Vol. 30, No. 8 (2018)
Synthesis, Characterization and Antimicrobial Activities of Organosilicon(IV) Complexes 1813
1H NMR spectra: The 1H NMR spectra of the synthesized
RESULTS AND DISCUSSION
compounds were recorded in DMSO using TMS as internal
standard. In the 1H NMR spectra of the free ligands the signals
appeared at δ 8.70-9.11 ppm due to the NH proton, which
completely disappeared from the spectra of the complexes
indicating the participation of NH group.Another sharp singlet
due to azomethine proton appeared at δ 11.90-12.03 ppm
which was shifted in the complexes indicating the participation
of azomethine proton in bond formation [14]. The aromatic
protons of the ligands appeared in the range δ 7.05-8.72 ppm
and OCH3 and OCH2 protons appeared in the range δ 3.82-
5.05 ppm. The signals due to OCH3 and OCH2 groups remains
unchanged in the spectra of complexes suggested the non-
involvement of these protons in bond formation. Further forma-
tion of complexes was confirmed by the appearance of new
signals at δ 1.20-1.23, 1.18-3.10, 1.20-3.11 and 6.52-7.45 ppm
due to methyl, ethyl, butyl and phenyl protons attached to
silicon atom.
All the ligands (HL1-HL3) and their complexes were
synthesized using the same synthetic procedure as perdicted
in Scheme-I. The ligands were prepared by the reaction of
substituted vanillin derivatives with thiophene-2-carboxylic
acid hydrazide. The final organosilicon(IV) complexes were
synthesized by reaction of R2SiCl2 with the sodium salt of
Schiff base ligands in equimolar ratio in the dry methanol.
The elemental analyses data of the synthesized compounds
were well in agreement with their corresponding molecular
formulae (Table-1). The molar conductance values of these
–1
complexes in the range of 7.18–12.4 Ω cm2 mol–1 suggested
non-electrolytic nature of complexes.
Electronic spectra: The electronic spectra of ligands and
their silicon complexes were recorded in dimethyl formamide.
The Schiff base ligands HL1-3 exhibited a maxima at 394-398
nm, respectively, which may be assigned to the n-π* transition
of the azomethine group and these bands showed blue shift in
the complexes suggesting the involvement of azomethine
nitrogen in complex formation. Some medium intensity bands
were also appeared at 238, 244 and 263 nm due to π-π* transi-
tion of benzene ring of Schiff base ligands, which remains
almost unchanged on complex formation.
IR spectra: The infrared spectra of compound were recorded
in the range of 4000-400 cm–1 using KBr pellets. The IR
spectrum of the ligands showed strong bands at 1698-1690
cm–1 [12] and 3392-3371 cm–1 due to ν(C=O) and ν(N-H)
stretching, respectively. These bands were disappeared in the
spectra of complexes indicating the involvement of these
groups in bond formation with silicon after deprotonation.
Another sharp band appeared at 1568-1565 cm–1 due to
azomethine ν(C=N) group of Schiff base ligands was shifted
to lower frequency in the complexes due to the decrease in the
electron density over nitrogen of azomethine group suggested
the participation of nitrogen of this group in coordination with
silicon atom. Other new bands appeared in the range 466-453
and 561-530 cm–1 were assigned to ν(Si-N) and ν(Si-O) modes,
respectively which confirmed the involvement of oxygen and
nitrogen as donor atoms to form bond with metal [13].
13C NMR: The 13C NMR spectra of ligands exhibited
signals at 160.3-164.2 and 152.0-154.1 ppm due to carbon of
carbonyl and azomethine groups, respectively, which was
shifted to the lower values in the spectra of complexes, indi-
cating the participation of these carbons in bond formation.
