Synthesis, characterization and antimicrobial activity of novel 3-ferrocenyl-2-pyrazolyl-1,3-thiazolidin-4-ones

Article abstract of DOI:10.1016/j.poly.2018.08.071

A new series of ferrocene-containing thiazolylpyrazoles – 3-ferrocenyl-2-pyrazolyl-1,3-thiazolidin-4-ones have been synthesized using convenient one-pot three component condensation. Twelve newly synthesized compounds were fully characterized by spectroscopic (IR and NMR) and electrochemical methods (cyclic voltammetry). Single crystal X-ray structure analysis were undertaken on two compounds. The twelve novel ferrocene derivatives were also evaluated for antimicrobial activity. The results showed moderate antimicrobial activity of synthesized compounds with better effect on Candida albicans and Gram-negative bacteria than Gram-positive bacterial strains.

Full text of DOI:10.1016/j.poly.2018.08.071

Accepted Manuscript
Synthesis, characterization and antimicrobial activity of novel 3-ferrocenyl-2-
pyrazolyl-1,3-thiazolidin-4-ones
Anka Pejović, Aleksandra Minić, Jovana Bugarinović, Marko Pešić, Ivan
Damljanović, Dragana Stevanović, Vladimir Mihailović, Jelena Katanić, Goran
A. Bogdanović
PII:
S0277-5387(18)30539-4
https://doi.org/10.1016/j.poly.2018.08.071
POLY 13394
DOI:
Reference:
Polyhedron
To appear in:
Received Date:
Accepted Date:
12 July 2018
27 August 2018
Please cite this article as: A. Pejović, A. Minić, J. Bugarinović, M. Pešić, I. Damljanović, D. Stevanović, V.
Mihailović, J. Katanić, G.A. Bogdanović, Synthesis, characterization and antimicrobial activity of novel 3-
ferrocenyl-2-pyrazolyl-1,3-thiazolidin-4-ones, Polyhedron (2018), doi: https://doi.org/10.1016/j.poly.2018.08.071
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Synthesis, characterization and antimicrobial activity of novel 3-
ferrocenyl-2-pyrazolyl-1,3-thiazolidin-4-ones
a
Anka Pejović, Aleksandra Minić,a Jovana Bugarinović,a Marko Pešić,a Ivan Damljanović,a
Dragana Stevanović,a Vladimir Mihailović,a Jelena Katanića and Goran A. Bogdanovićb
aDepartment of Chemistry, Faculty of Science, University of Kragujevac, R. Domanovića 12,
34000 Kragujevac, Serbia
bVinča Institute of Nuclear Sciences, University of Belgrade, PO Box 522, 11001 Belgrade,
Serbia
E-mail address: apejovic@kg.ac.rs (A. Pejović)
Abstract: A new series of ferrocene-containing thiazolylpyrazoles – 3-ferrocenyl-2-
pyrazolyl-1,3-thiazolidin-4-ones have been synthesized using convenient one-pot three
component condensation. Twelve newly synthesized compounds were fully characterized by
spectroscopic (IR and NMR) and electrochemical methods (cyclic voltammetry). Single
crystal X-ray structure analysis were undertaken on two compounds. The twelve novel
ferrocene derivatives were also evaluated for antimicrobial activity. The results showed
moderate antimicrobial activity of synthesized compounds with better effect on Candida
albicans and Gram-negative bacteria than Gram-positive bacterial strains.
Keywords: Ferrocene, 1,3-Thiazolidin-4-one, Pyrazole, Crystal structure, Electrochemistry,
Antimicrobial activity
 Corresponding author. e-mail: anka.pejovic@gmail.com

1. Introduction
Since the middle of the 20th century, the antimicrobial agents have been efficiently used
in health care. However, in the recent time the success in the application of antibiotics in
treating of infections has been eroded by the increased resistance of bacteria. Therefore, there
is a permanent interest in the development of novel substances exhibiting antimicrobial
activity [1, 2]. Certain organometallic compounds offer a promise in this respect, because
they are inert, often uncharged, and fairly lipophilic [3]. The metal centers exist in low
oxidation states, which limit the danger of oxidative damage inside cells [3]. It was found that
ferrocene and some of its derivatives (e.g., ferrocifen) have strong and useful
pharmacological properties and are considered as promising lead structures,particularly it is
found that ferrocene itself have some antiproliferative activity. Its mechanism of action
remains elusive despite several efforts; however, it is assumed by some researchers that the
redox chemistry (Fe2+/Fe3+) is the core of its activity [3].
On the other side, the sulfur and nitrogen fused heterocycles in their own are very
interesting due to their physicochemical properties, especially in the sense of designing new
drugs and new materials. The chemistry and pharmacology of thiazolidinone derivatives has
been of great interest to medicinal chemists lately [4–9]. The pyrazole ring is a prominent
structural moiety found in numerous pharmacologically active compounds. Pyrazole-based
derivatives have been regarded as anxiolytics [10], γ-gaminobutyric acid (GABA) receptor
antagonists and insecticides [11], potential positron emission tomography (PET) ligands for
cannabinoid receptors type 1 (CB1) [12], anti-inflammatory, antimicrobial [13], and growth
inhibition agents [14].
Several years ago, we described the efficient synthetic route to the 3-ferrocenyl-1-
phenylpyrazole-4-carboxaldehyde 3 (Scheme 1) [15]. Moreover, we discovered that this class
of compound represents an excellent starting material for the synthesis of a numerous novel
ferrocene-containing products [15]. In continuation of our ongoing interest in synthesis of
ferrocene-containing heterocyclic compounds of potential biological interest [16–19], and
having in mind the above considered, we have been prompted to synthesize new, possible
more potent, pharmacologically active compounds. We decided to evaluate the potential
application of the compound 3 (Scheme 1) in the synthesis of novel ferrocene-containing
thiazolylpyrazoles – via one–pot three–component condensation, expecting (based on their
structure) that the resulting novel ferrocene-containing heterocycles would be biologically
active. Therefore, in the present paper we aim to report the design and synthesis of the

