Synthetic, spectroscopic, and biological aspects of triorganosilicon(IV) complexes of tridentate schiff bases

Article abstract of DOI:10.1080/10426507.2012.686546

Hexacoordinated organosilicon complexes of type R₃Si(L) (R = ethyl, butyl, phenyl; HL = ligand, obtained by the condensation of 4-aminoantipyrine with 2-hydroxyacetophenone, 2-hydroxybenzophenone, 2-hydroxybenzaldehyde, and 2-hydroxynapthaldehy

Full text of DOI:10.1080/10426507.2012.686546

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Synthetic, Spectroscopic, and Biological
Aspects of Triorganosilicon(IV)
Complexes of Tridentate Schiff Bases
Jai Devi a , Suman Kumari a , Sonika Asijaa a & Rajesh Malhotra a
a Department of Chemistry, Guru Jambheshwar University of Science
and Technology, Hisar, 125001, Haryana, India
Accepted author version posted online: 17 May 2012.Version of
record first published: 06 Aug 2012.
To cite this article: Jai Devi, Suman Kumari, Sonika Asijaa & Rajesh Malhotra (2012): Synthetic,
Spectroscopic, and Biological Aspects of Triorganosilicon(IV) Complexes of Tridentate Schiff Bases,
Phosphorus, Sulfur, and Silicon and the Related Elements, 187:11, 1409-1417
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Phosphorus, Sulfur, and Silicon, 187:1409–1417, 2012
Copyright ꢀC Taylor & Francis Group, LLC
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426507.2012.686546
SYNTHETIC, SPECTROSCOPIC, AND BIOLOGICAL
ASPECTS OF TRIORGANOSILICON(IV) COMPLEXES
OF TRIDENTATE SCHIFF BASES
Jai Devi, Suman Kumari, Sonika Asijaa, and Rajesh Malhotra
Department of Chemistry, Guru Jambheshwar University of Science and
Technology, Hisar 125001, Haryana, India
GRAPHICAL ABSTRACT
Abstract Hexacoordinated organosilicon complexes of type R3Si(L) (R = ethyl, butyl, phenyl;
HL = ligand, obtained by the condensation of 4-aminoantipyrine with 2-hydroxyacetophenone,
2-hydroxybenzophenone, 2-hydroxybenzaldehyde, and 2-hydroxynapthaldehyde) have been
synthesized and characterized by elemental analysis, molar conductance, and spectroscopic
studies (IR, 1H, 13C, and 29Si NMR). The spectroscopic studies indicated that the ligands
acted as tridentate coordinating through azomethine nitrogen, carbonyl oxygen, and oxygen of
hydroxyl after deprotonation to the central silicon atom. The ligands and their organosilicon
complexes have been evaluated for antimicrobial activities against fungi (Aspergillus niger
and Candida albicans), and against Gram-positive bacteria (Staphylococcus aureus, Bacillus
subtilis), and Gram-negative bacteria (Escherichia coli). The aim of the present work is to syn-
thesize novel eco-friendly fungicides and bactericides and to study the effect of the biological
activity of ligands on the complexation with organosilicon moiety.
Received 6 March 2012; accepted 7 April 2012.
The authors are highly thankful to CSIR, New Delhi for providing financial assistance through letter no.
09/752(0014)/2007-EMR-1 which was provided to one of the authors (Suman Kumari) as Senior Research
Fellowship.
Address correspondence to Rajesh Malhotra, Department of Chemistry, Guru Jambheshwar University of
Science and Technology, Hisar 125001, Haryana, India. E-mail: malhotra ksrk@yahoo.co.in
1409

