Synthesis of manganese complexes of 4,5-dihydropyrazoles and metalation effect on the antimicrobial activity

Article abstract of DOI:10.14233/ajchem.2014.16428

1-Acetyl-5-aryl-3-(substituted thienyl)-4,5-dihydropyrazoles and their manganese complexes have been synthesized and characterized on the basis of elemental analysis, magnetic susceptibility, molar conductance, molecular weight determination and spectrosc

Full text of DOI:10.14233/ajchem.2014.16428

Asian Journal of Chemistry; Vol. 26, No. 19 (2014), 6443-6446
ASIAN JOURNAL OF CHEMISTRY
http://dx.doi.org/10.14233/ajchem.2014.16428
Synthesis of Manganese Complexes of 4,5-Dihydropyrazoles
and Metalation Effect on the Antimicrobial Activity
MAMTA AHUJA
Department of Chemistry, Government Meera Girls College, Mohan Lal Sukhadia University, Udaipur-313 001, India
Corresponding author: E-mail: mamdeep@rediffmail.com
Received: 26 September 2013;
Accepted: 7 January 2014;
Published online: 16 September 2014;
AJC-15936
1-Acetyl-5-aryl-3-(substituted thienyl)-4,5-dihydropyrazoles and their manganese complexes have been synthesized and characterized
on the basis of elemental analysis, magnetic susceptibility, molar conductance, molecular weight determination and spectroscopic data.
The IR data indicated that the coordination of the ligands to the manganese (II) ion was through the azomethine nitrogen and carbonyl
oxygen. The micro analytical, electronic spectral assignments and magnetic moment suggested an octahedral geometry around the metal
ion with 1:2 metal ligand stoichiometry. The complexes exhibited in vitro microbiocidal activity in comparison to ligands.
Keywords: Dihydropyrazoles, Manganese, Complexes, Microbiocidal activity.
biologically important moieties were coupled with the resulting
metals chelates having enhanced activity.
INTRODUCTION
Pyrazoles and their derivatives embedded with various
functional groups are versatile chelating agents towards metal
ions as well as important biological agents and a significant
amount of research activity has been directed towards this class.
In particular, they are used as, antitumor1, anti-antiangeogenic2,
antibacterial3, antifungal4, antiviral5, antidepressant6 and
anticonsulvant7,8 agents. Some of these compounds have also
anti-inflammatory9 properties. The recent success of pyrazole
COX-2 inhibitor has further highlighted the importance of these
heterocyclic rings in medicinal chemistry. The existence of
pyrazole core in biological active molecules has stimulated
the need for elegant and efficient ways to synthesize these
heterocyclic lead. Literature reports reveal that the activity of
a ligand can be altered several folds by coordination with
suitable metal ion10-12, apparently due to the accretion in
lipophilicity of the metal chelates13. In the present study, we
have investigated the interaction of Mn2+ with some newly
synthesized pyrazoles. The choice of manganese is promoted
by its valence electron configuration, which allow to use it in
different and unique ways that typically other elements cannot
be used in and its biological significance. Some of the major
biological roles of Mn-bound enzymes and proteins are
generation of O2 in photosynthesis, essentially involving
splitting of H2O and destroying the superoxide ion radical (by
the Mn enzyme super oxide dismutase) formed as a product
of oxidation by O2 in biological system. In order to synthesize
some microbiocidal compounds of high potency, these two
EXPERIMENTAL
Physico-chemical measurements: The 1H NMR spectra
in CDCl3 were recorded on aVarian EM390 (90 MHz) spectro-
meter in DMSO (d6) using TMS as internal reference. IR
spectra in nujol mulls on KBr plates (λmax in cm-1) were obtained
using a Perkin-Elmer 62, range 4000-200 cm-1 infrared spectro-
photometer. The electronic spectra of the complexes were
recorded on Beckmann DU-64 spectrophotometer using
DMSO as solvent. Conductance values were determined in
dry DMSO or DMF at 10-3 M concentration on a digital conduc-
tivity meter NDC 732. Molecular weight determinations were
carried out cryoscopically in dry nitrobenzene. Magnetic
susceptibilities were measured on a Sherwood magnetic
susceptibility balance, (accuracy 2 %) and carbon, hydrogen
and nitrogen analyses were performed on an automatic elemental
analyzer Carlo Erba Strumentazione model 1106 at RSIC
Chandigarh. Manganese was estimated by the atomic absor-
ption spectrophotometer (Varian Techtron, Model AA 120).
Synthesis of ligands: The ligands were synthesized by
two methods:
Conventional method
Synthesis of 3-aryl-1-(substituted thienyl)-2-propen-
1-one (1): In a solution of 0.01 mol of 2-acetylthiophene/2-
acetyl-5-chlorothiophene in methanol was added an aqueous
solution of 10-3 M sodium hydroxide to adjust the pH 10-11.