3-ferrocenyl-2-pyrazolyl-1,3-thiazolidin-4-ones. Furthermore, the results of antibacterial and
antifungal examinations of all newly synthesized compounds are presented, as well as the
results of electrochemical investigations and X-ray analyses.
Scheme 1. Synthesis of 3-ferrocenyl-1-phenylpyrazole-4-carboxaldehyde 3 [15]; (i) Ethanol,
reflux; (ii) POCl3 (3 equiv), DMF, r.t.;
2. Experimental Section
2.1 Materials and measurements
All chemicals were commercially available and used as received, except that the solvents
were purified by distillation. Chromatographic separations were carried out using silica gel
60 (Merck, 230–400 mesh ASTM), whereas silica gel 60 on Al plates, layer thickness 0.2
mm (Merck) was used for TLC. Electrochemical measurements were performed at room
temperature in a three-electrode cell using an Autolab potentiostat (PGSTAT 302N). The
working electrode was a glassy carbon disk (2 mm diameter). The counter electrode was a
platinum wire, and Ag/AgCl electrode was used as the reference electrode. Melting points
(uncorrected) were determined on a Mel-Temp capillary melting points apparatus, model
1
13
1001. The H and C NMR spectra of the samples in CDCl3 were recorded on a Bruker
13
Avance III 200 MHz (1H at 200 MHz, C at 50 MHz) NMR spectrometer. Chemical shifts
are expressed in  (ppm), relative to residual solvent protons as the internal standard (CDCl3:
7.26 ppm for 1H and 77 ppm for 13C). IR measurements were carried out with a Perkin–Elmer
FTIR 31725-X spectrophotometer.

2.2. Synthetic procedures
2.2.1. Synthesis of 3-ferrocenyl-1-phenylpyrazole-4-carboxaldehyde 3
3-Ferrocenyl-1-phenylpyrazole-4-carboxaldehyde 3 was synthesized according to
previously described procedure [15].
2.2.2 Synthesis of 2-(3-ferrocenyl-1-phenyl-1H-pyrazol-4-yl)thiazolidin-4-ones 5a–l
General procedure
An ice-cooled solution of the corresponding primary amine (1 mmol) and 3-ferrocenyl-1-
phenylpyrazole-4-carboxaldehyde 3 (356 mg, 1 mmol) in THF (2 mL) was irradiated in an
ultrasonic bath for 5 min, followed by the addition of thioglycolic acid (184 mg, 2 mmol).
After further irradiation for 5 min, DCC (206 mg, 1 mmol) was added to the resulting mixture
and irradiation continued for another 15 min under the same conditions. DCU was removed
by filtration, the solvent evaporated, and the residue taken up in EtOAc (30 mL). The organic
layer was washed with 5% aq. solution of citric acid, H2O, 5% aq. solution of NaHCO3 and
brine, successively, and dried overnight (anh. Na2SO4). After the evaporation of the solvent,
the crude mixture was purified by column chromatography (SiO2/hexane – EtOAc 7:3, v/v).
2-(3-Ferrocenyl-1-phenyl-1-H-pyrazol-4-yl)-3-methylthiazolidin-4-one (5a). Orange
solid. Yield (91 %), m.p. 136 °C; IR (KBr): ν = 3093, 2923, 1683, 1599, 1553, 1506 cm-1; 1H
NMR (200 MHz, CDCl3, ppm): δ 7.85 (s, 1H), 7.77 – 7.61 (m, 2H), 7.53 – 7.38 (m, 2H), 7.37
– 7.23 (m, 1H), 6.02 (s, 1H), 4.29 (s, 5H), 4.97 – 3.95 (m, 4H), 3.90 – 3.54 (m, 2H), 2.91 (s,
13
3H); C NMR (50 MHz, CDCl3, ppm): δ 170.9, 170.7, 149.6, 139.4, 129.3, 126.5, 125.9,
120.2, 118.7, 70.3, 69.8, 68.9, 67.9, 56.8, 32.9, 30.5.
3-Butyl-2-(3-ferrocenyl-1-phenyl-1-H-pyrazol-4-yl)thiazolidin-4-one (5b). Orange solid.
Yield (71 %), m.p. 133 °C; IR (KBr): ν = 3126, 2927, 1674, 1598, 1555, 1506 cm-1; 1H NMR
(200 MHz, CDCl3, ppm): δ 7.79 (s, 1H), 7.75 – 7.56 (m, 2H), 7.54 – 7.36 (m, 2H), 7.34 –
7.20 (m, 1H), 6.02 (s, 1H), 4.98 – 4.06 (m, 4H), 4.27 (s, 5H), 3.89 – 3.55 (m, 3H), 3.06 – 2.70
(m, 1H), 1.64 – 1.14 (m, 4H), 0.87 (t, J = 6.4 Hz, 3H); 13C NMR (50 MHz, CDCl3, ppm): δ
170.8, 149.4, 139.6, 129.4, 126.5, 125.6, 121.0, 118.7, 70.4, 69.8, 69.1, 67.8, 55.2, 43.2, 32.8,
29.4, 20.1, 13.8.
2-(3-Ferrocenyl-1-phenyl-1-H-pyrazol-4-yl)-3-hexylthiazolidin-4-one (5c). Dark red solid.
1
Yield (69 %), IR (KBr): ν = 3090, 2926, 1676, 1599, 1552, 1505 cm-1; H NMR (200 MHz,
CDCl3, ppm): δ 7.81 (s, 1H), 7.77 – 7.70 (m, 2H), 7.54 – 7.40 (m, 2H), 7.36 – 7.24 (m, 1H),
6.02 (s, 1H), 4.74– 4.60 (m, 2H), 4.43 – 4.32 (m, 2H), 4.19 (s, 5H), 3.87 – 3.60 (m, 3H), 2.88
(dt, J = 13.8, 6.8 Hz, 1H), 1.61 – 1.38 (m, 2H), 1.36 – 1.10 (m, 6H), 0.82 (t, J = 6.1 Hz, 3H).;
13C NMR (50 MHz, CDCl3, ppm): δ 170.8, 149.4, 139.6, 129.4, 126.5, 125.5, 121.0, 118.7,
69.6, 68.9, 68.5, 67.1, 55.1, 43.3, 32.6, 31.3, 27.2, 26.5, 22.5, 13.9.

