Synthesis, spectroscopic studies, and biological activity of organosilicon(iv) complexes of ligands derived from 2-aminobenzothiazole derivatives and 2-hydroxy-3-methoxy benzaldehyde

Article abstract of DOI:10.1080/10426507.2011.634465

The Schiff bases derived from the condensation of 2-aminobenzothiazole derivatives and 2-hydroxy-3-methoxybenzaldehyde and their silicon(IV) complexes with the general formula R₂Si(L)Cl (R = Et, Bu, Ph, L = 2-(2-hydroxy-3-methoxy) benzylideneam

Full text of DOI:10.1080/10426507.2011.634465

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Synthesis, Spectroscopic Studies, and
Biological Activity of Organosilicon(iv)
Complexes of Ligands Derived from 2-
Aminobenzothiazole Derivatives and 2-
Hydroxy-3-Methoxy Benzaldehyde
Jai Devi a , Suman Kumari a & R. Malhotra a
a Department of Chemistry, Guru Jambheshwar University of Science
and Technology, Hisar, Haryana, India
Version of record first published: 27 Mar 2012.
To cite this article: Jai Devi, Suman Kumari & R. Malhotra (2012): Synthesis, Spectroscopic Studies,
and Biological Activity of Organosilicon(iv) Complexes of Ligands Derived from 2-Aminobenzothiazole
Derivatives and 2-Hydroxy-3-Methoxy Benzaldehyde, Phosphorus, Sulfur, and Silicon and the Related
Elements, 187:5, 587-597
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Phosphorus, Sulfur, and Silicon, 187:587–597, 2012
Copyright ꢀC Taylor & Francis Group, LLC
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426507.2011.634465
SYNTHESIS, SPECTROSCOPIC STUDIES,
AND BIOLOGICAL ACTIVITY OF ORGANOSILICON(IV)
COMPLEXES OF LIGANDS DERIVED
FROM 2-AMINOBENZOTHIAZOLE DERIVATIVES
AND 2-HYDROXY-3-METHOXY BENZALDEHYDE
Jai Devi, Suman Kumari, and R. Malhotra
Department of Chemistry, Guru Jambheshwar University of Science
and Technology, Hisar, Haryana, India
GRAPHICAL ABSTRACT
R1
H
N
N
S
H3CO
O
Si
Cl
R
R
Abstract The Schiff bases derived from the condensation of 2-aminobenzothiazole deriva-
tives and 2-hydroxy-3-methoxybenzaldehyde and their silicon(IV) complexes with the general
formula R2Si(L)Cl (R = Et, Bu, Ph, L = 2-(2-hydroxy-3-methoxy) benzylideneaminobenzo-
thiazole) have been synthesized. These complexes have been characterized by elemental anal-
ysis, molar conductance, and spectroscopic studies including IR and NMR (1H, 13C, and 29Si)
spectroscopy. The analytical data suggest trigonal bipyramidal geometry around the silicon
atom in the resulting complexes. The ligands and their organosilicon complexes have also
been evaluated for in vitro antimicrobial activity against bacteria (Staphylococcus aureus,
Bacillus subtilis, and Escherichia coli) and fungi (Aspergillus niger and Candida albicans).
The complexes were found to be more potent as compared to the ligands.
Supplemental materials are available for this article. Go to the publisher’s online edition of
Phosphorus, Sulfur, and Silicon and the Related Elements to view the free supplemental file.
Keywords
Organosilicon complexes; spectroscopic studies; 2-aminobenzothiazole;
2-hydroxy-3-methoxybenzaldehyde
Received 13 June 2011; accepted 19 October 2011.
Authors are highly thankful to CSIR, New Delhi, for providing financial assistance through letter no.
09/752(0014)/2007-EMR-1.