6444 Ahuja
Asian J. Chem.
To this solution was added drop wise the corresponding
aldehyde (0.01 mol) with constant stirring at 0-5 °C which
was continued for another 2-4 h for the ligands L1-L3, 14-18 h
for the ligands L4-L6. The reaction mixture was then diluted
with water and acidified with conc. sulphuric acid. The solid
thus separated was washed with water, recrystallized from
methanol and dried under reduced pressure.
RESULTS AND DISCUSSION
Substituted 3-aryl-1-(substituted thienyl)-2-propen-1-one
(1) was synthesized by the condensation of substituted 2-
acetylthiophenes and substituted aldehydes in methanolic
sodium hydroxide solution, which were assigned trans confi-
guration on the basis of the coupling constant (16 Hz) between
1
two olefinic protons appearing at 7.40 and 8.10 ppm in H
Microwave method: Equimolar quantity of both reactants
was taken in a conical flask with 5 mL of alcohol. This was kept
in microwave for 30 s on low medium temperature. Thereafter,
5 mg of NaOH was added to it and was kept in microwave for
2 min. The reaction mixture was then diluted with water and
acidified with conc. sulphuric acid. The solid thus separated
was washed with water, recrystallized from methanol and dried
under reduced pressure.
NMR spectra. The reaction of (1) with hydrazine hydrate in
acetic acid gave the ligands L1-L6 ( Fig. 1). Nucleophilic attack
is the first step involved in the formation of ligands at the
carbonyl group forming unstable α,β-unsaturated hydrazones
which underwent intramolecular rearrangement at the double
bond followed by in situ acylation due to the presence of
acetic acid as solvent14,15. In these ligands, the trans C4-H was
observed at upfield (3.29-2.53 ppm) relative to its cis analogue
(3.16-382 ppm) (Table-1). Further, J4,5-trans was found to be
smaller in magnitude (4-6 Hz) as compared to J4,5-cis (12 Hz)
and was consistent with the literature value16. The integrated
proton ratio of different groups, as evident from the spectrum
of each ligand, was well in agreement with the proposed struc-
ture for these ligands.
Synthesis of 1-acetyl-5-aryl-2-(substituted thienyl)-4,5-
dihydropyrazoles (L1-L6): A solution of 3-(4-methoxy/3,4-
dimethoxy / 3,4,5-trimethoxy-1-(2-thienyl)-2-propene-1-one
(0.01 mol) in acetic acid (25 mL) was refluxed for 10-15 h
with excess of hydrazine hydrate (1.0 g, 0.02 mol). The resul-
ting mixture was cooled and poured onto ice water. The
separated solid was washed with water and recrystallized from
methanol.
Synthesis of Mn2+ complexes: Manganese chloride
(1.43 g, 0.01 mol), dissolved in minimum amount of water
(5 mL), was added to the solution of the corresponding ligand
(0.02 mol) in ethanol (25 mL) and refluxed for 3 h. The solid
thus separated, was filtered, washed with ethanol and dried
under vacuum.
All the metal complexes obtained were coloured and stable
at atmospheric conditions. These were insoluble in most of
the common organic solvents (ethanol, acetone, ether and
benzene) except in DMSO or DMF. The low conductance
values (5-20 ohm-1 cm2 mol-1) in dry DMSO or DMF indicated
their non -electrolytic nature. Manganese(II) generally forms
high spin complexes due to the additional stability of half filled
TABLE-1
PHYSICAL, ANALYTICAL AND 1H NMR DATA OF 1-ACETYL-5-ARYL-3-(SUBSTITUTED THIENYL)-2-PYRAZOLINES
Analytical data (%): observed (calculated)
m.p.
(°C)
Yield
(%)
Ligand
m.f.
1H NMR
C
H
N
2.09 (s, 3H, COCH3), 3.60 (s, 3H,
ArOCH3), 2.53-2.83 (dd, 1H, C4-H
trans), 3.26-3.56 (dd, 1H, C4-H cis)
5.43-5.63 (dd, 1H, C5-H), 6.60-
7.36 (m,7H, Ar-H and thienyl-H).
L1
95
75
65
70
80
84
82
C16H16N2O2S
63.5(64.0)
0(5.3)
9.1(9.3)
2.39 (s, 3H, COCH3), 3.83 (s, 6H,
ArOCH3), 2.82-3.14 (dd, 1H, C4-H
trans), 3.45-3.77 (dd, 1H, C4-H cis)
5.60-5.88 (dd, 1H, C5-H) 6.95-7.63
(m,6H, Ar-H and thienyl- H).
L2
L3
L4
L5
L6
160
115
110
180
123
C17H18N2O3S
61.9(61.8)
59.9(60.0)
57.1(57.4)
55.8(56.0)
54.9(54.7)
(5.4)
8.5 (8.7
7.3(7.8)
8.2(8.4)
7.9(7.7)
7.3(7.1)
2.43(s, 3H, COCH3), 3.80 (s, 9H,
ArOCH3), 2.56-2.86 (dd, 1H, C4-H
trans), 3.33-3.63 (dd, 1H, C4-H cis)
5.46-5.66 (dd, 1H, C5-H) 6.80-7.70
(m,5H, Ar-H and thienyl- H).
C18H20N2O4S
5.2(5.5)
4.7(4.5)
4.3(4.7)
4.7(4.8)
2.16 (s, 3H, COCH3), 3.50 (s, 3H,
ArOCH3), 2.73-3.16 (dd, 1H, C4-H
trans), 3.35-3.82 (dd, 1H, C4-H cis)
5.40-5.63 (dd, 1H, C5-H) 6.95-7.76
(m,6H, Ar-H and thienyl- H).
C16H15Cl N2O2S
C17H17 Cl N2O3S
C16H16N2O2S
2.25 (s, 3H, COCH3), 3.79 (s, 6H,
ArOCH3), 2.85-3.29 (dd, 1H, C4-H
trans), 3.51-3.70 (dd, 1H, C4-H cis)
5.65-5.87 (dd, 1H, C5-H) 6.82-7.41
(m,5H, Ar-H and thienyl-H).
2.36 (s, 3H, COCH3), 3.69 (s, 9H,
ArOCH3), 2.53-2.83 (dd, 1H, C4-H
trans), 3.16-3.43 (dd, 1H, C4-H cis)
5.63-5.86 (dd, 1H, C5-H) 6.94-7.45
(m,4H, Ar-H and thienyl-H).