2-(3-Ferrocenyl-1-phenyl-1-H-pyrazol-4-yl)-3-octylthiazolidin-4-one (5d). Orange solid.
Yield (60 %); IR (KBr): ν = 3123, 2925, 1676, 1599, 1555, 1506 cm-1; 1H NMR (200 MHz,
CDCl3, ppm): δ 7.79 (s, 1H), 7.74 – 7.60 (m, 2H), 7.53 – 7.38 (m, 2H), 7.36 – 7.25 (m, 1H),
6.01 (s, 1H), 5.03 – 4.05 (m, 4H), 4.29 (s, 5H), 3.87 – 3.51 (m, 3H), 3.05 – 2.67 (m, 1H), 1.67
13
– 1.38 (m, 2H), 1.38 – 1.03 (m, 10H), 0.84 (t, J = 5.9 Hz, 3H); C NMR (50 MHz, CDCl3,
ppm): δ 170.7, 149.3, 139.4, 129.3, 126.4, 125.5, 120.9, 118.6, 70.4, 69.8, 69.1, 67.7, 55.1,
43.3, 32.7, 31.6, 29.1, 29.0, 27.2, 26.7, 22.5, 14.0.
3-Benzyl-2-(3-ferrocenyl-1-phenyl-1-H-pyrazol-4-yl)thiazolidin-4-one (5e). Orange solid.
Yield (71 %); m.p. 169 °C; IR (KBr): ν = 3120, 2928, 1694, 1626, 1597, 1553, 1496 cm-1; 1H
NMR (200 MHz, CDCl3, ppm): δ 7.75 (s, 1H), 7.71 – 7.02 (m, 10H), 5.86 (s, 1H), 5.24 –
13
4.99 (m, 1H), 4.81 – 3.69 (m, 7H); 4.14 (s, 5H), C NMR (50 MHz, CDCl3, ppm): δ 171.1,
149.5, 139.4, 135.3, 129.3, 128.8, 127.8, 127.7, 126.4, 125.8, 120.1, 118.6, 70.3, 69.7, 68.8,
67.8, 54.8, 46.6, 32.7.
2-(3-Ferrocenyl-1-phenyl-1-H-pyrazol-4-yl)-3-(4-methoxyphenethyl)thiazolidin-4-one
(5f). Dark red solid. Yield (83 %), IR (KBr): ν = 3081, 2927, 1674, 1599, 1552, 1512 cm-1;
1H NMR (200 MHz, CDCl3, ppm): δ 7.72 (s, 1H), 7.71 – 7.60 (m, 2H), 7.54 – 7.36 (m, 2H),
7.34 – 7.21 (m, 1H), 7.02 (t, J = 8.4 Hz, 2H), 6.80 (t, J = 8.4 Hz, 2H), 5.78 (s, 1H), 4.82 –
4.64 (m, 1H), 4.50 – 4.31 (m, 3H), 4.19 (s, 5H), 3.99 – 3.68 (m, 3H), 3.74 (s, 3H), 3.19 – 2.92
13
(m, 1H), 2.89 – 2.65 (m, 2H); C NMR (50 MHz, CDCl3, ppm): δ 170.9, 158.3, 149.5,
139.5, 130.1, 129.6, 129.3, 126.5, 125.7, 120.7, 118.7, 114.0, 77.3, 69.9, 69.2, 69.1, 68.5,
67.4, 55.6, 55.2, 45.1, 32.7, 32.5.
2-(3-Ferrocenyl-1-phenyl-1-H-pyrazol-4-yl)-3-(furan-2-ylmethyl)thiazolidin-4-one (5g).
Orange solid. Yield (64 %), m.p. 187 °C; IR (KBr): ν = 3080, 2914, 1691, 1599, 1557, 1506
cm-1; 1H NMR (200 MHz, CDCl3, ppm): δ 7.89 – 7.56 (m, 3H), 7.55 – 7.20 (m, 4H), 6.31 –
6.23 (m, 1H), 6.22 – 6.14 (m, 1H), 6.01 (s, 1H), 5.16 – 3.98 (m, 6H), 4.24 (s, 5H), 3.97 – 3.56
(m, 2H); 13C NMR (50 MHz, CDCl3, ppm): δ 170.8, 149.5, 148.9, 142.4, 139.3, 129.3, 126.4,
125.8, 120.2, 118.6, 110.4, 108.9, 70.3, 69.6, 68.8, 67.8, 55.1, 39.7, 32.6.
2-(3-Ferrocenyl-1-phenyl-1-H-pyrazol-4-yl)-3-(thiophen-2-ylmethyl)thiazolidin-4-one
(5h). Yellow solid. Yield (69 %), m.p. 180 °C; IR (KBr): ν = 3131, 2922, 1679, 1599, 1553,
1
1505 cm-1; H NMR (200 MHz, CDCl3, ppm): δ 7.79 (s, 1H), 7.73 – 7.58 (m, 2H), 7.58 –
7.37 (m, 2H), 7.37 – 7.11 (m, 3H), 6.99 – 6.68 (m, 2H), 5.98 (s, 1H), 5.25 – 4.99 (m, 1H),
4.85 – 4.10 (m, 5H), 4.20 (s, 5H), 3.95 – 3.57 (m, 2H); 13C NMR (50 MHz, CDCl3, ppm): δ
170.8, 149.5, 139.3, 137.6, 129.3, 127.1, 126.8, 126.4, 126.0, 125.8, 119.9, 118.6, 70.2, 69.5,
68.7, 67.7, 54.6, 41.4, 32.6.
2-(3-Ferrocenyl-1-phenyl-1-H-pyrazol-4-yl)-3-(pyridin-2-ylmethyl)thiazolidin-4-one (5i).
Orange solid. Yield (58 %), m.p. 172 °C; IR (KBr): ν = 3128, 2927, 1680, 1597, 1555, 1505
1
cm-1; H NMR (200 MHz, CDCl3, ppm): δ 8.55 (s, 1H), 8.08 – 6.97 (m, 9H), 6.25 (s, 1H),
5.34 – 4.99 (m, 1H), 4.73 – 3.74 (m, 7H), 4.10 (s, 5H); 13C NMR (50 MHz, CDCl3, ppm): δ
171.5, 155.3, 149.6, 139.5, 136.9, 129.3, 126.4, 126.1, 122.6, 121.9, 120.1, 118.7, 70.1, 69.4,
68.5, 67.7, 55.6, 48.1, 32.6.

