV-Vis and
1H-NMR spectroscopy. The electrospray mass spectrometry (ES-MS) spectrometry confirms the forma-
tion of palladium(II) complexes in 1/2 (M/L) molar ratio. All the Pd(II) complexes 1, [Pd(SalAla)2]Cl2; 2,
[Pd(SalGly)2]Cl2; 3, [Pd(SalMet)2]Cl2; 4, [Pd(SalSer)2]Cl2; 5, [Pd(SalVal)2]Cl2; have shown antibacterial activ-
ity against Gram-positive bacteria Staphylococcus aureus and Gram-negative bacteria Escherichia coli.
Key words salicylaldehyde-amino acid Schiff base; Pd(II) complex; antibacterial activity
As bacterial drug resistance to currently administered test (micronucleus test in mouse) with tetraammine palladium
treatments is increasing, the need for the development of new hydrogen carbonate gave negative results.13) Staphylococcusꢀ
therapeutic strategies against bacterial infection becomes more aureus and Escherichiaꢀ coli are common bacteria affecting
stringent. Therefore, there is an urgent demand for the devel- mammalians. Although, there are a variety of drugs available
opment of new antimicrobial agents with improved properties for the treatment of their infections, strains of these bacteria
such as enhanced activity, reduced toxicity and shortened du- such as methicillin-resistant Staphylococcusꢀ aureus (MRSA),
ration of therapy.1)
extended-spectrum beta-lactamase (ESBL) producing E. coli,
Schiff bases have a variety of applications in chemical, and multi-resistant bacteria are now showing resistance to
biological and pharmacological fields.2,3) The bi-, tri- or these drugs. The present study describes the synthesis, charac-
tetra-dentate Schiff bases with N, O as donor atoms, can be terization and antibacterial activities of the next palladium(II)
obtained by condensation of aldehydes or ketones with vari- complexes of salicylaldehyde-amino acids (Gly, Ala, Met, Val,
ous amines, diamines or aminoacids.4) Schiff bases have been Ser) Schiff bases: 1, [Pd(SalAla)2]Cl2; 2, [Pd(SalGly)2]Cl2; 3,
used as organic chelating ligands in the synthesis of diverse [Pd(SalMet)2]Cl2; 4, [Pd(SalSer)2]Cl2 and 5, [Pd(SalVal)2]Cl2.
transition metal complexes.5,6) Due to their structural and All five Pd(II) complexes have been preliminary tested for
functional properties, sometimes similar with some biological their antibacterial activity against Gram-positive bacteria
systems, the coordination compounds with Schiff bases are (Staphylococcusꢀ aureus) and Gram-negative bacteria (Esch-
used in inorganic, organic and biological fields.7,8)
erichia coli). The mode of action of palladium ions and of
Amino acids play an important role in many biochemi- elemental palladium is not fully clear. Complex formation
cal processes. The metal complexes with amino acids play of palladium ions with cellular components probably plays a
an important role in understanding biological functions of basic role initially. Oxidation processes may also be involved,
macromolecules such as proteins in human body.9) The ami- due to the different oxidation states of palladium.13)
noacids form with salicilaldehyde heterodentate Schiff bases
that proved to possess catalytic activity10) or interact with
Experimental
All commercially available products were used without
DNA.11) During recent years, a series of aminoacids based
coordination compounds have been prepared and found to further purification unless otherwise specified. The Fourier
be active against several Gram-positive and Gram-negative transform (FT)-IR spectra were recorded on a JASCO FT-IR
bacterial strains.12) Owing to the ability of palladium ions to 660 Plus spectrometer using KBr pellets. The UV-VIS spectra
form complexes, they bind to aminoacids (e.g., L-cysteine, were recorded in EtOH solution, c=1mM, using a Cintra 101
1
L-cystine, L-methionine), proteins (e.g., casein, silk fibroin, device. The H-NMR spectra were recorded on a Bruker 500
many enzymes), DNA or other macromolecules (e.g., vitamin spectrometer. All chemical shifts are quoted on the δ-scale in
B6).13) Furthermore, it is very important to fully understand ppm. The mass spectra were recorded on a Excalibrus spec-
the physiological functions of the metal elements from human trometer using MeOH–CH2Cl2 (1:1, v/v) mixture, electrospray
body by studying their coordination behavior.14)
ionization (ESI) in the positive mode; m/z values are reported
Palladium compounds may interact with isolated DNA in Daltons.
inꢀ vitro. However, with one exception, mutagenicity tests
A study regarding the antibacterial activity of the Pd(II)
of several palladium compounds with bacterial or mamma- complexes was determined by diffusion method.15)
lian cells inꢀ vitro (Ames test: Salmonellaꢀ typhimurium; SOS
General Procedure for Synthesis of Pd(II) Complexes
chromotest: Escherichiaꢀ coli; micronucleus test: human lym- Salicylaldehyde (0.010mol) in ethanol (20mL) was added
phocytes) gave negative results. Also, an inꢀvivo genotoxicity to an aqueous solution of amino acids (0.010mol in 20mL
water). Then, an aqueous solution (10mL) of sodium acetate
(10mL, 0.020mol) was added. A solution of Pd(II) chloride
The authors declare no conflict of interest.