1410
J. DEVI ET AL.
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; spectroscopic studies; 4-aminoantipyrine
INTRODUCTION
Tridentate and tetradentate Schiff bases ligands with donor centers like N, O, S,
etc. showing broad biological activities are of particular interest not only for existing in
the keto or enol form but also to form complexes with unusual coordination numbers. It
is, however, noteworthy that the biological activity of Schiff bases was appreciably en-
hanced on complexation with organosilicon halide,1–4 where the coordination number of
silicon increases and the tetrahedral geometry changes to trigonal bipyramidal or octahe-
dral geometries. In general, the biochemical activity of organosilicon compounds is greatly
influenced by stereochemistry of the compound and the coordination number around sil-
icon atom. Moreover, organosilicon compounds have wide range of biological activities
including anticarcinogenic, antibacterial, antifungal, tuberculostatic, insecticidal, and aca-
ricidal activities and also used in pharmaceutical and chemical industries for the synthesis
of organic materials.5–9
In continuation of our earlier research on the isolation of antimicrobial com-
pounds,10–13 an attempt has been made to synthesize complexes of Schiff base derived
from 4-aminoantipyrine and aromatic aldehydes with triorganosilane and for studying the
effect of complexation on the biocidal activity to explore the possibility of their use as
potential biocides.
RESULTS AND DISCUSSION
Organosilicon (IV) complexes have been synthesized by refluxing chlorotriorganosi-
lanes with sodium salts of Schiff base ligands in 1:1 molar ratio in dry ethanol (see
Scheme 1). The isolated complexes were insoluble in most of the organic solvent except in
CDCl3, dimethyl sulfoxide (DMSO), and dimethylformamide (DMF). The molar conduc-
tance value of 10−3 M solution of these complexes was in the range of 8–15 ꢀ−1 cm2 mol−1
in DMSO, indicating nonelectrolytic nature of complexes. The geometry around silicon
atom in these complexes has been determined using various techniques Fourier transform
infrared (FT-IR) and NMR (1H, 13C, and 29Si) spectroscopic data.
IR Spectra
The infrared spectra of the compounds were recorded using KBr pellets in order
to find the coordination sites of ligand on complexation with chlorotriorganosilane on
the basis of shifting in the frequency of various groups and/or from the lowering in the
intensities of the absorptions. The broad band centered at 3350 cm−1 in spectra of the free
ligands are due to the hydroxyl group ν(OH). The broadness of the band suggested the
presence of hydrogen bonding between the azomethine nitrogen and the OH group. This
band was completely absent in the spectra of the complexes indicating the coordination of
the ligand to the silicon atom through the hydroxyl oxygen. A sharp high-intensity band
appeared at 1645 10 cm−1. It was due to the azomethine group ν(C N) in ligands and
was shifted to a lower frequency as compared to the free ligand by 10–15 cm−1 which

TRIORGANOSILICON(IV) COMPLEXES
1411
Scheme 1
indicated the donation of the lone pair of the azomethine nitrogen and inferred that the
ligands coordinate to the silicon atom through the azomethine nitrogen. In the pyrine ring
of free ligands, sharp bands appeared at 1680 cm−1 due to the ν(C O). It was shifted by
20–30 cm−1 toward lower frequencies on complexation with the silicon atom indicating the
participation of the carbonyl oxygen atom in coordination. The formation of the resulting
complexes were further supported by appearance of new bands at 624 10 cm−1 and 410
15 cm−1 due to ν(SiO) and v(SiN). The appearance of medium to strong intensity bands
around 1470–1410 cm−1, 1150–1075 cm−1, 850–785 cm−1, and 670–630 cm−1 have been
assigned to aromatic ring.
NMR Spectra
The 1H, 13C, and 29Si NMR spectra of the ligands and their corresponding organosil-
icon(IV) complexes were recorded in CDCl3 using tetramethylsilane (TMS) as the internal
standard. The chemical shift (ppm) and coupling constant (Hz) value are given in Table 1.
The 1H NMR spectra of the free ligand showed a singlet at 15.2 ppm due to an OH proton
which completely disappeared in the complexes indicating the coordination of the hydroxyl
oxygen to the silicon atom on complexation after deprotonation of this group. The sharp
signal at 10.8 ppm in the ligand due to the azomethine proton was shifted downfield in the