2-(3-Ferrocenyl-1-phenyl-1-H-pyrazol-4-yl)-3-(pyridin-3-ylmethyl)thiazolidin-4-one (5j).
Yellow solid. Yield (75 %), m.p. 187 °C; IR (KBr): ν = 3127, 2934, 1689, 1597, 1553, 1497
1
cm-1; H NMR (200 MHz, CDCl3, ppm): δ 9.57 (s, 1H), 8.05 – 7.00 (m, 9H), 5.88 (s, 1H),
5.27 – 4.87 (m, 1H), 4.75 – 4.59 (m, 1H), 4.41 – 4.04 (m, 4H), 4.10 (s, 5H), 3.92 – 3.67 (m,
13
2H); C NMR (50 MHz, CDCl3, ppm): δ 171.1, 149.6, 139.3, 129.3, 126.5, 119.7, 118.8,
69.5, 69.0, 68.8, 68.5, 67.3, 54.7, 32.6.
2-(3-Ferrocenyl-1-phenyl-1-H-pyrazol-4-yl)-3-m-tolylthiazolidin-4-one (5k). Orange
solid. Yield (53 %); IR (KBr): ν = 3127, 2919, 1680, 1599, 1558, 1504 cm-1; m.p. 167 °C; 1H
NMR (200 MHz, CDCl3, ppm): δ 7.90 (s, 1H), 7.71 – 7.60 (m, 2H), 7.52 – 7.38 (m, 2H), 7.33
– 7.16 (m, 2H), 7.14 – 6.98 (m, 3H), 6.44 (s, 1H), 4.68 – 4.50 (m, 2H), 4.33 (t, J = 1.8 Hz,
13
2H), 4.11 (s, 5H), 4.00 – 3.83 (m, 2H), 2.28 (s, 3H); C NMR (50 MHz, CDCl3, ppm): δ
170.8, 149.0, 139.5, 139.2, 137.4, 129.3, 129.0, 128.1, 126.4, 125.9, 125.7, 122.6, 121.1,
118.6, 69.4, 68.8, 68.3, 67.3, 57.7, 33.2, 21.4.
2-(3-Ferrocenyl-1-phenyl-1-H-pyrazol-4-yl)-3-p-tolylthiazolidin-4-one (5l). Orange solid.
Yield (34 %); IR (KBr): ν = 3115, 2921, 1683, 1599, 1555, 1503 cm-1; 1H NMR (200 MHz,
CDCl3, ppm): δ 7.92 (s, 1H), 7.77 – 6.95 (m, 9H), 6.44 (s, 1H), 4.67 – 4.50 (m, 2H), 4.41 –
13
4.26 (m, 2H), 4.23 – 3.78 (m, 2H), 4.12 (s, 5H), 2.27 (s, 3H); C NMR (50 MHz, CDCl3,
ppm): δ 170.7, 149.0, 139.4, 137.1, 134.8, 129.8, 129.3, 126.3, 126.0, 125.4, 121.0, 118.6,
69.4, 68.8, 68.2, 67.3, 57.6, 33.4, 21.0.
2.3. X-ray data collection and structure refinement for compounds 5k and 5g
Single-crystal X-ray diffraction data for both compounds were collected on an Oxford
Gemini S diffractometer equipped with a CCD detector, using monochromated Mo Kα
radiation. Data reduction and empirical absorption correction were performed with
CrysAlisPRO [20]. Crystal structures were solved by direct methods using SHELXS and
refined on F2 by full-matrix least-squares using SHELXL [21]. All non-H atoms were refined
anisotropically. All H atoms were placed in geometrically calculated positions and refined
using the riding model with Uiso values constrained to 1.2Ueq or 1.5Ueq of the parent C atoms.
Crystallographic details for structure analysis of the compounds 5k and 5g are summarized in
Table 1. Figures were produced using MERCURY [22]. The software used for the
preparation of the materials for publication: WINGX [23], PLATON [24], PARST [25].
Table 1. Crystallographic data and structure refinement for 5k and 5g.
Identification code
5k
5g
Empirical formula
Formula weight
Color, crystal shape
Crystal size (mm3)
Temperature
C29 H25 Fe N3 O S
519.43
Red, prism
0.26 x 0.43 x 0.45
298(2)
C27 H23 Fe N3 O2.5 S
517.39
Red, irregular
0.14 x 0.27 x 0.39
293(2)