Address correspondence to Jai Devi, Department of Chemistry, Guru Jambheshwar University of Science
and Technology, Hisar 125001, Haryana, India. E-mail: jaidevi gju@yahoo.com
587

588
J. DEVI ET AL.
INTRODUCTION
Organosilicon compounds with sulphur-containing ligands attract much attention
due to their wide range of biological activity. The sulphur-containing ligands as well as
their organosilicon complexes are known for their tuberculostatic, anticarcinogenic anti-
fungal, antibacterial, insecticidal, and acaricidal activities.1–6 The biological activities of
sulphur-containing ligands are considerably enhanced on complexation.7,8 The interest in
organosilicon(IV) complexes is further motivated by their versatile applicability in phar-
maceutical and chemical industries. The medical application and effectiveness of silatranes
in the treatment of wounds and tumors are related to the role of silicon in the growth of
epithelial and connective tissue and hair, where its function is to impart elasticity, strength,
and impermeability to water.9,10 It has been reported that the 2-aminobenzothiazole is a
structural unit in anti-inflammatory drugs, antibiotics, antioxidants, herbicides, and ther-
moplastic polymers.11 It acts as flavoring agent and is also present in the luciferine, where
it is responsible for the bioluminescence of fire flies.12
In the present work, the synthesis of new organosilicon complexes of Schiff bases
derived from 2-aminobenzothiazole derivatives and 2-hydroxy-3-methoxy benzaldehyde
as well as the results of a screening of these compounds for possible antimicrobial activity
is presented.
RESULTS AND DISCUSSION
Synthesis of the ligands is given in Scheme 1. The Schiff bases were prepared by con-
densation of 2-aminobenzothiazole derivatives with 2-hydroxy-3-methoxy benzaldehyde.
The purity of the compounds was checked by TLC.
R1
CHO
N
OH
N
H
S
N
EtOH
- H2O
NH2
S
+
OCH3
H3CO
OH
R1
L1H R1 = H
L2H
L3H
L4H
CH3
NO2
OCH3
Scheme 1
The reactions of diorganodichlorosilanes with the sodium salts of the Schiff bases
were carried out in 1:1 molar ratio in dry methanol. They proceed with the precipitation of
NaCl, which was removed by filtration. The complexes were isolated after the removal of
the solvent by evaporation in vacuo.
The complexes thus prepared were soluble in CDCl3, DMSO, and MeOH.
The molar conductance of 10−3 M solutions of these complexes has the low value

STUDIES OF ORGANOSILICON(IV) COMPLEXES
589
R1
R1
13
5
6
N
S
H
N
7
N
H
14
9
4
N
S
12
11
1
MeOH
8
3
+ R2SiCl2
+
NaCl
2
(1:1), reflux
H3CO
O
Si Cl
R
10
H3CO
ONa
R
R = Et, Bu, Ph
R1 = H, Me, NO2, OMe
Scheme 2
(6−20 ohm−1 cm2 mol−1), indicating the nonelectrolytic nature of the complexes. Informa-
tion about the geometry around the silicon atom in these complexes results from the FT-IR
and NMR (1H, 13C, and 29Si) spectroscopic data.
IR Spectra
The IR spectra of the ligands show a broadband at 3450–3200 cm−1 due to υ(O–H).
This band is completely absent in the spectra of the corresponding complexes indicating
coordination of the oxygen atom of ligand to the silicon atom (Table 1). A sharp band
around 1593 20 cm−1 due to υ(C N) of the free azomethine groups in the Schiff bases is
shifted to the lower frequency in the complexes indicating coordination of the azomethine
nitrogen atom to the silicon atom. The presence of silicon–oxygen and silicon–nitrogen
bonds is further evident from the appearance of new bands for υ(Si–O) at 518 15 cm−1
,
υ(Si N) at 426 10 cm−1, and υ(Si–Cl) at 442 15 cm−1. The practically unchanged
position of υ(C N) at 1623 cm−1 and υ(C–S–C) at 729 cm−1 for the thiazole ring confirms
that the thiazole moiety itself does not coordinate to the silicon atom by neither nitrogen
nor sulphur atoms.