© 2014 The Pharmaceutical Society of Japan
*To whom correspondence should be addressed. e-mail: aurel@uaic.ro

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Chart 1. Synthesis of Pd(II) Complexes
Fig. 1. Mass Spectrum of the Complex [Pd(SalMet)2]Cl2·3H2O
(0.0085mol) in MeCN (25mL) was added drop wise to the
reaction mixture while stirring at room temperature. The color
of the solution turns from yellow to brown. If the product pre-
cipitated from solution, it was separated by filtration and suc-
cessively washed with water and diethyl ether, if not, it was
allowed to crystallize, and successively washed with water
and diethyl ether. If necessary the product was recrystallized
from MeOH (75–85% yields).
Antimicrobial Testing The antibacterial activity of
the agents salicylaldehyde-amino acids Schiff bases ligands
1–5 has been tested using the diffusion method. Antimicro-
bial tests were made on standardized 24h bacterial cultures
Staphylococcusꢀ aureus ATCC 29213 (Gram-positive bacteria)
and Escherichiaꢀ coli ATCC 25922 (Gram-negative bacteria),
obtained in nutrient broth liquid medium, after thermostating
at 37°C, as described by guidelines in National Committee
for Clinical Laboratory Standards (NCCLS)-approved stan-
dard document M7-A7. Antimicrobial activities of the newly
Fig. 2. Molecular Structure Proposed for the [Pd(SalMet)2]Cl2·3H2O
Complex
synthesized chemical compounds were performed in Muel- 1 and their structure were investigated by ESI-MS, UV-VIS,
ler–Hinton medium (Oxoid) at a pH of 7.3. Both cultures IR and NMR.
were brought to a 0.5 McFarland turbidity standard, which
Mass Spectra of the Pd(II) Complexes The mass spec-
for most bacterial species corresponds to 2×108 colony form- trum of the complex 3, presents a molecular pattern with max-
ing unit (cfu)/mL. A build-in technique for solid media was imum intensity at m/z=739 (Fig. 1) that is in good agreement
used; thus, nutrient agar was melted and cooled to 45°C, then with [M+H]+ for the structure presented in Fig. 2.
poured in Petri plates and homogenized with 1mL bacterial
Similarly, all Pd(II) complexes show molecular patterns
culture obtained in nutrient broth. After the medium solidi- in electrospray (ES)-MS that confirm the general formula:
fied, a well was made in the plates with sterile borer (5mm). [Pd(SalAmac)2]Cl2·nH2O for these complexes. Thus, the
The test compounds (0.1mg) was introduced into the well and mass spectra of complex 1 shows a pattern with maximum at
plates were incubated at 37°C for 72h. Microbial growth was m/z=600 corresponding to [Pd(Sal Ala)2]Cl2·2H2O formula,
determined by measuring the diameter of zone of inhibition.16) the mass spectra of complex 2 shows a pattern with maximum
These zones of inhibition are the spaces in which microorgan- at m/z=554 for [Pd(Sal Gly)2]Cl2·H2O, complex 4 furnished a
isms have not replicated due to the action of the active sub- pattern with maximum at m/z=614 corresponding to [Pd(Sal
stance and therefore did not form bacterial colonies.
Ser)2]Cl2·H2O and complex 5 showing a pattern with maxi-
mum at m/z=638 for [Pd(Sal Val)2]Cl2·H2O.