Wavelength
0.71073
0.71073
Crystal system
Space group
Orthorhombic
Pbca
Orthorhombic
Pbca
Unit cell dimensions
a (Å)
b (Å)
11.1233(2)
30.5974(6)
14.2239(2)
4841.02(15)
8
10.9102(5)
30.6164(11)
13.8706(5)
4633.2(3)
8
c (Å)
V (Å3)
Z
Dcalc (Mg/m3)
1.425
0.737
1.483
0.775
μ (mm−1)
F(000)
2160
2144
θ range for data collection (°)
Index ranges
2.66-29.11
–12<=h<=14,
28419
5781, 0.0242
99.9 %
5781 / 0 / 317
1.072
0.0460, 0.1082
0.0563, 0.1140
0.580, –0.577
2.66-26.40
–13<=h<=13,
17406
4740, 0.0509
99.9 %
4740 / 0 / 316
1.077
0.0684, 0.1881
0.0901, 0.2019
0.359, –0.692
Reflections collected
Independent reflections, Rint
Completeness to θ = 26.00°
Data / restraints / parameters
Goodness-of-fit
Final R1/wR2 indices [I > 2σ(I)]
Final R1/wR2 indices (all data)
Largest diff. peak and hole (e.Å−3)
2.4. Antimicrobial activity determination
The ability of newly synthesized compounds to inhibit the growth of selected
microorganisms was determined using microdilution method described by Sarker et al. [26],
with some modifications described in our previous study [16].
The assays with tested compounds were performed on four bacterial ATCC cultures, two
Gram-negative: Salmonella enteritidis ATCC 13076 and Escherichia coli ATCC 25922, two
Gram-positive: Enterococcus faecalis ATCC 29212 and Staphylococcus aureus ATCC
25923; and one yeast Candida albicans ATCC 10231. All used microbial strains were
obtained from the Institute of Public Health, Kragujevac, Serbia. The bacteria were cultivated
on Nutrient agar at 37 °C for 24 h, while C. albicans was grown on Sabouraud dextrose agar
at 28 °C for 48 h before the experiment.
All the assays for evaluation of antibacterial and antifungal activities were performed
according to the NCCLS recommendations [27–29]. The growth of bacteria was monitored
using the indicator resazurin and growth of C. albicans was monitored visually. Minimal
inhibitory concentration (MIC) values, as the lowest concentration of compounds without
visible bacterial or fungal growth, were expressed in mg per mL. Antibiotic Ciprofloxacin

was used as a positive control for evaluation of antibacterial activity, while Clotrimazole was
a positive control for determination of antifungal potential.
3. Results and Discussion
3.1. Synthesis
One of the most prominent protocols for the 1,3-thiazolidin-4-ones synthesis is definitely
the one–pot three–component condensation. This condensation occurs in the presence of a
suitable dehydrating agent between amines, aldehydes and α-mercapto carboxylic acids [30–
32]. Therefore, we considered to synthesize a series of novel ferrocene-containing
2-pyrazolyl-1,3-thiazolidin-4-ones and at the same time to evaluate the synthetic potential of
3-ferrocenyl-1-phenylpyrazole-4-carboxaldehyde 3 previously obtained in our laboratory
[15]. Synthetic pathway for the preparation of 3-ferrocenyl-2-pyrazolyl-1,3-thiazolidin-4-
ones 5a-l included quite simple, elegant procedure, according to which mixture of primary
amines, 3-ferrocenyl-1-phenylpyrazole-4-carboxaldehyde 3 [15], α-mercapto carboxylic
acids and dehydrating agent in the appropriate solvent was irritated in ultrasound bath at low
o
temperature (around 0 C). Therefore, the realization of this synthetic plan began with the
preparation of the ferrocene containing 1-phenylpyrazole-4-carboxaldehyde 3 [see reference
15]. In the opening experiment, we performed the reaction by ultrasonic irradiation of the
methylamine, aldehyde 3 and α-mercapto carboxylic acids in a 1:1:2 ratio, in the presence of
N,N'-dicyclohexylcarbodiimide (DCC) using tetrahydrofuran as solvent. After the usual
workup and column chromatography (SiO2, n-hexane / EtOAc = 7:3, v/v), to our delight, the
desired 1-phenylpyrazole-4-carboxaldehyde 5a was obtained in an excellent yield (91%,
Table 2, entry 1). This result was very satisfactory to us, so we accepted these reaction
conditions as the optimal ones.
Table 2. Substrate scope of synthesis of 3-ferrocenyl-2-pyrazolyl-1,3-thiazolidin-4-ones 5a-l