1412
J. DEVI ET AL.
Table 1 1H NMR spectroscopic chemical shifts δ and coupling constants J (Hz) of ligands and organosilicon(IV)
complexes
Compound OH CH
N
H-aromatic
C-CH3, N-CH3
2.40, 3.20
Si-R
–
HL1
12.98
12.41
12.98
9.71
7.57–7.46(m, 3H), 7.42–7.38(m, 2H),
7.35(d, J = 8.2, 1H), 7.25(d, J =
8.1, 1H), 6.99(dd, J = 7.8, 2H)
7.78–7.76(m, 3H), 7.74–7.52(m, 2H),
7.39(d, J = 7.7, 1H), 7.21(d, J =
7.1, 1H), 7.11(dd, J = 9.6, 2H)
7.45–7.42(m, 3H), 7.35–7.29(m, 2H),
7.28(d, J = 12.0, 1H), 7.19(d, J =
8.1, 1H), 7.02(dd, J = 7.5, 2H)
HL2
–
–
2.41, 3.22
2.40, 3.20
2.38, 3.11
2.43, 3.21
2.41, 3.22
2.45, 3.25
2.42, 3.21
2.43, 3.20
2.40, 3.21
2.46, 3.19
2.45, 3.21
2.43, 3.20
2.45, 3.19
2.45, 3.10
2.47, 3.08
–
–
–
HL3
HL4
15.22 10.83 8.26(d, J = 1H), 7.78–7.70(m, 2H),
7.51–7.43(m, 3H),7.17(d, J =
1H), 7.40–7.25(m, 4H)
Et3Si(L1)
Bu3Si(L1)
Ph3Si(L1)
Et3Si(L2)
Bu3Si(L2)
Ph3Si(L2)
Et3Si(L3)
Bu3Si(L3)
Ph3Si(L3)
Et3Si(L4)
Bu3Si(L4)
Ph3Si(L4)
–
–
–
–
–
–
–
–
–
–
–
–
11.54 7.97–7.96(m, 3H), 7.89–7.72(m, 2H),
7.47(d, J = 7.9, 1H), 7.34(d, J =
0.86(t, 9H),
1.14(q, 6H)
8.2, 1H), 7.01(dd, J = 7.6, 2H)
12.18 7.99–7.94(m, 3H), 7.93–7.79(m, 2H),
7.54(d, J = 8.4, 1H), 7.33(d, J =
0.85(t, 9H),1.07–0.93
(m, 18H)
7.9, 1H), 7.24(dd, J = 8.2, 2H)
11.69 8.17–8.10(m, 3H), 7.92–7.77(m, 2H),
7.64(d, J = 8.9, 1H), 7.51(d, J =
7.74–7.66(m, 15H)
8.8, 1H), 7.30(dd, J = 8.1, 2H)
–
–
–
–
–
–
–
–
–
7.98–7.93(m, 3H), 7.81–7.66(m, 2H),
7.59(d, J = 12.3, 1H), 7.33(d, J =
8.4, 1H), 7.12(dd, J = 8.8, 2H)
7.89–7.86(m, 3H), 7.84–7.65(m, 2H),
7.51(d, J = 9.9, 1H), 7.42(d, J =
10.2, 1H), 7.21(dd, J = 10.8, 2H)
8.08–8.06(m, 3H), 7.94–7.71(m, 2H),
7.69(d, J = 11.7, 1 H), 7.58(d, J =
8.4. 1H), 7.38(dd, J = 9.9, 2H)
7.66–7.63(m, 3H), 7.57–7.38(m, 2H),
7.35(d, J = 8.8, 1H), 7.21(d, J =
8.5, 1H), 7.09(dd, J = 8.3, 2H)
7.89–7.82(m, 3H), 7.79–7.58(m, 2H),
7.49(d, J = 7.5, 1H), 7.55(d, J =
7.9, 1H), 7.37(dd, J = 8.9, 2H)
8.05–8.02(m, 3H), 7.95–7.74(m, 4H),
7.48(d, J = 8.6, 1H), 7.36(d, J =
7.6, 1H), 7.21(dd, J = 6.9, 2H)
8.33(d, J = 11.1, 1H), 7.86–7.65(m,
2H), 7.63–7.56(m, 3H), 7.19(d, J
= 11.1, 1H), 7.37–7.22(m, 4H)
7.81(d, J = 8.4, 1H), 7.76–7.67(m,
2H), 7.76–7.54 (m, 3H), 7.17(d,
J = 8.4, 1H), 7.42–7.33(m, 4H)
7.63(d, J = 11.4, 1H), 7.46–7.36(m,
5H),6.96–6.94(m, 4H), 6.82 (d,
J = 11.4, 1H)
0.80(t, 9H),1.10
(q, 6H)
0.82(t, 9H),1.08–1.22
(m, 18H)
7.65–7.57(m, 15H)
0.85(t, 9H),1.15
(q, 6H)
0.80(t, 9H),0.97–1.23
(m, 18H)
7.55–7.49(m, 15H)
0.81(t, 9H),1.12
(q, 6H)
0.96(t, 9H),1.32–1.19
(m, 18H)
7.34–7.23(m, 15H)