UV-VIS Spectra The electronic spectra of the Pd(II)
Results and Discussion
The complexes were synthesized by template method, Chart complexes exhibit bands in the UV range at 275–280nm and

14
Vol. 62, No. 1
Table 1. Main Electronic Spectral Bands of the Pd(II) Complexes
−1
−1
S. No.
Complexes
λ1 [nm]
ε [M cm−1
]
λ2 [nm]
ε [M cm−1
]
1
2
3
4
5
[Pd(Sal Ala)2]Cl2·2H2O
[Pd(Sal Gly)2]Cl2·1H2O
[Pd(Sal Met)2]Cl2·3H2O
[Pd(Sal Ser)2]Cl2·1H2O
[Pd(Sal Val)2]Cl2·1H2O
280
279
(0.618)
(0.33)
388
386
390
387
380
(0.269)
(0.519)
(0.453)
(0.089)
(0.38)
279
(1.303)
(0.256)
276
278, 226
(1.133), (1.0)
Table 2. Infrared Spectral Data of Pd(II) Complexes
Complexes
ν(N–H)
ν(C=N)
νasym/sym(COO–)
ν(M–N)
ν(M–O)
1
2
3
4
5
2921
3100
3048
3102
3106
1651
1628
1645
1640
1646
1526, 1292
1453, 1285
1582, 1321
1601, 1317
1612, 1218
555
592
523
589
546
463
465
464
428
421
Table 3. NMR Spectral Data for Pd(II) Complexes
Complexes
1H (δDMSO-d =2.50ppm)
13C (δDMSO-d =39.61ppm)
6
6
1
7.81 (s, 1H, C=N), 7.36–7.18 (m, 2H, Ar), 6.70–6.48 (m, 2H,
Ar), 5.30 (s, 1H, CHR), 1.46 (m, 3H, CH3)
181.6 (COOH), 162.9 (Car-OH), 156.9 (C=N), 134.5 (Car),
133.9 (Car), 120.4 (Car), 119.8 (Car), 114.2 (Car),
66.1 (C-R), 21.3 (CH3)
2
3
8.17 (s, 1H, C=N), 7.31–7.20 (m, 2H, Ar), 6.82–6.67 (m, 2H,
179.5 (COOH), 161.6 (Car-OH), 159.1 (C=N), 136.3 (Car),
Ar), 4.76 (d, 2H, CHR)
134.9 (Car), 121.1 (Car), 120.2 (Car), 115.4 (Car), 53.8 (C-R)
8.15 (s, 1H, C=N), 7.51–7.34 (m, 2H, Ar), 6.93–6.67 (m, 2H,
Ar), 4.58 (s, 1H, CHR), 2.49 (m, 2H, 2CH2), 2.17 (m, 3H,
CH3), 2.04 (m, 2H, 2CH2)
180.1 (COOH), 164.1 (Car-OH), 163.2 (C=N), 137.4 (Car),
135.8 (Car), 120.9 (Car), 119.7 (Car), 116.1 (Car),
69.5 (C-R), 37.8 (CH2), 33.9 (CH2-S) 19.2 (S-CH3)
4
5
8.26 (s, H, C=N), 7.53–7.20 (m, 2H, Ar), 6.75–6.62
178.1 (COOH), 162.8 (Car-OH), 161.4 (C=N), 135.1 (Car),
133.1 (Car), 122.7 (Car), 120.6 (Car), 112.9 (Car),
72.2 (C-R), 68.1 (CH2)
(m, 2H, Ar), 4.52 (s, 1H, CHR), 3.95 (d, 2H, CH2)
8.28 (s, H, C=N), 7.58–7.35 (m, 2H, Ar), 6.72–6.64
(m, 2H, Ar), 4.85 (s, 1H, CHR), 3.98 (s, 1H, CH),
2.40–2.50 (m, 6H, 2CH3)
181.9 (COOH), 165.6 (Car-OH), 163.9 (C=N), 134.3 (Car),
132.2 (Car), 124.9 (Car), 121.9 (Car), 118.8 (Car),
71.6 (C-R), 37.2 (C-R), 22.7 (CH3), 22.1 (CH3)
1
380–390nm, Table 1. All these Pd(II) complexes show a broad portant signals from H- and 13C-NMR spectra of the Pd(II)
d–d transition band in the region of 480–490nm assignable to complexes are presented in Table 3.
1
1B1g← A1g transition, typical to square planar geometry.17)
Magnetic susceptibility measurements suggest these com-
Infrared Spectra The infrared frequencies of Pd(II) plexes to be diamagnetic, that being confirmed by the sharp
1
complexes are given in Table 2. All the complexes exhibit signals in the H-NMR spectra.
broad bands in the range of 3440–3590cm−1, which can be at-
Antibacterial Activity The diameter of the inhibition
tributed to the presence of coordinated water molecules. The zones area has been measured. These inhibition zones provide
strong IR absorption band observed in the free Schiff base important information about the putative mechanism of bacte-
around 1600–1630cm−1 (1624–1647 cm−1), correspond to the rial resistance. All the tested palladium(II) complexes show a
ν(C=N) stretching vibration.18–20) Further, the shift observed for remarkable biological activity against Gram-positive Staphy-
νasym(COO−) vibration of ligands to higher frequency in the lococcusꢀaureus and Gram-negative Escherichiaꢀcoli bacteria.