Entry
1
2
3
4
5
6
7
8
R
Product
Yieldsa, b (%)
Methyl
n-Butyl
n-Hexyl
n-Octyl
Benzyl
91
71
69
60
71
83
64
69
58
75
53
34
5a
5b
5c
5d
5e
5f
5g
5h
5i
2-(p-
2-Furfuryl
2-Thenyl
2-Picolyl
3-Picolyl
m-Tolyl
p-Tolyl
9
10
11
12
5j
5k
5l
aIsolated yield after column chromatography.
bYields based on the starting 3-ferrocenyl-1-phenylpyrazole-4-carboxaldehyde 3.
With the conditions in hand, we expanded our studies on eleven other primary amines
(Table 2, Entries 2-12). All condensations were performed smoothly and the corresponding
thiazolidinones 5a–l were isolated pure in good to very high yields (see Table 2). As it is
show the best results were achieved for the methyl and 4-methoxyphenethyl derivatives 5a
and 5f (see Table 2, entry 1 (91%) and 6 (83%)), contrary to isolated N-aromatic compounds
5k and 5l which were obtained in yields of 53% and 34%, respectively (see Table 2, entry 11
and 12). Obtained results clearly indicate that structure of the starting primary amine has
major influence on the reaction outcome.
In order to rationalize obtained results, we considered the reaction mechanism. The
reaction of thiazolidinones formation has been the subject of study of the several research
groups [33]. A proposed mechanism for the reaction is outlined in Scheme 2 via initial imine
formation 4a-l followed by attack of the sulfur moiety of the thioglycolic acid on the imine
carbon to form the intermediate A. The next step involves intramolecular cyclization with the
expulsion of water (caused by the presence of appropriate dehydrating agent - DCC) giving
rise to the cyclized product thiazolidinone 5a-l. Based on this mechanism, it is highly
probable that lower yields obtained for the products 5k and 5l is caused by steric limitations
of the staring substrate.

Scheme 2. Plausible schematic mechanism for the synthesis of titled compound
3.2. Spectral characterization
All the synthesized thiazolidinones 5a–l were characterized by the standard spectroscopic
1
13
techniques (IR, and H- and C-NMR). The collected data were in complete agreement with
the proposed structures.
The main common feature of the IR spectra of synthesized thiazolidinones 5a–l is two
strong bands at 1674–1694 cm-1 and 1557-1559 cm-1, which are attributed to the C=O and
1
C=N stretching, respectively. The H NMR spectra of all twelve products contain signals of
the cyclopentadienyl rings protons appear at the similar positions (δ = 4.19 ppm for the
unsubstituted, and 3.69–5.16 ppm for substituted rings, respectively). Furthermore, in the 1H
NMR spectra of compounds 5a–l characteristic signals for –N-CH2, aromatic and aliphatic
1
protons appear in the expected regions. In addition, in the H NMR spectra of 5a–l, one
signal appears at about 6.02 ppm, origin of this signal corresponds to the methine protons of
the thiazolidinone moiety. Further proof to confirm proposed structures for these compounds
was provided by the X-ray crystal structure determinations of two representative
thiazolidinones – 5k and 5g.

3.3. Crystal and molecular structures of 5k and 5g
Only 5k and 5g were obtained as single crystals suitable for X-ray diffraction studies.
Both compounds crystallized in the same centrosymmetric space group Pbca with very
similar unit cell dimensions. Their molecular structures are shown in Figures 1 and 2. In both
molecules the Cp rings adopt nearly eclipsed geometry with the C1–Cg1–Cg2–C6 torsion
angle of –2.3 and 0.9° for 5k and 5g respectively (Cg1 and Cg2 are centroids of the
substituted and unsubstituted Cp rings).
The N1–N2–C11–C12–C13 five-membered ring is almost ideally planar in both
molecules (root-mean-square deviations of the atoms which define the plane is 0.0012 and
0.0005 Å in 5k and 5g respectively). This heterocyclic ring is nearly coplanar with N1
bonded phenyl ring [Dihedral angle between two rings is 2.4(1) and 5.9(3)° in 5k and 5g
respectively]. Dihedral angle between the N1 ring and the C1–C5 ciclopentadienyl ring is
significantly different in two molecules [17.6(1) and 26.3(2)° for 5k and 5g respectively].
Another heterocyclic ring, the S1–C14–N3–C15–C16, adopts an envelope conformation with
the S1 atom displaced from the mean plane defined by remaining four atoms for 0.619(3) and
0.566(8) Å in 5k and 5g respectively. The C14, N3, C15, C16 atoms which define the mean
plane in the S1-ring are approximately coplanar (The torsion angle C14–N3–C15–C16 is
3.9(3) and 1.4 (6)° in 5k and 5g respectively).
Figure 1. Molecular structure and numbering scheme of 5k shown with atomic displacement
parameters at the 30% probability level.