TRIORGANOSILICON(IV) COMPLEXES
1413
spectra of complexes indicating the involvement of nitrogen of azomethine group in coor-
dination. Aromatic protons of ligands appeared as multiplets in the range 8.26–6.74 ppm.
The singlets at 2.38 and 3.11 ppm due to the methyl protons of C-CH3 and N-CH3 groups,
respectively, remain unchanged on complexation. Furthermore, the formation of complexes
was supported by appearance of new signals at 0.83–1.55, 0.80–1.97, and 7.34–7.23 ppm
in the spectra of the complexes due to the ethyl, butyl, and phenyl proton attached to the
silicon atom. Integrated proton ratios were well in agreement with the proposed structure.
The 13C NMR spectra of the ligands and their corresponding organosilicon (IV)
complexes were recorded in CDCl3 with few drops of DMSO-d6 and the chemical shift
values are given in Table 2. In the 13C NMR spectra of the ligands, signal appeared due to
the azomethine carbon at 161.9 ppm was shifted downfield by 8–15 ppm on complexation
showed the involvement of this group in the coordination. The signal at 160.3 ppm due
to carbonyl carbon was shifted downfield by 5–10 ppm, confirming the involvement of
carbonyl oxygen in coordination with silicon atom. Signal due to carbon of ethyl group
attached to silicon appeared at 9.2–9.5 ppm whereas signal of carbon due to butyl group
attached to silicon atom appeared at 12.5–12.9, 13.3–14.0, 21.1–21.5, and 25.9–26.3 ppm,
respectively, while the carbon atom of the phenyl group attached to the silicon moiety
appeared at 146.9–136.9 ppm. Signals due to aromatic carbon atoms of the ligands appeared
in the range 156.9–109.1 ppm.
The 29Si NMRspectrareflect thecoordinationnumber of thenucleus inthe complexes.
In order to confirm the geometry of the complexes, 29Si NMR spectra were recorded. The
spectra showed one sharp singlet in each complex indicating the formation of a single
species. The value of chemical shift in 29Si NMR spectra depends upon the nature of R
group attached to silicon atom12 which is higher for alkyl group than aryl group because
of if R is phenyl, shielding of the 29Si nucleus is observed because the delocalized system
of phenyl group which allows for dπ-pπ interactions; thus the chemical shift is lowered
by 12–15 ppm in these complexes as compared to those which have greater σ donation
capacity, i.e., when R = alkyl group. This is well in agreement with the present isolated
organosilicon complexes, as the sharp singlet at −95 to −99 ppm was observed if R =
ethyl or butyl and −102 to −110 ppm if R = phenyl.
Antimicrobial Activity
The ligands and their corresponding complexes have been evaluated for in vitro
antibacterial activity against Gram-positive bacteria Bacillus subtilis and Staphylococcus
aureus and Gram-negative bacteria Escherichia coli and in vitro antifungal activity against
Candida albicans and Aspergillus niger fungi. The conventional bactericides tetracycline,
chloramphenicol, kanamycin, cefazoline sodium, and cefotaxime and fungicides cyclohex-
imide, carbendazim, and fluconazole were used as standards for comparing the activity
of compounds. Minimum inhibitory concentrations (MICs) were determined by means of
twofold serial dilution technique which is given in Table S1 (online Supplementary Materi-
als). Organosilicon (IV) complexes exhibited better antibacterial and antifungal properties
than those of the corresponding ligands. The complexes Ph3Si(L1) and Ph3Si(L3) were
most active antibacterial having MIC value of 3.12 µg mL−1 against E. coli and com-
plexes Ph3Si(L2) and Ph3Si(L4) having MIC value of 3.12 µg mL−1 against fungi A. niger
while complexes Bu3Si(L1), Bu3Si(L2), Bu3Si(L3), and Bu3Si(L4) were more potent against
E. coli and A. niger having MIC value in range of 6.25–25 µg mL−1. It may be concluded
that Si-Ph complexes are more potent than Si-Bu complexes. This is in agreement with our
earlier results,8 that if a bulkier substituent R is attached to the silicon atom the lipophilicity