NMR Spectra
1
The H NMR spectra of the ligands and the corresponding organosilicon(IV) com-
plexes were recorded in CDCl3 using TMS as the internal standard. Chemical shifts are
given in Table 2. The NMR spectra of the free ligands show a signal due to the OH pro-
ton at δ = 12.50 ppm, which is absent in the spectra of the complexes, indicating the
bonding of the hydroxyl oxygen atom to the silicon atom after the deprotonation of this
functional group. The sharp signal at δ = 9.27 ppm for the azomethine proton in the ligand
is shifted downfield in the spectra of the complexes indicating that the nitrogen atom of
Table 1 Characteristic IR frequencies (in cm−1) of Schiff base ligands and their metal complexes
Ligands/
υ(C N)
υ(C N)
υ(C S C)
complexes
υ(O H)
Thiazole
Azomethine
Thiazole
υ(Si O)
υ(Si N)
Si Cl
L1–4
H
3450(br)
1623–1625
1622–1625
1623–1625
1623–1626
1596–1599
1590–1595
1585–1588
1589–1594
729–731
728–731
729–732
728–730
—
—
—
Et2Si(L1–4)Cl
Bu2Si(L1–4)Cl
Ph2Si(L1–4)Cl
—
—
—
518–521
520–523
525–528
426–429
428–430
438–441
442–444
443–445
448–450

590
J. DEVI ET AL.
Table 2 1H NMR data of the ligands and their silicon(IV) complexes; δ (ppm), J (Hz)
Ligands/
complexes
N
CH OH
Aromatic-H
R1
—
Si R
L1H
9.30
12.48 7.99 (d, J = 8.1, 1H, 13-H), 7.86 (d,
J = 7.8, 1H, 10-H), 7.53 (t, J =
15.0, 1H, 6-H), 7.41 (t, J = 15.0,
1H, 4-H), 7.16 (dd, J = 7.5, 2H,
11-H, 12-H), 6.97 (t, J = 15.0, 1H,
5-H)
—
L2H
9.27
9.44
9.30
9.99
9.35
12.50 7.67–7.63 (m, 1H, 10-H), 7.28 (t, J = 2.74 (s, 3H)
6.6, 2H, 11-H, 12-H), 7.17 (dd,
J = 1.5, 1H, 5-H), 7.07 (d, J =
7.8, 1H, 4-H), 6.96 (t, J = 7.8, 1H,
6-H)
—
—
—
L3H
11.51 8.08 (d, J = 7.8, 1H, 10-H), 7.96 (d,
J = 6.0, 1H, 6-H), 7.55 (t, J =
14.1, 1H, 4-H), 7.46 (t, J = 14.7,
2H, 11-H, 12-H), 7.06 (t, J = 17.7,
1H, 5-H)
—
L4H
11.08 7.99 (d, J = 7.1, 1H, 10-H), 7.85 (d, 3.20 (s, 3H)
J = 7.5, 1H, 6-H), 7.77 (t, J = 6.8,
1H, 4-H), 7.24 (t, J = 7.9, 1H,
5-H), 7.08 (t, J = 6.7, 2H, 11-H,
12-H)
Et2Si(L1)Cl
Bu2Si(L1)Cl
—
7.99 (d, J = 8.1, 1H, 13-H), 7.60 (d,
J = 8.1, 1H, 10-H), 7.46 (d, J =
7.0, 1H, 6-H), 7.34–7.26 (m, 2H,
11-H, 12-H), 6.99 (d, J = 7.8, 1H,
4-H), 7.15 (dd, J = 9.3, 1H, 5-H)