Corresponding bond distances in both compounds are very similar (Table 3). There are
five N–C bonds in both molecules but only two of them (N3–C14 and N3–C23) are pure
single bonds while the remaining three could be accepted as π delocalized bonds (Table 3).
The S1 atom is bind to two sp3 hybridized carbon atoms but the S1–C14 bond is somewhat
longer than the S1–C16 (Table 3).
Although 5k and 5g do not contain any significant H-bond donor, both of them form
compact centrosymmetric dimers with molecules interconnected by two C13–H…O1i
hydrogen bonds (Fig. 3). This H-bond in both crystal structures has similar geometrical
parameters [H…O1i distance 2.40 and 2.34 Å, C13–H…O1i angle 156 and 167° in 5k and 5g
respectively; symmetry code: (i) x+1/2,−y+1/2,−z+1]. In both structures the dimer has similar
geometry (Fig. 3) and represents basic building unit in the crystal packing of two compounds.
However there is another structural characteristic that is more interesting in the packing of
both molecules: the formation of layers composed exclusively of ferrocene units. This is
illustrated in two projections in Figures S1 and S2 (see in supplementary material). One can
conclude that 5k and 5g regardless of significant dissimilarity in the N3-substituent, exhibit
very similar structural properties in solid state as formation of dimers and accumulation of
ferrocene units in separate sheets.
Figure 2. Molecular structure and numbering scheme of 5g shown with atomic displacement
parameters at the 30% probability level.
Table 3. Selected bond distances (Å) in crystal structures of 5k and 5g.
5k
1.799(3)
1.839(2)
5g
1.794(6)
1.832(5)
S1–C16
S1–C14

O1–C15
N1–C13
N1–N2
N1–C17
N2–C11
N3–C15
1.217(3)
1.355(3)
1.364(2)
1.419(3)
1.330(3)
1.354(3)
1.211(6)
1.352(6)
1.370(5)
1.418(6)
1.330(6)
1.349(6)
N3–C23
N3–C14
C1–C11
C11–C12
C12–C13
C12–C14
C15–C16
1.432(3)
1.467(2)
1.462(3)
1.422(3)
1.373(3)
1.492(3)
1.511(3)
1.447(6)
1.455(5)
1.460(6)
1.433(6)
1.354(6)
1.495(6)
1.489(8)
Figure 3. Centrosymmetric dimers of 5k (top) and 5g (bottom) formed in both crystal
structures by means of the same C13–H…O1 hydrogen bond. H atoms (which are not
involved in the formation of dimers) are omitted for clarity.

3.4. Electrochemistry
It is well known that the Fe2+/Fe3+ redox chemistry contributes to the bioactivity of
ferrocene derivatives [34]. Leaded by this fact, we decided to evaluate the electrochemical
properties of all the new compounds 5a–l. This was done by the means of cyclic voltammetry
in dichloromethane containing 0.1 mol/L tetrabutylammonium perchlorate as a supporting
electrolyte. As a representative example we choose compound 5a and the voltammograms
(collected on deferent scan rate) are presented on the Figure 4, whereas the data obtained for
all the other products are listed in the Table 4. As it has been shown in the Figure 4 (see
curves a) the 2-(3-ferrocenyl-1-phenyl-1-H-pyrazol-4-yl)-3-methylthiazolidin-4-one 5a
exhibit reversible one–electron redox couple. Oxidation wave (O1) appear at 0.684 V,
whereas the reduction one (R1) at 0.528 V, and we attributed them to the oxidation of
ferrocene unit by the forward potential sweep, and the reduction of the obtained ferricinium
ion at the back-potential sweep. These potentials are considerably more positive than the one
of the unsubstituted ferrocene, as a consequence of the presence of heterocyclic group. The
difference between those anodic and cathodic peak potentials (Table 4) is close to the
theoretical value and independent of the scan rate v. Both, the anodic (O1) and cathodic (R1)
peak currents are proportional to the square root of the scan rate, and their ratio is
independent of the scan rate, indicating a diffusion-controlled (Figure 5), also the ratio of
Figure 4. Cyclic voltammograms of 1 mM solution of 2-(3-Ferrocenyl-1-phenyl-1-H-
pyrazol-4-yl)-3-methylthiazolidin-4-one (5a) at the glassy carbon electrode by a 0.1 V s-1
scan rate in 0.1 M dichloromethane solution of Bu4NClO4: (a) ν = 100 mVs-1, (b) ν = 200
mVs-1, (c) ν = 300 mVs-1.

Table 4.
The electrochemical data for compounds 5a-l.
Compound
EO1 (V)a
0.684
0.681
0.665
0.665
0.668
0.668
0.690
0.674
0.650
0.656
0.668
0.656
ER1 (V)b
0.528
0.531
0.549
0.546
0.540
0.537
0.516
0.531
0.531
0.537
0.519
0.534
E1/2 (V)c
0.606
0.606
0.607
0.605
0.604
0.602
0.603
0.602
0.590
0.596
0.593
0.595
ΔE (V)
0.156
0.150
0.116
0.119
0.128
0.131
0.174
0.143
0.119
0.119
0.149
0.122
5a
5b
5c
5d
5e
5f
5g
5h
5i
5j
5k
5l
a EO1, potentials of anodic peaks O1 vs. Ag/AgCl, at scan rate 0.1 V s-1.
b ER1, potentials of cathodic peaks R1 vs. Ag/AgCl, at scan rate 0.1 V s-1.
c E1/2, half-wave potential of ferrocene’s redox couple vs. Ag/AgCl, at scan rate 0.1 V s-1, (E1/2 = (EO1+ER1)/2).
Figure 5. Anodic and cathodic peak currents obtained at different scan rates of 1 mM
solution of 2-(3-ferrocenyl-1-phenyl-1-H-pyrazol-4-yl)-3-methylthiazolidin-4-one (5a) at the
glassy carbon electrode in a 0.1 M dichloromethane solution of Bu4NClO4.
anodic and cathodic current was not unity and the difference between the oxidation and
reduction maxima (Ep =Epa− Epc) was 0.135.5 V, which confirms the quasi reversible nature
of the system. The electrochemical properties of compounds 5a–l are apparently interesting