1575–1590 cm−1 range and for νsym(COO−) to lower frequency The data are shown in Fig. 3.
in the 1340–1386cm−1 range visualizes the coordination of
The screening for the palladium chloride salts and new
carboxyl oxygen21) to the palladium ion. In the low frequency complexes were performed at the fixed amount of 0.1mg.
regions, bands detected around 420–460cm−1 ranges are as- Based on the obtained values of the relative zone inhibi-
signed to M–O (carboxylato oxygen atom) and the bands at tion,24,25) 1 and 2 complexes were found to be very effective
520–535cm−1 ranges are assigned to M–N (imino nitrogen).22) against Gram-positive Staphylococcusꢀaureus and Gram-nega-
1H-NMR Spectra The 1H-NMR spectra of the Pd(II) tive Escherichiaꢀcoli bacteria, Table 4.
complexes show the signals in the range of 8.28–8.17ppm
The obtained metal complexes showed increased activity
corresponding to the iminic protons (HC=N), 7.58–6.65ppm compared to the corresponding metal salt. The higher activity
assigned to aromatic protons.1,23) The signals of protons of the complexes may be due to the change in the structure of
from HCR are in the range of 5.30–4.58ppm and the signals compounds, as polarity of the metal. This can be explained
for protons from methylene and methyl groups appear at by partial sharing of the positive charge of the metal with
3.60–3.40ppm and 2.40–2.10ppm, respectively. The signals the donor groups of the ligands. Chelation of the metallic ion
corresponding to OH protons are broad and appear in the and –OH free groups from ligands increase the lipophilic
range of 11.96–10.27ppm. Chemical shifts for the most im- nature of the central metal atom, which favors its permeation

January 2014
15
Fig. 3. Zones of Inhibitions of Palladium Compounds against Escherichiaꢀcoli (a) and Staphylococcusꢀaureus (b) 1, PdCl2; 2, [Pd(Sal Val)2]Cl2; 3,
[Pd(Sal Ala)2]Cl2; 4, [Pd(Sal Gly)2]Cl2; 5, [Pd(Sal Ser)2]Cl2; 6, [Pd(Sal Met)2]Cl2
Table 4. Inhibitions Zones Data (mm) of Palladium Complexes
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Zones of inhibiton (mm)
Compounds
Staphylococcusꢀaureus
Escherichiaꢀcoli
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ATCC 29213
ATCC 25922
PdCl2/MeCN
6
22
19
13
13
14
13
12
7
20
24
14
8
1
2
3
4
5
11
8
Ampicilina)
Streptomycina)
6
a) Ref. 1.
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more efficiently through the lipid layer of the microorganisms.
Therefore, obtained complexes present remarkable biological
activity.
Conclusion
The salicylaldehyde and amino acids form inꢀ situ Schiff
bases that proved to be ligands for complexation of Pd(II) ion.
The coordination of metallic ion to the ligands, by imino nitro-
16) National Committee for Clinical Laboratory Standards, “Perfor-
mance Standards for Antimicrobial Disk Susceptibility Tests, 5th
ed., Approved Standard M2-A5,” NCCLS, Villanova, PA, 1993.
17) Lever A. B. P., “Inorganic Electronic Spectroscopy,” 2nd ed., Else-
vier, London, 1992.
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Nucl. Chem., 34, 2761–2765 (1972).
1
gen and carboxyl oxygen, was confirmed by IR and H-NMR
spectroscopic measurements. The square planar geometries
of the palladium(II) complexes were confirmed by UV-VIS
spectroscopy. The mass spectra confirm the formation of the
complexes in 1:2 (M:L) molar ratio. Complexes 1 and 2 were
found to possess a remarkable antibacterial activity against
Staphylococcusꢀaureus and Escherichiaꢀcoli bacteria.
20) Nakamoto K., “Infrared and Raman Spectra of Inorganic and Coor-
dination Compounds,” Wiley, New York, 1986.
21) Sallam S. A., Transit.ꢀMetalꢀChem., 31, 46–55 (2006).
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Acknowledgment The authors wish to thank CERNESIM
centre from “Alexandru Ioan Cuza” of Iasi University, Faculty
of Chemistry, for several determinations used in the study.
24) Rohini R., Muralidhar Reddy P., Shanker K., Hu A., Ravinder V.,
Eur. J. Med. Chem., 45, 1200–1205 (2010).
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