7.98 (d, J = 8.1, 1H, 13-H), 7.71 (d,
J = 6.0, 1H, 10-H), 7.35 (t, J =
12.0, 1H, 6-H), 7.29 (d, J = 5.1,
1H, 4-H), 7.24 (d, J = 9.8, 2H,
11-H, 12-H), 6.93 (t, J = 4.7, 1H,
5-H)
—
1.41 (t, J = 7.2, 4H),
0.89 (q, J = 7.2, 6H)
—
—
0.73 (t, J = 5.6, 6H),
1.02–0.98 (m, 8H),
0.88 (t, J = 4.5, 4H)
Ph2Si(L1)Cl
9.28
—
7.96 (d, J = 8.4, 1H, 13-H), 7.84 (dd,
J = 7.4, 1H, 10-H), 7.50–7.44 (m,
1H, 11-H), 7.43–7.24 (m, 1H,
12-H), 7.13 (t, J = 6.4, 1H, 6-H),
7.07 (t, J = 6.6, 1H, 5-H), 6.95 (d,
J = 7.9, 1H, 4-H)
—
7.63–7.61 (m, 10H)
Et2Si(L2)Cl
Bu2Si(L2)Cl
Ph2Si(L2)Cl
9.34
9.40
9.40
—
—
—
7.98 (d, J = 7.5, 1H, 10-H), 7.80
(J = 3.3, 1H, 6-H), 7.20 (d, J =
1.7, 1H, 4-H), 6.99 (d, J = 4.5,
1H, 11-H), 6.95 (d, J = 4.9, 1H,
12-H), 6.75 (d, J = 7.0, 1H, 5-H)
7.23 (d, J = 4.5, 1H, 10-H), 7.16
(J = 7.0, 1H, 6-H), 6.98 (t, J =
6.9, 1H, 11-H), 6.79 (d, J = 6.1,
1H, 12-H), 6.59 (d, J = 8.0, 1H,
4-H), 6.52 (d, J = 7.6, 1H, 5-H)
2.74 (s, 3H) 1.12 (q, J = 4.5, 4H),
0.85 (t, J = 4.5, 6H)
2.74 (s, 3H) 0.76 (t, J = 5.4, 6H),
1.06–0.95 (m, 8H),
0.85 (t, J = 4.5, 4H)
8.34 (d, J = 4.6, 1H, 10-H), 7.99 (d, 2.75 (s, 3H) 7.51–7.49 (m, 10H)
J = 4.5, 1H, 6-H), 7.79 (t, J = 7.2,
1H, 11-H), 7.65 (d, J = 4.9, 1H,
12-H), 7.56 (d, J = 3.8, 1H, 4-H),
7.52 (d, J = 8.9, d, 1H, 5-H)

STUDIES OF ORGANOSILICON(IV) COMPLEXES
591
Table 2 1H NMR data of the ligands and their silicon(IV) complexes; δ (ppm), J (Hz) (Continued)
Ligands/
complexes
N
CH OH
Aromatic-H
R1
—
Si R
Et2Si(L3)Cl
9.47
—
—
—
—
7.89 (d, J = 4.7, 1H, 10-H), 7.80 (d,
J = 3.2, 1H, 6-H), 7.74 (t, J = 8.7,
1H, 11-H), 7.58 (t, J = 5.8, 1H,
12-H), 7.45 (d, J = 1.7, 1H, 4-H),
7.34 (t, 11.5, 1H, 5-H)
1.13 (q, J = 4.7, 4H),
0.97 (t, J = 4.7, 6H)
Bu2Si(L3)Cl
Ph2Si(L3)Cl
Et2Si(L4)Cl
9.45
7.98 (d, J = 4.8, 1H, 10-H), 7.85 (d,
J = 7.1, 1H, 6-H), 7.75 (t, J = 5.8,
1H, 11-H), 7.47 (t, J = 14.3, 1H,
12-H), 7.21 (d, J = 4.8, 1H, 4-H),
7.13 (t, J = 5.5, 1H, 5-H)
—
—
0.77 (t, J = 5.1, 6H),
1.02–0.99 (m, 8H),
0.86 (t, J = 4.3, 4H)
9.48
9.35
8.23 (d, J = 4.7, 1H, 10-H), 8.07 (d,
J = 3.1, 1H, 6-H), 7.99 (t, J = 5.9,
1H, 11-H), 7.87 (d, J = 7.4, 1H,
12-H), 7.61 (d, J = 5.8, 1H, 4-H),
7.21 (t, J = 11.5, 1H, 5-H)