and surely deserve additional investigations, but both the extent and the type of the future
investigations exceed the scope of this work.
3.5. Antimicrobial activity
The synthesized compounds were screened for antimicrobial activity against a panel of
one yeast and four bacterial species. From the results of the microdilution method (Table 5),
most of the compounds inhibited examined bacteria and C. albicans growth at the highest
tested concentration. All synthesized ferrocene derivatives displayed inhibitory activity
against the Gram-negative strain S. enteritidis at a concentration of 2 mg/mL, while four
ferrocene derivatives were unable to inhibit the growth of Gram-negative strain E. coli at the
same concentration. Except for these four compounds, all studied ferrocene derivatives
showed MIC values at a concentration of 2 mg/mL for E. coli. Gram-positive strains E.
faecalis and S. aureus were less sensitive against synthesized compounds. MIC values (2
mg/mL) of 5a, 5f, 5g, 5i, and 5l were observed for both studied Gram-positive bacteria, while
compounds 5c, 5d, 5e, and 5h were inactive against these bacterial strains, with the MIC
values exceeding 2 mg/mL. However, some of the tested compounds (5a and 5j) displayed
inhibition at 1 mg/mL against C. albicans, suggesting that this yeast is the most sensitive
microorganism to the effects of studied ferrocenyl derivatives. The well-known antibiotic
ciprofloxacin and antimycotic clotrimazole showed lower MIC values (≤ 20 μg/mL)
compared to all synthesized compounds. Generally, according to the obtained results, fungal
strain and Gram-negative bacteria seem to be more sensitive to examined compounds than
Gram-positive bacterial strains. Similar results for antimicrobial activity were obtained in our
previous study with ferrocene containing quinolones [16].
Table 5. Minimal inhibitory concentrations (MIC) of synthesized compounds
MIC (mg/mL)
Compounds
E. faecalis
S. aureus
S. enteritidis
E. coli
C. albicans
2
2
2
>2
>2
>2
>2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
1
2
5a
5b
5c
5d
5e
5f
>2
>2
>2
2
2
>2
>2
2
>2
2
>2
>2
2
>2
2
2
2
5g
5h
5i
>2
2
>2
2
2
>2
2
2
>2
>2
2
2
1
5j
2
2
2
5k
5l
2
2
>2

MIC (μg/mL)
Ciprofloxacin
Clotrimazole
12.5
12.5
-
<3.125
-
6.25
-
-
a
-
20
4. Conclusion
In conclusion, twelve novel 3-ferrocenyl-2-pyrazolyl-1,3-thiazolidin-4-ones were prepared
using standard synthetic protocols in moderate to excellent yields (up to 91%). The structures
of all the obtained products were determined by the usage of standard spectroscopic methods.
Moreover, the proposed structures of products were also confirmed by single-crystal X-ray
diffraction analysis that was done for two representative thiazolidin-4-ones (5k and 5g). It
was revealed that 5k and 5g regardless of significant dissimilarity in the N3-substituent,
exhibit very similar structural properties in solid state as formation of dimers and
accumulation of ferrocene units in separate sheets. Further, electrochemical investigations,
performed by the cyclic voltammetry, showed that all products exhibit reversible one–
electron redox couple (attributed to the ferrocene unit) at the similar potential around 200 mV
higher than the potential of unsubstituted ferrocene.
The prepared heterocycles were tested against four strains of bacteria (E. faecalis, S.
aureus, S. enteritidis, E. coli) using microdilution method. It was found that all products
show better activity on the Gram-negative than on Gram-positive bacterial strains.
Furthermore, antifungal activity of the synthesized ferrocene derivatives tested on the human
pathogen yeast C. albicans showed that the obtained compounds have more pronounced
antifungal potential. Compounds 5a and 5j had the highest antimicrobial potential suggesting
that this kind of derivatives could serve as a basis for the production of possibly more potent,
ferrocene containing antimicrobials.
Supporting Information
X-ray crystalloraphic data in CIF format and copies of 1H NMR and 13C NMR spectra.
Acknowledgement
The authors are grateful to the Ministry of Education, Science and Technological
development of the Republic of Serbia for financial support (Grant Nos. 172034 and
III43004).

Appendix A. Supplementary data
CCDC 1854965 and 1854966 contains the supplementary crystallographic
data for 5k and 5g. These data can be obtained free of
charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or
from the Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail:
deposit@ccdc.cam.ac.uk.
Supplementary data to this article (Figures S1 and S2) can be found online.
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The preparation of a new series of 3-ferrocenyl-2-pyrazolyl-1,3-thiazolidin-4-ones by
standard synthetic protocol is described.
Twelve newly synthesized compounds were full spectrally and electrochemically
characterized. The structures of two compounds were corroborated by X-ray single-
crystal analysis. Antimicrobial activity of all synthesized compounds were evaluated.