Vol. 26, No. 19 (2014)
Synthesis of Manganese Complexes of 4,5-Dihydropyrazoles 6445
R
R'
R '
S
CH3
Cl
H3C
N
O
N
N
Mn
Cl
N
O
N
N
O
H3C
S
S
R'
R
R
Fig. 2. Structure of complexes
LIGANDS
R
H
H
R’
L1
L4
L5
L6
L9
L10
4-OCH3
The participation of the carbonyl oxygen in bonding was
revealed by a downward spectral shift in the ν(C=O) mode
from 1620-1660 cm-1 in the spectra of ligands to 1600-1640
cm-1 in the spectral of complexes. This thiophene ring stret-
ching vibration remained essentially unaltered in the spectra
of metal complexes. This strongly suggested that the sulphur
of thiophene ring was not involved in coordination. The new
absorption frequencies appearing in the far infrared region at
490-505, 405-420 and 290-295 cm-1 in the spectra of metal
complexes were attributed to M-O, M-N and M-Cl stretching
vibration, respectively19,20. Thus, the IR studies establishment
that the coordination of the ligands to the manganese(II) ion
was through the azomethine nitrogen and carbonyl oxygen.
The magnetic and electronic spectral assignments (Table-2)
suggested an octahedral geometry around the metal ion with
1:2 metal ligands stoichiometry.
3,4-(OCH3)2
3,4,5-(OCH3)3
4-OCH3
3,4-(OCH3)2
3,4,5-(OCH3)3
H
Cl
Cl
Cl
Fig. 1. Structure of ligands
3d subshell. High spin manganese(II) has a 6A1 ground state
in the octahedral configuration. This is the only spin sextet of
the configuration and departures from the spin only value of
the magnetic moment due to spin-orbit mixing of higher states
and low symmetry components of the crystal field are expected
to be negligible. The magnetic moments are found to be very
close to the spin only value of 5.92 BM and are independent
of temperature and stereochemistry. The observed effective
magnetic moments of the manganese(II) complexes of ligands
(L1-L6) (5.95-6.12 B.M. at 25 °C) were consistent with high
spin manganese(II).
Investigation of antimicrobial activity: The synthesized
ligands and their complexes were evaluated for their antimi-
crobial activity against four phytopathogenic fungi of economic
significance, viz., Alternaria alternata, Colletotrichum capsicum,
Fusarium oxysporum and Rhizoctoia solani and two bacteria,
viz., Gram-positive Bacillus subtilis and Gram-negative
Escherichia coli. Two fold serial dilution technique21 was
employed for the in vitro growth inhibitory studies against the
test organism. The concentration at which complete growth
inhibition observed, was taken as the minimum inhibitory
concentration (MIC) value expressed in µg mL-1. The bar plots
(MIC value v/s compounds) showed the comparative activity
pattern of different ligands and their complexes. The substituted
2-pyrazolines, viz., 1-acetyl-5-aryl-2-(substituted thienyl)-2-
pyrazolines (L1-L6) exhibited promising activity against the
entire test organisms. The compounds could effectively control
the growth of organisms at concentration ranging from >100
Electronic spectra: The electronic spectra of manganese(II)
complexes differ from those of other transition metal ion comp-
lexes because of the presence of large number of bands of
lower intensity and variable width. Four electronic spectral
bands were to be observed in the complexes under study
(Table-1). These bands near 19000, 22000, 25000 and 28000
cm-1 were assigned to 6A0g →4T1g(G), 6A1g → 4T2g(G), 6A1g →
4Eg (G) and 6A1g → 4T2g (D) transitions respectively, indicating
the octahedral environment (Fig. 2) around manganese(II)17,18
.
Infrared spectra: The infrared spectra of the complexes
showed a hypsochromic shifts in the ν(C=N) mode from 1610-
1580 to 1590-1555 cm-1. The ν(N-N) manifested a positive
shift of 10-20 cm-1. The negative shift in ν(C=N) and positive
shift in ν(N-N) mode were indicative of coordination of the
nitrogen of azomethine group of pyrazolines to the metal ion.
TABLE-2
MAGNETIC AND ELECTRONIC SPECTRAL DATA OF MANGANESE(II) COMPLEXES
OF 1-ACETYL-5-ARYL-3-(SUBSTITUTED THIENYL)-2-PYRAZOLINES
4
6A1g → 4T2g (G) (cm-1)
6A1g →4Eg(G) (cm-1)
6A1g → 4T2g (D) (cm-1)
µ
eff (B.M.)
Complex
6A1g T1g (G) (cm-1)
[Mn(L1)2Cl2]
[Mn(L2)2Cl2]
[Mn(L3)2Cl2]
[Mn(L4)2Cl2]
[Mn(L5)2Cl2]
[Mn(L6)2Cl2]
19345
19390
19240
19260
19510
19315
22390
22460
22540
22670
22495
22654
25225
25300
25315
25345
25280
25315
28411
28422
28432
28511
28425
28440
5.95
6.12
6.05
6.00
6.12
6.10