7.34–7.31 (m, 10H)
8.08 (J = 7.3, d, 1H, C10-H), 7.99
(J = 7.9, d, 1H, C6-H), 7.80 (J =
7.8, t, 1H, C11-H), 7.56 (J = 9.1, d,
1H, C12-H), 7.45(J = 7.1, d, 1H,
C4-H), 7.38 (J = 6.9, dd, 1H,
C5-H)
3.20 (s, 3H) 1.10 (q, J = 4.9, 4H),
0.95 (t, J = 4.9, 6H)
Bu2Si(L4)Cl
Ph2Si(L4)Cl
9.37
9.38
—
—
8.29 (d, J = 7.8, 1H, 10-H), 7.95 (d,
J = 6.9, 1H, 6-H), 7.71 (t, J = 4.5,
1H, 11-H), 7.64 (d, J = 8.1, 1H,
12-H), 7.37 (d, J = 6.5, 1H, 4-H),
7.31 (t, J = 6.9, 1H, 5-H)
3.21 (s, 3H) 0.79 (t, J = 5.7, 6H),
1.02–0.98 (m, 8H),
0.88 (t, J = 4.9, 4H)
8.18 (d, J = 7.8, 1H, 10-H), 7.97 (d,
J = 7.4, 1H, 6-H), 7.86 (t, J = 7.4,
1H, 11-H), 7.76 (d, J = 8.1, 1H,
12-H), 7.58 (d, J = 5.6, 1H, 4-H),
7.45 (d, J = 5.9, 1H, 5-H)
3.20 (s, 3H) 7.74–7.70 (m, 10H)
the azomethine group is also involved in coordination. The signals for the protons of the
benzothiazole ring appear at 8.34–7.50 ppm, while the signals of the protons of the ring of
substituted aromatic aldehyde are observed at 7.40–6.50 ppm. The signal for the protons
of the methoxy group appears as a sharp singlet at 3.98 ppm in the ligands and its position
is not changed on complexation. The signals in the spectra of the complexes at 0.80–1.23,
0.80–1.90, and 7.63–7.61 ppm are caused by the protons of the ethyl, butyl, and phenyl
groups attached to the silicon atom. The values of the respective coupling constants are
given in Table 2; they are in agreement with literature values.
The 13C NMR spectra of the ligands and the corresponding organosilicon(IV) com-
plexes were recorded in CDCl3. Chemical shifts are given relative to the TMS and are com-
piled in Table 3. In the 13C NMR spectra of the free ligands, the signal of the azomethine
carbon atom appears at δ = 163.0–165.6 ppm and is shifted by 10 ppm on complexation
to the diorganosilicon(IV) dichloride, confirming the involment of azomethine nitrogen in
coordination. The signals of the CH3 and CH2 carbon atoms of the ethyl group attached to
silicon are found at 8.2–8.4 and 20.1–19.3 ppm, respectively, while the 13C NMR signals
of the n-butyl group attached to silicon are observed at 8.6, 18.3, 19.9, and 27.3 ppm.

