2446 Devi et al.
Asian J. Chem.
Synthesis of the Schiff base ligands: The Schiff bases
2H2O. The Schiff-base ligands and their metal complexes are
stable at room temperature in solid state and these compounds
are generally soluble in DMF and DMSO. The colour, yield,
elemental analysis and molar conductance value of all the
compounds are presented in Table-1. The analytical data are in
agreement with the proposed stoichiometry of the complexes.
The metal:ligand ratio in the complexes was found to be 1:2.
The molar conductivity values for all the compounds in DMSO
was in the range 11-19 ohm-1 cm2 mol-1, suggesting non-elec-
trolyte nature ligand and complexes (Table-1) [22]. IR, NMR,
UV-visible and ESR data indicate the complexes of the forma-
tion of tridentate ligands.
(HL1) and (HL2) were formed by condensation of 4-methyl-
3-thiosemicarbazide (0.012 g, 1 mmol) with 2,5-dichloro-
salicylaldehyde (0.019 g, 1 mmol) and 2,5-dibromosalicy-
laldehyde (0.027 g, 1 mmol) in methanol solution. The reaction
mixture was refluxed for 3 h and the solution was filtered. The
yellow precipitate was washed several times with hot methanol
and dried under vacuum and all organic impurities were then
extracted by washing with diethyl ether. The purity of the
ligands was confirmed by thin layer chromatography and the
compositions of the ligands were confirmed by elemental
analysis and spectroscopic techniques.
Synthesis of metal complexes: The metal complexes
were prepared by adding Schiff base (HL1/HL2, 0.622/0.798,
2 mmol) to the appropriate metal salt, Cu(CH3COO)2·3H2O
(0.198 g, 1.0 mmol), Ni(CH3COO)2·7H2O (0.240 g, 1.0 mmol)
Co(CH3COO)2·4H2O (0.242 g, 1.0 mmol) and Zn(CH3COO)2·
2H2O (0.201 g, 1.0 mmol) in 20 mL aqueous methanol in 2:1
molar ratio. The reaction mixture was stirred in and heated
on a hotplate at 60 °C for 100 min. The solid precipitate was
obtained and volume of the obtained solution was reduced to
one half by evaporation and after this the coloured complexes
formed were purified by washing with hot ethanol and diethyl
ether and finally dried under vacuum (Scheme-I).
IR spectra: The IR spectra of compounds were recorded
in the range of 4000-400 cm-1 and band assignments are
reported in Table-2. The most characteristic vibrations are
selected by comparing the IR spectra of the ligands with those
of their metal complexes. The strong bands at 1595-1585 cm–1
for free ligand is due to the azomethine vibration mode,
ν(C=N) which gets shifted to the higher frequency in the range
of 1630-1608 cm–1 on complexation to metal atom. Ligands
display band at 3250-3238 cm–1 which are assigned to OH
vibration modes and this bands gets disappeared after comp-
lexation [23]. The sharp and distinct bands present in the far
infrared spectra of all complexes at 541-522 cm-1 and 457-
427 cm-1, 393-365 cm-1 provide a compelling evidence for the
presence of metal-oxygen, metal-nitrogen and metal-sulphur
bond, respectively [24].
RESULTS AND DISCUSSION
Condensation of the 2,5-dihalosalicylaldehyde with 4-
methyl-3-thiosemicarbazide readily gives the Schiff base
ligands which were identified by UV, IR, NMR, ESR and mass
spectra. Six-coordinate metal complexes were obtained from
2:1 molar ratio of Schiff base ligand with Cu(CH3COO)2·3H2O,
Ni(CH3COO)2·7H2O, Co(CH3COO)2·4H2O and Zn(CH3COO)2·
NMR spectra: 1H NMR spectra of Schiff bases and their
complexes were recorded in CDCl3 and DMSO. All chemical
shifts were reported in parts per million relative to TMS as
internal standard (Table-3). The free ligands showed singlet
at δ 8.03-8.05 ppm due to azomethine proton which gets shifted
H3C
H
HN
N
X
X
X
N
2
OH
1
6
S
OH
S
N
3
+
S
O
CH3
M(CH3COO)2.xH2O
methanol,
H2N
reflux
3 h
N
X
N
N
CHO
Cu
X
X
X
N
NH
CH3
9
4
8
H
H
2:1, stirr.
2 h
O
5
7
H
S
X= Cl,HL1
X=Br,HL2
X
N
H
NH
CH3
M = Co(II), Ni(II), Cu(II) and Zn(II)
Scheme-I: Synthesis of ligands and their metal complexes
TABLE-1
PHYSICAL AND ANALYTICAL DATA OF COMPOUNDS
Elemental analysis (%): Calcd. (found)
Yield
(%)
Ω × 10–3
Compd.
HL1
m.f.
C9H9N3OSCl2
m.w.
Colour
C
H
N
M
278.16 Yellow
613.33 Red brown
615.15 Green
618.80 Brown
620.34 White
367.00 Yellow
791.33 Red brown
793.15 Green
795.00 Brown
796.34 White
75
73
64
78
76
75
73
64
78
76
38.86 (38.78)
35.25 (35.17)
35.27 (35.19)
34.99 (35.92)
34.89 (35.82)
29.45 (29.38)
27.33 (27.21)
27.33 (27.23)
27.17 (27.11)
27.11 (27.07)
3.26 (3.17)
2.63 (2.57)
2.63 (2.57)
2.61 (2.55)
2.60 (2.54)
2.47 (4.87)
2.04 (2.01)
2.04 (1.97)
2.03 (1.96)
2.02 (1.95)
15.11 (15.07)
13.70 (13.65)
13.56 (13.48)
13.60 (13.53) 10.29 (10.22)
13.56 (13.49) 10.55 (10.49)
11.45 (4.87)
10.62 (10.55)
10.62 (10.54)
10.56 (10.51)
10.54 (10.49)
–
13
16
18
14
19
11
15
12
14
17
Co(L1)2 C18H16N6O2S2Cl4Co
9.61 (9.55)
9.57 (9.52)
Ni(L1)2
C18H16N6O2S2Cl4Ni
Cu(L1)2 C18H16N6O2S2Cl4Cu
Zn(L1)2
HL2
C18H16N6O2S2Cl4Zn
C9H9N3OSBr2
–
Co(L2)2 C18H16N6O2S2Br4Co
7.45 (7.38)
7.42 (7.35)
7.99 (7.92)
8.20 (8.13)
Ni(L2)2
Cu(L2)2 C18H16N6O2S2Br4Cu
Zn(L2)2
C18H16N6O2S2Br4Zn
C18H16N6O2S2Br4Ni
Ω × 10-3 = molar conductivity (Ohm-1 cm2 mol-1)
Devi, Jai