The aromatic carbons of ligands were observed in the range
114.0-152.1 ppm while OCH3 and OCH2 carbons appeared
at 55.5 and 73.7 ppm, respectively. The extra signals in the
complexes due to the carbons of methyl, ethyl and n-butyl
groups attached to the silicon atom appeared in the range of
8.52-8.61 ppm, 8.47-12.89 ppm and 8.25-27.11 ppm, respec-
tively and signals due to the carbons of phenyl ring attached
to the silicon atom appeared at 128.19-129.21 ppm.
29Si NMR spectra: The 29Si NMR spectra of the comp-
lexes were recorded in CDCl3 with small amount of dimethyl
sulfoxide (DMSO-d6) and chemical shift values for all the
organosilicon(IV) complexes were appeared in the range
-74.13 to -125.31 ppm with reference to TMS, indicated penta
coordinated geometry around silicon atom [15].
Antimicrobial evaluation: The Schiff base ligands and
their silicon complexes were evaluated for in vitro antimicrobial
activity against two Gram-positive bacteria viz. Klebsiella
TABLE-1
PHYSICO-CHEMICAL CHARACTERIZATION AND ELEMENTAL ANALYSIS OF
SYNTHESIZED SCHIFF BASES AND THEIR COMPLEXES
Elemental analysis (%): Calcd. (found)
Comp.
No.
Yield
(%)
Compounds
m.f.
m/z
C
H
N
Si
HL1
HL2
HL3
C20H18N2O3S
C20H17BrN2O3S
C20H17N3O5S
366.10
444.01
411.09
460.10
488.14
516.17
584.14
538.01
566.05
594.08
662.05
505.09
533.12
561.15
629.12
81
84
78
86
85
84
87
85
74
86
77
89
78
82
89
65.55 (65.46)
53.94 (53.98)
58.38 (58.24)
57.31 (57.48)
58.94 (58.73)
60.38 (60.62)
65.68 (65.32)
48.94 (48.65)
50.75 (50.90)
52.39 (52.45)
57.88 (57.36)
52.22 (52.34)
53.97 (53.78)
55.55 (55.68)
60.99 (60.78)
4.95 (4.69)
3.85 (3.78)
4.16 (4.32)
5.47 (5.34)
5.98 (5.88)
6.43 (6.64)
5.00 (5.23)
4.48 (4.44)
4.97 (4.88)
5.41 (5.35)
4.25 (4.48)
4.78 (4.74)
5.28 (5.21)
5.74 (5.35)
4.48 (4.32)
7.64 (7.34)
6.29 (6.27)
10.21 (10.36)
6.08 (6.21)
5.73 (5.62)
5.42 (5.34)
4.79 (4.63)
5.19 (5.34)
4.93 (4.72)
4.70 (4.49)
4.22 (4.34)
8.30 (8.44)
7.87 (7.82)
7.47 (7.32)
6.67 (6.44)
–
–
–
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
Me2Si(L1)Cl
Et2Si(L1)Cl
Bu2Si(L1)Cl
Ph2Si(L1)Cl
Me2Si(L2)Cl
Et2Si(L2)Cl
Bu2Si(L2)Cl
Ph2Si(L2)Cl
Me2Si(L3)Cl
Et2Si(L3)Cl
Bu2Si(L3)Cl
Ph2Si(L3)Cl
C22H25ClN2O3SSi
C24H29ClN2O3SSi
C26H33ClN2O3SSi
C32H29ClN2O3SSi
C22H24BrClN2O3SSi
C24H28BrClN2O3SSi
C26H32BrClN2O3SSi
C32H28BrClN2O3SSi
C22H24ClN3O5SSi
C24H28ClN3O5SSi
C26H32ClN3O5SSi
C32H28ClN3O5SSi
6.09 (6.11)
5.74 (5.65)
5.43 (5.15)
4.80 (4.68)
5.20 (5.34)
4.94 (4.67)
4.71 (4.34)
4.23 (4.64)
5.55 (5.46)
5.26 (5.48)
5.00 (5.11)
4.46 (4.52)