592
J. DEVI ET AL.
Table 3 13C NMR data of the ligands and their silicon(IV) complexes; δ (ppm)
Ligands/
complexes
C
N
OCH3
Aromatic
R
Si R
L1H
163.0
165.6
164.6
163.8
163.9
55.8
117.2 (C-1), 145.4 (C-2), 133.2 (C-3), 115.7
(C-5), 113.7 (C-4), 128.1 (C-6), 190.1 (C-8),
153.9 (C-9), 125.3 (C-10), 130.0 (C-11),
132.8 (C-12), 135.7 (C-13), 155.9 (C-14)
118.3 (C-1), 146.1 (C-2), 130.4 (C-3), 110.2
(C-4), 114.1 (C-5), 124.6 (C-6), 191.1 (C-8),
150.7 (C-9), 122.0 (C-10), 126.8 (C-11),
128.6 (C-12), 130.8 (C-13), 152.0 (C-14)
118.9 (C-1), 133.9 (C-2), 128.9 (C-3), 112.0
(C-4), 117.1 (C-5), 122.9 (C-6), 192.0 (C-8),
135.2 (C-9), 121.9 (C-10), 127.1 (C-11),
128.9 (C-12), 132.9 (C-13), 140.0 (C-14)
117.9 (C-1), 144.9 (C-2), 132.8 (C-3), 113.7
(C-4), 115.1 (C-5), 125.0 (C-6), 192.1 (C-8),
152.0 (C-9), 123.1 (C-10), 126.4 (C-11),
130.8 (C-12), 133.7 (C-13), 154.4 (C-14)
118.1 (C-1), 143.9 (C-2), 134.0 (C-3), 116.3
(C-4), 112.6 (C-5), 128.9 (C-6), 191.4 (C-8),
152.9 (C-9), 126.1 (C-10), 132.1 (C-11),
132.9 (C-12), 136.9 (C-13), 154.7 (C-14)
118.9 (C-1), 145.9 (C-2), 133.8 (C-3), 114.7
(C-4), 116.3 (C-5), 128.9 (C-6), 193.0 (C-8),
153.1 (C-9), 126.1 (C-10), 135.1 (C-11),
136.0 (C-12), 138.7(C-13), 154.1 (C-14)
117.6 (C-1), 144.1 (C-2), 132.9 (C-3), 115.9
(C-4), 113.1 (C-5), 129.0 (C-6), 192.4 (C-8),
153.0 (C-9), 126.9 (C-10), 134.0 (C-11),
135.1 (C-12), 137.2 (C-13), 153.9 (C-14)
119.1 (C-1), 145.6 (C-2), 131.2 (C-3), 111.9
(C-4), 114.8 (C-5), 123.9 (C-6), 192.0 (C-8),
150.9 (C-9), 123.1 (C-10), 127.7 (C-11),
128.9 (C-12), 132.1 (C-13), 152.9 (C-14)
117.9 (C-1), 146.0 (C-2), 130.4 (C-3), 113.4
(C-4), 116.2 (C-5), 123.9 (C-6), 193.1 (C-8),
152.0 (C-9), 125.9 (C-10), 127.5 (C-11),
129.4 (C-12), 132.6 (C-13), 153.2 (C-14)
118.9 (C-1), 145.9 (C-2), 132.1 (C-3), 112.5
(C-4), 114.2 (C-5), 123.1 (C-6), 193.0 (C-8),
150.1 (C-9), 124.4 (C-10), 126.9 (C-11),
128.4 (C-12), 131.5 (C-13), 154.2 (C-14)
119.2 (C-1), 134.0 (C-2), 129.2 (C-3), 114.6
(C-4), 117.6 (C-5), 123.6 (C-6), 196.2 (C-8),
135.4 (C-9), 121.4 (C-10), 126.7 (C-11),
127.0 (C-12), 135.4 (C-13), 140.3 (C-14)
119.3 (C-1), 135.1 (C-2), 128.0 (C-3), 118.1
(C-4), 119.1 (C-5), 125.1 (C-6), 192.0 (C-8),
134.4 (C-9), 121.6 (C-10), 126.5 (C-11),
130.8 (C-12), 134.9 (C-13), 150.4 (C-14)
119.8 (C-1), 135.9 (C-2), 128.9 (C-3), 115.7
(C-4), 121.0 (C-5), 125.8 (C-6), 193.1 (C-8),
135.9 (C-9), 122.9 (C-10), 127.8 (C-11),
130.5 (C-12), 134.4 (C-13), 143.0 (C-14)