6446 Ahuja
Asian J. Chem.
to 3.15 µg mL-1. Among the fungi, Alternaria alternata was
the most affected organism against which the compounds had
the least MIC values (12.5 µg mL-1). The mode of action of 2-
pyrazolines may involve the inhibition of certain specific
biochemical reactions like formation of RNA, ATP, translation
and transcription, as a consequence of which the normal
cellular processes of growth and divisions are affected22. The
presence of different functional groups on nucleus and side
chain length have marked effect on the magnitude of biological
activities. The introduction of methoxy group at 5-position of
pyrazolines ring increased the biological activity of these
compounds23. A perusal of Fig. 3 revealed that among the
variously substituted 2-pyrazolines, the compounds with 3,4,5-
trimethoxy phenyl group at position-5 and thienyl group at
position -3 of pyrazoline ring (L3 and L6) were the most active
(MIC = 12.5 µg mL-1) against Alternaria alternata and Escheri-
chia coli followed by 3,4-dimethoxyphenyl substituted 2-
pyrazolines (L2 and L4) which exhibits the MIC values up to
25 µg mL-1 and 4-methoxy substituted pyrazolines were the
least active (MIC = 50 µg mL-1). Following structure activity
order could be drawn from the activity data:
60
50
40
30
20
10
0
[Mn](L1)2Cl2
[Mn](L2)2Cl2
[Mn](L3)2Cl2
[Mn](L4)2Cl2
[Mn](L5)2Cl2
[Mn](L6)2Cl2
]
]
]
]
]
]
Alternaria Colletotrichum Fusarium
alternata capsicum oxysporum
Rhizoctoia
solani
Bacillus
subtilis
Escherichia
coli.
Fig. 4. Microbiocidal activity data of manganese complexes of 1-acetyl-
5-aryl-3-(substituted thienyl)-4,5-dihydropyrazoles
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