1414

TRIORGANOSILICON(IV) COMPLEXES
1415
and, hence, the bioactivity is increased. A marked enhancement of in vitro biocidal studies of
ligands was exhibited in coordination with silicon atom against all microorganisms strains
under tested identical experimental conditions. The increase in antimicrobial activity may
be explained on the basis of fact that on chelation, the polarity of silicon atom is reduced
due to overlap of ligand orbital and sharing of positive charge of silicon atom with donor
groups. Further, it increases delocalization of chelate ring and increases the lipophilicity
of complexes.14 This increased lipophilicity enhances penetration of complexes there by
disturbing the respiration process of cell and blocking the synthesis of proteins, which
further restricts growth of organisms. The conventional fungicides and bactericides showed
the inhibition at concentration less than 3.12 ppm.
EXPERIMENTAL
Analytical grade chlorotriorganosilane, 4-aminoantipyrine, 2-hydroxybenzaldehyde,
2-hydroxyacetophenone, 2-hydroxybenzophenone, and 2-hydroxynaphthaldehyde pur-
chased from Aldrich were used as such. All the experimental work was carried out under
the dry nitrogen using a vacuum line as chlorotriorganosilanes and their complexes were
highly moisture sensitive. Solvents were purified according to the standard procedures. All
the ligands and organosilicon (IV) complexes were analyzed for C, H, and N on Perkin
Elmer 2400 elemental analyzer (Waltham, Massachusetts) while silicon was estimated
gravimetrically as silicon dioxide (SiO2). Molar conductance were measured with a
conductivity bridge type Model-306 Systronics (Mumbai, India) in DMSO as solvent. IR
spectra were recorded using Spectrum BX Series FT-IR spectrophotometer (Perkin Elmer,
New Jersey) in range 400–4000 cm−1 as KBr pellets. Nuclear magnetic resonance spectra
(1H, 13C, and 29Si) were recorded on Bruker Avance II 300 MHz NMR Spectrometer
(Bruker, Switzerland) and all chemical shifts were reported in ppm relative to TMS as an
internal standard in CDCl3.
Synthesis of Ligands
Schiff base ligands were prepared by refluxing the 4-aminoantipyrine (10 mmol)
and 2-hydroxyacetophenone/2-hydroxynaphthaldehyde (10 mmol) in the dry ethanol in
equimolar ratio for 2 h. Yellowish solids separated out at room temperature which were
filtered and recrystallized in the same solvent. The purity of the compound was monitored
through thin layer chromatography.
Synthesis of Organosilicon Complexes
Organosilicon complexes were prepared in inert atmosphere using dry nitrogen.
Sodium salts of the ligands were prepared by stirring the respective ligand (10 mmol) and
sodium metal (10 mmol) in 30 mL dry ethanol in equimolar ratio. A calculated amount of
triorganosilicon(IV) chloride was slowly added to the sodium salt of the Schiff base ligand
in 1:1 molar ratio and the resulting solution was refluxed for 3 h. The precipitate of NaCl
separated out during the course of reaction was removed by filtration. The excess solvent
was removed under vacuum to yield the desired compound which was repeatedly washed
with dry hexane. The solid was recrystallized using a mixture of dry ethanol and dry hexane
to ensure the purity of compound and finally dried under reduced pressure. The elemental
analyses of these complexes are given in Table 3.

1416

TRIORGANOSILICON(IV) COMPLEXES
1417
Antimicrobial Activity
Antimicrobial activities of ligands and their corresponding organosilicon complexes
were carried out using a twofold serial dilution technique. The stock solution was prepared
by dissolving the compounds in dry DMSO with a concentration of 100 µg mL−1. For in
vitro antimicrobial activity, these compounds were screened against Gram-positive bacteria
(S. aureus and B. subtilis) and Gram-negative bacteria (E. coli) and fungi (A. niger a◦nd
C. albicans). The incubation period of A. niger and C. albicans was 7 d at 25 ◦ 1 C
◦
and 36 h at 37 1 C, respectively, whereas for bacteria it was 24 h at 37 1 C. The
conventional bactericides tetracycline, chloramphenicol, kanamycin, cefazoline sodium,
and cefotaxime and fungicides cycloheximide, carbendazim, and fluconazole were used as
standards for comparing the activity of compounds.
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