—
—
L2H
56.0
55.1
56.8
57.0
55.5
56.4
56.2
55.6
56.7
55.8
56.2
56.4
18.4
—
—
—
—
L3H
L4H
43.5
—
Et2Si(L1)Cl
8.7, 17.4
Bu2Si(L1)Cl 162.9
—
8.4, 17.9, 18.1,
26.9
Ph2Si(L1)Cl
Et2Si(L2)Cl
162.3
166.1
—
136.3, 133.6,
131.9, 128.1
19.8 8.2, 17.3
Bu2Si(L2)Cl 165.9
19.9 8.3, 17.3, 18.6,
26.9
Ph2Si(L2)Cl
Et2Si(L3)Cl
167.7
168.7
19.3 135.9, 133.6,
129.9, 127.5
—
—
—
8.2, 20.7
Bu2Si(L3)Cl 165.8
8.6, 18.3, 19.8,
27.3
Ph2Si(L3)Cl
168.3
136.9, 132.4,
131.9, 129.8

STUDIES OF ORGANOSILICON(IV) COMPLEXES
593
Table 3 13C NMR data of the ligands and their silicon(IV) complexes; δ (ppm) (Continued)
Ligands/
complexes
C
N
OCH3
Aromatic
R
Si
R
Et2Si(L4)Cl
164.4
57.3
118.4 (C-1), 145.0 (C-2), 131.6 (C-3), 115.4
(C-4), 117.9 (C-5), 126.2 (C-6), 193.7 (C-8),
153.9 (C-9), 124.1 (C-10), 128.4 (C-11),
132.5 (C-12), 135.1 (C-13), 153.7 (C-14)
120.4 (C-1), 145.3 (C-2), 133.2 (C-3), 112.9
(C-4), 122.5 (C-5), 126.3 (C-6), 193.2 (C-8),
154.0 (C-9), 124.7 (C-10), 131.8 (C-11),
133.5 (C-12), 137.0 (C-13), 158.4 (C-14)
118.9 (C-1), 144.6 (C-2), 132.5 (C-3), 113.1
(C-4), 120.6 (C-5), 127.3 (C-6), 194.1 (C-8),
153.9 (C-9), 125.4 (C-10), 130.8 (C-11),
131.6 (C-12), 135.6 (C-13), 155.7 (C-14)
44.5 8.5, 20.1
Bu2Si(L4)Cl 164.3
58.6
56.9
45.4 8.4, 17.7, 18.1,
24.9
Ph2Si(L4)Cl
165.1
45.9 137.4, 134.9,
128.3, 126.6
Signals due to the carbon atoms of the phenyl group appear in the range of 126.6–137.4
ppm. The 13C NMR signals of the carbon atoms of substituted aromatic aldehyde ring are
observed in the range of 117.2–129.0 ppm and those of the benzothiazole moiety at 193.1–
121.4 ppm.
In the 29Si NMR spectra of the complexes, a sharp signal appears in the range of
−96.2 to −98.9 ppm13 with respect to TMS indicating a pentacordinated environment
around the silicon atom with the nitrogen atom occupying equatorial position and the most
electronegative atom occupying axial position14,15. On the basis of the spectroscopic studies,
the pentacordinated structure of the complexes shown in Figure 1 has been proposed.
Antimicrobial Activity
The ligands and their complexes were tested for antimicrobial activities using a
twofold serial dilution technique. The stock solution was prepared by dissolving the com-
pounds in dry DMSO to give a concentration of 100 µg/mL. For in vitro antimicrobial
activity, these compounds were screened against gram-positive bacteria [Staphylococcus
Figure 1 Proposed structure of the silicon(IV) complexes.

594
J. DEVI ET AL.
Figure 2 Biological activity of the silicon(IV) complexes.
aureus(a) and Bacillus subtilis(b)] and gram-negative bacteria [Escherichia coli(c)] and
against fungi [Aspergillus niger(d) and Candida albicans(e)]. The incubation period of
Aspergillus niger and Candida albicans was 7 days at 25 ◦C 1 ◦C and 36 h at 37 ◦C
1
◦C, respectively, whereas for bacteria it was 24 h at 37 ◦C 1 ◦C. The conventional bacte-
ricides tetracycline and chloramphenicol and fungicide fluconazole were used as standards
for comparing the activity of the compounds (MIC values are given in Figure 2).
From the data of the antimicrobial activity it may be concluded:
(i) In vitro biocidal studies indicate that the organosilicon(IV) complexes of type
R2Si(L)Cl were more potent than their parent ligands against the same microor-
ganisms under identical experimental conditions, but due to their poor water solubility
in vivo studies remain untouched. The increased antimicrobial activity may be ex-
plained on the basis of chelation theory,16,17 according to which the polarity of the
metal ion is reduced when the positive charge of the metal ion is shared with a donor
group. The increased lipophilicity enhances the penetration of the metal complexes
into the lipid layer of cell membrane resulting in interference with normal cell process.
(ii) Among the complexes, Ph2Si(L1–4)Cl was found to be more active than Bu2Si(L1–4)Cl
and Et2Si(L1–4)Cl. This may be due to the fact that the bulkiness of substituent R
increases the lipophilicity coupled with the polarity of Si–C bond, which increases the
bioactivity of these complexes in the order Ph > Bu > Et.

595

596
J. DEVI ET AL.
(iii) The conventional fungicide and bactericide show the inhibition at the concentration
less than 3.12 ppm. None of the compounds shows a better inhibition than the conven-
tional fungicide and bactericide used. Some of the compounds have toxicity similar to
that of the conventional fungicide against Bacillus subtilis and fungi Aspergillus niger
and Candida albicans.
EXPERIMENTAL
All the experimental work was carried out under dry nitrogen using vacuum line.
All chemicals used in this work were purchased from Aldrich and used without further
purification. Solvents were purified according to the standard procedures. IR spectra were
1
obtained using a Spectrum BX Series FT-IR spectrophotometer as KBr pellets. H NMR
and 13C NMR spectra were recorded with a Bruker Avance II 400 MHz NMR spectrometer
and all chemical shifts are reported in ppm relative to TMS as an internal standard in CDCl3.
Elemental analyses were carried out on Perkin Elmer 2400 instrument. Molar conductances
were measured with a conductivity bridge type Model-306 Systronics in DMSO as solvent.
The content of silicon and chlorine were determined gravimetrically.
Synthesis of the Ligands
The Schiff bases were prepared by condensation of equimolar quantities of the respec-
tive 2-aminobenzothiazole derivative (0.001 mol) and 2-hydroxy-3-methoxy benzaldehyde
(0.001 mol) in dry ethanol as solvent. The reaction mixture was stirred at room temperature
and then refluxed for 3 h. The resulting yellowish solid thus obtained was filtered and
recrystallized from ethanol. The purity of the compounds was checked by TLC.
Synthesis of the Organosilicon(IV) Complexes
Thecomplexes werepreparedininert atmosphereusingdrynitrogen. First, thesodium
salts of the Schiff bases were prepared by allowing an equimolar ratio of the ligand and
sodium metal to react in dry methanol (15 mL). An equimolar amount of the diorganosil-
icon(IV) dichloride was then slowly added to the sodium salt of the respective ligand and
the reaction mixture was stirred for about half an hour and then refluxed for two hours.
Solid sodium chloride formed during the reaction was removed by filtration. The excess of
solvent was removed under reduced pressure to give the crude product. The compound was
repeatedly washed with dry hexane and the solid was recrystallized using a mixture of dry
methanol and dry hexane and finally dried in vaccuo. All the other organosilicon(IV) deriva-
tives of Schiff base ligands were synthesized by applying this procedure. The elemental
analyses of the complexes are given in Table 4.
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